Myriade (CNES Microsatellite Program)

Myriade is the name of the CNES (French Space Agency) small satellite series, the development of which started in 1999 with the DEMETER spacecraft.

At the start of the 21 century the Myriade program represents the new microsatellite concept of CNES (France), taking advantage of low-cost satellites for technology demonstrations and to serve specific needs of the scientific community. The goal is to provide missions of about 15 MEuro (launch included) at a rate of two missions/year. In general, there are simply more frequent launch opportunities available for microsatellites as secondary payloads on a number of launch vehicles.

The objectives are to take advantage of these new enabling capabilities (innovation and introduction of technologies and common spacecraft structures, operations, management methods, building partnerships, improving access to space, etc.). Myriade offers the opportunity for a rearrangement of agency/industry functions such as to relieve CNES of the recurrent provisioning and integration tasks. It is also considered a structure for international cooperation (training for prime contractorship). The platform provides a large range of pointing capabilities: Earth pointing, inertial pointing. It also allows attitude slew maneuvers. ^{1) 2) 3) 4) 5)}

Original Myriade platform concept design was conducted by CNES. Today, the Myriade organization consists of:

• CNES is prime (design authority) of the Myriade series

• Satellite AIT (Assembly, Integration and Test) is performed by CNES or French industry. There is also a partnership between CNES, Alcatel Alenia Space (AAS), and EADS/Astrium SAS.. Astrium has participated to system test bench development, software specification and validation, some of the AOCS modes development, etc. While Alcatel has participated to PCDU (Belgium) and system bench development.

• CNES signed partnership agreements with AAS, and EADS/Astrium SAS which permits them to use the Myriade bus design and products for their own applications/missions (defense, commercial market, etc.). ^{6) 7)}

As of 2006, 16 Myriade-based satellites have been either launched (6 satellites) or are under development (10 satellites), see Table 2. The key for the success lies in the platform design were all elements have been carefully considered in terms of simplicity and robustness, while making no compromise with design and manufacturing quality. The most innovative initiative has been the decision to carefully review and minimize the redundancy principle, in association with a very efficient and well tested FDIR (Failure detection, Isolation and Recovery). A further key element in the program is the willingness of all partners to develop a real product line, i.e. carefully monitor the platform configuration, sign long term agreements with the equipment suppliers, make strategic stocks of elementary parts, etc.

Figure 1: Illustration of the Myriade platform architecture (image credit: CNES)

Myriade platform and subsystems:

The Myriade platform consists of a quasi cubic structure of 60 cm side length containing all subsystems. The overall architecture of the Myriade series S/C is based around generic avionics in support of the following functional chains: ^{8) 9)}

• AOCS (Attitude and Orbit Control Subsystem). The system employs 3 coarse sun sensors (for acquisition and safe mode); a 3-axis magnetometer and a star tracker for precise attitude measurement; and three raw gyros (1-axis gyros) for attitude information during S/C maneuvers. Attitude control is provided by up to 4 reaction wheels and by magneto torquers (to unload the reaction wheels). An optional orbit control system (hydrazine propulsion with four 1 N thrusters) may be used for orbit maneuvering capabilities and/or 3-axis attitude control. A pointing performance of < 5º is provided in raw attitude control mode while the stellar sensor provides accuracies of better than 0.1º (3σ) with a knowledge of better than 0.02º.

• Power generation, regulation, and distribution function. This includes: a) one steerable AsGa solar array (two deployable panels), b) one 14 Ah lithium-ion battery, c) a PCDU (Power Conditioning and Distribution Unit) in charge of: S/C separation, battery regulation, power distribution, thrusters and magneto-actuator commands. The solar array may be fixed (sun-pointing mission) or oriented with a one-axis mechanism (normal to the orbit plane).

• The onboard control function is provided by an OBC (On-board Computer) based on a transputer T805 (with 1 Gbit memory) with FPGA and PIC microcontrollers for equipment interfaces (I²C bus). - The onboard software and its FDIR (Failure Detection, Isolation and Recovery) function have already successfully recovered many problems [SEU (Single Event Upset) on wheels, on star tracker, on DRAM memory].

• Thermal control is based on the use of passive systems (paints, MLI, SSM coatings) and heaters.

Figure 2: The mechanical layout of the Myriade platform (image credit: EADS Astrium, CNES)

• Two S-band Rx/Tx chains and two antennas provide a quasi-omnidirectional coverage. The downlink data rate is 400kbit/s in operational modes and 25 kbit/s in safe mode. The S-band link is fully compliant with the CCSDS standard, whereas the X-band link complies with this standard at the packet level. The X-band is devoted to payload telemetry, whereas the S-band is being used for housekeeping and technological telemetry. The X-band downlink rates are in the range 18-72 Mbit/s. The S-band may be used for payload telemetry with limited data transmission requirements (up to 1 Gbit/day).

• The S/C design makes use of a generic and modular structure (aluminum and honeycomb). The X-panel includes the launcher adapter and the propulsion subsystem with its hydrazine tank. The payload is located on the X+ side of the bus. The mass budget of the series is < 130 kg. The power budget may be up to 200 W (max). The payload mass is limited to 40-60 kg, and a mean payload power of < 50 W. The design life is 2 years.

• Satellite redundancies are limited to :

- 2 transmitters (Tx) in cold redundancy

- 2 receivers (Rx) in hot redundancy

- 2 electric motors/electronics for solar generator rotating system

- Internal OBC parts (controllers, flash EPROM memories, DRAM memories for central software, FRAM memories).

Figure 3: Typical DEMETER platform structure in the Myriade program (image credit: CNES)

Operational support modes:

Four AOCS operational modes have been developed (some modes might not used):

• MAS (Mode Acquisition et Survie) or Acquisition and Safehold mode, which performs spacecraft solar pointing; it is used either for the first acquisition or the safe mode

• MGT (Mode Grossier de Transition) or Coarse Transition Mode, used to perform a coarse pointing support

• MCO (Mode Controle d'Orbite) or Orbit Control Mode, dedicated to orbit control maneuvers; it uses a set of thrusters

• MNO (Mode Normal Operational), used for fine attitude pointing (0.1º pointing performance); it is based on a star sensing measurements.

Some lessons learnt from operational missions: ¹⁰⁾

• The choice of high-level performance for the Myriade platform was rather favorable for the program. Although COTS equipment is being used whenever suitable there is no compromise on overall quality.

• AOCS: The pointing accuracies are better than 0.05º, due to good performances of the star tracker.

• The OBC has been developed internally at CNES with the support of an R&D program. The FDIR system was qualified on a test bench. The OBC provides a performance of 5 MIPS (1 Gbit memory) and is reprogrammable in orbit.

• The in flight operations (for all missions: DEMETER, PARASOL, Essaim and their payloads) have been completely successful so far (fall 2006). The onboard software and its FDIR function have successfully recovered many problems (SEU on wheels, on Star Tracker, on DRAM memory). The reloading function of onboard software has been used extensively to correct bugs or optimize functions or services. DEMETER has demonstrated very good autonomous orbit control function performance (satellite position maintained at ± 50 m wrt onboard commanding) thanks to efficient GPS and propulsion subsystem. (62 ms pulses generating a ΔV of < 1 cm/s).

• The aging of the solar generator and the battery induced losses of efficiencies in the range of 0.5 % to 8% after 15 months of in-flight operation.

• Some events (equipment resets due to SEU or software bugs/tunings), have induced satellite safe mode on the three in flight missions (around fifteen in total).

• There is a high probability of demonstrating in orbit a mission lifetime of > 3 years (design life of 2 years, the 2 years were already demonstrated as of August 2006).

As of 2006, the Myriade product line is considered to be operational. The design has been qualified in flight and the system can be used in many ways. As of fall 2006, there is a high probability that a demonstration of > 3 years in orbit operations will be achieved with the DEMETER mission (launch June 29, 2004) which surpassed already in August 2006 the design life of 2 years.

Figure 4: Illustration of Myriade series spacecraft (image credit: CNES, Astrium)

Table 1: Main performance characteristics of the Myriade satellite series

Table 2: Overview of missions using the Myriade platform

Figure 5: Artist's view of the DEMETER spacecraft in orbit (image credit: CNES)

Figure 6: Artist's view of the PARASOL spacecraft (image credit: CNES)

Figure 7: Artist's view of the Essaim constellation (image credit: EADS Astrium)

Figure 8: The Microscope spacecraft (image credit: CNES)

Figure 9: One SPIRALE satellite in flight configuration (image credit: Alcatel Alenia Space)

Figure 10: Artist's view of the Picard spacecraft >(image credit: CNES)

Figure 11: Illustration of the AlSAT-2 spacecraft (image credit: EADS Astrium SAS)

Figure 12: Artist's rendition of the SARAL spacecraft on an ISRO minisatellite bus (image credit: CNES)

Note: In the timeframe 2005/6, the AltiKa instrument was planned to fly on a Myriade platform. However, on Feb. 23, 2007 CNES and ISRO signed a MOU (Memorandum of Understanding) of the cooperative SARAL (Satellite with ARgos and ALtiKa) mission in which ISRO became the provider of the spacecraft (AltiKa is provided by CNES).

Figure 13: Illustration of the TARANIS satellite (image credit: CNES)

Figure 14: The Myriade HRG spacecraft (image credit: CNES)

Figure 15: Four secondary payloads (PARASOL+4 Essaim) on the Ariane-5 ASAP ring (image credit: CNES)

Myriade ground segment:

The multi-mission Myriade ground segment consists of:

• CCC (Command and Control Center), located at CNES in Toulouse, France. It performs the monitoring function [TT&C (Telemetry, Tracking & Command)] for the various Myriade missions. CCC can operate up to five Myriade missions in parallel.

• A ground station network (with 3.1 m diameter antenna S-band stations in Toulouse and northern Europe, and an X-band station in Toulouse for payload data acquisition). The network operates automatically.

• Another CNES station network can be used for punctual tasks (e.g., localization, etc.).

• The CCC interfaces with the scientific (or technological) Mission Center which send the payload programming messages and receives the payload data.

2) Information provided by Bernard Tatry of CNES, Toulouse, France

3) J. P. Aguttes, "High Resolution (metric) SAR Microsatellite Based on the CNES Myriade bus," Proceedings of IGARSS-2001, July 9-13, 2001, Sydney Australia

4) M. H. Thoby, "MYRIADE: CNES Microsatellite Program," AIAA/USU Small Satellite Conference, Aug. 13-16, 2001, Logan, UT, SSC01-I-8

5) 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

6) P. Luneau, M.-A. Clair, S. Pradalie, "MYriade CNES/Alcatel/Astrium Partnership," Proceedings of the 4S Symposium: `Small Satellite Systems and Services,' Chia Laguna Sardinia, Italy, Sept. 25-29, 2006, ESA SP-618

7) B. Tatry, M.-A. Clair, "Myriade CNES Small Satellite Program - Performances, Missions, Tool for education and cooperation," Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B5.1.01

8) M. Le Du, J. Maureau, P. Prieur, "Myriade: an adaptive AOCS concept," 5th International ESA Conference on Guidance Navigation and Control Systems, Frascati, Italy, Oct. 22-25, 2002, ESA SP-516

9) http://smsc.cnes.fr/MYRIADE/GP_plateforme.htm

10) B. Tatry, M.-A. Clair, "Myriade CNES Small Satellite Program: Performances, Missions, Tool foe education and cooperation," Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B5.1.01