GIOVE-B (Galileo In-Orbit Validation Element-B)
GIOVE-B is the second spacecraft of the Galileo In-Orbit Validation phase. The primary aim of GIOVE-B is to flight test the technologies needed for the Galileo constellation. Specific requirements call for: a) To transmit the required navigation signals to secure payload frequencies for the Galileo system, b) to characterize the MEO environment, c) to investigate the on-board clock performance, and d) to perform signal in space experiments.
The GIOVE-A and -B technology demonstrator spacecraft were built in parallel to provide in-orbit redundancy; their capabilities are complementary. Experience from the GIOVE missions will support the development of the IOV system, thereby reducing risk and helping to guarantee the success of the Galileo mission. The four spacecraft of the follow-on IOV (In-Orbit Validation) system are to be developed by ESNI (European Satellite Navigation Industries) formerly known as GaIn (Galileo Industries SA). 1)
Figure 1: GIOVE-B in ESA's test facility at ESTEC, in The Netherlands (image credit: ESA)
The GIOVE-B spacecraft (formerly referred to as GSTB-V2/B), designed and developed by ESNI (the European industrial team was led by Astrium GmbH, with Astrium Ltd., Thales Alenia Space, France, and Thales Alenia Space Italy performing integration and testing in Rome). The spacecraft is 3-axis stabilized using the Proteus bus. The modular design of the S/C consists of two cubes, one dedicated to the payload and the other, known as the platform module, to the spacecraft's control and operations subsystems. The spacecraft has a stowed size of about 0.955 m x 0.955 m x 2.4 m (aluminum honeycomb structure), and a launch mass of about 530 kg. The nominal experimental life time is two years. 2) 3) 4)
Figure 2: Overview of the GIOVE-B spacecraft architecture (image credit: EADS Astrium)
• Avionic subsystem: The avionics provides all satellite control and data handling functions, required for safe operation of the satellite during all mission phases including also non-nominal situations. The avionics is an integrated control and data system which consists of the AOCS (Attitude and Orbit Control System) with its sensors and actuators, the DH (Data Handling) and the OBSW (Onboard Software).
- ICDU (Integrated Control and Data Unit)
- Two Earth Sensors, 3 FSS (Fine Sun Sensors) internally redundant: 2º yaw, 0.35º roll & pitch
- Four reaction wheels
- Two torque rods (or 8 thrusters) for momentum dumping
- Two 3-axis gyros
Figure 3: Block diagram of the AOCS architecture (image credit: ESA, Astrium)
• EPS (Electrical Power Subsystem):
- Power is being generated by two symmetric solar panel wings, each 4.34 m in length, the panel wings are steerable.
- Standard silicon cells: minimum orbit average power of 925 W @ 36 V (EOL), max supply of up to 1.1 kW
- Li-ion battery
- The EPS uses an unregulated bus with a voltage of 23 - 37 V for power distribution.
The EPS is centralized on a single computer, the ICDU (Integrated Control and Data management Unit) as shown in Figure 4.
Figure 4: Photo of the ICDU (image credit: EADS Astrium)
• Propulsion subsystem (monopropellant):
- The propulsion subsystem has a single tank carrying up to 28 kg of hydrazine.
- Eight thrusters (2 x 4)
• The thermal control subsystem is dimensioned to withstand the worst-case thermal loads. The thermal design employs passive radiators and active regulation by heaters, which are monitored by the central computer to ensure the safety and health of the satellite. In particular, the thermal control subsystem achieves precise thermal control of the atomic clock panel to within 0.4º C, which maximizes the timing stability of the clocks.
RF communications: The TT&C communications for S/C operations are provided in S-band (ESA standard). A dual mode TT&C transponder unit provides the functions in both standard and spread spectrum mode: uplink telecommand demodulation, downlink telemetry transmission, coherent frequency turn-around and ranging turn-around function. GIOVE-B operations are being carried out from Telespazio's Fucino Space Center in Italy. 5)
Compared with GIOVE-A, GIOVE-B is flying new on-board technologies such as:
• A passive hydrogen maser, used as a precise time reference
• A different type of navigation signal generator
• Solid-state radio transmitters
• A different design of L-band antenna to broadcast the navigation signals
• An alternative type of radiation monitor to continue characterizing the environment in medium-Earth orbit.
Figure 5: Artist's conception of the deployed GIOVE-B satellite (image credit: ESA)
Launch: TheGIOVE-B satellite was launched on April 27, 2008 from the Baikonur Cosmodrome (Kazakhstan) on a Soyuz-Fregat launcher (launch provider: Starsem). The Fregat upper stage performed a series of maneuvers to reach a circular orbit at an altitude of about 23,200 km, inclined at 56º to the equator, before safely delivering the satellite into orbit some 3 hours and 45 minutes later. The two solar panels deployed correctly. 6)
The launch of the GIOVE-B satellite experienced several delays from its original launch date in late 2005/early 2006. The spacecraft arrived at ESA/ESTEC in September 2007 for final testing. GIOVE-B had been ready for launch in December 2007; however, a Soyuz Fregat launcher was not available at that time. - In the meantime, ESA had taken advantage of the delay and upgraded the spacecraft's navigation signal generator so that it can broadcast the MBOC (Multiplex Binary Offset Carrier) signal on the Open Service, applying the latest agreement between the United States and the European Union on a common interoperable signal design. 7)
Orbit: MEO (Medium Earth Orbit) near-circular ground track repeat orbit of 17 revolutions in (approximately) 10 sidereal days, altitude ~ 23,200 km, inclination = 56º, period of 14 hours and 3 minutes. - Note: Both spacecraft, GIOVE-A and -B, use the same orbital plane.
Figure 6: Bottom view of GIOVE-B showing the phased array antenna elements (image credit: ESA)
• In July 2012, ESA's GIOVE-B experimental navigation satellite is gradually raising its orbit as it prepares for a well-earned retirement at the end of its four-year mission paving the way for Europe's Galileo constellation. On July 24, 2012, an initial thruster firing raised GIOVE-B's orbit by about 30 km. This will be followed by others in the next three weeks so that by mid-August the satellite will be in a graveyard orbit some 600 km above its original 23,222 km orbit. 8)
GIOVE-B, like its predecessor GIOVE-A, performed excellent work testing Galileo hardware, securing Europe's rights to the radio frequencies set aside for Galileo and gathering data on a MEO (Medium-Earth Orbit) configuration.
• The GIOVE-B spacecraft is operating nominally in 2012.
• In April 2011, three years after ESA's Galileo prototype GIOVE-B reached orbit, the passive hydrogen maser at its heart is still ticking away as the most precise atomic clock ever flown in space for navigation – that is, until the first Galileo satellites join it later this year. 9)
• GIOVE-B is operating nominally as of fall 2009. It carries the first maser clock ever flown and it provides a signal first with MBOC (Multiplexed Binary Offset Carrier).
Open Service Performance (2009): Applying the specified ranging accuracy performance (i.e. 130 cm 95%) and the related nominal and degraded segment constellations, the following global Galileo Open Service DF (Dual-Frequency, E5a-L1) Availability performance is achieved over system lifetime. The service is declared available if both accuracy requirements are temporally and locally met, i.e. 4 m horizontal and 8 m vertical positioning performance. 10)
Table 1: Summary status of the segment key performance drivers and the related and derived system performances (Ref. 10)
• In the summer 2009, the GIOVE-B mission has achieved its main objective of maintaining the Galileo frequencies. The clock characterization results assessed over the last 12 months confirm that Europe has the most accurate clock in space, the highly stable Passive Hydrogen Maser. - For the first time the agreed interoperable GPS-Galileo composite L1 1575.42 MHz CBOC signal has been transmitted from space enabling early signal experimentation. Field experimentation has confirmed the CBOC (Composite Binary Offset Carrier) signal is better than BOC (1,1) leading to a 20-25% improvement in multipath mitigation (Ref. 3). 11) 12) 13) 14) 15)
• After being offline for two weeks, apparently due to space radiation effects, GIOVE-B began transmitting again on Sept. 25, 2008. According to ESA, the spacecraft stopped operating between Sept. 9-24, entering automatic shutdown to protect delicate circuitry.
• Nominal operations started after ITR (In-orbit Test Review) assessment on July 3, 2008. 18) 19)
• GIOVE-B began transmitting its first dual-frequency navigation signals on May 7, 2008- referred to as SiS (Signal in Space) broadcast. This represents a great step for satellite navigation since GIOVE-B is now, for the first time, transmitting the GPS-Galileo common signal using a specific optimized waveform, MBOC (Multiplexed Binary Offset Carrier), in accordance with the agreement drawn up in July 2007 by the EU and the US for their respective systems, Galileo and the future GPS III. 20)
After early orbit operations and platform commissioning, GIOVE-B's navigation payload was switched on and signal transmission commenced on May 7, 2008. The 25 m station at Chilbolton, UK, makes it possible to analyze the characteristics of GIOVE-B signals with great accuracy and to verify that they conform to the Galileo system's design specification.
GIOVE-B features a signal-generation system, an atomic clock, and an antenna design that all differ from the one on GIOVE-A. The double-redundant navigation payload transmits a Galileo signal on three separate frequency channels (L1, E5, and E6). The main payload elements are:
Table 2: Summary of the payload performance parameters and features
NAVANT (Navigation Antenna):
NAVANT is a phased array of individual L-band elements, illuminating the whole visible Earth below. NAVANT is based on array technologies with an integrated beam forming network. The NAVANT unit for GIOVE-B was designed and developed by EADS CASA Espacio, Madrid, Spain.
The main antenna requirements are the isoflux corrected pattern in the Earth coverage (12.67º semi-cone angle), circular polarization, (around 1 dB axial ratio in the coverage), two independent self-diplexed L-band (1.15-1.6 GHz) transmit bands operation (two channels E5 and E6 in Low Band 2 x 50 W and one channel L1 in High Band 75 W) and high parameters stability (including phase centre location and group delay). 21)
The selected design is a 1.4 m diameter flat array of 45 stacked patches with a hexagonal lattice structure and a mass of < 14 kg. The design includes array design, beam forming networks, structural design and thermal control.
Figure 7: Photo of the navigation antenna (image credit: EADS CASA)
Figure 8: Earth-Facing Panel of GIOVE-B showing the LRR panel and the navigation antenna (image credit: EADS, ESA)
NSGU (Navigation Signal Generation Unit):
The objective is to create different types of representative Galileo signals. One unit has a mass of < 1.4 kg and a power consumption of < 20 W. The device was developed by Saab Space of Sweden. The on-board timing signal, derived from the atomic clocks, is incorporated into the navigation message by the NSGU together with various correction terms and other message data uploaded from the ground.
Figure 9: Illustration of the NSGU device (image credit: ESA, Saab)
GIOVE-B's upgraded NSGU supports the latest navigation signal waveforms agreed between the EU and US authorities in July 2007, which are designed to allow the Galileo system to be compatible, and largely inter-operable, with GPS. GIOVE-B was the first satellite to transmit the upgraded MBOC (Multiplexed Binary Offset Coding) modulation standard from space, thus paving the way for its future roll-out on Galileo and GPS III satellites.
After leaving the NSGU, the channelized navigation signals are translated to L-band frequencies and then amplified by 50 W solid state power amplifiers before being broadcast by the navigation antenna. The antenna is a 42-element planar array that provides an "isoflux" beam across the visible surface of the Earth.
Navigation signals: GIOVE-B has three fully redundant channels operating in the allocated E5, E6 and E2L1E1 frequency bands. The flexibility to provide both single and dual-channel (E2L1E1+E5 or E2L1E1+E6) operation is possible, as operational needs dictate.
Like its predecessor, GIOVE-B carries two redundant small-size rubidium atomic clocks [2 RAFS (Rubidium Atomic Frequency Standard)], each with a stability of 10 ns per day. But it also features an even more accurate payload: the PHM (Passive Hydrogen Maser), with a stability better than 1 ns per day. - Two PHM units will be used as primary clocks onboard the operational Galileo satellites, with two rubidium clocks serving as backup.
PHM (Passive Hydrogen Maser):
PHM has a stability of ≤ 1 ns per day. PHM is the first of its kind and the most accurate clock so far flown in space. PHM has a mass of 18 kg and a power consumption of < 60 W. The PHM clock was developed by Selex Galileo (formerly Galileo Avionica) of Italy (electronics), the Neuchatel Observatory and the company Temex TNT (Temex Neuchatel Time) Switzerland, funded by ESA (Figure 11). 22) 23) 24) 25) 26) 27) 28) 29)
PHM composition: The PHM is a self-standing instrument, essentially composed of two hardware packages:
- The PP (Physics Package) including the MCSA (Microwave Cavity and internal Shields Assembly), the HSA (Hydrogen Supply Assembly) and the External Shield Assembly
- The EP (Electronics Package) including the MEM (Main Electronics Module) and the HDO (Hydrogen Dissociator Oscillator). Figure 7 illustrates the overall configuration of the PHM instrument. Both the EP and the PP are housed in self-standing separate subassemblies, with the PP lying over the EP Main Electronics Module. The latter provides the electrical, thermal and mechanical interfaces with the spacecraft.
The total mass of the instrument is 18 kg and the volume is 28 liters.
Figure 10: PHM outline (PP fixation cradle and MLI removed), image credit: SpectraTime, Selex Galileo, ESA
Figure 11: Photo of the PHM engineering model (image credit: Swiss Space Office)
Table 3: Clock performances achieved for the GIOVE-A and -B spacecraft
Figure 12: Photo of the PHM flight model onboard GIOVE-B (image credit: ESA)
In addition, there are two compact RAFS (Rubidium Atomic Frequency Standard) clocks, each with a stability of ≤ 10 ns per day; one of which will be in hot redundancy for the PHM, and the other in cold redundancy. The RAFS units are the same as flown on GIOVE-A. 30)
Figure 13: Photo of the RAFS flight model onboard GIOVE-B (image credit: ESA)
PHM clock characterization: After almost 10 years of continuous development in Europe the first PHM space maser has been launched on 27 April 2008. Its extremely good performance makes it the most stable of all clocks currently in orbit.
The test results from data collected on PHM on ground and in flight are demonstrating the suitability of the maser design for space missions, exceeding the Galileo satellite requirements.
The life-demonstration testing on ground is confirming the capability of the PHM to operate in vacuum over the given Galileo mission life of 12 years. The data so far collected shows no appreciable degradation. This is also re-affirmed by the extremely good in-flight performance of PHM on-board GIOVE-B. The excellent satellite performance is consistent with that expected from the design and measured on PHM on ground.
The on-ground performance of the IOV (In Orbit Validation) flight models so far produced are promising as they improve on their precursors for GIOVE-B, while industry has already improved on their production rate capability (Ref. 29).
FGUU (Frequency Generation and Up-converter Unit):
The main functions of this unit are: receiving signals from NSGU, generation of LO signals for frequency translation, conditioning and filtering of signals to meet system requirements, generation of the NSGU reference clock signal.
Figure 14: Photo of the FGUU device (image credit: Norspace)
SSPA (Solid State Power Amplifier):
SSPA generates four signals (carriers) in two different navigation frequency bands (upper and lower band, center frequencies at 1575 MHz and at 1191 MHz). This unit is amplifying the output RF carriers of the RNSS (Radio Navigation Satellite Service) signals. The output power level for each individual signal is about 50 W in L-band, PAE (Power Added Efficiency) > 35%@ 2 dB Comp. The mass of SSPA is 1.3 kg. SSPA was developed by Galileo Avionica, Italy.
Figure 15: Photo of the SSPA device (image credit: Galileo Avionica)
OMUX (Output Multiplexer):
Each OMUX combines the output signals from two SSPAs.
Figure 16: Block diagram of the GIOVE-B navigation payload (image credit: EADS Astrium, Ref. 2)
Figure 17: Block diagram of the GIOVE-B (older version) navigation payload (image credit: ESA) 31)
Additional payloads on GIOVE-B: (LRR, MEO GPS receiver, SREM)
LRR (Laser Retroreflector):
LRR is an integrated array of 67 coated corner cube reflectors is located on the nadir side of the S/C. The objective is laser ranging for POD (Precise Orbit Determination). The RRA (Retroreflector Array) is of size: 305 mm x 305 mm x 42 mm and trapezoidal in shape. 32)
GIOVE-B follows a yaw steering law such that the body +Z axis points continuously to nadir, together with a rotation performed around the Z axis that maintains the S/C Y axis perpendicular to the sun. The +X spacecraft panel is maintained away from the sun.
High-precision SLR (Satellite Laser Ranging) measurements, independent of the navigation signal generation, are being used for POD (Precise Orbit Determination). By taking into consideration a more accurate orbit, the pseudorange error can be minimized.
Figure 18: Location of LRR array on GIOVE-B (image credit: ESA, NASA)
MEO GPS receiver:
A GPS navigation receiver to experiment with autonomous localization in the MEO orbit. GPS receivers have been used successfully on low Earth orbit (LEO) satellite missions for several years. The use of a GPS receiver at altitudes higher than LEO, however, is non-trivial as the receiver will be outside the main lobe of the GPS broadcast signals, and it will have to track signals from GPS satellites transmitting from the other side of the Earth.
SREM (Standard Radiation Environment Monitor):
SREM is an ESA device (a nearly identical SREM instrument was previously flown on STRV-1c (launch Nov. 16, 2000), PROBA-1 (launch Oct. 21, 2001), Integral (Oct. 17, 2002), and on Rosetta (launch March 2, 2004)). The objective is to characterize the MEO (Medium Earth Orbit) radiation environment. The instrument was developed at Oerlikon/Contraves & PSI, Switzerland. SREM has a mass of 2.5 kg and a power consumption of < 2 W. 33) 34)
SREM has 3 x Si surface barrier detectors. It monitors the following constituents: 35)
- > 0.5 MeV electrons
- > 10 MeV protons
- >150 MeV ions.
Figure 19: Photo of the SREM instrument (image credit: ESA)
Ground system architecture:
After its successful launch by a Soyuz Rocket from Baikonur (April 27, 2008) and accurate insertion into its target orbit by the Fregat autonomous upper stage, GIOVE-B completed its LEOP (Launch and Early Operations Phase), followed by a successful IOTP (In- Orbit Test Phase) and is currently in its Nominal Operations Phase (2009). A network of global ground stations were needed for LEOP since 24 hour coverage is required during the initial critical operations after launch.
The main Mission Control Centre is Telespazio's TT&C Ground Station at the Fucino Space Center in Italy. This is being used throughout the lifetime of the spacecraft to command & control it and assess its health. It is also supported by the TT&C ground station of Kiruna, Sweden operated by SSC.
• MCS (Mission Control Center), Fucino, Italy (operated by Telespazio) 36)
• Communications network: (Ref. 18)
- S-band TT&C network, with a telecommand uplink data rate of 2 kbit/s and a telemetry downlink data rate of 31.250 kbit/s
- For LEOP (Launch and Early Orbit Phase): Dongara (Australia), Kiruna (Sweden), Santiago de Chile (Chile), South Point Hawaii (USA), and Fucino (Italy)
- For commissioning and routine operations: Kiruna (Sweden) & Fucino (Italy)
Nominal routine operations are carried out during nominally scheduled passes on Kiruna and Fucino ground stations. The typical pass duration is of 2.5 hours, which are sufficient to:
1) Perform overall satellite health checks and platform operations if any (Earth sensor blinding configuration, time adjustment and correlation, eclipses verification)
2) Download mass memory stored telemetry
3) Payload management
4) Data gathering on request by customer/manufacturer.
• In-orbit testing facility in Redu, Belgium and Chilbolton, UK.
Two L-band IOT (In-Orbit Test) stations were used, one at Redu, Belgium and the other in Chilbolton, UK. The Chilbolton site provided greater sensitivity since it had a 25 m dish as shown in Figure 20.
Figure 20: Photo of the Chilbolton antenna (image credit: ESA)
Figure 21: Overview of the GIOVE-B ground system architecture (image credit: Telespazio)
CONGO (COoperative Network for GIOVE Observation)
In 2008, the DLR (German Aerospace Center) and BKG (German Federal Agency of Cartography and Geodesy) started an initiative to deploy a small global network of GNSS receivers capable of GIOVE tracking. Both institutions have long experience in GPS and GLONASS processing and already operate numerous GNSS stations within and outside Europe. All of these stations provide real-time data transmission, enabling continuous signal monitoring and processing. Most stations offer a 1-Hz measurement rate; DLR's Experimental Verification Network (EVnet) can support up to 100 Hz for ionospheric studies and scintillation monitoring. 37) 38)
For the new CONGO network, some of the existing BKG/DLR sites have been supplemented with GIOVE-capable receivers, and additional sites have been established with the local support of international partners. Eight stations have now been deployed at carefully selected sites around the world. This network offers global coverage with at least a dual-station visibility for most areas and serves as a starting point for routine processing of GIOVE data.
Figure 22: Overview of the CONGO network in 2009 (image credit: DLR)
Legend to Figure 22: The thumbnail pictures indicate the corresponding receivers and antennas for each site.
The network was later joined by various other national and international partners, providing GNSS reference stations, network services, data archives and processing services.
• Deutsches Zentrum für Luft- und Raumfahrt/German Space Operations Center (DLR/GSOC)
• Federal Agency for Cartography and Geodesy (BKG)
• Technische Universität München/Institute of Astronomical and Physical Geodesy (TUM/IAPG)
• Deutsches Zentrum für Luft- und Raumfahrt/Institute of Communication and Navigation (DLR/IKN)
• Deutsches GeoForschungsZentrum Potsdam (GFZ), Section GPS/Galileo Earth Observation
• Geoscience Australia (GA)
• Centre National des Etudes Spatiales (CNES), France.
Key aspects that distinguish CONGO from other GNSS networks include the capability of GIOVE-A/B (Galileo) and QZSS [Quasi-Zenith Satellite System (NICT, JAXA, Japan)], QZSS signal tracking, the support of multiple frequencies (L1/E1, L2, L5/E5), the full global coverage and the real-time capability.
Figure 23: The CONGO network sites as of Feb. 2012 (image credit: DLR)
As of February 2012, CONGO comprises a total of 23 different sites around the world (Figure 23). The distribution of sites was originally chosen such as to achieve a continuous GIOVE tracking coverage with the minimum of eight stations (Maui, Concepcion, Fredericton, Wettzell, Hartebeesthoek, Singapore, Chofu, Sydney) deployed in the first design stage of Sept. 2009. Since then the number of stations has increased by more than a factor of two. In particular, a dense coverage in the African and Asian region was achieved through stations contributed by the GFZ real-time GNSS network. Besides improving the common-view statistics the large number of stations now offers an enhanced robustness against station failures and communication gaps. Various CONGO stations are co-located with stations from other networks (such as IGS, EUREF, or EVNet) and common antennas are used where feasible. QZSS tracking is presently supported by five CONGO stations in the Asian-Pacific region.
Data Processing and Products: Measurements at a 1 s sampling interval as well as auxiliary data are transmitted in real-time from all receivers to a central server at BKG, Frankfurt, where they can be accessed by multiple concurrent users. The NTRIP (Networked Transport of RTCM via Internet Protocol) is used for transmission to and from the central server, but receiver-specific raw data formats are employed in the respective data streams. A transition to the RTCM3 format is foreseen once a standardized set of high precision messages becomes available for all signals and constellations. A permanent CONGO data archive is hosted by the TUM (Technische Universität München), where all data streams are received and decoded in real-time. Besides the original data transmitted by each receiver, RINEX3 observation files at 10 s sampling data and navigation files in SP3 format are generated and stored for off-line analyses on a daily basis (Figure 24).
Figure 24: CONGO processing and data flow (image credit: DLR)
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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 (firstname.lastname@example.org).