SPIRALE (The French Spaceborne Early Warning Demonstrator) Mission
SPIRALE is a spaceborne early warning demonstrator program of DGA (Ministry of Defense of France), composed of two microsatellites in GTO (Geostationary Transfer Orbit) and of a ground segment for satellite control and for image processing. The French acronym of SPIRALE (Systeme Preparatoire Infra-Rouge pour l'Alerte) stands for: ”Preparatory System for IR Early Warning.”
The primary objective of the infrared imagery demonstration is to pave the way for a future operational space based EW (Early Warning) system by in orbit validation of its key requirements. SPIRALE will collect infrared Earth background images, which will allow to derive from simulations the technical characteristics of the future space based EW system (sensor, platform stability and processing chain).
The industrial team consists of EADS Astrium SAS as prime contractor of the program (responsible for system engineering, launch segment and ground segment, operations management and image processing). TAS (Thales Alenia Space) is responsible for the space segment, i.e. the definition and development of the demonstrator satellites. 1) 2) 3) 4) 5) 6)
The SPIRALE program started in January 2004. The CDR (Critical Design Review) took place in July 2006.
Each microsatellite is based on the Myriade platform developed by CNES (licensed to industry). The SPIRALE space segment has to comply with the following main observation requirements:
• Use of two microsatellites in GTO and compatibility with its associated environments (eclipses, radiation). The GTO implies the crossing of the Van Allen radiation belts twice during each orbit. These are regions located at about 1.4-1.5 RE (inner) and 4.5-6 RE (outer) where many energetically charged particles from the solar wind are trapped in the Earth's magnetic field. Note: RE is the Earth radius generally defined as 6378.140 km (mean equatorial radius).
• Use of the Ariane-5/ASAP launch system
• S-band communications for the TT&C services of each spacecraft and X-band transmissions for payload data
• Ensure a depointing capacity for reaching any point of the terrestrial disk
• Provision of a maneuvering capability for orbit control
• Use of an infrared specific payload to collect Earth background signature according to early warning requirements
• Ensure a long duration level of autonomy with a low number of contacts with the ground.
Figure 1: Overview of the operational concept during the various orbital periods (image credit: Astrium)
The spacecraft platform is based on the maximum reuse of already developed Myriade equipments - leading to a low-cost/short planning development capability compatible with a demonstrator philosophy. The bus is of cubical shape based on an aluminum structure on the bottom side of the S/C (accommodation of launcher interface and propulsion subsystem). The platform lateral faces are composed of four aluminum honeycomb panels linked by an aluminum truss on which the majority of the satellite components are mounted. Each lateral panel can be independently opened leading to a simple integration process of the platform.
For SPIRALE, the internal accommodation has been redesigned, mainly for radiation hardening reasons or for integration optimization. As a consequence, the SPIRALE satellite platform accommodation is very compact. This has led to a strong level of optimization of the satellite harness that presents a compact routing.
Figure 2: Artist's rendition of the SPIRALE spacecraft (image credit: TAS)
The payload is directly linked to the platform through the upper panel. This permits to get a compact satellite configuration minimizing thermal gradients and deformation and as a consequence, optimizing the pointing performances of the payload line of sight.
The thermal subsystem is based on passive cooling of the platform and payload components by the use of radiators (4 for the platform and 2 for the payload) associated to active heating lines (7 for the platform and 6 for the payload). The external thermal coating has also been adapted with regard to Myriade because of the hard GTO environment constraints. The MLI (Multi-Layered Insulation) has been redesigned and is compatible with low altitude atox and high altitude ESD (Electro-Static Discharge) constraints. Mainly, a conductive silicone black paint is used for external insulation. The stellar sensor head thermal control has also been fully redesigned, with a specific radiator and an associated heating line.
EPS (Electric Power Subsystem): The power supply makes reuse of the Myriade subsystem consisting of:
• A solar array, composed of GaAs cells and adapted to the GTO constraints ensuring power generation when illuminated by the sun
• A PCDU (Power Conditioning and Distributing Unit) using an unregulated power bus
• A Li-ion battery, similar to the Myriade one, and dedicated to power storage and energy source during eclipse or high consumption phases
• SADA (Solar Array Drive Assembly) also similar to the Myriade one and used for orienting the solar array with regard to energetic needs and mission constraints.
OBC (Onboard Computer): The command and control architecture of the SPIRALE satellites is an adapted version with the following main evolutions:
• Use of a centralized architecture. The payload specifics (simple interface, few functions) induced to centralize each payload management function inside the OBC.
• Modification of the image chain architecture of the OBC.
AOCS (Attitude and Orbit Control Subsystem): The concept deviates from the traditional LEO system due to the required GTO service support. The concept implemented is based on:
• Four distinct AOCS modes with a specific transition strategy
• Attitude sensing/estimation is using a star tracker, 3 sun sensors, and three 1-axis gyrometers
• Actuation is provided by 4 reaction wheels, 2 magnetotorquer bars, and four 1 N hydrazine thrusters.
Figure 3: AOCS modes of the SPIRALE transition modes (image credit: TAS)
The AOCS mode transition logic is illustrated in Figure 3. For SPIRALE, each AOCS mode is auto-coded with the Matlab-Simulink® simulator and directly encapsulated in the onboard software.
- The MAS mode, is the survival mode used after launcher separation and after anomaly. This mode differs from the Myriade one: it uses thrusters as main actuators and gyros and sun sensors for attitude estimation
- The MGT mode aims at ensuring a guided pointing to permit the transition towards nominal mode, and to initiate gyro-stellar attitude estimation. This mode also uses thrusters
- The MNO mode is the nominal mode. Attitude actuation is based on reaction wheels with an optimized hybrid unloading strategy based on low altitude magnetic actuation and high altitude kinetic momentum optimization through specific pointing
- The MCO mode is the orbital control mode, based on thruster actuation and gyro-stellar estimation. It is used to deliver a ΔV impulse needed for orbit acquisition (mainly perigee rising) and orbit control.
The MCO mode can be reached from both MNO and MGT mode. This is specific to SPIRALE because of the GTO orbit: the perigee altitude after injection is so low that MNO cannot be used (limitation of the reaction wheels capacity). This is why the first perigee rising maneuver with MCO has to be executed from MGT.
RCS (Reaction Control Subsystem): A monopropellant subsystem, based on four 1 N thrusters and with a tank enabling a 4.7 kg hydrazine capacity.
Figure 4: SPIRALE thruster qualification model (image credit: EADS-ST)
Figure 5: The propulsion subsystem of SPIRALE (image credit: TAS)
Table 1: Overview of key parameters of the SPIRALE spacecraft
Figure 6: Spacecraft view of the SPIRALE launch configuration (image credit: TAS)
Figure 7: Final checkout of the SPIRALE satellites at Kourou (image credit: EADS Astrium SAS)
Launch: The two SPIRALE spacecraft, SPIRALE-A and SPIRALE-B, were launched on Feb. 12, 2009 on Ariane-5 ECA from Kourou. The SPIRALE spacecraft were secondary payloads to the primary communication satellites of Hot Bird 10 (4892 kg) of EutelSat and NSS-9 (2290 kg) of SES New Skies. The two SPIRALE spacecraft fitted on the ASAP (Ariane Structure for Auxiliary Payloads) for microsatellites. 7) 8)
Orbit: Ariane GTO injection orbit, perigee = 600 km, apogee = 36,000 km, inclination = 2º. The SPIRALE operational orbit is referred to as HEO (Highly Elliptical Orbit) with a perigee of 600 km and an apogee of 36000km, inclination = 7º.
In GTO (Geosynchronous Transfer Orbit) - considering that unloading can only be executed by using the Earth magnetic field at low altitude - a specific pointing strategy, based on roll reversal maneuvers, is implemented at high altitudes to minimize the solar pressure disturbing torques effect on the unsymmetrical satellites solar array configuration. This concept is illustrated in Figure 9.
During parts of the orbit dedicated to mission and low altitudes phases, the solar array is kept pointed towards the sun and is oriented in the same half space as the sun with regards to the orbital plane. During standby phases of the orbit, at middle altitude range, the satellites are inertial pointed to the sun, but with their solar array oriented in the opposite direction. This leads to a ‘natural’ unloading of the reaction wheels because of the change of the sign of the solar disturbing torque, with regards to the satellite body.
Figure 8: A SPIRALE spacecraft in flight configuration (image credit: TAS, EADS Astrium)
Figure 9: ‘Roll Reversal’ pointing strategy (image credit: TAS)
Status of the mission:
• Fall 2012: One year and a half after the end of the operations, and thanks to the JSpOC public catalog of TLE, the project has access to the cumulative orbital bulletins of both SPIRALE satellites. It is observed, that the apogee of SPIRALE-A has dropped by more than 10,000 km in 19 months. 9)
- The SPIRALE satellites were the first French objects de-orbited after the implementation of the French Space Act. Risks of collision with the ISS were completely mitigated during the operations, and the GEO belt was left in less than one month after passivation. The probability for the two spacecraft to fall to Earth's in less than 25 years is greater than 90% (Ref. 9).
• After nearly two years of mission, the mission goals were achieved (the mission of the demonstrator has been completed in 20 months). End of life operations of the two SPIRALE demonstration spacecraft in February and March 2011. The perigee altitude was lowered to ~200 km. Simulations show compliance with the 25-year rule. 10)
- After more than 16 months in orbit, the two SPIRALE satellites health is such that a life prolongation is envisaged. Despite a very severe radiation environment, the initial mission has been fulfilled without any major anomaly, collecting millions of elementary observation sequences of the Earth background. - In addition the payload hardware has hardly been affected by the environment and shows today a nominal behavior very similar to the beginning of life behavior (very limited number of defective pixels, stabilized temperatures, no increase of power demands, etc.).
- The orbit determination performed through angular measurements by the SPIRALE station shows a very stable orbit even during encountered safe mode phases. Station keeping maneuver are performed rarely, 2 or 3 times a year nominally.
- Reaction Wheels: The particular off-loading strategy leads to keep significant margins on the wheel velocity (or kinetic momentum) which remains under 1500 rpm (~half of the wheel capacity) since the beginning of life. This “in orbit velocity” includes mission depointing conditions, yaw flip & roll steering maneuver, and special calibration configurations; it has been measured on the four reaction wheels.
- The DTU star tracker, used nominally in normal mode, is the corner stone of the attitude control subsystem. Although its availability is most of the time greater than 95% (measured on a slippery window of one orbit), it can be disturbed for a certain sky portion distributed randomly in space. In such cases, the gyro-stellar estimator switches automatically on the FOG gyro and the mission continues without any significant attitude perturbation, up to the next star tracker availability.
- SADM (Solar Array Drive Mechanism), commanded several times per orbit to optimize its pointing towards the sun, didn’t show any weakness since the beginning of life (for SPIRALE-A and -B).
- Power subsystem: Despite long eclipse periods, the power subsystem fits with the needs: the maximum encountered Depth Of Discharge (20% after a 2 hours eclipse) is far from the “under voltage” FDIR thresholds even when transmitting payload data to the ground during eclipse. Moreover, with the adverse spatial environment (4 paths through the Van Allen radiation belts per orbit), the solar array and battery degradations are lower than foreseen and open the door for a possible life extension.
- FDIR efficiency: When the satellite functioning is disturbed by radiation effects (perturbations of data exchange between the OBC and AOCS equipment most of the time), the FDIR strategy automatically resets (off and on) the on-board equipment (gyro, reaction wheels, star tracker) without ground intervention. This kind of phenomena regularly occurs in the Van Allen belts for an altitude lower than 6000 km at a mean rate of typically 1.5 events per day and per satellite.
- After the LEOP phase and a few reconfigurations in safe mode, the tanks are still fuelled with more than 50% of the propellant. The thruster behavior looks nominal, even station keeping maneuver are performed very rarely. In safe mode, the propellant consumption is far weaker than foreseen.
- The remaining propellant should be sufficient to perform a “de-orbitation” maneuver on the two satellites although this specific operation had not been envisaged in the nominal mission (Ref. 11).
• The initial GTO of the two spacecraft after deployment had an apogee at GEO altitudes whereas the perigee was around 250 km. Thus the instrument can be fully tested at GEO altitude, and observations performed on the lower parts of the orbit are corrected by a scale factor. It was however necessary to increase the perigee altitude to reduce the disturbing torques due to air drag effect at low altitude, thus allowing the use of Normal Mode (attitude controlled without the thrusters). This perigee rising from 250 km to 600 km has been achieved during the LEOP phase with very little propellant (~2 kg). Inclination of separation orbit was about 2º (near-equatorial orbit) and did not require any correction for the mission (Ref. 9).
Sensor complement: (Infrared Imager)
The instrument accommodation is on the upper platform honeycomb panel of the spacecraft. A payload control unit (the BECI) interfaces the platform electronics inside the lower structure of the satellite.
The overall objective of the imager is gather and analyze infrared images of terrestrial backgrounds and assess the technical requirements to detect ballistic missiles during their powered phase, just after launch.
The IR instrument is composed of a telescope, a filter wheel (ensuring images spectral selection), and an optical focal plane assembly (FPA). All the instrument optical components are mounted on a dedicated CeSiC (Carbon-fiber reinforced Silicon Carbide) structure for ensuring a strong optical stability and low dependency to thermal gradients. Further elements of the IR imager are:
- The focal plane assembly, composed of a dedicated cryostat homing the focal plane and cooled by an active cooling subsystem
- The video proximity electronics unit (BVP) providing conversion of the analog signal coming from the focal plane assembly into digital signals. BVP provides also the focal plane power supply.
- The payload control unit (BECI) is used to provide the command and control interface between the platform and the payload. BECI provides also the following functions: power supply and thermal control algorithm of the cooling subsystem, as well as the power supply and command for the instrument filter wheel, the reception and formatting from the BVP (of the digital signal with associated telemetry), and the processing of the payload telemetry prior to ground transmission
- The X-band payload telemetry chain, based on the Myriade 8-PSK transmitter, associated to a high gain low aperture antenna.
Figure 10: SPIRALE telescope CeSiC main mirror prior to polishing (image credit: TAS)
Figure 11: Illustration of the infrared imager payload (image credit: TAS)
The imager uses a MCT (Mercury Cadmium Telluride or HgCdTe) -type infrared detector array assembly in a staring configuration. The detector is provided by Sofradir of Grenoble, France.
Pyrotechnics were used for the deployment of the solar panels: On this mission, a pyrotechnic system was used to open both satellites' solar panels. This technique has advantages over mechanical systems (actuators) in terms of smaller bulk (volume) and mass. The Pyrosoft system, developed by Lacroix Defence & Security, also reduces the shock level caused by the opening of the panels and thus avoids any risk of damage. 15)
With this new procedure, the solar panels remained folded inside the satellite fairing until the proper time prior to the firing of the system, which enabled them to be deployed as planned. This represents the first in-orbit use of the low shock Pyrosoft release.
EADS Astrium, the prime contractor, is in charge of the ground segment for satellite control and image processing, as well as the integration and in-orbit exploitation of the satellites, while Alcatel Space is responsible for the satellite development.
The communications with the ground have been simplified: a unique dual antenna (TM/TC and payload data downloading) located on the Astrium site (Figure 12) is supported by the 2 GHz CNES antenna network (LEOP phase or back-up operations). In addition, the orbit determination is only based on angular measurements performed regularly by the station during the different satellite visibilities.
The TM/TC and payload data antenna is dedicated to the SPIRALE satellites and is operated by an Astrium team located in the C2 (Control Command) center at Astrium Toulouse. This C2 center includes a complete mission programming kit with demand deposit, mission operation planning and verification tools & facilities. Once the demand are validated and included in the mission planning, they are directly transferred to the TM/TC facilities to be translated and uploaded to the satellite.
When the payload telemetry is downloaded and received by the ground, it is processed in a protected and dedicated area which is very close to the other ground entities (Mission planning, TM/TC facilities). This global collocation of all the ground components in less than 100 m2 permits an active dialog and interface of all the ground entities and is a key of mission efficiency and success (Ref. 11).
Figure 12: The SPIRALE TM/TC and payload telemetry antenna on the top of W building in the Astrium Toulouse Space Center (image credit: EADS Astrium)
1) L. Frécon, A. Clauss, R. Dupré, “SPIRALE: The French Space-Based Early Warning Demonstrator,” 58th IAC (International Astronautical Congress), International Space Expo, Hyderabad, India, Sept. 24-28, 2007, IAC-07-B4.6.15
2) D. Alary, H. Lambert, “The Myriade Product Line, A Real Success Story,” Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B5.4.01, also in Acta Astronautica, Vol. 61, Issues 1-6, June-August 2007, pp. 223-227
3) A. Carucci, E. Montfort, R. Zecchini, C. Laborde, E. Suc, J-P. Gontan, Y. Delclaud, S. Mankai, “SPIRALE: Enlarging the Myriade Platform Flight Domain for the French Space-based Early Warning Demonstrator,” Proceedings of the 4S Symposium: Small Satellite, Systems and Services,Chia Laguna, Sardinia, Italy, Sept. 25-29, 2006
4) Didier Alary, Daniel Galindo, Eric Maliet, “SPIRALE: An optical space demonstrator for early warning,” 56th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law, Fukuoka, Japan, Oct. 17-21, 2005, IAC-05-B1.1.05
5) Didier Alary, “Early Warning Optical Demonstrator,” Conference on Missile Defense, AAAF, June 7-9, 2009, Seville, Spain
6) Philippe Chèoux-Damas, Laurent Boulogne, “SPIRALE demonstrator for Optical Early Warning from Space,” 2nd International Conference Military Space, Paris, France, Sept. 17-19, 2007, URL: http://www.aaafasso.fr/DOSSIERSAAAF/ACTES_COLLOQ.LIBRES/Ac-tColloq.07/Mil.Space07.NonCompress/44.pdf
7) “HOT BIRD 10, NSS-9, SPIRALE A And B In orbit,” Space Mart, Feb. 14, 2009, URL: http://www.spacemart.com/reports/HOT_BIRD_10_NSS_9_SPIRALE_A-And_B_In_orbit_999.html
8) “Two Spirale satellites are ready for their liftoff as auxiliary passengers on Ariane 5's next mission,” February 10, 2009, URL: http://www.arianespace.com/news-mission-update/2009/566.asp
9) Francois Bonaventure, Slim Locoche, Anne-Helene Gicquel, “De-orbitation studies and operations for SPIRALE GTO satellites,” Proceedings of the 23rd International Symposium on Space Flight Dynamics, Pasadena, CA, USA, Oct. 29- Nov. 2, 2012, URL: http://issfd.org/ISSFD_2012/ISSFD23_CRSD1_3.pdf
10) Fernand Alby, “Overview on 2011 Space Debris Activities in France,” Proceedings of the 49 Session of UN COPUOS (Committee on the Peaceful Uses of Outer Space), STSC (Scientific and Technological Subcommittee), Vienna, Austria, Feb. 6-17, 2012, URL: http://www.oosa.unvienna.org/pdf/pres/stsc2012/tech-23E.pdf
11) Emmanuel Giraud, Philippe Cheoux-Damas, Laurent Boulongne, Dominique Pawlak, “Two Small Satellites in GTO Orbit,” Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010
12) “Spirale, the space-borne early warning system: two million images collected in its first year of operations by Astrium,” EADS Astrium, June 15, 2010, URL: http://www.astrium.eads.net/de/pressezentrum/spirale-the-space-borne-early-warning-system-two-million-images-collected-in.html
13) “SPIRALE accepted by the French Armament Procurement Agency (DGA),” May 15, 2009, EADS Astrium, URL: http://www.astrium.eads.net/node.php?articleid=267
14) “French Missile-Alert Demo Passes On-Orbit Checkout,” Space News, May 18, 2008, p. 3
15) “Successful Cooperation between Egide and Lacroix Defence & Security,” March 30, 2009, URL: http://www.euroinvestor.co.uk/news/story.aspx?id=10237973
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.