Galileo FOC Series
Galileo navigation program: FOC (Full Operational Capability)
Galileo is a joint initiative of the European Commission (EC) and the European Space Agency (ESA). Galileo will be Europe's own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It will be inter-operable with GPS and GLONASS, the two other GNSS (Global Navigation Satellite Systems). The complete system consists of:
• A space segment of 30 MEO satellites in 3 planes inclined at 56º
• A launch segment to place the satellites into their operational orbits
• A control ground segment for monitoring and control of the satellites
• A mission ground segment managing all mission specific data
• A user ground segment of equipment capable of receiving and using Galileo signals
Figure 1: The Galileo constellation of 30 spacecraft (image credit: ESA)
The Galileo program has been structured into two phases:
1) IOV (In-Orbit Validation) phase: IOV consists of tests and the operation of four satellites and their related ground infrastructure. The first two IOV satellites were launched on Oct. 21, 2011. The second pair of IOV satellites, IOV-3 and IOV-4, were launched on Oct. 12, 2012.
2) FOC (Full Operational Capability) phase: FOC consists of the deployment of the remaining ground and space infrastructure. It includes an initial operational capability phase of 18 operational satellites. The full system will consist of 30 satellites, control centers located in Europe and a network of sensor stations and uplink stations installed around the globe.
Contract kick-off with OHB as the prime of the FOC space segment WP1 (Work Package 1) took place in late January 2010; two years later, again after highly competitive bidding and based on the performance in WP1, OHB was also able win the space segment Work Order 2 contract, thus increasing the total number of satellites to be built to 22. The Galileo FOC project is characterized by an extremely challenging schedule, which foresees a delivery of a finalized satellite every six weeks. 4)
Development of the Galileo Satellites
For the Galileo FOC development phase, OHB System of Bremen, Germany teamed with SSTL (Surrey Satellite Technology Ltd.), UK. Within this team, OHB-System is the prime contractor and is responsible for the development of the 22 spacecraft. SSTL is fully responsible for the satellite payloads. The system level activities will be led by OHB-System, making use of the experience gained by SSTL through its GIOVE-A activities. 5)
ESA is already procuring 4 satellites from Astrium through its IOV (In-Orbit Validation) program which brings the number of operational Galileo satellites now under contract to 18.
Figure 2: Illustration of the Galileo FOC spacecraft (image credit: OHB System)
Spacecraft of the FOC series:
The production of the spacecraft series, with a delivery schedule of each pair of satellites in periods of 3 months, requires an assembly line production technique to meet the time table. This can only be achieved by implementing a modular satellite design.
The FOC satellites, 22 in total, provide the same operational services as their predecessors, but they are built by a new industrial team: OHB in Bremen, Germany build the satellites with Surrey Satellite Technology Ltd in Guildford, UK contributing the navigation payloads.
Figure 3: Galileo FOC solar wing deployment being checked at ESA/ESTEC (image credit: ESA)
Legend to Figure 3: The navigation satellite's pair of 1 m x 5 m solar wings, carrying more than 2500 state-of-the-art gallium arsenide solar cells, will power the satellite during its 12 year working life. 6)
The design of the 22 Galileo FOC satellites is quite different with respect to that of their four Galileo IOV (In-orbit Verification) counterparts. For technical and cost reasons, only half of the units aboard were re-used from IOV. Part of the rationale are also more demanding requirements for FOC compared with IOV, e.g. an increased RF signal output power level, tougher radiation requirements, and harsher launch load requirements to name a few. Hence, qualification on subsystem and system level had to be done from scratch (Ref. 4).
Fulfilment of ESA's FOC satellite requirements was achieved through a simple and robust design, leading to a satellite of ~720 kg with a provided power production of 1.9 kW (end of life), which provides navigation signals in L1, E5, and E6 bands, as well as Search-and-Rescue services. The FOC satellite design is depicted in Figure 4.
Figure 4: Galileo FOC satellite design with "plug-in" propulsion module (image credit: OHB System)
A lot of OHB's development work went into optimizing the design for series production. It was identified that in order to meet the production cadence requirement of six weeks, parallelization of work on each satellite would have to be achieved. As this is very hard to achieve at late stages of integration and testing of a satellite, the focus was put in particular on the early stages of MAIT (Manufacturing, Assembly, Integration, and Testing).
The satellites are integrated in seven modules, depicted also in Figure 5:
• the propulsion module (integrated at the propulsion supplier, Moog Inc.)
• the solar generator module (integrated at the solar generator supplier)
• clock, antenna, and payload core module (integrated at SSTL, OHB's co-prime, responsible for the payload, located in Guildford, UK)
• the center and the platform core modules (integrated at OHB's premises in Bremen, Germany).
Work on these seven modules can be executed independently from each other and in parallel to each other. A good example for that is the propulsion module. While in most satellites, the propulsion system is distributed over the entire spacecraft, the modularity intended for Galileo FOC let OHB designers to mount all the propulsion-related systems on one panel, which can be integrated and replaced also late in the MAIT process (as depicted in Figure 4). The big access panel in the launch dispenser-facing side of the satellite increases the ease of access into the satellite, also at late stages of the assembly.
In the next step of integration after module integration, the seven modules form the platform and the payload (see Figure 2), which also can be treated independently from each other and in parallel to each other (payload at SSTL, platform at OHB). Final integration of payload and platform and system level tests are then also carried out at OHB. Subsequently, the integrated and tested satellites are shipped to ETS (European Test Services) in Noordwijk, The Netherlands, for the environmental test campaigns.
Development work was based on early availability of functional models of the board computers. These were used in subsystem breadboards to facilitate software development with hardware in the loop as early on as possible. Further development work was carried out in two thermal development models that focussed on the two thermally critical areas: firstly, the clock panel, where clock temperature stability was demonstrated, and secondly the area of the travelling wave tubes, where sufficient high dissipation on limited radiator area was validated.
On system level, a flat-sat engineering model was employed to check out inter-subsystem compatibility and interaction. A further payload-only engineering model was employed by SSTL at their premises for payload-level development work.
Figure 5: Illustration of satellite modules (image credit: OHB System)
MAIT (Manufacturing, Assembly, Integration, and Testing): The MAIT approach picks up on the design of the satellite and focuses on the series production and the production cadence as well. The production is based on an island mode, while the check-out equipment and ground support equipment stays in place, it is the satellites that move from station to station. The activities that are executed at a given station are trimmed to give all stations more or less the same stay duration. After that duration, all satellites move forward one station. Primary goal is to keep the flow of satellites going, meaning to avoid "clogging" the production pipeline, as this would have impact on all previous islands, which cannot turn to the next satellite in line, whereas all succeeding islands or stations would "run dry". Hence margin for trouble shouting must be taken into account. For larger issues in the production pipeline, there is a so-called "recovery island" foreseen, which is equipped with all types of ground support equipment which can handle problems that take several days or even weeks to resolve while the rest of the pipeline continues normally.
Table 2: Key parameters of the Galileo spacecraft 7)
Figure 6: The main antenna of the FM2 satellite is being inspected at ESTEC prior to mass property testing in August 2013 (image credit: ESA, Anneke Le Floc'h) 8)
Nominal orbit of the Galileo constellation: The Galileo constellation is composed of a total of 30 MEO (Medium Earth Orbit) satellites, of which 6 are spares (Figure 1). Each satellite will broadcast precise time signals, ephemeris and other data. The Galileo satellite constellation has been optimized to the following nominal constellation specifications:
- Circular orbits (satellite altitude of 23,222 km), orbital inclination of 56°, three equally spaced orbital planes.
- Eight operational satellites, equally spaced in each plane, two spare satellite (also transmitting) in each plane.
Launch: On August 22, 2014, the first two Galileo FOC satellites,FOC-1 (FM1) and FOC-2 (FM2), were launched from Kourou on the Soyuz ST-B vehicle (flight VS09), operated by Arianespace. 9)
Unfortunately, the orbit injection of the FOC spacecraft didn't occur as planned and the satellites did not reach their intended orbital position.
The liftoff and first part of the mission proceeded nominally, leading to the release of the satellites according to the planned timetable, and reception of signals from the satellites. It was only a certain time after the separation of the satellites that the ongoing analysis of the data provided by the telemetry stations, operated by ESA and the French space agency, CNES, showed that the satellites were not in the expected orbit.
The targeted orbit was circular, inclined at 56º with a semi major axis of 29,900 km. The satellites are now in an elliptical orbit, with an eccentricity of 0.23, a semi major axis of 26,200 km and inclined at 49.8º.
According to the initial analyses, an anomaly is thought to have occurred during the flight phase involving the Fregat upper stage, causing the satellites to be injected into a noncompliant orbit.
Figure 7: Photo of the Galileo FOC-3 and FOC-4 satellites fitted onto dispenser (image credit: ESA/CNES/ARIANESPACE-Service Optique CSG) 10)
Figure 8: Illustration of satellite configurations in various mission phases (image credit: OHB System)
Launch: The seventh and eighth Galileo satellites (FOC-3 and FOC-4) were successfully launched together on March 27, 2015 (21:46:18 UTC) atop a Soyuz-STB/Fregat vehicle (VS11) from Europe's Spaceport (Kourou, ELS) in French Guiana. 11) 12)
All the Soyuz stages performed as planned, with the Fregat upper stage releasing the satellites into their target orbit close to 23, 500 km altitude, around 3 hours 48 minutes after liftoff. Shortly thereafter, the two satellites sent their first signals from orbit, which were received by the CNES control center in Toulouse. 13)
Following initial checks, run jointly by ESA and France's CNES space agency from the CNES Toulouse center, the two satellites will be handed over to the Galileo Control Center in Oberpfaffenhofen, Germany and the Galileo in-orbit testing facility in Redu, Belgium for testing before they are commissioned for operational service. This is expected in mid-year.
Figure 9: Artist's view of the protective launcher fairing which jettisoned at 3 min 29 sec after launch, revealing the two Galileo satellites attached to their dispenser atop the Fregat upper stage (image credit: Arianespace, ESA)
All the Soyuz stages performed as planned, with the Fregat upper stage releasing the satellites into their target orbit close to 23,500 km altitude, around 3 hours and 48 minutes after liftoff. Shortly thereafter, they sent their first "sign of life" to ESOC (European Space Operation Center) in Darmstadt, Germany. Over the next few days, the two satellites will also be undergoing preliminary function testing.
Two further Galileo satellites are still scheduled for launch by end of 2015. These satellites have completed testing at ESA/ESTEC in Noordwijk, the Netherlands, with the next two satellites also undergoing their own test campaigns.
Next year the deployment of the Galileo system will be boosted by the entry into operation of a specially customized Ariane 5 launcher that can double, from two to four, the number of satellites that can be inserted into orbit with a single launch.
Launch: The Galileo-11 and -12 satellites (FOC-7 and FOC-8) were launched atop a Soyuz STB/Fregat rocket at 11:51:56 GMT on December 17, 2015 from Kourou, Europe's Spaceport in French Guiana. 17)
Launch: The Galileo-13 and -14 satellites (FOC-9 and FOC-10) lifted off together at 08:48 GMT on May 24, 2016 atop a Soyuz rocket from French Guiana. The twin Galileo spacecraft were deployed into orbit close to 23,522 km altitude, at 3 hours and 48 minutes after liftoff. The coming days will see a careful sequence of orbital fine-tuning to bring them to their final working orbit, followed by a testing phase so that they can join the working constellation later this year. 18)
- "Today's launch brings Europe's Galileo constellation halfway to completion, in terms of numbers," remarked Paul Verhoef, ESA's Director of the Galileo Program and Navigation-related Activities. "It is also significant as Galileo's last flight by Soyuz this year before the first launch using a customized Ariane 5 to carry four rather than two satellites each time – which is set to occur this autumn."
- Known by their nicknames Danielè and Alizée, another two Galileo FOC satellites developed and built by OHB System AG, have been successfully launched on board a Soyuz launcher, which lifted off from the Kourou Space Center in French Guiana. 19)
Launch: On November 17, 2016 (13:06 :48 UTC), a quartet of Galileo satellites (Galileo 15-18), each with a mass between 715 kg and 717 kg, and a combined liftoff mass of 2,865 kg, was launched and deployed by Ariane 5 into a circular orbit during a mission lasting just under four hours. The Ariane 5 launch, designated Flight VA233 in Arianespace's numbering system, was from the Kourou Spaceport in French Guiana. 20) 21)
Flight VA233 marked Arianespace's first use of its heavy-lift Ariane 5 to loft Galileo satellites, following seven previous missions with the company's medium-lift Soyuz. The Soyuz vehicles carried a pair of Galileo spacecraft on each flight, delivering a total of 14 navigation satellites into orbit since 2011.
The Galileo satellites are at their target altitude, after a flawless release from the new dispenser designed to handle four satellites. Over the next few days, engineers will nudge the satellites into their final working orbits and begin tests to ensure they are ready to join the constellation. This is expected to take six months or so. This mission brings the Galileo system to 18 satellites.
Figure 10: Above Earth's atmosphere, Ariane's aerodynamic fairing is jettisoned and the four Galileo satellites ‘see' space for the first time (image credit: ESA, P. Caril) 22)
Launch: On December 12, 2017, a quartet of Galileo satellites (Galileo 19-22), each with a mass between 715 kg and 717 kg, were launched on Ariane-5 ES in Kourou at 18:36 GMT (flight VA240). The first pair of satellites was released almost 3 hours 36 minutes after liftoff, while the second pair separated 20 minutes later. 23) 24)
- The satellites were released into their target 22,922 km altitude orbit by the dispenser atop the Ariane-5 upper stage. In the coming days, this quartet will be steered into their final working orbits. There, they will begin around six months of tests – performed by the European Global Navigation Satellite System Agency (GSA) – to check they are ready to join the working Galileo constellation.
- This mission brings the Galileo system to 22 satellites. Initial Services of the constellation began almost a year ago, on 15 December 2016.
- "Today's launch is another great achievement, taking us within one step of completing the constellation," remarked Jan Wörner, ESA's Director General. "It is a great achievement of our industrial partners OHB (DE) and SSTL (GB) for the satellites, as well as Thales Alenia Space (FR, IT) and Airbus Defense and Space (GB, FR) for the ground segment and all their subcontractors throughout Europe, that Europe now has a formidable global satellite navigation system with remarkable performance."
- Paul Verhoef, ESA's Director of Navigation, said: "ESA is the design agent, system engineer and procurement agent of Galileo on behalf of the European Commission. Galileo is now an operating reality, so, in July 2017, operational oversight of the system was passed to the GSA. Accordingly, GSA took control of these satellites as soon as they separated from their launcher, with ESA maintaining an advisory role. This productive partnership will continue with the next Galileo launch, by Ariane-5 in mid-2018."
- "Meanwhile, ESA is also working with the European Commission and GSA on dedicated research and development efforts and system design to begin the procurement of the Galileo Second Generation, along with other future navigation technologies."
- Next year's launch of another quartet will bring the 24-satellite Galileo constellation to the point of completion, plus two orbital spares.
• July 4, 2017: Investigators have uncovered the problems behind the failure of atomic clocks onboard satellites belonging to the beleaguered Galileo satnav system, the European Commission said on July 2. 25)
- For months, the European Space Agency — which runs the program — has been investigating the reasons behind failing clocks onboard some of the 18 navigation satellites it has launched for Galileo, Europe's alternative to America's GPS system.
- Each Galileo satellite has four ultra-accurate atomic timekeepers, two that use rubidium and two hydrogen maser. But a satellite needs just one working clock for the satnav to work — the rest are spares.
- Three rubidium and six hydrogen maser clocks were not working, with one satellite sporting two failed timekeepers.
- "The main causes of the malfunctions have been identified and measures have been put in place to reduce the possibility of further malfunctions of the satellites already in space," commission spokeswoman Lucia Caudet said.
- ESA found after an investigation that its rubidium clocks had a faulty component that could cause a short circuit, according to European sources. The investigation also found that operations involving hydrogen maser clocks need to be controlled and closely monitored, the same sources said.
- The agency has taken measures to correct both sets of problems, the sources added, with the agency set to replace the faulty component in rubidium clocks on satellites not yet in orbit and improve hydrogen maser clocks as well.
- "The supply of the first Galileo services has not and will not be affected by the malfunctioning of the atomic clocks or by other corrective measures," Caudet said, and that the malfunctions have not affected service performance.
• June 22, 2017: Europe's Galileo navigation constellation will gain an additional eight satellites, bringing it to completion, thanks to a contract signed today at the Paris Air and Space Show. - The contract to build and test another eight Galileo satellites was awarded to a consortium led by prime contractor OHB, with Surrey Satellite Technology Ltd overseeing their navigation platforms. 26)
- This is the third such satellite signing: the first four In Orbit Validation satellites were built by a consortium led by Airbus Defence and Space, while production of the next 22 FOC (Full Operational Capability) satellites was led by OHB.
- These new batch satellites are based on the already qualified design of the previous Galileo FOC satellites, except for changes on the unit level – such as improvements based on lessons learned and reacting to obsolescence of parts.
- ESA's Director of the Galileo Program and Navigation-related Activities, Paul Verhoef, signed the contract with the CEO of OHB, Marco Fuchs and OHB Navigation Director Wolfgang Paetsch, in the presence of ESA Director General Jan Woerner and the EC's Deputy Director-General for Internal Market, Industry, Entrepreneurship and SMEs, Pierre Delsaux.
• June 8, 2017: Two further satellites have formally become part of Europe's Galileo satnav system, broadcasting timing and navigation signals worldwide while also picking up distress calls across the planet. These are the 15th and 16th satellites to join the network, two of the four Galileos that were launched together by Ariane 5 on 17 November, and the first additions to the working constellation since the start of Galileo Initial Services on 15 December. 27)
- The growing number of Galileo users around the world will draw immediate benefit from the enhanced service availability and accuracy brought by these extra satellites.
- The launch into space and the maneuvers to reach their final orbits still left a lot of rigorous testing before the satellites could join the operational constellation.
- Their navigation and search and rescue payloads had to be switched on, checked and the performance of the different Galileo signals assessed methodically in relation to the rest of the worldwide system.
- This lengthy testing saw the satellites being run from the second Galileo Control Center in Oberpfaffenhofen, Germany, while their signals were assessed from ESA's Redu center in Belgium, with its specialized antennas. The tests measured the accuracy and stability of the satellites' atomic clocks – essential for the timing precision to within a billionth of a second as the basis of satellite navigation – as well as assessing the quality of the navigation signals.
- Oberpfaffenhofen and Redu were linked for the entire campaign, allowing the team to compare Galileo signals with satellite telemetry in near-real time. — Making the tests even more complicated, the satellites were visible for only three to nine hours a day from each site.
- The satellites are now broadcasting working navigation signals and are ready to relay any Cospas–Sarsat distress calls to regional emergency services.
- Now that these two satellites are part of the constellation, the remaining pair from the Ariane 5 launch is similarly being checked to prepare them for service.
• June 02, 2017: Europe's Galileo satellite navigation system has undergone its first performance report since it started work at the end of last year – passing with flying colors. The European GNSS Agency, GSA, through its GNSS Service Centre, has published the first of its regular quarterly performance reports on Galileo. This ‘European GNSS (Galileo) Initial Services Open Service' report, now available online, covers the first three months of 2017, and documents the good performance of Galileo Initial Services to date. 28)
- The report shows the 11 satellites then operating in the Galileo constellation were able to provide healthy signals 97.33% of the time on a per satellite basis, with a ranging accuracy better than 1.07 m and disseminating global UTC time within its signal to within 30 billionths of a second on a 95 percentile monthly basis.
Table 3: Galileo reported constellation information
- "Galileo Initial Services were declared by the European Commission on 15 December 2016," comments Joerg Hahn of ESA's Galileo System Office. "It was thanks to the tremendous effort of ESA's Galileo team working closely together with colleagues from the Commission and GSA that this milestone could be achieved: the key pillars for reaching are the currently deployed Galileo satellites in combination with the global Galileo ground segment infrastructure, defined and implemented by the ESA team with their respective industry partners."
- The Initial Service performance levels (Figure 11) achieved by the system are monitored using two complementary monitoring platforms: the Time and Geodetic Validation Facility, an independent precision time-measuring system accurate to a billionth of a second – using an ensemble of atomic clocks located at ESA/ESTEC in Noordwijk, the Netherlands – and the GALSEE (Galileo System Evaluation Equipment), based in Rome, Italy.
Figure 11: SISE constellation 95%, March 2015-April 2017 (image credit: EC, ESA)
- In the future, the independent monitoring of the services will be carried out by GSA's Galileo Reference Centre, currently taking shape beside ESTEC in Noordwijk. The results for the first quarter of 2017 show the measured performances are generally far better than the minimum performance levels identified in the Service Definition Documents.
- "Looking back over the ranging accuracy of the Galileo constellation from the time of the very first positioning fix in 2014 to the present, the overall performance trend for the Open Service is very positive," adds Joerg Hahn. "It has reached values of less than 1 m in recent months, being already competitive with other satellite navigation systems. The high-quality ranging service enables user level positioning with a typical accuracy of around 3 m on the ground and 5 m in altitude during periods when four satellites are visible. With the limited infrastructure so far deployed, current horizontal position fixes can be achieved during more than 80% of the time with accuracies better than 10 m. This user level performance is expected to improve with the launch of more satellites making the provided Galileo services more accurate, more available and more robust for end users."
• March 28, 2017: Eutelsat and GSA (GNSS Agency) Europe signed an 18-year contract covering the preparation and service provision phases for the EGNOS (European Geostationary Navigation Overlay Service) GEO-3 payload that will be hosted on the Eutelsat 5 West B satellite that is due for launch late in 2018. The new payload marks a replenishment of current EGNOS capacity and is scheduled to start service in 2019 for 15 years. 29)
- EGNOS V3, the second generation of the EGNOS System, will implement a second protected frequency (L5) to offer to the dual frequency safety of life users a more robust and accurate vertical guidance service (increased robustness with respect to the ionosphere), according to the ESA's EGNOS V3 Phases C/D - Summary Statement of Work. Version 3 of EGNOS will add a second frequency (L5) and a second GNSS constellation (Galileo) to the GPS L1 corrections currently being provided by EGNOS.
- With the deployment of Galileo and the introduction of new capabilities in GPS, EGNOS V3 will offer improved SoL (Safety of Life) services to the civil aviation community as well as potential new applications for maritime or land users, thus showcasing the system's increased potential to become a leading edge GNSS system in the future.
- EGNOS operational messages are currently broadcast via navigation payloads on-board two GEO satellites, including an Inmarsat-3F2 satellite that is fast approaching end-of-life. The GEO-3 services will replenish the EGNOS SBAS payloads, guaranteeing EGNOS SIS availability and supporting the transition to the dual-frequency multi-constellation-capable EGNOS V3.
• February 28, 2017: Vidal Ashkenazi, a leading authority and advocate of Europe's Galileo satellite navigation system has been named as an Officer of the Order of the British Empire by Her Majesty Queen Elizabeth II for services to science. 30) 31)
- Vidal Ashkenazi is an expert in geodesy, the science of Earth measurement, as well as satellite navigation, with a 33 year academic career at the University of Nottingham in the UK.
- As the founding Director of the Institute of Engineering Surveying and Space Geodesy, Prof. Ashkenazi has supervised about 50 PhD students, most of whom occupy senior positions in industry and academia worldwide. In 1998 he left the University to found Nottingham Scientific Ltd, a company that specializes in Global Navigation Satellite System critical applications, involving safety, security and national policy.
- In 1976 he was invited by the US National Academy of Science to come to the US National Geodetic Survey to help develop a standardized coordinate system for mapping and satellite navigation. He contributed to the development of a standard geodetic coordinate system for satellites, known as WGS84 (World Geodetic System 1984), today in common global use.
- In the late 1990s this led to the resurveying of all the major airports around the world to the WGS84 standard. At the request of Eurocontrol, he was directly involved in carrying out this task across the UK and in continental Europe.
- Prof. Ashkenazi also played an important role as Galileo began to take shape, for example by leading the group of academic experts in the European Commission's GNSS-2 Forum in 1998, gathering together all the major European experts in the field. His direct advocacy proved influential, when in 2003 he addressed the European Parliament's Industry, External Trade, Research and Energy Committee on why Europe needed its own satellite navigation system.
- Drawing on his transatlantic contacts, he was invited to the US Department of Commerce to address a senior US government and industrial audience on Galileo. He did this by giving a presentation entitled "Galileo: Friend or Foe?" promoting an approach of partnership rather than competition between GPS and Galileo that has indeed come to pass.
- In 2005, the then Director of Navigation at ESA invited Prof. Ashkenazi and his company to make the first study on Galileo evolution, known today as the post-2020 Galileo Second Generation.
• January 19, 2017: ESA is reporting that anomalies have been noted in the atomic clocks serving Europe's Galileo satellites. Anomalies have occurred on five out of 18 Galileo satellites in orbit, although all satellites continue to operate well and the provision of Galileo Initial Services has not been affected. 32)
- Highly accurate timing is core to satellite navigation. Each Galileo carries four atomic clocks to ensure strong, quadruple redundancy of the timing subsystem: two RAFS (Rubidium Atomic Frequency Standard) clocks and two PHM (Passive Hydrogen Maser) clocks. - The current Galileo constellation consists of 18 satellites in orbit, adding up to a total of 36 RAFS clocks and 36 PHM clocks.
- RAFS clocks: In recent months a total of three RAFS clocks unexpectedly failed on Galileo satellites – all on FOC (Full Operational Capability) satellites, the latest Galileo model. These failures all seem to have a consistent signature, linked to probable short circuits, and possibly a particular test procedure performed on the ground, with investigations continuing to identify a root cause.
- No RAFS clock failures have occurred aboard the four Galileo IOV (In Orbit Validation) satellites, the original Galileo model. In addition the RAFS clock on ESA's very first test navigation satellite, GIOVE-A launched in 2005, has been checked, and was reactivated successfully.
- Continuing investigations on the ground have identified potential weaknesses in the RAFS clock design, but no root cause has yet been yet established.
Figure 12: Photo of the Galileo rubidium clock (image credit: ESA, Temex)
- PHM clocks: In the past two years, there have been five PHM clock failures on the IOV satellites and one PHM failure on the FOC satellites. These failures are better understood, linked to two apparent causes. One is a low margin on a particular parameter that leads, on some units, to a failure. The second is related to the fact that when some healthy PHM clocks are turned off for long periods, they do not restart because of a change in clock characteristics in orbit. To date, two PHM clocks have failed owing to the first mechanism, and four to the second.
Figure 13: Photo of the Galileo PHM (Passive Hydrogen Maser) clock (image credit: ESA)
- Corrective actions: For the remaining 33 RAFS clocks in orbit, the risk of failure is believed to be lower owing to different testing procedures on the ground before launch. In addition, new operational measures have been put in place to further mitigate the risk. All these measures have no effect on Galileo's overall performance.
- While investigations by ESA and its industrial partners are continuing, there is consensus that some refurbishment is required on the remaining RAFS clocks still to be launched on the eight Galileo satellites being constructed or tested, and awaiting launch.
- For the remaining 30 PHM clocks working in orbit, operational procedures are being studied to significantly reduce the risk of future failure. These measures are being validated, ahead of their planned introduction in a few weeks.
- Looking forward: Overall, three out of four IOV satellites have experienced clock anomalies, and two out of 14 FOC satellites. — As ESA Director General Jan Woerner commented during his 18 January press briefing, no individual Galileo satellite has experienced more than two clock failures, so the robust quadruple redundancy designed into the system means all 18 members of the constellation remain operational. This includes one satellite that supports only the Open Service for mass market applications, and two satellites in elliptical orbits that are nevertheless expected to be reintegrated into the full constellation for use from these orbits.
- Similarly, Galileo's Initial Services, which began on 16 December, have been unaffected by these anomalies.
- The impact of RAFS and PHM clock refurbishment on Galileo's launch schedule is under study, but ESA is confident that the clock issues will be resolved and remains committed to launch the next four Galileo FOC satellites before the end of this year.
• January 17, 2017: Brad Parkinson, hailed as the father of GPS, has visited ESA's technical heart to meet the team behind Europe's Galileo satellite navigation system. — Brad Parkinson was awarded the 2016 Marconi Prize for his part in developing satellite navigation. In 1972, then a US Air Force Colonel, he was put in charge of ‘Program 621B', which became the Global Positioning System. Over one long September weekend in 1973 he and his team decided all key GPS elements. The first satellite was launched in February 1978. 33)
- Paul Verhoef, ESA's Director of the Galileo Program and Navigation-related Activities, invited Prof. Parkinson to ESA's facility in the Netherlands to address the Directorate's annual gathering on 11 January. Also present were members of the European Global Navigation Satellite System Agency – set to oversee newly operational Galileo services – and the European Commission.
- Brad Parkinson congratulated the Galileo team on their achievement: "Back around the turn of the century, there were elements in the US government who were extremely opposed to Galileo. I didn't see it that way. Instead, I regarded it as largely inevitable, and that another constellation of reliable navigation satellites would be of enormous value to users across the entire world. That's really the noble goal behind all this, not GPS or Galileo individually, but PNT – providing position, navigation and timing services to everyone.
- "I was privy to some early discussion on Galileo, and there were some Europeans saying they didn't want anything like GPS. But there were a number of experts pointing out all the time and effort that had gone into defining GPS and frankly they responded that the GPS system was pretty near optimal. — Today, all four current constellations work much the same way. Even the Russians, with GLONASS, who went with a different signal structure – using FDMA (Frequency Division Multiple Access) with separate satellites using different frequencies – are moving towards the CDMA (Code Division Multiple Access) used by the other systems, meaning different satellites use the same frequencies with coding to differentiate them."
- GPS and Galileo utilize complementary signal modulations to enable seamless combined use. Negotiations are under way to provide the US government with access to Galileo's PRS (Public Regulated Service), the most precise and secure class of the Galileo signal.
- Satellite navigation has come far from its military origin: there are billions of satnav receivers in use today. Brad Parkinson is not surprised: as a Professor at California's Stanford University, he and his students pioneered many applications, including the first GPS-steered tractor for precision agriculture, and a blind GPS-guided aircraft landing back in 1992.
- "There are misapprehensions on this, but from day one GPS was conceived as a military/civil system – I testified to Congress accordingly back in 1975."
- "The sheer diversity of uses has been surprising. Surveyors were early adopters, with land surveying that used to take days or weeks being performed within hours, while geologists have gained enormous understanding of earthquakes and the like by measuring ground motion of a few millimeters annually."
- "Looking ahead, within 15 years many human-operated vehicles – automobiles, trucks, aircraft and ships – will be self-driving, with one essential element being satellite navigation." - One pleasant surprise has been the extremely precise ranging achieved through ‘realtime kinematics' and other advanced signal processing methods.
- But Brad Parkinson warned that such achievements will be at risk if adjacent radio frequencies are turned over to terrestrial users, potentially leading to overlapping interference several billion times stronger than the faint satellite signals.
- "It's a creeping obligation, internationally, to defend the radio spectrum in order to assure PNT (Position Navigation and Timing) to users worldwide, in order to nurture and support new uses of satellite navigation."
Figure 14: Photo of Brad Parkinson (image credit: B. Parkinson)
• December 15, 2016: Europe's Galileo satellite navigation system has entered its initial operational phase, offering positioning, velocity and timing services to suitably equipped users around the globe. Today, the European Commission, owner of the system, formally announced the start of Galileo Initial Services, the first step towards full operational capability. 34)
- After five years of launches there are now 18 satellites in orbit. The most recent four, launched last month, are undergoing testing ahead of joining the constellation next spring. The full Galileo constellation will consist of 24 satellites plus orbital spares, intended to prevent any interruption in service.
- ESA Director general Jan Woerner noted, "For ESA, this is a very important moment in the program. We know that the performance of the system is excellent. The announcement of Initial Services is the recognition that the effort, time and money invested by ESA and the Commission has succeeded, that the work of our engineers and other staff has paid off, that European industry can be proud of having delivered this fantastic system."
- Paul Verhoef, ESA's Director of the Galileo Program and Navigation-related Activities, added, "Today's announcement marks the transition from a test system to one that is operational. We are proud to be a partner in the Galileo program. Still, much work remains to be done. The entire constellation needs to be deployed, the ground infrastructure needs to be completed and the overall system needs to be tested and verified. In addition, together with the Commission we have started work on the second generation, and this is likely to be a long but rewarding adventure."
- Initial Services: Galileo is now providing three service types, the availability of which will continue to be improved. Galileo Initial Services are a result of cooperation between the EC (European Commission), GSA (GNSS Supervisory Agency, Europe), and ESA (European Space Agency).
1) The Open Service is a free mass-market service for users with enabled chipsets in, for instance, smartphones and car navigation systems. Fully interoperable with GPS, combined coverage will deliver more accurate and reliable positioning for users.
2) Galileo's PRS (Public Regulated Service) is an encrypted, robust service for government-authorized users such as civil protection, fire brigades and the police.
3) The SAR (Search and Rescue) Service is Europe's contribution to the long-running COSPAS–SARSAT international emergency beacon location. The time between someone locating a distress beacon when lost at sea or in the wilderness will be reduced from up to three hours to just 10 minutes, with its location determined to within 5 km, rather than the previous 10 km.
- Finding your way: Like all satnav systems, Galileo operations rely on the extremely precise measurement of time – around 10 billionths of a second on average. Because all electromagnetic waves, including radio, travel at a fixed speed – just under 30 cm each billionth of a second – the time it takes for Galileo signals to reach a user receiver yields distance measurements. All the receiver has to do is multiply the travel time by the speed of light. — A minimum of four satellites must be visible to pinpoint position: one each to fix latitude, longitude and altitude, with another to ensure synchronized timings. More satellites provide a greater level of service coverage and precision.
Figure 15: The complete Galileo constellation will consist of 24 satellites along three orbital planes, plus two spare satellites per orbit. The result will be Europe's largest-ever fleet, providing worldwide navigation coverage (image credit: ESA, P. Carril)
• December 8, 2016: Galileo satellites 13 and 14 have begun transmitting navigation signals as fully operational members of Europe's satnav constellation. The two satellites were launched together from Europe's Spaceport in French Guiana on 24 May. Their flight into space, and subsequent maneuvers to reach their final orbital altitude, was only the start of their quest to join the operational constellation. 35)
- Next, their navigation and search and rescue payloads were methodically switched on, checked out and their performance assessed in relation to the rest of the worldwide Galileo system. This lengthy test phase saw the satellites being run from the second Galileo Control Center in Oberpfaffenhofen, Germany, while their payloads' output was assessed from ESA's Redu center in Belgium, equipped for the tests with specialized antennas for receiving and uplinking signals. The test campaign measured the accuracy and stability of the satellites' atomic clocks – essential for the timing precision to within a billionth of a second as the basis of satellite navigation – as well as assessing the quality of the navigation signals.
- Oberpfaffenhofen and Redu were linked for the entire campaign, allowing the team to compare Galileo signals with satellite telemetry in near-real time. These two satellites were visible in the sky above Redu for a limited time each day, ranging from three to nine hours, so tests were scheduled accordingly. Now that in-orbit testing is completed, the satellites are transmitting working navigation signals and are ready to relay any Cospas-Sarsat distress calls to emergency services.
- The next four satellites, launched together on 17 November, are beginning the same in-orbit testing activity, with the aim of joining the network next spring.
• December 5, 2016: With Europe's Galileo satnav constellation soon to provide initial services, ESA is looking further ahead: its next-stage navigation research program received strong backing during last week's Council at Ministerial level. 36)
- In partnership with the EU (European Union), ESA has overseen the creation of two satnav systems: first EGNOS, which improves the precision of US GPS signals over most European territory, in general operation since 2009 and for ‘safety of life' uses since 2011; and now Galileo, with initial services due to be declared soon. Both program are on a steady footing, with their future construction and evolution being supported through the EU's Global Navigation Satellite System and Horizon 2020 Programs.
- Meanwhile, ESA's Directorate of the Galileo Program and Navigation-related Activities has put together NAVISP (Navigation Innovation and Support Program), which will apply ESA's hard-won expertise from Galileo and EGNOS to new satellite navigation and, more widely, positioning, navigation and timing challenges. — ESA Director General Jan Woerner won strong Member State backing for the optional NAVISP during last week's Ministerial Council in Lucerne, Switzerland.
- NAVISP will boost Member State industrial competitiveness and innovation priorities in the upstream and downstream navigation sector and it will include investigating the integration of satellite navigation with non-space technologies and complementary positioning and communication techniques.
- NAVISP is structured into three elements, with the first developing new satnav technologies and concepts, the second focused on industrial competitiveness and the third offering support to Member State national programs and activities.
- In a world where satnav-based positioning, navigation and timing services are becoming ubiquitous – underpinning everything from automated drones to precision farming to electricity grids and financial networks – NAVISP will investigate novel ways of making these services more robust and reliable, to facilitate the emergence of competitive European actors.
• November 23, 2016: On 17 November, an Ariane 5 rocket launched four new Galileo satellites (Galileo satellites 15–18), accelerating deployment of the new satellite navigation system. The first pair was released 3 hours 36 minutes later, while the second separated 20 minutes later at the target altitude. 37)
- At the Toulouse space center of France's CNES space agency, a joint ESA–CNES team is now working around the clock to shepherd the four through the critical early orbits, lasting nine days for one pair and 13 days for the other.
• August 9, 2016: Europe's fifth and sixth Galileo satellites, which were salvaged from their faulty launch into working orbits, are set to begin broadcasting working navigation signals for test purposes. This activation will allow satnav receiver manufacturers, service providers and scientific researchers to make use of these test signals. A decision on whether these satellites will become part of the operational Galileo constellation is due to be taken by the European Commission. 38)
- A malfunction in their Soyuz-Fregat upper stage during their 22 August 2014 launch placed the Galileo-5 and -6 into highly elliptical – or elongated orbits – instead of their planned circular medium-Earth orbits. — A team based at ESA/ESOC in Darmstadt, Germany, then performed a complex series of maneuvers to raise and circularize their orbits.
- The satellites lacked sufficient fuel to reach their originally envisaged orbits, but the salvage meant that their navigation payloads could then be operated on an ongoing basis; their initial orbits dipped the satellites too close to Earth to keep their antennas properly locked on the planet. "Once their orbits were modified, their navigation payloads could be turned on and in-orbit testing could take place," explains Marco Falcone, Head of the Galileo System Office "The good news was their performance was excellent." For the corrected orbits see Figure 19.
- Now they will be tested on a more sustained basis, along with the rest of the Galileo satellites. A pair of ‘Notice Advisory to Galileo Users' (NAGUs) informing the user community of their availability for testing purposes have been published on the European Global Navigation Satellite System Service Center website. Users are welcome to provide feedback on their usage of GSAT0201 and GSAT0202 by contacting the GSC helpdesk.
- The navigation signals will include a signal health status reading that ‘signal component currently in test' and its navigation data validity status will be ‘working without guarantee'. In this way, these signals will not disturb the performance of any receivers using the Galileo signals coming from the other satellites.
• June 22, 2016: A sea-based test is demonstrating the potential of extending satnav augmentation coverage into north polar regions, offering a safety-of-life standard of navigation performance to users including shipping or aircraft in flight. The Norwegian research vessel Gunnerus, owned by the Norwegian University of Science and Technology, is equipped to pick up satnav signals from GPS and GLONASS as well as augmentation signals specially generated for the test, modelled on Europe's existing EGNOS (European Geostationary Navigation Overlay System). 39)
- Gunnerus is making use of the signals during five days of sailing off Trondheim. The demonstration is part of the Arctic Test Bed project, conducted within the European Global Navigation Satellite System Evolutions Program (EGEP) of ESA. The ESA-designed EGNOS improves the precision of US GPS signals over most European territory, while also providing continuous and reliable updates on their ‘integrity'.
- A 40-strong network of ground monitoring stations perform an independent measurement of GPS signals, so that corrections can be calculated and then passed to users immediately via a trio of geostationary satellites. The result is a several-fold increase in precision.
- "Simply due to Earth's curvature, EGNOS signals are not visible above about 70º north, but they are needed to support polar routing," explains Marco Porretta, overseeing the Arctic Test Bed project.
- To investigate possible methods for improving SBAS ( Satellite-Based Augmentation System) performance in this Arctic region, the test campaign will assess the benefits of augmentation for various types of satnav signals: single-frequency GPS; dual-frequency GPS; and dual-constellation dual-frequency, where GPS signals are combined with those of its Russian counterpart, thus increasing the number of observations.
- "The planned next-decade upgrade of EGNOS, along with other augmentation systems operated over other continents (such as the US equivalent WAAS (Wide Area Augmentation System), will perform multi-constellation augmentation as standard," adds Marco. "That means data from this test case should be especially valuable to support interoperability between future augmentation systems."
- The Arctic Test Bed makes use of some EGNOS reference stations along the north of Europe, along with additional stations in locations including Greenland, Jan Mayen Island, Spitsbergen and Norway.
Figure 16: EGNOS covering Europe (image credit: ESA)
• April 29, 2016: The Galileo-11 and -12 satellites, launched on Dec. 17, 2015, have been officially commissioned into the Galileo constellation, and are now broadcasting working navigation signals. The satellites' navigation payloads were submitted to a gamut of tests, centered on ESA's Redu center in Belgium, which possesses a 20 m-diameter antenna to analyze the satellites' signals in great detail. 40)
- The satellites' onboard atomic clocks – while the most precise ever flown for navigation purposes – must be kept synched by Galileo's global ground segment, which also keeps track of the satellites' exact positions in space. The tests were therefore essential to ensure these latest additions to the fleet met their performance targets while also meshing with the global Galileo system.
- Coordinated from the Galileo Control Centers in Oberpfaffenhofen in Germany (which controls the satellite platforms) and Fucino in Italy (which oversees the provision of navigation services to users), the success of these tests mean these satellites have now been integrated into the Galileo constellation.
Figure 17: Artist's rendition of a FOC (Full Operational Capability) Galileo satellite inMEO (Medium Earth Orbit), image credit: ESA
• February 2, 2016: The Galileo-9 and -10 satellites, launched on Sept. 11,2015, have started broadcasting working navigation messages. 41)
- Once safely in orbit and their systems activated, their navigation payloads and search and rescue transponders were subjected to a rigorous process of in-orbit testing, to ensure their performance reached the necessary specifications to become part of the Galileo system. Radio-frequency measurements of the Galileo signals were made from ESA's Redu Center in Belgium. The site boasts a 20 m diameter dish to analyze their signal shape in high resolution.
- Along with assessing that the satellites themselves were functioning as planned, the test campaign also confirmed they could mesh properly with the worldwide Galileo ground network.
- The testing was coordinated from the Galileo Control Centers in Oberpfaffenhofen in Germany – performing the command and control of the satellites – and Fucino in Italy – overseeing the provision of navigation messages to users.
- The operations team, successfully led by SpaceOpal GmbH, completed the testing campaign few days ahead of schedule, with the satellites beginning to broadcast valid navigation signals on 29 January, 2016.
• December 1, 2015: The Galileo-7 and -8 satellites (FOC-3 and FOC-4), launched on March 27, 2015, completed their commissioning activities and were declared operational, broadcasting navigation signals and, from today, relaying search and rescue messages from across the globe. 42)
- The RF (Radio Frequency) measurements were made from ESA's Redu center in Belgium. The site boasts a 20 m diameter dish to analyze Galileo signals in great detail. Last but not least, security testing has ensured that Galileo's PRS (Public Regulated Service) – a maximum precision service restricted to authorized users – is as secure as required.
- The checks carried out from the Galileo Control Centers in Oberpfaffenhofen in Germany and Fucino in Italy, as well as from Redu, prove the performance of these two satellites is excellent for navigation purposes. New onboard features such as seamlessly swapping between the different atomic clocks – a unique feature in global satnav systems – has been verified, which translates into more robust navigation services.
• November 9, 2015: Europe's fifth and sixth Galileo satellites – subject to complex salvage maneuvers following their launch last year into incorrect orbits – will help to perform an ambitious year-long test of Einstein's most famous theory. 43)
- The Galileo-5 and -6 satellites (also referred to as FOC-1 and FOC-2) were launched together by a Soyuz rocket on 22 August 2014. But the faulty upper stage stranded them in elongated orbits that blocked their use for navigation. ESA's specialists moved into action and oversaw a demanding set of maneuvers to raise the low points of their orbits and make them more circular.
- "The satellites can now reliably operate their navigation payloads continuously, and the European Commission, with the support of ESA, is assessing their eventual operational use," explains ESA's senior satnav advisor Javier Ventura-Traveset.
- "In the meantime, the satellites have accidentally become extremely useful scientifically, as tools to test Einstein's General Theory of Relativity by measuring more accurately than ever before the way that gravity affects the passing of time."
- Although the satellites' orbits have been adjusted, they remain elliptical, with each satellite climbing and falling some 8500 km twice per day. It is those regular shifts in height, and therefore gravity levels, that are valuable to researchers.
- Albert Einstein predicted a century ago that time would pass more slowly close to a massive object. It has been verified experimentally, most significantly in June 1976, when a hydrogen maser atomic clock on GP-A (Gravity Probe A) of NASA and SAO (Smithsonian Astrophysical Observatory) was launched 10,000 km into space, confirming the prediction to within 140 parts in a million.
- Atomic clocks on navigation satellites have to take into account they run faster in orbit than on the ground – a few tenths of a microsecond per day, which would give us navigation errors of around 10 km per day. "Now, for the first time since GP-A, we have the opportunity to improve the precision and confirm Einstein's theory to a higher degree," comments Javier.
- This new effort takes advantage of the passive hydrogen maser atomic clock aboard each Galileo, the elongated orbits creating varying time dilation, and the continuous monitoring thanks to the global network of ground stations. "Moreover, while the GP-A experiment involved a single orbit of Earth, ESA will be able to monitor hundreds of orbits over the course of a year," explains Javier.
• October 2015: Preliminary in-orbit payload performance results: A look is taken at at the initial results of navigation and search and rescue payload operation in orbit of the satellites FOC-1 (FM01) and FOC-2 (FM02) and compared with the predictions and performance in ground tests. Operations for Galileo FOC are supported by OHB experts from early on in the design and development process (Ref. 7).
- Initial Measurements (Output power modulated): The FM01 PL IOT sequence started with slowly increasing the modulated output power of the E1 / E5 / E6 signals. At each output power level the output power was verified against the predictions based on the on-ground measurements of the spacecraft - as measured in TVAC (Thermal Vacuum) and of the antenna as measured both by the supplier and after the integration onto the S/C. These predictions shown against the measured output power in the nominal operating point as provided by the IOT (In-Orbit Testing) station in REDU are shown in the following table.
Table 4: Modulated EIRP (Effective Isotropic Radiated Power)
- Timing Subsystem performance : A key feature of the FOC satellite is the so-called seamless switching capability between the prime PHM (Passive Hydrogen Maser) and the hot redundant clock RAFS (Rubidium Atomic Frequency Standard). The CMCU (Clock Monitoring and Control Unit) gives the ground segment the capability to adjust the frequency and the phase of the 10.23 MHz reference frequency (used inside the navigation payload and the spacecraft) derived from either of the two clocks (nominally PHM) by measuring the relative phase between these two frequencies over time (phase meter). This allows to iteratively align the frequencies and finally the phase of the two reference frequencies (if successful resulting in a zero phase difference). Then, the reference clock can be swapped without affecting the quality of the navigation signal, i.e. with minimum impact on both the code and the carrier phase. A seamless switch from RAFS to PHM as seen by the users (in this case the test receivers) on ground is shown in Figure 18. The Galileo receivers were continuously tracing the signals; the clock swap resulted in a position ‘jump' of less than 3 cm.
Figure 18: Seamless switch from RAFS-B to PHM-B resulting in a position ‘jump' of less than 3 cm (image credit: OHB)
- Initial measurements of SART (Search and Rescue Transponder): The objective of this (secondary) payload is to receive distress signals transmitted in UHF (406.1 MHz) and upconvert them to L-band and forward them to the earth stations from which then the rescue is organized. The results of the SART measurement have been affected by interference, especially the group delay measurement and the gain measurement in fixed gain mode. The main results of the SART testing are summarized in Table 5. Considering the impact of the interferers on the measurements, the SART showed very similar performance to the on-ground test results.
Table 5: SART Test results
- The first six FOC satellites have been deployed in orbit, and are all in perfect health, despite the anomalous orbit of satellites FOC-1 (also called FM01) and FOC-2 (FM02) due to an injection failure of the upper stage of the launch vehicle. Further satellites are delivered in the months to come, with the next pair of satellites to be launched in December 2015 (Ref. 7).
• Sept. 25, 2015: Europe's latest pair of Galileo satellites, Galileo-9 and -10, launched on Sept. 11, 2015, have passed its initial check out in space, allowing control to be handed over to the main control center and join the growing fleet. 44)
- The satellites fired their thrusters to drift towards their target orbital positions at around 23, 222 km altitude – helped along in this case by a near-perfect orbital injection to begin with. Firings will resume around the end of October to stop the drift and achieve fine positioning in orbit, guided by ESOC's specialist flight dynamics team.
- Once on their way, the satellites were handed over on 19 and 20 September, respectively, to the Galileo Control Center in Oberpfaffenhofen, Germany managed by SpaceOpal. The navigation payloads on Galileo-9 and -10 still need to undergo detailed testing, led from ESA's Redu center in Belgium with the support of both Oberpfaffenhofen and the second Galileo Control Center in Fucino, Italy, which has oversight of Galileo's navigation mission.
• March 13, 2015: ESA is reporting that the sixth Galileo satellite of Europe's navigation system has now entered its corrected target orbit, which will allow detailed testing to assess the performance of its navigation payload. 45)
Table 6: Overview of the recovery actions taken by the various teams
Figure 19: Corrected orbits of satellites 5 and 6 (image credit: ESA)
Legend to Figure 19: The original (in red) and corrected (in blue) orbits of the fifth and sixth Galileo satellites, along with that of the first four satellites (green). The first four satellites, launched in pairs in 2011 and 2012, were released into circular 23,222 km altitude orbits in two planes. The fifth and sixth satellites, launched by Soyuz–Fregat on 22 August 2014, ended up in an incorrect orbit because of a problem with the upper stage. This elongated orbit took them up to 25 ,900 km above Earth and back down to 13,713 km – too low for their navigation payloads to operate throughout. So, during November 2014 and January–February 2015, the satellites respectively underwent a series of maneuvers to raise the low point of their orbits by 3500 km while also making their orbits more circular. So now their navigation payloads are operable, and undergoing testing, while the European Commission – the Galileo system owner – prepares to decide whether the salvaged satellites will be incorporated into the constellation.
• Dec. 03.2014: Europe's fifth Galileo satellite, one of two delivered into a wrong orbit by VS09 Soyuz-Fregat launcher on August 22, has transmitted its first navigation signal in space on Saturday, 29 November 2014. It has reached its new target orbit and its navigation payload has been successfully switched on. A detailed test campaign is under way now the satellite has reached a more suitable orbit for navigation purposes. 46)
- The fifth and sixth Galileo satellites, launched together on 22 August, ended up in an elongated orbit of 25,900 km x 13,713 km. A total of 11 maneuvers were performed across 17 days, gradually nudging the fifth satellite upwards at the lowest point of its orbit. As a result, it has risen more than 3500 km and its elliptical orbit has become more circular.
- The commands were issued from the Galileo Control Center by Space Opal, the Galileo operator, at Oberpfaffenhofen in Germany, guided by calculations from a combined flight dynamics team of ESA/ESOC, in Darmstadt, Germany and France's CNES space agency. The commands were uploaded to the satellite via an extended network of ground stations, made up of Galileo stations and additional sites coordinated by CNES. Satellite manufacturer OHB also provided expertise throughout the recovery, helping to adapt the flight procedures.
- In the new orbit, the satellite's radiation exposure has also been greatly reduced, ensuring reliable performance for the long term.
- The revised, more circular orbit means the fifth satellite's Earth sensor can be used continuously, keeping its main antenna oriented towards Earth and allowing its navigation payload to be switched on. Significantly, the orbit means that it will now overfly the same location on the ground every 20 days. This compares to a normal Galileo repeat pattern of every 10 days, effectively synchronizing its ground track with the rest of the Galileo constellation.
- Navigation test campaign: The satellite's navigation payload was activated on 29 November, to begin the full ‘In-Orbit Test' campaign. This is being performed from ESA's Redu center in Belgium, where a 20 m diameter antenna can study the strength and shape of the navigation signals at high resolution.
- The first Galileo FOC navigation signal-in-space transmitting in the three Galileo frequency bands (E5/E6/L1) was tracked by Galileo Test User Receivers deployed at various locations in Europe, namely at Redu (B), ESTEC (NL), Weilheim (D) and Rome (I). The quality of the signal is good and in line with expectations.
- The SAR (Search And Rescue) payload will be switched on in few days in order to complement the in-orbit test campaign.
• Nov. 10, 2014: ESA's fifth Galileo navigation satellite, one of two left in the wrong orbit this summer, will make a series of maneuvers this month as a prelude to its health being confirmed. The aim is to raise the lowest point of its orbit – its perigee – to reduce the radiation exposure from the Van Allen radiation belts surrounding Earth, as well as to put it into a more useful orbit for navigation purposes. - Should the two-week operation prove successful then the sixth Galileo satellite will follow the same route. 47)
- The Galileo pair, launched together on a Soyuz rocket on August 22, 2014, ended up in an elongated orbit travelling out to 25,900 km above Earth and back down to 13,713 km. The target orbit was a purely circular one at an altitude of 23,222 km. In addition, the orbits are inclined at 49.8º while the nominal inclination should be 56.
- The two satellites have only enough fuel to lift their altitude by about 4000 km – insufficient to correct their orbits entirely. But the move will take the fifth satellite into a more circular orbit than before, with a higher perigee of 17,339 km.
- The recovery is being overseen from the Galileo Control Center in Oberpfaffenhofen, Germany, with the assistance of ESA/ESOC, in Darmstadt, Germany.
Figure 20: "Galileo orbits viewed from above," ESA, released on Sept. 16, 2014 48)
• Oct. 8, 2014: The Independent Inquiry Board, formed to analyze the causes of the launch anomaly, came up with the following conclusion: The root cause of the anomaly on flight VS09 is a shortcoming in the system thermal analysis performed during stage design, and not an operator error during stage assembly. 49)
The system thermal analyses have been reexamined in depth to identify all areas concerned by this issue. Given this identified and perfectly understood design fault, the Board has chosen the following corrective actions for the return to flight:
- Revamp of the system thermal analysis.
- Associated corrections in the design documents.
- Modification of the documents for the manufacture, assembly, integration and inspection procedures of the supply lines.
These measures can easily and immediately be applied by NPO Lavochkin to the stages already produced, meaning that the Soyuz launcher could be available for its next mission from the Guiana Space Center as from December 2014. - Beyond theses corrective actions, sufficient for return to flight, NPO Lavochkin will provide Arianespace with all useful information regarding Fregat's design robustness, which is proven by 45 successful consecutive missions before this anomaly.
• Oct. 6, 2014: EU Government and officials are debating how to proceed now. The options are to continue, as scheduled, with the December launch of two more Galileo satellites aboard a Soyuz Fregat rocket, or to wait until next spring or summer and launch four Galileo satellites on a heavy-lift Ariane 5 vehicle. - One argument for waiting until mid-2015 for the next launch is that it would give ESA and OHB additional time to put the satellites through a rigorous in-orbit test campaign to debug them before launching additional satellites. 50)
• On September 27-28, 2014, the two satellites, FM1 and FM2, launched on 22 August were handed over from ESA/ESOC, in Darmstadt, Germany, to the Galileo Control Center, Oberpfaffenhofen, which will care for them pending a final decision on their use. 51)
Fregat injection anomaly and LEOP (Launch and Early Operations Phase):
While the lower stages of the Soyuz worked flawlessly, soon it became clear that some time after the first of two firings of the final stage of the launcher, an anomaly occurred aboard the Fregat. While the second of these two firings was apparently with nominal thrust and nominal duration, the attitude of the Fregat during the second firing was significantly off the mark. While the exact reasons for the anomaly are subject to ongoing investigations of both ESA and Arianespace, the impact on the deployment orbit where identified within a few hours (Ref. 4).
The OHB support team present at ESOC together with their ESA/ESOC colleagues spent the next five days stabilizing the satellites despite their unexpected environment. At the end of these intense days, both satellites were thermally stable with a stable attitude pointing, solar arrays deployed, and reaction wheels run-in completed.
Both satellites – despite the different environment with different orbit period, harsher radiation environment, and in an unforeseen highly elliptical orbit - are in perfect health, no redundancy in any unit or subsystem was lost so far.
Mission recovery and re-definition:
The current assessment of the situation is as follows: While the satellites are operating in perfect health, the orbit is significantly different compared to what was anticipated. The FOC satellites do carry substantially more propellant with them than they would need for a nominal mission. The reason are the ESA requirements: the satellite design shall be identical for nominal and spare satellites. Spare satellites must – on top of what nominal satellites have to achieve in terms of ΔV – be able to quickly transition from the spare position into the position of the satellite that they are supposed to replace.
None of the "Work Package 1" satellites is likely to ever carry out this task (all satellites are to be nominal satellites, the spare role is reserved for latter satellites); however, their design must allow for this role as per requirements and they are fuelled accordingly. While this is good news for the recovery at first look, a second look reveals that even with this large propellant margin aboard, the satellites cannot correct the major injection failure that altered both eccentricity and inclination of the orbit. But navigation can be made possible from a variety of different orbits, including highly elliptical orbits (as e.g. also investigated currently in the scope of ESA's Galileo 2nd Generation studies) and lower-than-MEO orbits. Hence being in a different-than-expected orbit – quite contrary to e.g. communication satellites that do not inject properly into their geostationary orbit – does not mean the end of the mission.
The two most urgent problems that FM1 and FM2 are currently facing are identified as:
• Earth sensor field of view
• Radiation environment
The first issue is a result of simple geometry: the Earth sensors aboard are designed for missions in orbits in or above MEO. However, in the part close to the orbit's perigee, the Earth appears too large in the sensor's field of view, which means that it will lose track. The current onboard software is designed to interpret this as a severe issue / potential failure in the AOCS (Attitude and Orbit Control System). It reacts as its designers: by ending AOCS normal mode and entering an AOCS safe mode. While this approach made sense for the intended circular MEO orbit, it means that in the current orbit continuous normal mode is not possible. This again means that also continuous payload operation is not possible.
The second issue is a result of the fact that the current orbit transfers through parts of the Earth's Van Allen belts which feature higher levels of radiation than the originally foreseen MEO for Galileo. This means that on average per week, the satellites now endure the radiation dose they should have received in the course of one average month. While these elevated radiation risks do not pose an immediate risk, experts do agree that these levels should be decreased in the not too distant future.
With this background in mind, the following tasks are currently being investigated by OHB for ESA to support the re-definition and recovery of the first FOC mission from its injection anomaly caused by Fregat:
• Firstly, in order to allow the onboard Earth sensors to become fully operative, it is planned to invest most of the propellant onboard of FM1 and FM2 to raise the orbit's perigee. In this way, the Earth will be small enough in the sensors' field of view to enable uninterrupted normal mode of the AOCS. This task is not as straightforward, as the orbit change mode required, for this maneuver normally already would require the Earth sensor to be part of the AOCS control loop. Hence a work-around is needed and will be developed.
• Raising the perigee will also decrease the radiation levels, thereby addressing also the second problem identified. The inclination will likely not be changed (at least not significantly).
• Even with a raised perigee, the Earth will still appear larger in the Earth sensor's FOV than anticipated. Hence, the AOCS controller and the failure detection, identification and recovery algorithms need to be updated. This will be achieved through a combination of parameter setting changes and onboard software changes.
• Once the perigee is raised and the AOCS normal mode is stable in the "repaired" orbit, payload commissioning can commence. Impacts of the altered, but still other than originally designed for orbit onto payload operation needs also to be assessed.
• While currently the satellites are working fine, over-the-lifetime effects of the different environment (e.g. thermal, radiation, etc.) need to be analyzed. This must take into account the time in the original flawed orbit as well as the remainder of the mission in the altered orbit.
• Flight control procedures, satellite user manuals, as well as other documentation need to be updated accordingly.
• The impact on other segments needs to be analyzed as well, by their respective primes and ESA.
All these activities will be carried out in the coming weeks (rather than months), which could bring the perigee raising maneuver forward as early as the end of September 2014. With all of the above-mentioned activities successfully implemented, current analyses indicate that the two satellites could still carry out over 90% of their navigation services, which would turn this near-disaster into a great success for Europe.
SSTL's role in the OHB contract covers all phases from design through to support to in-orbit operations. The key deliverables under SSTL responsibility are 2 EM (Engineering Model) payloads, 22 FM (Flight Model) payloads and the associated EGSE (Electrical Ground Support Equipment).
The starting point for the SSTL payload design was the GIOVE-A payload but with major enhancements to meet the Galileo FOC requirements which are far more stringent than those for the GIOVE-A mission (Ref. 5).
The main driving requirements are:
• Lifetime: 12 years in MEO for FOC (whereas GIOVE-A was a 27 month mission)
• Launch scenario: Dual launch on Soyuz or 4 x launch on Ariane-5 with an effective mass limit of ~730 kg/spacecraft.
• Services: The FOC satellites must offer all five Galileo services – Open, Commercial, Safety-of-Life, Public Regulated, and Search & Rescue
• Capability: The FOC satellites carry the highly performant clocks: PHM (Passive Hydrogen Maser) in addition to the RAFS (Rubidium Atomic Frequency Standard). Each spacecraft carries a hydrazine propulsion system for constellation maintenance which was not a requirement on the test bed.
• Interfaces: The FOC satellites must comply to the operational interfaces with the GCS (Galileo Control Segment) in S-band and the GMS (Galileo Mission Segment) in C-band.
• Security: The FOC satellites must comply with the on-board bus and payload security requirements as well as those of the main ground-space segment interfaces.
• Production & industrialization: In order to minimize the recurring costs of production and generate satellites at the required cadence, the payload procurement and AIT (Assembly, Integration and Test) processes have been designed with production optimization as a key driver.
Payload design: The design consists of the following subsystems:
1) Timing subsystem: The timing subsystem is the heart of the navigation payload – it generates the master timing reference. The reference signal is generated at 10.23 MHz by one of 4 clocks – a redundant pair of RAFS (Rubidium Atomic Frquency Standard) with excellent short term stability and a redundant pair of PHMs (Passive Hydrogen Masers) with excellent short and long term stability. The nominal 10.23 MHz reference frequency can be offset with a small correction in-orbit to take account of relativistic effects and clock drift. The satellite design ensures that the temperatures of these clocks are maintained within a narrow band to further improve their stability. The control and monitoring of the clocks is provided by a CMCU (Clock Monitoring & Control Unit) with internal redundancy.
Figure 21: Photo of the two atmomic clocks, the PHM (left) and the RAFS (right), image credit: Galileo GNSS 52)
2) Mission uplink subsystem: The MSU (Mission Uplink Subsystem) receives the encrypted data messages coming from the ERIS (External Regional Integrity System) and the Galileo GMS (Ground Mission Segment). The uplink signals are a multiplex of a maximum of 6 simultaneous CDMA channels received by the receive-only antenna and demodulated by the receiver. The data messages are then passed on to the CSU (Common Security Unit) for decryption.
3) Signal generation subsystem: The SGS (Signal Generator Subsystem) is responsible for the generation of ranging and spreading codes, storage and buffering of navigation data obtained from the mission receiver through the CSU and generating the appropriate modulated L-band signals. The navigation signals supported by the payload will be compliant with the latest signal agreements between the EU and the US. The binary signal components of the navigation signals are modulated with the relevant subcarriers according to the Galileo Navigation Signal in Space ICD to create the three complex navigation signals by the NSGU (Navigation Signal Generation Unit). They are then upconverted to the E5, E6, and L1-bands, respectively.
Kongsberg Norspace of Norway is the supplier of two key elements within the satellites: the FGUUs (Frequency Generator and Upconverter Units) and SARTs (Search and Rescue Transponders). The redundant shoebox-sized FGUU takes the outputs of the satellite's adjacent NSGU (Navigation Signal Generator Unit) and converts them into L-band signals across Galileo's three spectral bands. It is these signals that end up guiding Galileo users through their receivers.
Figure 22: Photo of the FGUU (image credit: Galileo GNSS)
4) RF amplification subsystem: The RAS (RF Amplification Subsystem) is designed to meet the following requirements:
• Meet the EIRP requirements specified for the Galileo FOC navigation mission
• Operate three L-band channels with center frequencies at 1191.795 MHz, 1278.75 MHz and 1575.42 MHz.
• Comply with the out-of-band spurious emissions requirements of the Galileo FOC navigation mission
• Comply with 2:1 redundancy requirement
• Comply to the gain and phase variation requirements of the Galileo FOC navigation mission.
The objective of the RAS is to transmit the navigation signals to the ground at a quality and power level high enough for the receiver to track them and prevent interference with the radio astronomy bands and other existing navigation systems. The RF amplification subsystem contains 3 redundant pairs of MPM (Microwave Power Module) LCs (L-band Channels), with one of each pair in cold standby per band, switches, output filters for the L1-channel and an OMUX (Output Multiplexer) for the E5/E6 channels. The RF signal is transmitted through a phased array antenna mounted on the Earth-facing panel of the satellite.
5) Search & rescue subsystem: The SAR (Search and Rescue) payload's key function is to receive distress beacon signals at 406.05 MHz, band limit the signal, control its dynamics, convert it to L-band at 1544.10 MHz, and amplify it up to a 5 W output signal. The key design challenge is to overcome external interfering signals, combined with any internally generated spurs, in combination with large in-band signal dynamics to maintain the payload gain stability. The SAR payload system will include a transponder that translates the UHF distress beacon signal to the SAR payload output at L-band, for transmission to the MEO system local user terminals, in conjunction with the Galileo SAR antennas and two test couplers.
6) Laser retroreflector array: The laser retroreflector array consists of fused silica corner cubes, which have the geometrical property of turning incoming light rays through 180 degrees so that they return to their source. The ground portion of the ranging system consists of a highly calibrated pulse laser, a telescope and associated timing electronics.
Figure 23: The navigation payload consists of 3 panels per satellite: Clock module, Antenna module, and Core module (image credit: SSTL) 53)
Status of Galileo FOC satellite payloads:
Figure 24: Photo of the Galileo FOC FM2 main L-band antenna (image credit: ESA, Anneke Le Floc'h)
• On June 2013, SSTL delivered the first four Galileo FOC navigation payloads to the Galileo GNSS system to prime contractor OHB System AG. The payloads were shipped to OHB in Bremen, Germany for integration of the payload to platform and the start of the satellite integration and test activities. 55)
SAR/Galileo (Search And Rescue) payload
Background: The LEOSAR system, developed by the International COSPAR-SARSAT Program, currently provides accurate and reliable distress alert and location data to help search and rescue (SAR) authorities to assist persons in distress. In 2000, consultations started between the COSPAS-SARSAT Program and the European Commission on the feasibility to install 406 MHz SAR instruments on the Medium Orbit navigation satellites systems in order to develop a 406 MHz MEOSAR component to the COSPAS-SARSAT system. The main benefits of the MEOSAR system will be the near instantaneous global coverage with accurate independent location capability (in opposition with the current LEO system which has a higher latency to provide location information).
The inclusion of a SAR (Search And Rescue) payload in the Galileo satellites represents a major opportunity to dramatically enhance the performance provided by this system, it marks a significant expansion of the COSPAS-SARSAT program, a satellite-based network designed to bring help to air and sea vessels in distress. The international COSPAS–SARSAT satellite relay system has been making air and sea travel safer for 30 years, saving 24,000 lives along the way. 57) 58)
• The first Galileo SAR demonstration payload has been successfully tested. The second pair of Galileo IOV (In-Orbit Validation) satellites, launched aboard a Soyuz rocket from Kourou on October 12, 2012, are the first of the European Galileo constellation of navigation satellites to host a SAR payload. Both IOV satellites carry a search and rescue repeater, consisting of a SAR transponder and a combined UHF receiving and an L-band transmitting antenna.
SAR repeaters on the satellites can acquire UHF signals emitted from emergency beacons aboard ships, aircraft, or even carried by individuals. Ground stations, known as Local User Terminals, locate the source of distress calls using signals relayed by participating satellites and then alert local authorities for rescue. 59)
Figure 25: Galileo search and rescue repeater signal (image credit: ESA)
The SAR repeaters on these two Galileo satellites are the first of a new class of ‘MEOSAR' repeaters, combining broad field of views with the ability to quickly determine positions. Galileo's satellites are also the first with the capability to despatch return link messages via their navigation signals, assuring those in distress that help is on the way.
An additional advantage of this new MEOSAR system is that less ground infrastructure is required – just three to four terminals are sufficient to serve all European territory.
This initial SAR unit's transponder was built by Mier Comunicaciones in Spain, with its combined receiving and transmitting antenna developed by Spain's Rymsa company. 60)
Figure 26: Photo of the Galileo search and rescue transponder for the IOV satellites (image credit: Mier Comunicaciones, ESA)
• The first two Galileo-FOC satellites,FOC-1 (FM1) and FOC-2 (FM2), launched on August 22, 2014, are also equipped with a SAR payload — as will be all future Galileo-FOC satellites.
Figure 27: Galileo FOC FM1 Search and Rescue antenna supplied by Rymsa of Spain (image credit: SSTL) 61)
Note: Galileo's UHF search and rescue antenna is located next to the satellite's main circular navigation antenna.
Mier Comunicaciones and Rymsa, both of Spain, provided the hardware on the SAR-equipped IOV satellite pair now in orbit, with Kongsberg Norspace of Norway selected to provide the SARTs (SAR Transponders) on the follow-on Galileo-FOC satellites. 62)
Figure 28: Photo of the SART (Search And Rescue Transponder), image credit: ESA, Kongsberg Norspace
The shoebox-sized SART picks up emergency distress calls from the ground or sea and relays them to the nearest rescue center, while also sending a return-link message that help is on the way. Galileo's search and rescue capability marks a significant enlargement of the international COSPAS-SARSAT system. 63)
The search and rescue package on each Galileo satellite, with its receive–transmit antenna housed next to the larger navigation antenna, is only 8 kg and consumes just 3% of satellite power.
Founded by Canada, France, Russia and the US, Cospas–Sarsat began with payloads on LEO (Low Earth Orbit) satellites, whose rapid orbital motion allowed Doppler ranging of distress signals, to pinpoint their source. - The drawback is that they fly so close to Earth that their field of view is comparatively small.
Now Galileo satellites, along with two other constellations orbiting at MEO (Medium Earth Orbit) altitudes, have joined Cospas–Sarsat. Because Galileo satellites fly at heights of 23,222 km, they combine broad views of Earth with the ability to quickly determine the position of a distress signal.
Status of Galileo's SAR (Search & Rescue) Service
• April 6, 2017: Europe's Galileo satnav network does more than let us find our way – it is also helping to save lives. Today sees a spotlight cast on Galileo's Search and Rescue service, which pinpoints people in distress on land or sea. 64)
- The service is Europe's contribution to the COSPAS–SARSAT international satellite-based locating system that has helped to rescue more than 42,000 people since 1982 – the only system that can independently locate a distress beacon wherever it is activated on Earth.
- The Galileo SAR service is being formally premiered today (April 6, 2017), a date chosen to highlight the COSPAS–SARSAT 406 MHz signal.
- This new system has already proven its worth, as Tore Wangsfjord, Chief of Operations at Norway's Joint Rescue Coordination Center recounted to a satnav meeting in Munich, Germany, last month. His center's responsibility extends from 55ºN to the North Pole: "The results with Galileo have been good so far, and will improve with more satellites."
- A recent rescue was triggered by a distress signal from a crashed helicopter in the far north of Norway. The distress signal via Galileo arrived at his center 46 minutes before the alert from the existing COSPAS–SARSAT, and the identified position proved to be within 100 m of the crash, rather than the current system's 1.5 km. — "This is just one of several real-life distress situations where it has already shown improved accuracy and timing. Galileo will undoubtedly contribute to saving lives."
- As Xavier Maufroid of the European Commission told the Munich summit: "The service represented just 1% of total Galileo program costs, but should result in thousands of lives being saved." 65)
Figure 29: A helicopter airlift during a Norwegian search and rescue exercise on the Svalbard archipelago (image credit: Sysselmannen på Svalbard–Birgit Adelheid Suhr) 66)
Figure 30: Galileo within new system: Like the US GPS and Russian GLONASS, European Galileo satellites are carrying COSPAS–SARSAT MEOSAR (Medium Earth Orbit Search and Rescue) transponders (image credit: NOAA) 67)
Galileo's Ground Segment:
• GMS (Ground Mission Segment): The GMS must provide cutting-edge navigation performance at high speed around the clock, processing data from a worldwide network of stations. GMS has two million lines of software code, 500 internal functions, 400 messages and 600 signals circulating through 14 different elements.
The GMS is responsible for the determination and uplink of navigation data messages needed to provide the navigation and UTC time transfer service. For this purpose, it will use a global network of GSS (Galileo Sensor Stations) to monitor the navigation signals of all satellites on a continuous basis, through a comprehensive communications network using commercial satellites as well as cable connections in which each link will be duplicated for redundancy. The prime element of the GSS is the Reference Receiver.
The GMS communicates with the Galileo satellites through a global network of mission ULS (UpLink Stations), installed at five sites, each of which will host a number of 3 m antennas. The ULSs will operate in the 5 GHz Radionavigation Satellite (Earth-to-space) band.
The GMS will use the GSS network in two independent ways. The first is the OD&TS (Orbitography Determination and Time Synchronization) function, which will provide batch processing every 10 minutes of all the observations of all satellites over an extended period and calculates the precise orbit and clock offset of each satellite, including a forecast of predicted variations, SISA (Signal-in-Space Accuracy), valid for the next hours. The results of these computations for each satellite will be up-loaded into that satellite nominally every 100 minutes using a scheduled contact via a mission ULS. The OD&TS operation thus monitors the long-term parameters due to gravitational, thermal, ageing and other degradations.
• GCS (Ground Control Segment): The GCS monitors and controls the constellation with a high degree of automation.
The GCS s responsible for satellite constellation control and management of Galileo satellites. It provides the telemetry, telecommand and control function for the whole Galileo satellite constellation. Its functional elements are deployed within the Galileo Control Centers (GCC) and the five globally distributed Telemetry Tracking and Control (TT&C) stations. To manage this, the GCS will use a global network of nominally five TTC stations to communicate with each satellite on a scheme combining regular, scheduled contacts, long-term test campaigns and contingency contacts.
A hybrid Communication Network interconnects the remote stations (ULS, GSS, and TTC stations) with the GCC by different means of standard and special radio, wired data and voice communication links, assuring the communication between all the sites. The two Ground Control Centres (GCCs) constitute the core of the Ground Segment. There are two redundant elements located at Fucino (Italy) and Oberpfaffenhofen (Germany).
• TTC (Telemetry, Tracking and Command Stations): There are two, at Kiruna in Sweden and Kourou in French Guiana. The TTC Stations will include 13 m antennas operating in the 2 GHz space operations frequency bands. During normal operations, spread-spectrum modulation, similar to that used for TDRSS (Tracking and Data Relay Satellite System), and ARTEMIS data relay applications, will be used, to provide robust, interference free operation. However, when the navigation system of a satellite is not in operation (during launch and early orbit operations or during a contingency) use of the common standard TTC modulation will allow non-ESA TTC stations to be used.
• ULS (Uplink Stations): These consist of a network of stations to uplink the navigation and integrity data.
• GSS (Galileo Sensor Stations): a global network providing coverage for clock synchronization and orbit measurements.
• DDN (Data Dissemination Network): The DDN is interconnecting all Galileo ground facilities.
Figure 31: Photo of the Galileo TT&C antenna in Kiruna, Sweden (image credit: SSC) 70)
Ground System of SAR/Galileo (Search And Rescue/Galileo):
The Galileo Program involvement into COSPAS-SARSAT goes beyond the space component of the MEOSAR (Medium Earth Orbit Search And Rescue) system. Indeed, the European Union has deployed a significant Ground Segment infrastructure, which provides localization services for distress alerts transmitted by SAR beacons over a wide area comprising continental Europe, and vast oceanic areas around the continent (Figure 33). The SAR/Galileo Ground Segment can receive and process SAR distress signals relayed by any operational Galileo spacecraft or other satellite of the COSPAS-SARSAT MEOSAR constellation and determine thereby the location of the beacon within the coverage area (Ref. 56). 71)
Figure 32: Overview of the Search And Rescue function within Galileo (image credit: ESA)
The ground segment of the Search and Rescue Service of Galileo consists of 3 receiving ground stations, called MEOLUTs (Medium Earth Orbit Local User Terminal), which receive the distress signals relayed by the Galileo Search and Rescue repeater in the 1544 MHz band. Each MEOLUT includes a minimum of 4 antennas tracking different Galileo satellites.
Receiving the signal relayed from four different satellites makes it possible to determine the distressed beacon position by triangulation using TOA (Time of Arrival) and FOA (Frequency of Arrival) techniques. The MEOLUT then decodes the distress signal message, determines the beacon localization and provides this information to the COSPAS-SARSAT MCCs (Mission Control Centers).
The 3 European MEOLUTs are located in Svalbard (Norway), Makarios (Cyprus) and Maspalomas (Spain) and provide the SAR/Galileo service over the ECA (European Coverage Area) as shown in Figure 33. Each site is equipped with four antennas to track four satellites. Each MEOLUT is connected to a central facility, the MEOLUT MTCF (Tracking Coordination Facility) located at the SAR/Galileo control center in Toulouse, France, which optimizes the MEOLUT tracking plan of the 3 European MEOLUT in order to achieve the best location accuracy and availability over the European Coverage Area.
As a component of the COSPAS-SARSAT MEOSAR system, agreed to at the COSPAS-SARSAT 2012 conference, the SAR/Galileo ground segment is also capable of receiving the distress signal relayed by the MEOSAR payloads embarked on the GLONASS and GPS satellites (SAR/GLONASS and GPS/DASS payloads).
The performances achieved by the SAR/Galileo Service when the full Galileo constellation is operational are indicated in Table 7. The SAR/Galileo ground segment also includes the RLSP (Return Link Service Provider), which is responsible for providing Return Link Acknowledgment Messages to the COSPAS-SARSAT distress beacons equipped with a Galileo receiver. The Return Link Messages are embedded within the navigation message of the E1 signal.
Table 7: SAR/Galileo service performance recorded at ground segment
Figure 33: SAR/Galileo European coverage area and ground facilities (image credit: ESA)
Figure 34: Photo of the MEOLUT station on Spitsbergen Island (Svalbard, Norway), showing the 4 antennas (image credit: ESA, Ref. 71)
Legend to Figure 35: The ESA-built Maspalomas MEOLUT on Gran Canaria, is part of an extension of the international COSPAS-SARSAT search and rescue program into MEO , spearheaded by Galileo. Each site is equipped with four antennas to track four satellites. There are three sites in all: Maspalomas and Spitsbergen will combine with a third station at Larnaca in Cyprus, currently approaching completion. These three sites are monitored and controlled from the SAR Ground Segment Data Service Provider site, based at Toulouse in France. The stations are networked to share raw data, effectively acting as a single huge 12-antenna station, achieving unprecedented detection time and localization accuracy in relaying search and rescue signals to local authorities.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).