ARTEMIS (Advanced Relay and Technology Mission Satellite)

ARTEMIS is ESA's first GEO data relay communication satellite with the objective to demonstrate new communication technologies, principally for data relay and mobile services. The technology demonstrations include an optical intersatellite link, first European operational use of an electric ion propulsion system, and a transponder for the support of EGNOS (European Geostationary Navigation Overlay Service) for signal enhancement of the GPS/GLONASS navigation satellite constellations. ^{1) 2) 3)}

Spacecraft:

The S/C structure consists of a box-shaped three-axis bus of Italsat heritage (Alenia Spazio bus family, Alenia is also the prime contractor to ESA). The primary structure consists of the central cylinder (aluminum honeycomb skinned with carbon fiber), the main platform, the propulsion platform, and four shear panels. The secondary structure is made up of the N/S radiators, the E/W panels, and the Earth-facing panel. The central propulsion module houses the propellant tanks, LAE (Liquid Apogee Engine), the East panel with the L-band antenna feed, the West panel with the IOL (Inter Orbit Link) antenna. The two antenna reflectors (2.85 m diameter) for IOL support are dominant features of the S/C structure.

Figure 1: Illustration of the ARTEMIS spacecraft (image credit: ESA)

S/C attitude is measured by Earth/sun sensors and gyros. Reaction wheels serve as actuators. Thrusters of an RCS (Reaction Control System) are used for wheel off-loading. The UPS (Unified Propulsion System) employs a bi-propellant system of a single 400 N LAE for insertion into GEO. The propellants are stored in two Cassini-type 700 liter tanks. E/W positioning is maintained by a 10 N RCS (Reaction Control System) engine. N/S positioning is maintained by electric ion thrusters. The IPS (Ion Propulsion System) comprises two thruster assemblies, RIT (RF Ion Thruster) and EIT (Electro-bombardment Ion Thruster). Each is powered and monitored separately, but a common propellant supply is used (40 kg of xenon), 600 W of input power is needed for operation..

S/C electric power of 2.8 kW (at equinox after ten years to a 42.5 VDC bus) is provided by two solar wings (span of 25 m). BSR (Back Surface Reflecting) solar cells are mounted on each of the two solar wings. Two NiH₂ batteries provide energy of 60 Ah for eclipse protection. A S/C design life of 10 years is provided. The S/C launch mass is 3100 kg (550 kg payload, 1538 kg propellant). ⁴⁾

At the platform level, ARTEMIS combines all the tasks classically associated with the data handling and attitude and orbit control subsystems into one subsystem, the ICDS (Integrated Control and Data System). The ICDS in turn is comprised of the following elements (all units have a redundancy):

• OBCU (On Board Computer Unit)

• IRES (InfraRed Earth Sensor)

• RIGA (Rate Integrating Gyro Assembly)

• PSSA (Precision Sun Sensor Assembly)

• MWA (Momentum Wheel Assembly)

• RU-A (Remote Unit A), RU-B (Remote Unit B)

• OBDH (On Board Data Handling) bus (ESA standard). The OBDH bus provides the communications media between the OBCU, and all platform and payload units.

The attitude and orbit control functions within ICDS make use of additional actuators which are not part of the ICDS, namely the RCS, the LAE, and the ion thrusters plus their alignment platform (ITAM).

Table 1: Overview of major ARTEMIS parameters

Launch: A launch of ARTEMIS, along with BSAT-2B of BSS (Broadcasting Satellite System) Corp. of Tokyo, Japan as co-passenger, took place on July 12, 2001 on an Ariane-5 vehicle from Kourou.
However, an upper stage malfunction of Ariane-5 resulted in a lower orbit than GEO (the two satellites were left in an orbit with a perigee of 592 km and an apogee of 17,518 km instead of the intended GTO of 858 km x 35,853 km). ⁵⁾

ARTEMIS rescue mission:

ESA immediately started recovery actions after the launch to get ARTEMIS into GEO using the kick motor as well as two experimental electric propulsion systems. In a first step, the kick motor (with a 400 N main engine in 5 successive perigee passes) achieved an orbital altitude of about 31,000 km. In a second step on July 24, the orbit was circularized with three apogee typical boost maneuvers. This provided an orbital period of 19 hours (referred to as "parking orbit") at an altitude of 31,000 km. By Sept. 2001, a four-station ground network consisting of Fucino (Italy), Dongara (Australia), Southpoint (Hawaii), and Santiago (Chile) was fully operational in support of ARTEMIS. Several months between ARTEMIS arrival in parking orbit and the start of the orbit raising maneuvers (via ion propulsion) were used to carry out commissioning and payload performance verification. In a third step, after the solar arrays were fully deployed, the RITA (Radio-frequency Ion Thruster Assembly) of Astrium GmbH was activated and tested. The actual ion-propulsion phase started on Feb 19, 2002; the final geostationary orbit at about 36 000 km was reached by the end of January, 2003. The ARTEMIS experience demonstrates the value of the flexible AOCS system using a combination of chemical propulsion with an ion propulsion system featuring an inherent high specific impulse. ^{6) 7) 8)}

ARTEMIS is the first spacecraft in history whose mission was salvaged by the availability of electric propulsion(flexible propulsion architecture using bi-propellant and ion propulsion).An upper stage malfunction of Ariane-5 resulted in a useless orbit with a perigee of 592 km, an apogee of 17,518 km, and an inclination close to 3º (the nominal GTO called for a perigee of 857 km, an apogee of 35,837 km, and an inclination of about 2º). The failure orbit represented a shortfall of about 500 m/s in injection velocity. -- The liquid apogee kick motor then raised the orbit into a 31,000 km circular parking orbit (5 near-perigee and 3 apogee maneuvers were performed, with sufficient fuel left for S/C east-west stationkeeping and attitude control using a 10 N RCS over the design life), and still about 5000 km short in altitude to GEO. The parking orbit was achieved on Aug. 24, 2001.

The final orbit raising maneuver employed an IPP (Ion Propulsion Package), carried on board for north-south stationkeeping and maintenance functions and not for any orbit boost functions of the S/C. In stationkeeping configuration, the thrust direction of the ion engines is perpendicular to the orbital plane. The rescue operation, however, required thrust to be generated in the orbital plane. This could only be realized by rotating the satellite in the orbital plane by 90º with respect to its nominal orientation. The actual ion-propulsion phase started on April 4, 2002 using a single functional ion thruster (RITA) and gaining between 12-14 km of altitude every day; the final geostationary orbit at about 36 000 km was reached by the end of January, 2003. This is a remarkable accomplishment of ion propulsion considering the S/C launch mass of 3,100 kg (550 kg payload, and 1,538 kg of bi-propellant). RITA-10 was used to retrieve the Artemis satellite from a total loss to a full recovery, after thrusting for 6,430 hours. - All orbit raising maneuvers were performed by a dedicated team of ESA, Alenia Spazio S. p. A., and EADS Astrium GmbH at the Telespazio center in Fucino, Italy. The deorbiting maneuvers of the S/C after mission end (a design life time of 10 operational years) will make use of the ion propulsion thruster with the remaining 25 kg of xenon. ^{9) 10)}

Figure 2: Maneuver strategy/geometry for ARTEMIS salvage mission

Orbit: Geostationary orbit at 21.4º eastern longitude.

Status of the mission: The ARTEMIS spacecraft is fully operational as of 2008 providing its communication services to a number of missions.

• In December 2006, ARTEMIS successfully relayed optical laser links from an aircraft in early December. These airborne laser links, established over a distance of 40 000 km during two flights at altitudes of 6000 m and 10 000 m, represent a world first. The relay was set up through six two-way optical links between a French Mystère 20 equipped with the airborne laser optical link LOLA (Liaison Optique Laser Aéroportée - Airborne Optical Laser Link) and the SILEX laser link payload on board ARTEMIS in its geostationary orbital position at 36 000 km altitude. These tests were made by Astrium SAS (France), the prime contractor for both LOLA and SILEX, as part of the airborne laser optical link program funded by the DGA (Délégation Générale pour l'Armement - the French Arms Procurement Agency). ¹¹⁾

Figure 3: LOLA telescope assembly, as fitted to aircraft used in the Artemis laser link trials (image credit: Astrium SAS)

• On Dec. 9, 2005, a first bi-directional optical link (data and command transmission) between Kirari (OICETS) of JAXA and ARTEMIS of ESA was established. OICETS (launch Aug. 23. 2005) was in a sun-synchronous LEO orbit and equipped with LUCE (Laser Utilizing Communications Equipment). ¹²⁾

• Since April 2003, ARTEMIS has been routinely providing high-data-rate links to SPOT-4 of CNES and to Envisat of ESA. Both the optical and Ka-band links are providing very-high-quality image transmission. SPOT-4 has been using one link session per day to transmit its data via ARTEMIS to CNES in Toulouse. Envisat is using a microwave link (8 links per day on two channels) for its ASAR and MERIS instrument image data, which Artemis transmits directly to the Envisat Processing Center at ESRIN in Frascati, Italy.

• ARTEMIS operations/services are being provided by a consortium which is made up of Alenia Spazio-Telespazio and ESA. Other companies involved in the project are Alcatel Espace, Astrium, Austrian Aerospace, Bosch Telecom, Casa, Fiar, Fiat Avio, Fokker, Laben, Saft and Top-Rel. ¹³⁾

• ARTEMIS finally reached its geostationary orbit at the end of January 2003 at 21.5º E (completing a most remarkable satellite recovery operation which has lasted for 18 months) and began to provide its new communication services. ¹⁴⁾

• In Nov. 2001, Artemis made a world premiere by establishing a laser link with the French Earth Observation satellite SPOT-4: imaging data was sent by SPOT-4 using a laser beam as signal carrier to Artemis and from there by radio waves to the ground. ¹⁵⁾

Payload/experiment complement:

The S/C data relay payload provides feeder links between Artemis and the ground as well as IOLs (Inter Orbit Links) between ARTEMIS and the S/C in LEO (SPOT-4). The feeder links operate at 20/30 GHz, while the IOLs can operate in S-band (2 GHz), Ka-band (23/26 GHz), and optical frequencies.

The feeder link, S-band and Ka-band elements jointly comprise the SKDR (S/Ka-band Data Relay) payload, while the optical IOL payload element is called SILEX (Semiconductor Intersatellite Link Experiment). ARTEMIS data relay service support (via RF links) is planned to be provided to ENVISAT of ESA.

SKDR (S/Ka-band Data Relay). The objective of the IOL antennas (2.85 m diameter) is to track a LEO user satellite via either loaded table values and/or error signals - and to receive up to 450 Mbit/s of data in the Ka-band, or up to 3 Mbit/s in S-band for relay via the feeder link to Earth (return link operation). Up to 10 Mbit/s in Ka-band and 300 kbit/s in S-band may be transmitted by ARTEMIS to the LEO satellite (forward link operation). In addition, ARTEMIS broadcasts a 23.540 GHz beacon to help the LEO satellite to track it. ¹⁶⁾

• A single Ka-band transponder (plus one backup) provides return/forward frequencies of 25.25 - 27.5/23.2 - 23.5 GHz links in Rx/Tx, adjustable EIRP (Effective Isotropic Radiated Power) of 45-61 dBW, G/T of 22.3 dB/K, up to 150 Mbit/s each of the three channels LEO to ARTEMIS (return link), and up to 10 Mbit/s from ARTEMIS to LEO (forward link). RH/LHCP on command.

• One S-band transponder (plus one backup) provides return/forward frequencies of 2.200-2.290/2.025-2.110 GHz links in Rx/Tx, adjustable EIRP 25-45 dBW, G/T of 6.8 dB/K. The bandwidth is 15 MHz. Up to 3 Mbit/s of data can be transmitted in a single channel from LEO to ARTEMIS (return link), and up to 300 kbit/s can be transmitted from ARTEMIS to LEO (forward link). RH/LHCP on command.

Feeder link of SILEX and SKDR: Three transponders (plus one backup) act as ground-ARTEMIS links for SILEX and SKDR. The feeder Ka-band frequencies are: 27.5-30/18.1-20.2 GHz for Rx/Tx. The EIRP is 43 dBW, G/T of 0 dB/K, use of 234 MHz bandwidth, linear vertical polarization.

Technology demonstrations:

SILEX (Semiconductor Intersatellite Link Experiment), an ESA laser experiment built by MMS, France (now EADS Astrium SAS). SILEX consists of two optical terminals, namely OPALE (Optical Payload for Intersatellite Link Experiment) located on ARTEMIS, and PASTEL (PAssager SPOT de Técommunication Laser) on-board SPOT-4. The objective is to beam data at rates of 50 Mbit/s (bit error rate of <10^-6) from the transmitter terminal on SPOT-4 in LEO - to the receiver (OPALE) on ARTEMIS for subsequent relay via feeder link to the SPOT ground segment in Toulouse. The SILEX terminal on-board ARTEMIS is also being used to support a second LEO experiment, namely an IOL between ARTEMIS and OICETS (Optical Inter-orbit Communications Engineering Test Satellite) of NASDA. ¹⁷⁾

• PASTEL (PAssager SPOT de Técommunication Laser). A joint ESA/CNES passenger demonstration experiment. PASTEL is a prototype high data-rate intersatellite transmission system based on laser technology. The objective is to transmit imaging data from SPOT-4 to ARTEMIS. The aim of the experiment is to validate the PASTEL concept design in an operational environment. PASTEL is a gimbal-mounted assembly consisting of a telescope, an optical bench with a fine pointing system, communication detectors with avalanche photodiodes, a thermal control system for precision temperature control, a two-axis gimbal mechanism, and the launch locking mechanisms needed during the launch phase. The telescope mirrors and main structural elements are made of Zerodur. The acquisition and tracking sensors use CCD detectors. The laser diodes are of the GaAlAs type. The SPOT-4 - ARTEMIS optical links operate at wavelengths of 830 nm. The peak output power is 160 mW (60 mW continuous operation), the beamwidth is 0.0004º. Data to be transmitted include: HRVIR image data, pseudo-noise (PN) code, PASTEL telemetry.

• OPALE (Optical Payload for Intersatellite Link Experiment) terminal, mounted on the geostationary satellite ARTEMIS (a GEO terminal). The receiver employs Si-APD (Silicon Avalanche Photodiode) detectors and a low-noise trans-impedance amplifier of 1.5 nW useful receiver power.

Figure 4: Schematic illustration of a SILEX laser terminal (image credit: ESA, EADS Astrium SAS)

Each SILEX terminal features a telescope of 25 cm diameter (which is mounted on a coarse pointing mechanism), and provides an `antenna' gain of well above 100 dB. The disadvantage of these extreme antenna gains is the very narrow width of the transmitted beam, requiring very accurate pointing. The divergence tolerance of the optical communication beam for the SILEX configuration is 8 μrad (or about 0.00046º). PASTEL and OPALE use a dedicated acquisition sequence. Initially, both terminals (OPALE and PASTEL) coarsely point to each other. This is done when OPALE scans a wide-angle (750 μrad) beacon beam in the direction of PASTEL. On illumination of PASTEL by the beacon beam, it rapidly corrects its line of sight and directs in turn a narrow communication beam towards OPALE. Similarly, OPALE detects the incoming PASTEL signal, aligns its line of sight, and transmits its narrow communication beam towards PASTEL. The two terminals then remain locked on each other in closed-loop tracking, permitting subsequent communication.

SILEX terminal characteristics (OPALE): total mass: 150 kg; moving part: 70 kg; telescope diameter: 25 cm; power: 130 W; laser diodes: 60 mW power, 0.8 to 0.86 µm; pointing accuracy: better than 1 arc second.

Figure 5: LEO-GEO data transmission of SILEX (image credit: ESA)

A first data transmission test, using the laser link (SILEX) between ARTEMIS and SPOT-4, was realized on Nov. 21, 2001 (with ARTEMIS in parking orbit) on four consecutive SPOT-4 orbits for contact periods between four and 20 minutes each. The SILEX terminal on-board ARTEMIS activated its optical beacon to scan the area where SPOT was expected to be. When contact was made, SPOT-4 responded by sending its own laser beam to ARTEMIS. On receiving the SPOT-4 beam, ARTEMIS stopped scanning and the optical link was maintained for a pre-programmed period lasting from 4 to 20 minutes. Data rates of 50 Mbit/s were reached transmitting test data from SPOT-4 via ARTEMIS to the ground. An extremely low bit error rate of the data stream was confirmed (better than 10^-9) at ESA's test station in Redu (Belgium) and the SPOT 4 receive station in Toulouse, France. On Nov. 30, 2001, the first-ever transmission of an image by laser link took place from one S/C to another. ^{18) 19)}

Note: The art of establishing the optical data link consists of pointing a laser beam so accurately that the partner satellite is illuminated. The laser beam has a width of only 300 m after travelling 40,000 km through space while the LEO satellite has a relative velocity of several km/s. While in parking orbit, 26 optical links were attempted and established, all of them were successful. Once the link was acquired it was always maintained for the pre-programmed time slot, and no loss has ever occurred.

Figure 6: Overview of mission elements between ARTEMIS and SPOT-4 (image credit: CNES)

LLM (L-band Land Mobile) payload. The objective of the communications payload is to permit two-way communications, via satellite, between fixed Earth stations and land mobiles, such as trucks, trains or cars, anywhere in Europe and North Africa. The LLM package is fully compatible with the EMS (European Mobile System) payload developed by ESA and flown on Italsat-2. Hence, full redundant support is provided. - The LLM receives the signals transmitted by the fixed users in Ku-band (14.2 GHz) and transmits them at L-band (1550 MHz) to the mobile users (forward link). The return link establishes the connection from the mobile user at L-band (1650 MHz) to the S/C, and at Ku-band (12.75 GHz) from the S/C to the fixed user in the ground segment. About 400 bi-directional user links can be established simultaneously.

ARTEMIS carries two antennas of 2.85 m diameter and a multiple element feed for pan-European coverage and three European spot beams. Three 1 MHz plus three 4 MHz SSPA (Solid-State Power Amplifier) channels, provide 400 2-way circuits with an EIRP>19 dBW. The on-board L-band transmits to terminals (users) in the ground segment at 1550 MHz and receives data at 1650 MHz. A Ku-band feeder link at 14.2/12.75 GHz Rx/Tx transmits the data to the home stations. All channels are fully tunable and most commandable for LH/RHCP support.

IPP (Ion Propulsion Package). The electric propulsion system of ESA on-board ARTEMIS was developed under the leadership of EADS Astrium GmbH to provide the ΔV required for north-south stationkeeping maneuvers (inclination control throughout the lifetime of the S/C). IPP consists of a redundant pair of thruster assemblies, one mounted on each of the north and south faces. Each assembly comprises ITAM (Ion Thruster Alignment Mechanism) upon which two redundant thrusters from different sources are mounted. The entire IPP (2 RITA+ 2 EITA) assembly has a mass of 84 kg (without propellants).

RITA and EITA are both "gridded" ion thrusters and for both engines the ion-beam neutralization is provided by electrons delivered by a so-called "neutralizer" electron source. ESA pursued two separate developments by industry to avoid the possibility of the incorrect application of a single technology thereby jeopardizing the future utilization of electric propulsion in its space programs. The complete IPP consists of the following assemblies: ^{20) 21)}

Figure 7: Block diagram of IPP on ARTEMIS (image credit: Astrium GmbH)

• RITA (Radio-frequency Ion Thruster Assembly) of Astrium GmbH, Ottobrunn, Germany. The RIT-10 thruster for ARTEMIS has a beam diameter of 9.8 cm and delivers 15 mN thrust with an Isp of 3500 s and a system power of 560 W. The system provides 15,000 hours of operation at 15 mN. A single RITA unit has a mass of 13.9 kg. Positively charged particles are fed into the exhaust beam by a neutralizer to avoid satellite charging. The RITA-10 system device was used on the ARTEMIS spacecraft.

Table 2: Characteristics of the RIT-10 device ²²⁾

Figure 8: Illustration of the RIT-10 device (image credit: EADS Astrium)

• EITA (Electron-bombardment Ion Thruster Assembly). EITA is an ion engine system mainly developed by DERA, UK. The UK-10 has an exit diameter of 10 cm diameter, it provides a maximum thrust of 18 mN at an Isp of 3400 s. The input power is 700 W. Thrust is provided by the acceleration of xenon ions, produced by a DC electrical discharge in a diverging axial magnetic field, and a three grid system used to extract the positive ions from the resulting plasma. The positive charge of the ion beam is neutralized by electrons from an external cathode. The EITA version for ARTEMIS was provided by MMS, UK (now Astrium Ltd.). It delivers 18 mN of thrust. A single EITA unit has a mass of 15.2 kg. ^{23) 24) 25) 26)}

• PSDA (Propellant Storage and Distribution Assembly) built by Astrium Ltd.; including XST (Xenon Storage Tank), PSME (Electric Pressure Regulator Mechanism), an electronic xenon pressure regulator, and EPRE (Electric pressure Regulator Electronics) which is physically included in the EITA box.

• One Ion Thruster Alignment Assembly consisting of two ITAM (Ion Thruster Assembly Mechanism) by Austrian Aerospace, and ITAE (Ion Thruster Alignment Electronics) of Astrium GmbH. A gimbal range of >6º half-cone range from nominal orientation is provided.

Figure 9: RITA ion thruster aboard ARTEMIS prior to fairing encapsulation (image credit: Astrium)

EGNOS (European Geostationary Navigation Overlay Service) payload.

The objective is to provide enhanced navigation performance in terms of accuracy and integrity (with the required levels of availability and continuity) over the ECAC (European Civil Aviation Conference) region. The service may later be extended to neighboring regions. ^{27) 28)}

The EGNOS payload on ARTEMIS uses the Ku-band in the uplink and downlink (for S/C - fixed user communication in the ground segment). The uplink frequency is allocated at 13.875 GHz, and separate from the LLM feeder link frequency, while the downlink of the navigation payload is shared with the LLM channels (12.748 GHz). The transmitted EGNOS wide-area service signal is the GPS L1 frequency at 1575.42 MHz (L-band).

The total mass of the navigation payload, including structure, thermal control hardware and the DC harness, is 25 kg. Its total power consumption is about 110 W.

Table 3: Some performance characteristics of the ARTEMIS navigation payload

The EGNOS payload on ARTEMIS serves as a geostationary wide-area augmentation system for all GPS signals in its large area of coverage by transmitting:

• GPS-like signals (ranging function)

• GPS health and integrity conditions obtained by ground monitoring stations. This is the RAIM (Receiver Autonomous Integrity Monitoring) function.

• Ranging errors (differential correction function) - these are the conventional DGPS services.

The ESA-developed EGNOS payload on ARTEMIS is part of the overall EGNOS system.

2) A. Wilson, "ARTEMIS," ESA publication BR-142 with the title: More than Thirty Years of Pioneering Space Activities, 1999, pp. 156-161

3) A. Dickinson, S. Greco, I. La Rosa, M. Protto, "The ARTEMIS Program: Near Term Advanced Communications Technology," Proceedings of 47th AIAA Congress, Beijing, China, Oct. 7-11, 1996

4) J. E. Haines, "Design and Performance of the ARTEMIS Power System," 1st International Energy Conversion Engineering Conference, August 17-21, 2003, Portsmouth, VA, USA, AIAA 2003-5975

5) http://www.esa.int/artemislaunch/

6) G. Oppenhaeuser, A. G. Bird, L. van Holtz, "ARTEMIS -- 'A Lost Mission' on Course for Full Recovery," ESA Bulletin Nr. 110, May 2002, pp. 9-16, URL: http://www.esa.int/esapub/bulletin/bullet110/chapter1_bul110.pdf

7) R. Killinger, M. Surauer, R. Kukies, A. Tomasetto, L. van Holtz, "ARTEMIS Orbit Raising Inflight Experience with Ion Propulsion," Proceedings of 53rd IAC and World Space Congress, Oct. 10-19, 2002, Houston, TX, IAC-02-S.4.04

8) http://www.esa.int/esaCP/ESA9BAVTYWC_Improving_0.html

9) R. Killinger, H. Gray, R. Kukies, M. Surauer, G. Saccoccia, A. Tomasetto, R. Dunster, "ARTEMIS Orbit Raising In-Flight Experience with Ion Propulsion," 28th International Electric Propulsion Conference (IEPC), Toulouse, France, March 17-21, 2003

10) G. Oppenhäuser, A. G. Bird, "ARTEMIS Finally Gets to Work," ESA Bulletin 114, May 2003, pp. 50-53, URL: http://www.esa.int/esapub/bulletin/bullet114/chapter5_bul114.pdf

11) http://www.esa.int/esaCP/SEMN6HQJNVE_index_0.html

12) http://www.esa.int/esaCP/SEMIJVVLWFE_index_0.html

13) http://www.aerospace-technology.com/projects/artemis/

14) http://www.esa.int/esaCP/SEMQEK1A6BD_index_0.html

15) http://www.esa.int/esaCP/ESAI8MZ84UC_index_0.html

16) Note: In very elaborate communication systems with intermediate geostationary transmission satellites, the term `uplink' is usually replaced by `forward link' to avoid confusion. Similarly, the term `downlink' is usually replaced by `return link.'

17) T. Tolker-Nielsen, J.-C. Guillen, "SILEX: The First European Optical Communication Terminal in Orbit," ESA Bulletin No 96, Nov. 1998, URL: http://www.esa.int/esapub/bulletin/bullet96/NIELSEN.pdf

18) G. Planche, V. Chorvalli, "SILEX in-orbit performances," Proceedings of the 5th International Conference on Space Optics (ICSO 2004), March 30 - April 2, 2004, Toulouse, France. Ed.: B. Warmbein. ESA SP-554, Noordwijk, The Netherlands

19) http://spot4.cnes.fr/spot4_gb/pastel.htm

20) Information provided by R. Killinger of Astrium GmbH, Ottobrunn, Germany

21) "RITA - The Ion Propulsion System for the Future," URL: http://cs.space.eads.net/sp/PDF/rita.pdf

22) http://cs.space.eads.net/sp/SpacecraftPropulsion/Rita/RIT-10.html

23) D. G. Fearn, "Low Cost Missions Using Ion Propulsion," Proceeding of the British Interplanetary Society Symposium on `The search for life on Mars,', London, Nov. 11, 1998

24) Information provided by C. Edwards of DERA

25) H. L. Gray, "Development of Ion Propulsion Systems," GEC Review, Vol. 12, No 3, 1997, pp. 154-168

26) http://www.esa.int/esaCP/ESANO40VMOC_index_0.html

27) S. Badessi, C. F. Garriga, J. Ventura-Traveset, J. M. Pieplu, "The European ARTEMIS Satellite Navigation Payload: Enhancing EGNOS AOC Performance," ION GPS 1998, Nashville, TN (USA), Sept. 15-18, 1998.

28) The ECAC coverage area is from 30º W to 45º E and from 25º N to 75º N