GIOVE-A (Galileo In-Orbit Validation Element-A)
GIOVE-A (Galileo In-Orbit Validation Element-A)
GIOVE-A is the first element of the Galileo In-Orbit Validation phase. This pilot satellite marks the very first step towards Europe's new global navigation satellite system, a partnership project involving the European Space Agency (ESA) and the European Commission (EC).
The GIOVE-A spacecraft, designed and developed by SSTL (Surrey Space Technology Ltd.), UK, employs the GMP (Geostationary Minisatellite Platform), a newly developed minisatellite bus program of SSTL (developed for the UK MOSAIC program in 2002) having a stowed envelope of 1.3 m x 1.3 m x 1.8 m (h), and 3-axis stabilization. GMP is designed to be tolerant of a single point failure in any of the subsystems (Figure 8). The bus structure is modular with separate propulsion, avionics and payload bays for ease of AIT (Assembly, Integration and Test). The structural frame uses aluminium honeycomb and thermal control is achieved mainly passively with the north/south Y panels being used as radiator surfaces for the payload. The bus features fully redundant avionics units, a butane propulsion system with two tanks each holding 25 kg, and two deployable sun-tracking wings with 2 panels per wing (each panel of size: 1.74 m x 0.98 m). The deployed solar arrays measure 7 m from tip to tip.
Note: In a general SSTL platform renaming convention of 2007/8, the GMP bus was renamed to SSTL-900, designed for MEO, GEO, HEO and interplanetary orbits.
Figure 1: Artist's conception of the deployed GIOVE-A satellite (image credit: ESA)
The OBDH (Onboard Data Handling) system employes an OBC695 that makes use of the single chip implementation of the ERC32 processor (ATMEL) along with a dual-redundant CAN (Controlled Area Network) bus. For GIOVE-A the OBC695 is being used as a payload computer; the central OBC is SSTL's OBC386 based on an Intel microprocessor. The GIOVE-A spacecraft has a launch mass of about 602 kg. 1) 2) 3) 4) 5)
The electric power system features scalable power levels from 500 W to 2 kW. A dual power bus is used consisting of a regulated main bus (50 V) for the payload, and an unregulated battery bus (28 V, from the battery) for the platform. The spacecraft's deployed silicon solar arrays interface to the spacecraft via SADM (Solar Array Drive Mechanism), built by SNECMA (also referred to as SEPTA®). The total power output per wing at summer solstice, EOL is 667 W at 51.5 V (assuming one string failure). For eclipse operation, the power to the payload is supplied from the Li-ion battery (battery capacity is 60 Ah or 1950 Wh). The power system is fully autonomous and fault tolerant. This is achieved through the use of majority voting circuits for critical control signals and command lines and the use of redundant systems. 6) 7)
Figure 2: Block diagram of the GIOVE-A power system (image credit: SSTL)
The AOCS (Attitude and Orbit Control Subsystem) uses a "normal mode" design. The requirements call for the spacecraft to follow an attitude profile that keeps the spacecraft-sun vector nominally in the X-Z body plane, thus allowing the solar arrays to maintain normal solar incidence by a rotation of the panels (nominally constrained to be about the Y axis). Significant roll rotations can not be used to achieve this due to the nadir pointing requirement. The design uses a momentum bias approach coupled with feed-forward 'momentum steering' to drive the required yaw maneuver.
The AOCS employs Earth horizon sensors, sun sensors and gyroscopes (2 sets of 3 QRS11 MEMS gyros of Systron Donner Inertial, Walnut Creek, CA, USA) for attitude rate sensing. The QRS11 is a MEMS technology, solid-state "gyro on a chip." Actuation is provided by reaction wheels, magnetorquers and thrusters (a butane propulsion system providing a ΔV of about 90 m/s for orbit maintenance). Pointing: pitch/roll capability: ±0.1º (3σ), yaw capability: ±1.0º (3σ). The operational attitude of the spacecraft in normal mode is such that: 8) 9)
Figure 3: The AOCS architecture of GIOVE-A (image credit: SSTL, ESA)
• The antenna boresight, nominally aligned with the spacecraft Z-axis, is always nadir pointing
• The solar arrays (with a rotation axis nominally aligned with the spacecraft Y-axis) are articulated to point always into sun for optimal power generation.
• For thermal reasons, the +X face of the spacecraft is always deep-space pointing.
Figure 4: Schematic layout of the GIOVE-A spacecraft components (image credit: SSTL)
RF communications: The TT&C communications for S/C operations are provided in S-band (SSTL standard). The RF subsystem has SSTL-built S-band receivers and transmitters with antennas positioned for full 4π coverage. It is fully redundant and the receiver allows direct commanding of any equipment connected to the CAN bus. - The GIOVE-A satellite control center is based at SSTL in Chilbolton (making use of existing facilities), with ground stations at RAL and in Kuala Lumpur, Malaysia.
Table 1: Overview of key GIOVE-A characteristics
Figure 5: High-level block diagram of GIOVE-A (image credit: SSTL)
GIOVE-A was designed, built and launched in less than 30 months. AIT on the GIOVE-A spacecraft started in November 2004. The CDR (Critical Design Review) was held successfully in March 2005, and in July 2005 the satellite was shipped to ESA/ESTEC for the environmental test campaign. The ESA furnished navigation and timing payloads were delivered to SSTL between January and June 2005. SSTL has demonstrated in this project that its low-cost rapid-response approach to satellite development can be applied to a higher class of mission than had previously been thought possible.
Figure 6: Image of the GIOVE-A spacecraft (image credit: SSTL)
Figure 9: Photo of GIOVE-A on top of the Fregat upper stage (image credit: SSTL, ESA)
Launch: The GIOVE-A spacecrft was launched on December 28, 2005 (5:19 UTC) from the Baikonur Cosmodrome on a Soyuz-Fregat launcher (launch provider: Starsem). A prime objective of the mission was to secure vital frequency filings, generate the first Galileo navigation signals in space, characterize a prototype rubidium atomic clock, and model the radiation environment of Medium Earth Orbit (MEO) for future Galileo spacecraft. 10)
Orbit: MEO (Medium Earth Orbit) near-circular ground track repeat orbit of 17 revolutions in (approximately) 10 sidereal days, altitude = 23,258 km, inclination = 56º, period of 14 hours and 22 minutes.
Status and events of the GIOVE-A mission
• November 24, 2021: Surrey Satellite Technology Ltd (SSTL) has successfully de-commissioned GIOVE-A, the pathfinder satellite for Europe's Galileo constellation, after 16 years of operations in Medium Earth Orbit (MEO). The decision to de-commission the satellite was made due to the obsolescence in computing systems required for the operation of GIOVE-A, and de-commissioning of the spacecraft took place on 24 November 2021. The procedure involved transitioning the satellite to Earth pointing mode , turning off the reaction wheels and setting the attitude and orbit control system to standby mode, before finally switching off the on board computer and transmitter. 11)
- GIOVE-A was designed, built and tested by SSTL in only 30 months for the European Space Agency (ESA) and was launched on 28 December 2005 with a mission to secure vital frequency filings, generate the first Galileo navigation signals in space, characterise a prototype rubidium atomic clock, and model the radiation environment of MEO for future Galileo spacecraft. GIOVE-A was the first European satellite launched into the demanding MEO radiation environment, where it greatly out-performed its 27 month design lifetime.
- Sir Martin Sweeting, SSTL's Executive Chairman, summed up GIOVE-A's achievements by commenting "GIOVE-A was a milestone mission for SSTL that demonstrated how our pragmatic approach and innovative, low cost, small satellites could deliver critical mission requirements for landmark space programs, such as Galileo. GIOVE-A secured the vital frequency filings with the International Telecommunications Union (ITU) on the 12th January 2006 and completed its original mission for ESA in 2008. GIOVE-A also over-delivered on its original lifetime and mission goals, and hosted experimental SSTL hardware which provided additional valuable data about the MEO environment for more than ten years – an inspiring and game-changing mission on so many levels."
- "If not for GIOVE-A the 26 Galileo satellites in orbit today would not exist," comments Paul Verhoef, Director of ESA's Directorate of Navigation. "Its speedy development and launch opened the way for our working constellation to follow."
- After completion of its mission for ESA, GIOVE-A was maneuvered into a higher "graveyard" orbit at 23,300 km above the Earth to make way for the first fully operational capability Galileo satellites. In 2012 SSTL took over operations from ESA and GIOVE-A continued to provide valuable in-orbit data on the MEO environment. The Merlin radiation monitor on-board GIOVE-A collected a unique 10+ year record for the MEO orbit and data analysis at the Surrey Space Centre, supported by ESA, showed some interesting features such as the "electron desert" in 2008/9 during what was the lowest solar minimum of the space era, and one of the largest electron storm events on record in April 2010. Several scientific journal papers have been published from the radiation data generated by GIOVE-A and a new model of the outer Van Allen belt electron fluxes, ‘MOBE-DIC', has been produced to help improve future satellite designs.
- Monitoring of the rubidium clocks on board GIOVE-A, key data for the Galileo constellation that followed, revealed no issues during the 6.5 years that the navigation payload was operational.
- Also onboard GIOVE-A was an experimental SSTL GPS receiver, the SGR-GEO, which in 2012 achieved a GPS position fix at 23,300km altitude - the first position fix above the GPS constellation on a civilian satellite. This demonstrated a new timing and orbital positioning solution for satellites operating in orbits higher than 20,000km which can only receive a few of the weaker GPS signals that "spill over" from the far side of the Earth.
- The SGR-GEO was subsequently used to track many signals from GPS satellites, including the sidelobes not normally visible to ground-based systems. The performance of GNSS receivers at high altitudes is very sensitive to sidelobe strength, and data from the SGR-GEO helped to map out the antenna patterns of GPS satellites for use in planning navigation systems for future high altitude missions in geostationary orbit , lunar orbit, and beyond into deep space.
- In 2010 SSTL was awarded a contract to assemble, integrate and test the first navigation payloads for the full operational capability Galileo spacecraft, and over a period of 10 years SSTL supplied a total of 34 navigation payloads for Galileo, Europe's global navigation positioning constellation.
- GIOVE-A carries an engraved plaque in tribute to SSTL engineer, Tom Fairburn, who tragically lost his life in the 2004 Indian Ocean Boxing Day Tsunami. Tom worked as a payload mechanical engineer on GIOVE-A and is warmly remembered by colleagues at SSTL.
• On Dec. 28, 2015, SSTL has marked the 10th year of on orbit operations from its GIOVE-A satellite, the pathfinder mission for Europe's Galileo satellite navigation program. GIOVE-A has greatly exceeded its 27 month design lifetime, and is still in operational use. Since retirement from its original mission for the European Space Agency, SSTL has operated the spacecraft at 23,300 km where it provides valuable data about the radiation environment in MEO, and an on-board experimental GPS receiver,SGR-GEO, is used to map out the antenna patterns of GPS satellites for use in planning navigation systems for future high altitude missions in GEO, and beyond into deep space. 12) 13)
- The Merlin radiation monitor on-board GIOVE-A has collected a unique 10 year record for the MEO orbit and data analysis at the SSC (Surrey Space Centre), supported by ESA, has shown some interesting features such as the "electron desert" in 2008/9 during what was the lowest solar minimum of the space era, and one of the largest electron storm events on record in April 2010. Several scientific journal papers have been published from the radiation data generated by GIOVE-A and a new model of the outer Van Allen belt electron fluxes, ‘MOBE-DIC', has been produced to help improve future satellite designs.
• April 2013: ESA's retired GIOVE-A navigation mission has become the first civilian satellite to perform GPS position fixes from high orbit. Its results demonstrate that current satnav signals could guide missions much further away in space, up to geostationary orbit or even as far as the Moon. GIOVE-A has been able to fix its position, velocity and time from GPS signals, despite orbiting more than 1000 km above the downward-pointing US satellites. 14)
- Any satellite orbiting above the GPS constellation can only hope to detect signals from over Earth's far side, but the majority are blocked by the planet. For a position fix, a satnav receiver requires a minimum of four satellites to be visible, but this is most of the time not possible if based solely on front-facing signals.
• ESA formally ended GIOVE-A's mission at the end of June, 2012, although it will go on being operated for now by prime contractor SSTL (Surrey Satellite Technology Ltd) of Guildford, UK, to gather radiation data and performance results from a GPS receiver. GIOVE-A had a design life of only 27 months, it continued operating for 78 months. 15)
- After decommissioning GIOVE-A was moved into a graveyard orbit about 100 km above Galileo's orbital altitude of 23,222 km, control was passed to its prime contractor SSTL.
• On January 12, 2012 was the 6th anniversary that GIOVE-A transmitted its first Galileo signals from orbit. Built with a design life of 27 months, its mission was to secure the radio frequency filing for the Galileo satellite system with the ITU (International Telecommunications Union), test the critical Galileo payload equipment, and perform tests to characterize the radiation environment of MEO (Medium Earth Orbit). GIOVE-A remains fully operational having been declared a full mission success by ESA in 2008. 16)
• On Dec. 28, 2010, the GIOVE-A spacecraft was 5 years in orbit and continues to operate. 17)
• GIOVE-A is operating nominally in 2010. In January 2010, the satellite has been in orbit for 21 months beyond its original 27 month mission design life and continues to provide critical data to all of the ground users experimenting with Galileo navigation signals. 18) 19)
• GIOVE-A is operating nominally in 2009. Navigation payload has performed very well in orbit. SSTL expects the spacecraft to continue operating well beyond the current extension period 20)
• GIOVE-A was placed into a higher orbit of ~ 23,300 km. In the summer of 2009, the operating team at SSTL executed a series of precisely planned maneuvers during July and August 2009 that have repositioned the satellite 113 km above the nominal MEO orbit that the 27 operational Galileo navigation satellites will occupy. 21)
• In June 2009, ESA approved an extension of the GIOVE-A mission for a further twelve months, which provides for operations to be supported to the end of March 2010. 22)
• August 14, 2008: Switch to redundant payload to monitor the long-term performance of the redundant RAFS.
• During 2007 and 2008, the GIOVE-A mission has transitioned from an experimental mission into what is effectively an operational mission. Since the end of the first phase of experimentation the GIOVE-A satellite has behaved remarkably well and validated both its development approach and many of the techniques and technology applicable to a Geo comms mission. 23)
• GIOVE-A has been transmitting Galileo-like signals from January 12, 2006. Its payload is able to generate and transmit all Galileo modulations and multiplexing schemes including the Binary Offset Carrier (BOC) modulated signal as well as the wideband Alternative BOC (AltBOC) modulation. Note that Giove-A can only transmit two signals at a time (either L1+E5 or L1+E6). The in orbit testing of Giove-A has been successfully accomplished demonstrating the good performance of the payload and the generated Signal-in-Space. Giove-A now offers a fully representative Galileo Signal-in-Space. 24)
• Mission extension of 1 year: In April 2008, GIOVE-A passed its "nominal mission duration of 27 months" (3 years of design life). ESA confirmed that GIOVE-A is a "full mission success" and has contracted SSTL to continue operations for an additional year as the satellite continues to perform and provide valuable Galileo services. - Since commissioning the satellite has achieved a remarkably high operational availability with signals being broadcast for 99.8% of the time over the last year. The primary atomic clock, fundamental to all future Galileo satellites in providing highly accurate positioning and time reference signals, has been operating continuously since June 2007. 25)
• The final orbit was reached 222 minutes after lift-off. Two minutes after spacecraft separation from the Fregat, the Guildford control center established contact with the satellite, performed an initial checkout and initiated the deployment of the solar arrays within two hours
• The very next day, the radiation monitoring payloads (Merlin, CEDEX) were activated. GIOVE-A was operating successfully in the Galileo orbit. These payloads have been operated near-continuously since launch.
• During the following week, the platform commissioning was completed and the satellite was ready for payload operations.
• Payload operations: The navigation payload commissioning commenced on Jan. 10, 2006 with a complete checkout of the low-power equipment. On Jan. 12, 2006, the high-power equipment was activated and GIOVE-A broadcast its first navigation SIS (Signal-In-Space) (L1 and E5) message. The signals were received at Chilbolton and Redu ground stations -as well as by other observers using various makes of Galileo test receivers, as listed in the "GIOVE ground segment."
• In the weeks following the initial signal, all of the Galileo signals were exercised - thereby securing Europe's frequency allocation filing for Galileo, granted by the ITU (International Telecommunications Union). ITU was informed in early March 2006 that all frequencies of Galileo had been put into use. This major milestone of the Galileo program has been achieved in a record time of 33 months from contract award.. 26) 27)
• In early April 2006, the first laser-ranging test was successful providing results well within expectations. The measurements are being used for POD (Precise Orbit Determination) analysis.
• On May 11, 2006 the SGR-GEO 24 channel C/A code GPS receiver on GIOVE-A was powered up for the first time. During the 90 minutes of operation, the GPS receiver successfully tracked the signals of 4 GPS satellites for most of the time, briefly reaching 5 satellites, and collected a complete set of GPS Almanac data. The GPS signals tracked appeared to be considerably stronger than expected and it is believed that some signals from sidelobes of the GPS satellite antennas were tracked.
Unfortunately the incompatibility with the main transmitted payload has meant only very short periods of operations at the beginning of the satellite's life have been possible so far. During these operations, the SGR-GEO tracked 5 GPS satellites, including two from the side-lobes of the GPS transmit antenna. It is hoped that there will be a further opportunity for the operation of the SGR-GEO at the end of GIOVE-A's life to give a more thorough demonstration. 28)
• In addition to the primary payload, the radiation monitoring payloads (Merlin, CEDEX) have been operated successfully. Both radiation monitoring payloads have been operated near-continuously since launch. The data has been analyzed and is of good quality.
• An SLR (Satellite Laser Ranging) campaign took place from May 22 to July 24 to provide data for the characterization of the satellite's onboard clock. The campaign was coordinated by the International Laser Ranging Service (ILRS) and the GPS (GIOVE Processing Center) at ESA/ESTEC. Fourteen SLR stations took part in the campaign.
• In June 2006, the first clock characterization campaign was successfully carried out by Galileo Industries S.A.. The measurements showed that the rubidium clocks are operating as expected. - Work still to be done includes full characterization of the clock and the radiation experiments.
• Interoperability between the GPS and Galileo signals has also been demonstrated with GETR.
• In early May 2007, GIOVE-A successfully transmitted its first navigation message, containing the information needed by user receivers to calculate their position. Prior to reaching this milestone, the satellite had been broadcasting only the data needed for measuring the receiver-to-satellite distance. The first Galileo navigation message was created by the navigation signal generator unit on board GIOVE-A, using content prepared by the GIOVE Mission Segment. The objective of the test was to demonstrate an end-to-end link between the Mission Segment and the user receivers. The navigation message is being generated for demonstration purposes only - no service guarantee is provided. 29)
Figure 10: Evolution of the GIOVE-A operation phases (image credit: ESA)
The GIOVE-A payload configuration is of a regenerative design, generating navigation signals on either the E5a, E5b, E6, or on the E2, L1, and E1 frequency bands. The objective of the regenerative navigation payload is twofold:
1) To generate the appropriate navigation messages, ranging signals and spreading codes and then modulate them onto one of the four navigation channels.
2) To retrieve data uploaded to the satellite via the platform's S-band RF communication system.
The payload configuration includes dual signal generation chains and up-conversion stages (Figure 11), one is based on a SSTL design and the other one on ESA furnished GALILEO payload equipment. Together with the switched transmitter stage, the payload provides the necessary redundancy for single channel transmissions, thereby fulfilling the primary mission requirements.
The back-up payload shown, has SSTL substitute designs to replace any or all of ESA's CFI (Customer Furnished Items) units comprising the signal generation chain, and is carried only as a mitigation against schedule risk should any of the CFI units not be available on time. The triple-redundant navigation payload is capable of transmitting the Galileo signals in two separate frequency channels.
Note: The GIOVE-A navigation payload deviates somewhat from the navigation payload of GIOVE-B, representing a parallel signal generation chain developed by SSTL to promote experimentation. 30) 31) 32)
• GIOVE-A carries only the two RAFS clocks, while GIOVE-B carries the two RAFS clocks plus the two PHM clocks. Of the two RAFS clocks on GIOVE-A, one is operational and one serves as cold spare.
• GIOVE-A introduced two SSTL-developed elements in the navigation chain, namely NMGU and MFUU (modulator and two upconverter units) which are not part of the general Galileo navigation chain. The Galileo navigational message generator (NMGU) feeds the modulator with representative navigation messages synchronized with the Galileo system time. When connected, the signal generator and message generator produce truly representative Galileo signals.
The navigation payload comprises the following main elements:
CFI Signal Generator Chains - for the generation, storage and buffering of navigation data, ranging and spreading codes. The navigation signal generator provides control and programmability of the generation, content and format of the navigation messages and selection of the data rates, spreading and ranging codes. The navigation data is then modulated to generate the desired signal and up-converted to the final output frequency. For redundancy, two CFI chains are embarked.
SSTL Signal Generator Chain - the 3rd signal generation chain for the generation of a navigation signal sufficient to fulfil the frequency filing protection requirements. As with the CFI signal generator chain, the SSTL chain performs the navigation message generation, modulation and up-conversion.
Transmit chain: provides switching, channel amplification, gain control, high power amplification and filtering of the navigation signal. Two transmit chains are employed, one for the upper band and the other for the lower. The transmit chains are configured so that they can be switched to provide redundancy in single-channel operation.
NAVANT (Navigation Antenna):
NAVANT is a phased array of individual L-band elements (isoflux), illuminating the whole visible Earth below. NAVANT is based on array technologies with an integrated beam forming network (Figures 7 and 12). NAVANT provides a) an isoflux pattern to equalize the received power level on ground, and b) a broadband frequency response to cover all the Galileo frequency bands with high performance. The NAVANT unit for GIOVE-A was designed and developed by TAS (Thales Alenia Space) formerly AAS-I (Alcatel Alenia Space Italia). 33) 34) 35) 36)
The key drivers/requirements of the antenna design are:
- Dual-band RHCP (Right Hand Circular Polarization) transmit antenna (1156-1300/1555-1596 GHz)
- Gain: 15 dBi at EOC (Edge of Coverage), with isoflux shape in the field of view of ±12.2º and admissible ripple of 2 dB
- Axial ratio: < of 1.5 dB
- Phase center and group delay stability: ±5 mm and 50 ps respectively
- Stiffness: > 100 Hz
- Mass: 16 kg
NAVANT consists of an array of 36 self-diplexed and stacked patch radiators fed by two independent BFNs (Beamforming Networks) operating at low and high frequency band respectively. The array grid is a mixed lattice, optimized for the dual band functionality. A radiating element is a dual-band patch composed by four stacked layers assembled in a compact, light and detachable stand-alone unit. The patches are copper-gold on kapton supported by honeycomb spacers. All patch layers are properly grounded to prevent ESD (Electrostatic Discharge). A dedicated PIM (Passive Intermodulation) test confirmed also the PIM-free design. The feeding of the radiating element has been optimized at the low and high frequency band for broadband performance and for a low axial ratio at the radiating element level.
Figure 13: Layout of a radiating element of NAVANT (image credit: AAS-I)
Figure 14: NAVANT element layout showing the quadrant symmetry (image credit: AAS-I)
The measured performance parameters of NAVANT can be summarized as follows:
- EOC gain: 15 dBi at both frequency bands, only at the high band the ripple exceeded the 2 dB
- Axial ratio: < 0.6 dB at low band, and < 0.7 dB at high band
- Phase center and group delay stability: worst case of ±3.5 mm and of 21 ps, respectively
- Stiffness: 145 Hz
- Mass: 21.9 kg.
NSGU (Navigation Signal Generation Unit):
The objective is to create two representative Galileo signals. There are two units onboard, one unit has a mass of < 1.4 kg and a power consumption of < 20 W. The main tasks performed by the NSGU are: reception of navigation message information, generation of navigation message modulating a set of PRN (Pseudo Random Noise) codes, multiplexing of the various signal components for base-band/IF interface with the up-conversion unit. The NSGU instrument was provided by Laben, Italy. 37)
Figure 15: Illustration of an NSGU device (image credit: Laben)
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. The FGUU was developed by Norspace of Norway [Note: As a consequence of the decision to close Alcatel Space Norway (ASN) in April 2003, a new privately owned company was created in June 2003 under the name of AME SPACE]. GIOVE-A carries 2 FGUU devices, one able to generate a simple Galileo signal and the other, more representative Galileo signals.
Figure 16: Illustration of the FGUU device (image credit: ESA)
RAFS (Rubidium Atomic Frequency Standard):
These are two clocks (redundant), compact rubidium atomic clocks with a stability of ≤ 10 ns per day. The clock system technique employed is RAFS, the same technique is also being used by the GPS and GLONASS spacecraft series. A RAFS unit has a mass of 3.3 kg. The RAFS clocks were designed and developed by Temex TNT (Temex Neuchatel Time), Switzerland. - Overall, the timekeeping system consists of two rubidium atomic frequency standards and a CMCU (Clock Monitoring & Control Unit) containing a phase lock loop to generate the 10.23 MHz output clock. 38) 39)
Figure 17: Illustration of the RAFS clock (image credit: ESA)
CMCU (Clock Monitoring & Control Unit):
CMCU is designed and developed by Alcatel Espacio, Spain. The CMCU interfaces the onboard Atomic Frequency Standards and performs the following tasks: 40)
- Generates the onboard reference frequency at 10.23 MHz (Master Clock)
- Implements a "phase comparison" system for the control of the redundant RAFS clock with the main S-PHM clock (Note: the GIOVE-A spacecraft is only equipped with the 2 RAFS clocks).
- Interfaces with the OBDH (Onboard Data Handling System)
Figure 18: Illustration of the CMCU device (image credit: ESA)
Figure 19: Block diagram of the general navigation payload chain (image credit: ESA)
NMGU (Navigation Message Generation Unit):
NMGU is designed and developed at SSTL (Figure 11). The device generates the Galileo signals (RF carriers) in two different navigation frequency bands (upper and lower band, center frequencies at 1575 MHz and at 1191 MHz).
MFUU (Modulator, Frequency Generator & Upconverter Unit):
MFUU is an SSTL device, developed and built by Alcatel Alenia Space, Italia. This unit (there are two) is amplifying the output RF carriers of the RNSS (Radio Navigation Satellite Service) signals. The upper band provides the signals, E2, L1, and E1; the lower band has the signals E5a/b, and E6). The output power level for each individual signal is about 50 W in L-band.
Platform S-band TT&C transmitter and receiver (SSTL): the regenerative navigation payload makes use of the platform TT&C receivers and transmitters to uplink, via the platform OBC, non-real time payload data which is then converted into the navigation messages by the payload processor, and to acknowledge correct receipt of the navigation data.
GTRF (Galileo Terrestrial Reference Frame):
The European Galileo radionavigation system opted for UTC/TAI (Universal Time Coordinated/International Atomic Time) and ITRF (International Terrestrial Reference Frame) as its time and coordinate references. In practical terms, the GTRF is an independent realization of the ITRS (International Terrestrial Reference System).
Environmental/experimental payloads: (Merlin, CEDEX, LLR, MEO GPS)
Two radiation monitors are being used to characterize the MEO environment. The radiation environment in the Galileo orbit is one of the most severe in the vicinity of the Earth, with the spacecraft encountering the high (and variable) flux of energetic electrons in the heart of the outer trapped radiation belt. Both instruments are mounted on the outside surface of Giove-A. Note: The radiation environment of MEO is rather poorly documented by civilian satellites. And the data of the GPS satellite series is not available in the public domain. 41)
Status of Merlin and CEDEX: 2008
The Giove-A results show that the mean AE-8 fluxes (AE-8 is a model for trapped electron fluxes developed at NASA) agree well with GIOVE observations at around > 0.8 MeV but tend to under-predict at higher energies. This is in contrast to experience at geostationary orbit, where AE-8 is generally conservative. Nevertheless, the fluxes are within the reported uncertainties of the AE-8 model (a factor 2 to 3) over the long term. 42) 43)
Merlin is a compact space weather monitor designed and developed at QinetiQ, UK. The instrument was selected by ESA to fly on GSTB-V2/A (GIOVE-A) as an auxiliary payload. Merlin is a small, low power (2.5 W) instrument with a mass of 1.7 kg; it can be fitted on all operational and experimental spacecraft. The objective is to measure the radiation hazards in MEO, including internal charging currents (and thus ambient energetic electron fluxes), energetic proton fluxes, cosmic ray ion linear energy transfer spectra, and total ionizing dose rates in silicon. 44) 45)
Merlin is of CREDO (Cosmic Radiation Environment and Dosimetry Experiment) and SURF heritage. CREDO was initially flown on UoSat-3 of SSTL (launch January 22, 1990), the CREDO-II instruments were flown on STRV-1a (launch June 17, 1994) and on STRV-1c (launch Nov. 16, 2000), both missions of DERA.
Merlin incorporates the SURF and CREDO measurement functions into one package (complete with total dose monitoring) and includes the necessary power conditioning and data handling/storage functions.
Merlin consists of two `card-frame' modules, each housing printed circuit boards, along with a lid and a base. There is also an EMC screen between the two modules. The upper module accommodates the `sensor board' on which the detectors and their associated front-end electronics are located. The lower module houses the `generic processor board' which accommodates the housekeeping elements including power conditioning, microprocessor, analog-to-digital converter, memory, I/O ports and data interfaces.
Table 2: Performance capability/characteristics of Merlin
Given its exposed location and the uncertainties of MEO orbit characterization at present, Merlin has been allocated additional mass to increase its box-level radiation shielding. After accounting for the three card frames, each with thicker walls than standard, as well as thicker base and lid, the Merlin GSTB-V2/A unit has a total mass of approximately 1.7 kg.
Figure 20: Cutaway view of the Merlin instrument (image credit: ESA, QinetiQ)
Total dose is measured within Merlin at two shielding depths using calibrated RadFETs. These devices are left unbiased except during readout so that interruptions in power do not affect their readings. The shielding in the space-facing direction is 3 mm and 6 mm of equivalent Aluminium.
Protons and Ion LET (Linear Energy Transfer) monitor: Merlin houses two particle telescopes based on PIN diode detectors, one for heavy ion LET measurements and one for proton counting. The telescopes comprise two large-area circular silicon diodes of radius 9.8mm, separated by 25 mm. Pulse height analysis is applied to determine LET values, and simple thresholds applied for particle counting purposes. The telescope arrangement provides three main benefits: directionality, which enables the particle flux per steradian to be determined. Proton fluxes of > 40 MeV energy and ion LET spectra are measured, with ion LET values binned into 32 logarithmically spaced channels with the range of 95 MeV cm2 g-1 to > 28,500 MeV cm2 g-1.
CEDEX (Cosmic ray Energy Deposition Experiment):
CEDEX is designed and developed at the University of Surrey. The instrument is of CRE (Cosmic-Ray Experiment) heritage which was first flown on KITSat-1 (launch 1992), then on PoSat-1 (launch 1993), as on TiungSat of Malaysia (launch Sept. 26, 2000, CEDEX was operational to 2003), and on AMSAT-OSCAR-40 (launch 2000). The objective of CEDEX is to monitor high-energy (40 - 50 MeV) proton fluxes, to characterize the MEO orbital radiation environment in terms of the observed particle LET (Linear Energy Transfer) spectrum at the spacecraft. As a secondary function, dose rate-induced photocurrents are monitored by PIN diodes at 4 different shielding depths (2 mm and 4 mm Aluminum, and 2 mm and 4 mm of Copper shielding), enabling characterization of the dose-depth curve. The instrument mass is 2.05 kg (including radiation shielding), and has a power consumption of 4.2 W at 38 V.
The primary sensor consists of a 30mm x 30mm PIN diode detector 300 microns in depth, housed in a separate screened aluminium unit mounted on the CEDEX module box (three area PIN-diode detectors are mounted in a "telescopic" arrangement; hence, information pertaining to directions of the energy particles detected can be derived.). This is connected to a charge amplifier and a pulse-shaping circuit which, in turn, are connected to an event-driven, hardware-logic controlled pulse-height multi-channel analyzer. CEDEX is controlled autonomously by a CAN-microcontroller with its own data-storage RAM and built-in data-compression software. This sends data to an internal CAN-controller which formats and sends them on to the primary OBC via the spacecraft's CAN (Controlled Area Network) bus. CEDEX is a multichannel analyzer with 512 channels and a 0.5 pC (picocoulomb) charge resolution. The instrument charge range is between 0.2 -24 pC, equivalent to a normal incidence particle LET range of about 60 - 7500 MeV cm2 g-1 (200,000 particles/s).
Figure 21: Concept illustration of the CEDEX instrument (image credit: SSTL)
Figure 22: The CEDEX instrument (image credit: SSTL)
Data from GIOVE-A's radiation environment payload indicates that both instruments are working well, and are returning valuable information. The MEO environment is observed to be highly, dynamic as would be expected for this orbital regime, mainly due to occurrences of electron enhancement events. The charging, dose and dose rate effects from these electron enhancement events have been clearly recorded. The most significant event seen in the first five months was the April 2006 enhancement of the outer electron belt (14-21 April). A major solar particle event has yet to be observed, though a minor event July 2006 was recorded. Further data analysis from the Giove-A mission is on-going.
LRR (Laser Retroreflector):
An integrated array of 76 coated corner cube reflectors (each of 27 mm diameter) is located on the nadir side of the spacecraft (Figure 23). The objective is laser ranging for POD (Precise Orbit Determination) analysis [independent tracking by the SLR station network managed by the ILRS (International Laser Ranging Service)]. The LLR array is of size: 308 mm x 408 mm x 48 mm and trapezoidal in shape having 76 corner cubes, manufactured by IPIE (Institute for Precision Instrument Engineering) of Moscow, Russia. 46)
The GIOVE-A AOCS normal mode must maintain the spacecraft attitude such that the payload line of sight (nominally aligned with the spacecraft +Z Body axis) is always nadir-pointing and the solar array panels (aligned with the spacecraft body Y axis) can always achieve normal solar incidence by a rotation of the solar panels around the body Y axis.
Fourteen laser ranging stations participated in a campaign to track ESA's GIOVE-A satellite during the spring and summer of 2006, providing valuable data for the characterization of the satellite's onboard clock. The campaign was coordinated by ILRS (International Laser Ranging Service) and the GPC (GIOVE Processing Center) at ESA/ESTEC. The performance characterization of the onboard clocks is significantly enhanced by the use of SLR, a high precision technique for orbit determination that is independent of the navigation signal generation. The technique is based on a global network of stations that measure the round flight time of ultra short laser pulses to satellites equipped with LLR. Laser ranging provides instantaneous range measurements of millimeter-level precision which can be used to derive accurate orbit data. The use of SLR data allows a more robust orbit determination, and thus a more accurate clock characterization. In addition, certain satellite properties relevant to navigation, such as the offset between the center of mass and the center of the navigational phase center, can be verified and calibrated. 47) 48)
MEO GPS receiver:
A GPS navigation receiver to experiment with autonomous localization in the MEO orbit. The GPS receiver used is an adaptation of the SGR-GEO GPS receiver developed at SSTL. In this SGR-GEO demonstration on the GIOVE-A spacecraft, the GPS measurements are being recorded and downloaded to a ground station for post-processed orbit recovery.
The SGR-GEO device is a 24-channel GPS receiver. The receiver design also has several specialized features that will help it operate in GEO/MEO orbits: a) medium gain patch antenna, b) OCXO (Oven Controlled Crystal Oscillator), c) radiation mitigation, and d) FARM (Ferro-electric Random Access Memory). A larger patch antenna with a parasitic element was developed for use on GIOVE-A. - The MEO/GEO altitudes provide a very unfavorable environment for the reception of GPS signals. The receiver has to track the signals from the GPS satellites positioned on the far side of the Earth, implying weaker signal reception.. 49)
Special software to allow the tracking of weak GPS satellite sidelobe signals has been included and an orbit estimator is used to help the receiver provide position fixes for most of the time. The small physical size of the receiver permits overall shielding without incurring a prohibitive mass increase. A receiver radiation tolerance of around 13 kRads was achieved when powered, but up to 50 kRads was achieved with intermittent operation, which means that COTS technology can be applied with caution for GEO missions (Ref. 28).
Figure 24: Use of GPS signals outside the GPS orbit (left), SGR-GEO GPS receiver (right), image credit: SSTL
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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).