GIOVE (Galileo In-Orbit Validation Element)
As the name implies, GIOVE is the "Galileo In-Orbit Validation Element", a forerunner and technology demonstration project of the European Galileo constellation, a long-term navigation program in parallel to the US GPS and the Russian GLONASS constellations, created as a joint undertaking by the EC (European Commission) and by the European Space Agency (ESA).
The large Galileo program was structured in phases or subprograms for management reasons. Both the `Definition Phase', dealing with services definition, and the `System Design and the Critical Technology Development Phase', were successfully completed in May 2003.
• Secure the use of the frequencies allocated by the International Telecommunications Union (ITU) for the Galileo system
• Verify the most critical technologies of the operational Galileo system, such as the on-board atomic clocks and the navigation signal generator
• Characterize the novel features of the Galileo signal design, including the verification of user receivers and their resistance to interference and multipath
• Characterize the MEO (Medium Earth Orbit) radiation environment of the planned Galileo constellation.
GIOVE system architecture:
The overall GIOVE system architecture consists of:
1) Space segment: consisting of the GIOVE-A and GIOVE-B spacecraft (both missions are defined in separate files on the eoPortal)
2) Ground control segment:
• GIOVE-A Control Center operated by SSTL in Guildford (UK)
• GIOVE-A TT&C stations located at Oxford (UK), Santiago (Chile), and Kuala Lumpur (Malaysia)
• GIOVE-B Control Center operated by GAIN (Galileo Industries SA) in Fucino (Italy)
• GIOVE-B TT&C stations located at Fucino (Italy) and Kiruna (Sweden)
3) Ground mission segment:
• GPC (GIOVE Processing Center) operated by GAIN in Noordwijk (The Netherlands)
• A global network referred to as GESS (Galileo Experimental Sensor Stations)
Figure 1: Overall architecture of the GIOVE system (image credit: ESA)
Figure 2: Schematic view of the GIOVE architecture with emphasis on the GESS elements (image credit: ESA)
GMS (Ground Mission Segment):
The GPC (GIOVE Processing Center), initially working with GPS observables (Figure 1), has been upgraded to process the GIOVE satellite measurements. For this purpose, a global GESS (Galileo Experimental Sensor Station) network has been deployed. This activity, referred to as GIOVE Mission segment (GIOVE-M), started in 2005. - When the GIOVE-A signals were first broadcast (starting on January 12, 2006), GIOVE-M, which was still under deployment, was used to perform preliminary experimentation in the key areas of sensor station characterization and orbit and onboard clock assessment. 4) 5) 6) 7) 8)
The payload ground segment of GIOVE (GSTB-V2) consists of a worldwide network of sensor stations, referred to as GESS (Galileo Experimental Sensor Stations), collecting high quality Galileo data at 1 Hz, and a GPC (GIOVE Processing Center) at ESA/ESTEC in Noordwijk, The Netherlands. The GPC provides the functions of: orbit-determination, integrity and time-synchronization algorithms, and generation of navigation and integrity core products. The GPC receives in addition satellite telemetry and other ancillary data from the GIOVE-A Ground Satellite Centre (GSC) located in Guildford (UK). SLR stations are sending ranging data to the GPC as well. The GSC receives from the GPC the satellite navigation message to be uplinked to the satellite and broadcast to the user.
The GIOVE-A satellite is always in simultaneous view of at least 2 GESSs. This condition is necessary mainly to monitor the on-board clocks accurately and continuously. Each GESS consists basically of a newly developed dual GPS/Galileo receiver and a newly developed dual antenna. The receivers are normally connected to a commercial Rubidium clock. One GESS serves as the E-PTS (Experimental - Precision Timing Station), providing the reference time scale using UTC/TAI (Universal Time Code/International Atomic Time). This latter GESS is located at IEN (Istituto Elettrotecnico Nazionale), Torino, Italy, connected to an AHM (Active Hydrogen Maser), located in a controlled environment [note: IEN is also referred to as INRiM (Istituto Nazionale di Ricerca Metrologica)]. The IEN time reference is being used as the basis for GST (Galileo System Time) in GSTB-V2. 9) 10)
All clocks in the GIOVE-A system are synchronized to this AHM. The AHM output signals, both the 10 MHz frequency and 1 pulse per second (1 PPS), are fed to the IEN station as an external reference time scale. The AHM is continuously monitored versus IEN's ensemble of atomic clocks and also compared to external reference time scales such as Coordinated Universal Time (UTC) realized by the Bureau International des Poids et Mesures (BIPM) in Sevrès, France.
Figure 3: Galileo System Time and Geodetic Reference Frame Standards (Ref. 2)
Table 1: Requirements of Galileo service performance standards
The GESS stations were manufactured by Indra Espacio for ESA under Galileo Industries contract. 11)
Table 2: Overview of the GESS station network
Figure 4: GESS stations used in the GIOVE experimentation for the year 2007 (image credit: ESA)
Data from the GESS network were processed, with a sampling interval of one second, and satellite laser ranging (SLR) data from the International Laser Ranging Service (ILRS) collected between October 28, 2006, and January 17, 2007, when 12 GESS stations were operational. During this data analysis period, GIOVE-A was configured to transmit the L1 and E5 signals using the nominal payload chain, driven by the RAFS Flight Model 4 (RAFS FM4), one of the two onboard clocks. The GETR (Galileo Experimental Test Receiver) within all GESSs uses seven Galileo channels, configured to generate the observables shown in Table 3.
Table 3: GIOVE-A observables
Galileo test receivers and facilities:
GETR (Galileo Experimental Test Receiver):
GETR is a prototype device (GPS/Galileo receiver) designed by Septentrio Satellite Navigation NV, Leuven, Belgium with ESA funding (Septentrio is an offspring of Leuven University). The purpose of the GETR is to verify the acquisition, tracking and noise characteristics of all Galileo signals in the frame of the current Galileo demonstration and frequency filing activities. The GETR device is capable to track simultaneously 4 general Galileo signals + 1 wideband AltBOC (Alternate Binary Offset Carrier) signal + 9 GPS satellites (L1+L2). GETR units were installed at the Redu (ESA) and Chilbolton (UK) In-Orbit-Test-Stations and at the SSTL facility in Guildford, UK - receiving the first ever Galileo RNSS signals broadcast from GIOVE-A. 15) 16) 17)
GETR consists of an all-in-view dual-frequency GPS receiver (PolaRx2) combined with a Galileo receiver module, which in addition integrates a 9 channel GPS C/A code receiver. The latter allows very close synchronization and bias estimation between the PolaRx2 receiver and the Galileo receiver module, although both are using their own frontends. The Galileo receiver module can simultaneously track up to 7 GIOVE-A/B and/or Galileo signals.
Figure 5: GETR instrument supporting GIOVE-A (image credit: ESA)
The Signal-In-Space (SIS) experimentation provided results for the payload characterization, supporting data for frequency filing, performance characterization of the Galileo signals in a real environment and confirmation of the GETR design. A dedicated test-bench was set-up in the Navigation Laboratory at ESA/ESTEC consisting of two key components, namely the RF constellation signal simulator GSVF (Galileo Signal Validation Facility), and GETR. The test set included also a Galileo/GPS antenna (of Space Engineering), an active RHCP (Right Hand Circular Polarized) device, able to receive L1, L2, L5, E5, E6 and E1 signals. The analysis work was divided into three parts: 18)
- Intensive functional verification, validation, characterization and calibration of the test-bench
- Characterization and validation of receiver/signal performance against the GSVF
- Initial characterization and validation with the real GIOVE-A SIS.
The flexible GETR device is fully compliant with GIOVE-A as well as with Galileo SIS ICDs (Interface Control Documents) in terms of modulations, code length, chip rate and BOC (Binary Offset Carrier) modulations, including also BOC (15,2.5) and BOC (10,5) signals. The tests included different controlled multipath and interference environments as well as inter-system interference (GPS) analysis. As of fall 2006, the laboratory infrastructure is also available to receiver manufacturers for validation of their products.
GTR (Galileo Test Receiver):
GATE (Galileo Test Environment):
GATE is a ground-based test bed developed by a consortium under the lead of IfEN (Gesellschaft für Satellitennavigation mbH), Poing, Germany and funded by DLR, Germany. The objective is to monitor Galileo signals from the GIOVE, IOV, and Galileo spacecraft series. GATE has the capability to receive the Galileo bands E6, E5a,b and L1 in full bandwidth. In particular, GATE is a ground-based realistic test environment for developers of receivers and service applications for the future satellite navigation system Galileo.
GATE is operationally available for public users from August 2008 onwards. The GATE test site is near Berchtesgaden, Germany (with 6 earth-fixed Galileo transmitters). The facility is being operated by the German Aerospace Center (DLR), Oberpfaffenhofen. This provides a unique opportunity for Galileo receiver and application developers to perform realistic field-tests of hardware and software for Galileo operations at an early stage (several years prior to full Galileo constellation operations). In this way GATE will also support European products for Galileo entering the market. GATE has successfully passed the final system performance acceptance testing and pre-operational optimization phase in early July 2008 and has entered the commercial operation phase starting on August 1, 2008. 22) 23) 24) 25) 26) 27) 28) 29)
There are three major mission objectives to be covered by GATE: a) signal experiments, b) receiver testing, and c) user applications. The GATE system architecture is subdivided into four segments: (Figure 6)
1) GATS (GATE Transmit Segment), consisting of six earth-fixed GATE Transmit Stations enclosing the service area.
2) GAMS (GATE Mission Segment), consisting of two GATE Monitoring Stations (GMS) and the GATE Processing Facility (GPF) located within the test area.
3) GCS (GATE Control Segment). GCS includes the GMCF (GATE Monitoring & Control Facility), the GADS (GATE Archiving & Data Server), and the GTF (GATE Time Facility).
4) .The Support Segment comprising the facilities and functionalities for preparing and supporting the GATE missions. These are in particular the mobile GATE User Terminal (GUT) with the user receiver, the GATE Mission Support Facility (GMSF), and the GATE Signal Laboratory (GSL).
The ground-based transmitters, which are part of the GATS, emit all frequencies foreseen for Galileo. Hence, they have to be flexible in signal generation and adaptive to changes in signal structure. As GATE is a real-time system, it is necessary to feed the navigation message in real-time to the transmitters. The transmitters are also equipped with stable atomic clocks. The GATE signal generators, developed by Astrium GmbH, are designed to generate the Galileo navigation signals in the E5, E6 and L1 band simultaneously. its major building blocks are the control computer, the RRC (Rubidium Reference Clock) unit, and three SGUs (Signal Generation Units), one for each Galileo frequency band, followed by an RF amplifier section.
Some key features of each GATE signal generator are:
• Independent setup of Doppler frequency shifts for each channel and programmable code to carrier phase ratio.
• Four independent code generators for primary and secondary codes per unit with programmable chip rates and BOC (Binary Offset Carrier) subcarrier modulations.
• Look-up table based signal composition enabling advanced Galileo-specific modulation schemes like AltBOC and CASM (Coherent Adaptive Subcarrier Modulation), also referred to as Interplex.
Due to this modular approach, based mainly on FPGA and DSP (Digital Signal Processing) hardware, the GATE signal generator provides a flexible architecture; it is adaptive to future system evolutions.
GAMS monitors the navigation signals by using two GMS, performs the time synchronization of all system clocks, and generates navigation messages and steering commands to be sent to the six transmitters.
GCS includes the functions and facilities needed to monitor and control the entire GATE system (Figure 7, including mission planning and data archiving).
Figure 6: Overview of the GATE infrastructure (image credit: IfEN GmbH)
Figure 7: Overview of GCS (image credit: IfEN GmbH)
During the GATE system testing phase significant interference effects on the E5 and E6 band were observed. Intensive investigations revealed a military air surveillance radar station as the source of the interfering signals. The presence of this radar interference can lead to a degradation of receiver positioning performance on the E5 and E6 bands in the GATE area (Ref. 22).
At the end of July 2008, GATE has completed its design, development and testing phase and thus reached its Full Operational Capability (FOC). The commercial operations of GATE, open to all users worldwide, have been started on August 1st, 2008. In parallel to the commercial operations of GATE, a system upgrade has been started on October 1, 2008, in terms of Galileo signal structure compliance with the latest version of the ESA Galileo SIS ICD and the inclusion of the new Galileo Open Service (OS) signal structure (CBOC), as published by the GSA at the end of April 2008 in the latest version of the Public GSA Galileo OS SIS ICD. 30)
After completion of the signal upgrade in early 2010, the GATE test infrastructure will be capable of transmitting the Galileo OS, the Galileo SoL Service (on functional basis), the Galileo CS (only C/NAV-0 without encryption) and a Galileo PRS dummy signal according to the latest versions of the ESA Galileo SIS ICD and the Public GSA Galileo OS ICD.
Beside this signal upgrade, another important goal of this system upgrade is the possible certification of GATE as accredited open-air test infrastructure to perform the necessary tests needed for the certification process to certify Galileo Safety-of-Life (SoL) equipment.
GALANT (GAlileo/GPS multi-ANTenna receiver) demonstrator measurement campaign at GATE:
The GATE (Galileo Test Environment) facility offers the unique opportunity to test this receiver with Galileo signals under real conditions. In 2010, the ICN (Institute of Communications and Navigation) of the German Aerospace Center (DLR) developed GALANT, a multi-antenna receiver platform, to test the signal array processing technology for mitigation and detection of radio interference in receivers of satellite navigation systems. The goal was to demonstrate the performance gain of digital beamforming (DBF) for GNSS receivers. It was clear at the very beginning that the impact of interference on the system had to be considered at each stage of the receiving chain from the single antenna element to the positioning solution. For this reason a completely new receiver design was necessary, which provides access to each part in the receiver to take appropriate counter measures against almost any kind of interference and jamming. 31) 32)
In the current generation of the GPS/Galileo E1/E5a receiver, a two by two antenna array is employed. The DBF algorithm manages to reach the theoretical bound of signal power boost which is approximately 6 dB for a square 2 x 2 antenna array if the mutual coupling effect is taken into account. The adaptive beamforming together with time and frequency domain adaptive filtering (FDAF) give the receiver the unique capability of mitigating interference in the spatial, in the frequency and also in the time domain. The direction of arrival of impinging GNSS satellite signals can be estimated within a few degrees of accuracy thanks to a novel calibration method. This is a very powerful and effective tool for detecting and mitigating spoofing, interference and multipath. Due to its real-time capability, the effects of multipath signals, interference and other kind of distortions can be directly detected and assessed online.
The multi antenna receiver hardware mainly consists of four components (Figure 8):
1) E1/E5 active antenna array with calibration network,
2) E1/E5a frontend with four channels for each frequency band
3) A FPGA board with integrated analog to digital and digital to analog converters
4) An embedded Intel 2 GHz dual core PC card.
Figure 8: Main components of multi antenna receiver (image credit: DLR)
The antenna array (1) consists of 4 single antenna elements arranged as a planar 2x2 array with center-to-center interelement spacing of 90 mm. The single antenna elements are designed for the reception of the Galileo E1/E5 bands and utilize a stacked patch architecture. An E1/E5a frontend box (2) converts each of the received single antenna signals down to an intermediate frequency and performs signal amplification and filtering. Also two up converters for the E1/E5 calibration signal generation are integrated. The FPGA/ADC card and an embedded PC are integrated in a cPCI chassis. The FPGA/ADC board samples the analog IF signals and performs the IF signal processing. Also the PRN correlation and calibration signal generation are done by the FPGA.
The current generation of the demonstrator operates in dual-band mode, namely in the E5 and E1-band of the European Galileo GNSS system. In conventional dual frequency navigation antennas a combined output is used for both frequency bands. In the case of interference, especially for the well known E5 DME and radars it is not desirable to share one analogue signal path for both frequency bands. Due to the high power of such interference a signal distortion caused by saturation effects in the LNA may harm both bands. Therefore a novel antenna has been designed which allows a frequency separation with high isolation without the need of a diplexer (Figure 9).
The main parameters for the optimization of this antenna are also indicated in the picture. The signals received in the E1-band are coupled to the HF Ports, whereas those in the E5a/E5b are delivered to the LF Ports. Circular polarization operation is obtained by combining both HF and both LF Ports using 90º-hybrids that can be optimized for the E1 and E5a/E5b-bands, respectively. For RHCP reception the E5 to E1 port isolation is better then 20 dB and the E5 to E1 port is higher then 40 dB.
Figure 9: Schematic top view of the dual-band antenna (E5a/E5b and E1) with highly isolated outputs (image credit: DLR)
Array signal processing: Several signal processing algorithms are implemented in order to take advantage of the multi antenna setup. The following most important ones should be mentioned:
• Minimum Mean Squared Error (MMSE) beamforming
• DOA (Direction of Arrival) estimation.
Results of GALANT campaign: Interference and multipath robustness are key requirements in SoL (Safety-of-Life) applications. Array processing is a very powerful technique to meet these requirements and, therefore, is a key technology for the development of robust SoL receivers with improved and reliable signal reception. The estimated directions of arrival correspond very well with the true ones. The digital beamforming pointed the main beam for each satellite towards the direction with most signal energy. FDAF results showed the gain of frequency domain adaptive filtering for high power DME signals in the E5 band.
One of the main roles of GIOVE satellites is to test new code modulations foreseen for Galileo. The GIOVE-A signal in space is fully representative of the operational GALILEO system in terms of radio frequency and modulations, as well as chip rates and data rates. However, GIOVE-A codes are different from GALILEO codes. The navigation message of GIOVE-A is consistent with the transmission protocol (including interleaving and FEC encoding) but, in the summer of 2006, the frames do not contain any ephemeris parameters. The ranging signals of GIOVE-A can be used to generate a complete set of GNSS measurements: ranges, carrier phases, Doppler measurements and carrier-to-noise ratios, but they cannot be included in any positional solution, unless some external measurements of GIOVE orbits, such as laser ranging, will become available. 33) 34)
The ranging signals of Galileo are using advanced code modulation schemes, which are expected to provide significant improvement of the tracking and multipath performance as compared to current GPS. Figure 10 consists of 4 frequency bands: E5a, E5b, E6 and L1. The design of the Galileo signal structure presents significant user advantages compared to the signals of current GPS:
• Each Galileo signal includes a so-called "pilot" data-less component, which offers several benefits with respect to a data-bearing signal like the GPS CA code, including reduced noise and better tracking robustness at low signal power.
• The novel modulation schemes will result in significant reduction of both tracking and multipath noise for all the code ranges. One of the new modulations, E5-AltBOC will have exceptionally low noise characteristics. 35)
• A more reliable and robust 3-step coding scheme for navigation bits will be used (FEC-encoding, interleaving and improved CRC).
Figure 10: Galileo signals baseline overview (image credit: ESA)
Although the principles of Galileo are quite similar to the principles of GPS, Galileo offers a much greater variety of signals and services. Main parameters of Galileo signal components are presented in Table 4. The complex signal structure, which includes as many as 10 signal components, will serve the needs of 4 Galileo positioning services:
- OS (Open Service)
- SoL (Safety-of-Life) service
- CS (Commercial Service)
- PRS (Public Regulated Service)
Galileo observables are user measurements provided by Galileo receivers. Each Galileo observable is (similarly to GPS) a set of 4 measurements, which includes a code pseudorange, a carrier-phase measurement, a Doppler (or a range rate), and a C/N0 (carrier-to-noise ratio).
Table 4: Overview of Galileo signal components
The GIOVE-A satellite is transmitting ranging signals modulated with Galileo spreading codes in all the Galileo frequency bands. The data collected by the custom-built receiver (GETR) indicate stable reception and tracking of all the foreseen Galileo signals. In accordance with theoretical expectations, high rejection of long-range multipath and superior multipath performance has been demonstrated for wide-band Galileo signals, in particular for open-service E5AltBOC and also for L1A and E6A intended for PRS.
• GIOVE-A SIS (Signal-In-Space) is fully representative of Galileo SIS: 36)
- RF and modulations
- Chip rates and code lengths
- Data rate
• GIOVE-A can only transmit two signals at a time (L1+E5 or L1+E6)
• GIOVE-A codes are different from Galileo codes
• GIOVE-A Navigation Message not representative from structure and contents viewpoint (demonstration only purpose).
Agreements on GPS and Galileo navigation signal standards:
• In June 2004, the United States (US) and the European Union (EU) have completed negotiations on an agreement to harmonize their respective satellite navigation systems: the existing US Global Positioning Satellite (GPS) system and the planned European Galileo system. The US-EU agreement was signed on June 26, 2004, in Shannon, Ireland by the Secretary of State Colin Powell and the European Commission Vice President, Loyola de Palacio. 37) 38)
The essence of the agreement was to switch to a range of frequencies and modulations known as BOC 1.1 (Binary Offset Carrier 1.1), allowing both EU and US forces to block each others signals in the battlefield without disabling the entire system. The European Union also agreed to address the "mutual concerns related to the protection of allied and US national security capabilities."
As a result of achieving harmonization, the signals emitted by the two systems' open services "are going to become the de-facto world standard in civilian satellite radio navigation" and will permit users to use either system - or both at the same time - with a single receiver. The civil user level interoperability of the GPS and Galileo signals consists of:
- Geodesy nearly identical ~ 2 cm
- Timing different but each system will transmit timing offsets
- Radio frequency compatible.
This allows manufacturers to build "dual system" civil receivers where the civil users can choose to use GPS, Galileo, or a combination of signals based on their needs.
• On July 26, 2007, the United States and the European Union announced an agreement for a common GPS-Galileo signal, called MBOC (Multiplexed Binary Offset Carrier) for civilian use. In the future, this will allow receivers to track the GPS and/or Galileo signals with higher accuracy, even in challenging environments. These signals will be implemented on the Galileo Open Service and the GPS-IIIA new civil signal. 39)
Building on the historic cooperative agreement on GPS and Galileo signed between the two parties in June 2004, a joint compatibility and interoperability working group overcame technical challenges to design interoperable optimized civil signals that will also protect common security interests.
The resulting GPS L1C signal and Galileo L1F signal have been optimized to use the MBOC waveform. Future receivers using the MBOC signal should be able to track the GPS and/or Galileo signals with higher accuracy in challenging environments that include multipath, noise, and interference.
The GIOVE-B satellite is the first satellite to transmit its signals in the new MBOC modulation scheme standard.
Some background of the Galileo and GIOVE programs:
Galileo is Europe's civilian-managed GNSS (Global Navigation Satellite System) program which had its beginnings in the latter part of the 1990s. The enormous success of the US GPS constellation along with its vast infrastructure of civil equipment manufacturers was the main reason for the EU to start its own space-based navigation system - to create jobs in Europe and to participate in a technology with a global appeal. Repeatedly, the dependency on the US military GPS constellation was stated as an argument (as opposed to a civilian-owned and managed constellation). In 1998 the European Union decided to develop its own satellite navigation system, which it called "Galileo." In 1999, the different concept proposals (from Germany, France, Italy and the United Kingdom) for Galileo were compared and reduced to one by a joint team of engineers from all four countries.
Also in 1999, the responsibility of a systematic Galileo program definition was given to two bodies: the EC (European Commission) and to ESA (European Space Agency). The design approach involved the adoption of "structured phases." Both the `Definition Phase', dealing with services definition, and the `System Design and the Critical Technology Development Phase', were successfully completed and finalized in May 2003.
In the early phases of the Galileo program, the US government expressed repeated concerns about the European Galileo initiative - regarding the signal compatibility of the two systems and also becoming increasingly aware of the unwanted competition by a newcomer. The ambitious goals of the Europeans implied automatically a speed-up in GPS constellation modernization goals and a provision of better signal services for the civil user community. As a consequence, the SA (Selective Availability) feature, which degraded the L1 signal location accuracy of the GPS constellation, was removed on May 2, 2000. The Presidential Directive (of US President Bill Clinton) permitted now civil users worldwide general access to the highest‐possible accuracy of GPS signals. Only ionospheric effects were now the major remaining source of signal error.
The funding of the Galileo program, estimated to be over € 3.4 billion for a 30 satellite constellation and its ground segment infrastructure, turned out to be a major challenge for the European Union. The plan was for private companies and investors to invest at least two-thirds of the cost of implementation, with the EU and ESA dividing the remaining cost. However, this plan simply didn't work out. In early 2007, the EU had yet to decide how to pay for the Galileo program. On November 30, 2007, the 27 EU transportation ministers involved finally reached a funding agreement - which hinged on the national interests of all EU member states backing the program. The current goal is that the Galileo constellation should be operational by 2013 (a delay of 3 years from initial goals). 40)
GIOVE is the official name selected for the first two Galileo spacecraft in preparation for the in-orbit demonstration and validation phase of the European Navigation Program, Galileo. The naming ceremony took place on Nov. 9, 2005 at ESA/ESTEC. 41) 42)
Note: The selected abbreviation "Giove" is also the Italian word for "Jupiter". Hence, the naming commemorates aptly the discoveries of Galileo Galilei (1564-1642) in the fields of astronomy and navigation. On January 7, 1610, Galileo discovered the first four moons of Jupiter with his telescope which were later given the names of: Io, Europa, Ganymede, and Callisto. With regard to navigation, Galileo discovered in retrospect, that the four moon orbits of Jupiter could be used as a universal clock to obtain the longitude of a point on the Earth's surface.
Prior to the GIOVE naming ceremony in November 2005, the first step in the Galileo program was known as GSTB (Galileo System Test Bed). In this setup, the experimental ground segment of Galileo was referred to as GSTB-V1 (Galileo System Test Bed-V1). while the experimental space segment of Galileo was called GSTB-V2 (Galileo System Test Bed-V2).
The GIOVE subprogram involves the launch of two satellites for in-orbit testing of critical technologies such as the atomic clocks and the characterization of the novel navigation signals specifically developed for the subprogram (early signal experimentation). An important GIOVE/GSTB goal is to secure Europe's frequency allocation filing for Galileo, granted by the ITU (International Telecommunications Union). Under current ITU regulatory provisions, the so-called RNSS (Radio Navigation Satellite Service) signal allocations of the GALILEO system must be brought into use by mid-2006 or risk being lost to other users.
Already in 2002, ESA began the development of a system consisting of a ground segment and two satellites. In July 2003, ESA awarded two Galileo contracts to procure two experimental satellites in parallel.
1) GIOVE-A has been built by SSTL (Surrey Space Technology Ltd.), UK. The former name of this spacecraft was GSTB-V2/A.
2) GIOVE-B has been developed by GAIN (Galileo Industries S. A.), Brussels, a European consortium of Alcatel Alenia Space (France, Italy), Astrium GmbH (Germany), Astrium Ltd (UK) and Galileo Sistemas y Servicios (Spain) as the main members of the consortium. The former name of this spacecraft was GSTB-V2/B.
GIOVE-A and -B were built in parallel to provide in-orbit redundancy; their capabilities are complementary. The SSTL satellite carries a rubidium atomic clock and transmits a signal through two separate channels at a time. The GAIN satellite adds a hydrogen-maser clock and will transmit a signal through three separate channels.
GSTB (Galileo System Test Bed) refers to an initial test and validation phase of the ground segment and a space segment of the Galileo navigation system [validation of new technologies for operational use, characterization of the radiation environment in MEO (Medium Earth Orbit), experimentation with live Galileo signals]. GSTB is an integral part of the Design Development and In-Orbit Validation Phase to mitigate program risks.
The GSTB is subdivided into two main development steps, Version 1 (V1) and Version 2 (V2). The V2 part deals with the satellites, while the V1 part deals with such issues as integrity, orbit determination and time synchronization, algorithms, security issues, etc. The space segment of GSTB consists of 2 experimental satellites of the European Galileo navigation system - they are now being called GIOVE-A and -B. Both satellites have a design lifetime of three years for a nominal mission duration of 27 months.
In GSTB-V2, ESA has direct responsibility for the procurement of all of the GSTB-V2/B payload units and some of the GSTB-V2/A payload units. The other part of the GSTB-V2/A payload is a parallel signal generation chain developed by SSTL, the satellite prime contractor. The units procured by ESA include:
• Rubidium frequency standards (both satellites)
• A passive hydrogen maser (GIOVE-B)
• Clock monitoring and control units (both satellites)
• Navigation signal generation units (both satellites)
• Frequency generation and up-conversion units (both satellites)
• Solid state power amplifiers (GIOVE-B)
• Output filters and diplexer (GIOVE-B)
• Navigation antennas (both satellites)
The development and IOV (In-Orbit Validation Phase) subprogram were started in December 2003. Also, GJU (Galileo Joint Undertaking) was set up in 2003 to manage the development/validation phase. The two GIOVE satellites will be followed by the first four operational satellites in 2011, also referred to as IOV (In-Orbit Validation). GIOVE and its four GalileoSat successors will pave the way for the deployment of the full Galileo system constellation, providing for unprecedented satellite positioning, navigation and timing in the 21st century. 43) 44) 45) 46)
Galileo – Space Segment:
The planned space segment architecture consists of a MEO constellation of 30 satellites placed in three orbital planes with an inclination of 56º and a mean altitude of 23616 km. 27 The satellites are organized to give a 27/3/1 Walker constellation. One satellite per plane will act as an active spare for the other 9 MEO satellites in the same plane (quick recovery in case of failure). The orbital period is 5/3 revolutions/day.
• Walker constellation: 27/3/1
• 3 in-orbit spares (1 /plane)
• Semimajor axis: 29600 km (mean altitude of 23616 km)
• Inclination: 56º
• Period: 14 hours, 22 minutes
• Ground track repeat: ~ 10 days
• Orbit graveyard at MEO (+ 300 km)
Figure 11: Artist's rendition of the fully deployed Galileo navigation constellation (image credit: ESA)
Status of the GIOVE ground segment:
The GIOVE Ground Segment has been successfully deployed and operated routinely, providing the infrastructure needed by GIOVE experimenters to undertake their activities, in terms of data collection and distribution, and algorithm implementation. Within these activities, extensive ODTS / IONO [Orbit Determination & Time Synchronization / IONO: is a module which is in charge of estimating the IFB (Inter-frequency Biases), the BGD (Broadcast Group Delays) and the parameters of the NeQuick ionospheric model for single-frequency users] experimentation has been carried out, and very interesting results have been obtained: 47) 48)
• The deployed ODTS (Orbit Determination & Time Synchronization) has shown the ability to provide accurate estimation of GIOVE and GPS orbits and clocks with very good performances, considering the limitations of the available tracking network
• During the whole experimentation, orbit, clocks ionospheric delay and interfrequency biases estimation performances have been assessed using different techniques, all of them showing a good level of agreement between them: orbit and clock consistency, comparison with IGS (International GPS Service) products, residual analyses, clocks and system noise characterization, use of SLR (Satellite Laser Ranging) data
• The orbit prediction shows a good stability for both satellites with typical daily prediction errors lower than 20 cm
• Both RAFS on-board GIOVE-A and PHM onboard GIOVE-B have demonstrated excellent performances in terms of frequency stability
• Clock predictability for the two technologies embarked on GIOVE satellites (Rubidium and Passive Hydrogen Maser) are promising with errors at 100 minutes clearly below the nanosecond, in line with the Galileo mission requirements
• GGTO (GPS to Galileo Time Offset) results show feasible compliance to the Galileo requirements.
Over more than 3 years of operation, the GIOVE segment reached an advanced level of maturity, through the upgrade of its infrastructure and the experience gathered along the experimentation. 49)
As far as sensor station raw data characterization is concerned, this latest phase of experimentation allowed to perform in-depth validation of Galileo Mission Segment key technology and design aspects - for both IOV (In-Orbit Validation) and FOC (Full Operational Capability) phases.
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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).