PARASOL of CNES Myriade Series

PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar) is a French (CNES) science mission of the Myriade series, put forward by CNRS/LOA [Laboratoire d'Optique Atmosphérique at USTL (Lille University of Science and Technology), Lille, France)], with the general objective to study the Earth's atmosphere. Of particular interest in this study is the role of clouds and aerosols (fine particles in suspension in the air) in climate mechanisms (which effect has global warming on cloud cover? How do clouds and aerosols interact?).

The PARASOL mission flies the POLDER-derived instrument as its main payload with the objective to improve the microphysical and radiative property characterization of clouds and aerosols for model improvement. Clouds and aerosols play an important role in direct and indirect radiative forcing of the Earth's energy budget. The study of the dynamic processes and interactions between liquid and solid particles (radiative properties of clouds and aerosols) in the atmosphere is therefore a major goal. PARASOL (POLDER) complements CALIPSO (Lidar) and NASA's Aqua and Aura (MODIS, CERES) missions, it also makes use of the CloudSat (CPR) radar data. The instruments form a so-called "train of satellites," all in the same orbits and in a PM sequence (a loosely-coupled constellation), provide a unique opportunity of near-simultaneous observations, by collecting data in different spectral domains (optical and microwave regions) and at various FOVs (Field of Views). ^{1) 2) 3)}

Spacecraft:

The PARASOL microsatellite of CNES consists of a box-like structure (size: 60 cm x 60 cm x 80 cm) with deployable solar panels (Myriade microsatellite bus). The S/C is three-axis stabilized, providing a pointing accuracy of 0.1º, and a knowledge of 0.02º. Attitude sensing is provided by a star sensor, 3 sun sensors, a magnetometer, and gyros, actuation is provided by 4 reaction wheels and 3 magnetic torquers. The solar power is about 180 W (BOL). The solar array consists of two hinged panels folded against the platform during launch (array length of 1.7 m, triple junction AsGa solar cells are used with an efficiency of 26%, providing 180 W of power). The solar array is articulated for maximum power generation. The Li-ion battery has a capacity of 15 Ah (commercial product of Sony). Regulation and distribution of power is provided by PCDU (Power Control and Distribution Unit). Orbit control (maintenance) is provided by a blow-down hydrazine system (four 1 N thrusters, using 4.5 kg of hydrazine, Isp of 210 s). The S/C mass is 120 kg at launch, the mission design life is one year with two years as goal.

OBC: Centralized onboard data handling is provided by a 10 MIPS T805 microprocessor (a radiation-hardened transputer at 15 krad). The OBC power consumption is < 6 W with a mass of 3 kg. Serial asynchronous links are used for onboard data handling.

The PARASOL mission is the second one (after DEMETER) in the Myriade microsatellite series of CNES (see Myriade overview at end). Myriade also marks the beginning of an innovative partnership between CNES, Alcatel Space and EADS-Astrium. These two companies helped develop Myriade bus, so they can use the concept for their own missions.

Spacecraft construction was provided mainly in-house. CNES also called on its network of industry partners, including Toulouse-based aero-structures manufacturer Latécoère to assemble and integrate the microsatellite.

Figure 1: Artist's view of the PARASOL spacecraft (image credit: CNES)

Orbit: Sun-synchronous circular orbit, altitude = 705 km, inclination = 98.21º, the local equator crossing time is at 13:30. PARASOL will be part of the so-called "A-train" consisting of the Aqua, CloudSat, CALIPSO, PARASOL, and Aura spacecraft.

Note: At the launch of PARASOL, the A-train is consisting of the Aqua (launch 2002) and Aura (launch 2004) spacecraft of NASA. CALIPSO and CloudSat are going to follow in 2005. In addition, the OCO (Orbital Carbon Observatory) minisatellite of NASA was assigned to the A-train, with a planned launch in 2007/8.

Launch: A launch of PARASOL as a secondary payload on Ariane-5G+ from Kourou took place on Dec. 18, 2004. Another secondary payload was NanoSat1 of INTA, Spain.

The primary payloads on this flight were Helios-2A (launch mass of 4200 kg), a military reconnaissance mission of DGA, France, in partnership with Spain and Belgium (the first satellite in France's second-generation military surveillance system); and four microsatellites called Essaim-1, -2, -3, and -4. Essaim means `swarm' in French, they are also of DGA (Délégation Générale pour l'Armement - French Arms Procurement Agency). The objective is analysis of the electromagnetic environment (military use). The Essaim satellites use also the Myriade microsatellite bus of CNES.

Figure 2: Illustration of the A-train constellation (image credit: CNES)

Figure 3: Illustration of the A-train observations (image credit: NASA)

Communications: A payload mass memory provides a capacity of 16 Gbit. RF communications are provided in S-band (frequency = 2.2 GHz) at a downlink data rate of 400 kbit/s and an uplink rate of 20 kbit/s. In addition, there is an X-band downlink for payload data (8 GHz, 16.8 Mbit/s data rate) with a modulation scheme in QPSK. The communication protocol is in CCSDS standard.

Mission status: The PARASOL spacecraft is operating nominally as of 2008. ⁴⁾

• The PARASOL spacecraft reached its final 705 km orbit in March 2005 and joined the A-train of NASA flying with Aqua, CALIPSO, CloudSat, and Aura).

• As of April 2005, the spacecraft and its instruments completed the commissioning phase. Important maneuvers were conducted during the commissioning phase to get into the final operational orbital slot (SSO, 705 km altitude, LTAN of 13:30 hours). Now, the PARASOL orbit control imposes to keep the S/C in a prescribed "box". The needed maneuvers are being conducted nearly each month - with the consequence that each time the scientific mission is "off" for two orbits.

• As of fall 2005, PARASOL was set to begin its mission to observe clouds and aerosols in the A-train.

• As of fall 2005, the hydrazine left in the propulsion system is sufficient to maintain station keeping for > 10 years.

• On-orbit operational lessons learnt: The DEMETER operational experience and feedback has permitted to upload corrections on the PARASOL platform software very soon after its launch (both S/C are of the same family).

Sensor complement:

POLDER-P (Polarization and Directionality of the Earth's Reflectances - PARASOL), a CNES instrument (radiometer/polarimeter) of ADEOS and ADEOS-II mission heritage. POLDER-P is an improved version of POLDER1/2, designed at the LOA atmospheric optics laboratory in Lille (CNRS-USTL) and built by EADS-Sodern. The objective is the measurement of multidirectional and polarized solar VIS/NIR radiation reflected by the atmosphere and Earth's surface, and aerosols. POLDER-P consists basically of a digital staring camera composed of a 274 x 242 pixels CCD detection array, wide field-of-view telecentric optics (± 51º cross-track and ± 43º along-track) and of a rotating wheel carrying spectral and polarized filters. ^{5) 6) 7) 8)}

Table 1: POLDER-P instrument parameters

Figure 4: Illustration of the PARASOL payload (image credit: CNES)

The sensor concept is based upon the application of polarization filters and measures in a number of spectral bands and at various observation angles (multiangle viewing capability). Compared to POLDER-1/2, the telecentric optics array has been turned 90º to favor multidirectional viewing over daily global coverage. Likewise, a 1020 nm waveband has been added to conduct observations for comparison with data acquired by the lidar on the CALIPSO mission, one of its companion satellites on the A-train. POLDER-P also relies on the innovative techniques developed to calibrate the POLDER1/2 instruments, using in particular the sun's reflection from the ocean surface, clouds and desert areas as targets to validate inflight performance.

A motorized filter wheel enables the successive study of different wavebands of visible light. By adding a band in the near-infrared, POLDER-P expands the spectrum being analyzed, and provides an even richer harvest of scientific data.

POLDER has nine spectral channels, three of which are implemented with polarized filters (total of 15 channels, three channels are needed for each polarized band). The spectral region is from 443 to 910 nm. For the polarized wavelengths, 3 filters measure the linear polarization of the incoming radiation in three directions separated by 120º. The observed data permit the derivation of the Stokes parameters and the total irradiance. The acquisition of an image sequence takes 20 seconds, enabling the observation of one natural target from different directions. The radiometric performances of POLDER-P are similar to those of POLDER-1/2 flown on ADEOS and ADEOS-II, respectively. The vicarious calibration methods, based on the observation of natural targets (no calibration system on board PARASOL), are also the same as those of the POLDER-1/2 instruments.

Table 2: Spectral bands of POLDER (comparison of bands with heritage instruments)

The saturation levels are given in units of normalized radiance, i.e., the maximum spectral radiance divided by the solar spectral radiance at nadir and multiplied by π. The dynamic reflectance range is subsequently obtained by dividing the range by cos θ_s, where θ_s is the solar zenith angle.

A scene (a 2D image) has a size of about 1600 km (along-track) x 2400 km (cross-track) with a ground spatial resolution of 6 km x 7 km at nadir.

Instrument accommodation: POLDER-P has a mass of about 32 kg, a size of about 80 cm x 50 cm x 25 cm, the power consumption = 50 W in imaging mode (mean power of 29 W). The data rate is 883 kbit/s at 12 bit quantization.

Figure 5: Cut-away view of the POLDER-P instrument (image credit: ICARE)

Figure 6: Overview of POLDER-P instrument filter settings (image credit: CNES)

Figure 7: POLDER-P instrument response (image credit: ICARE)

Ground segment:

Mission operations and payload data acquisition are being performed at CNES (Myriade Control Center). The MIcrosatellite Ground Segment (MIGS) designed by CNES comprises: ⁹⁾

• CCC (Command and Control Center) in Toulouse

• Network of S-band (2 GHz) satellite tracking stations

• Two S-band stations at Aussaguel, near Toulouse, and Kiruna in Sweden providing telemetry/telecommand links

• The TETX (Telemetry Earth Terminal X-band), receiving station in Toulouse for science data reception

• Communication network

• Mission Center in Toulouse

• ICARE Data Processing and Management Center hosted at USTL (Université des Sciences et Techniques de Lille), Lille, France. Note: ICARE is a research structure set up in 2003 on a national level and consisting of CNES, INSU, USTL, etc. (all research laboratories) - to study aerosol-cloud-radiation interactions and the water cycle (cloud properties, atmospheric chemistry) and using data from various missions. ¹⁰⁾

Figure 8: Overview of the MIGS reference architecture, presently with PARASOL and DEMETER (image credit: CNES)

Figure 9: Overview of the PARASOL ground segment (image credit: CNES)

Figure 10: Data products of PARASOL (image credit: CNES)

2) C. Bastien-Thiry, M. Bach, A. Lifermann, "PARASOL a Microsatellite in the A-Train for Earth Atmospheric Observations," Proceedings of the 5th IAA Symposium on Small Satellites for Earth Observation," April 4-8, 2005, Berlin, Germany

3) P. Lier, M. Bach, "PARASOL a microsatellite in the A-Train for Earth atmospheric observations," Proceedings of the 56th IAC 2005, Fukuoda, Japan, Oct. 17-21, 2005, IAC-05-B5.4.03

4) C. Bastien Thiry, F. Serene, H. Darnes, F. Duchevet, "DEMETER and PARASOL after 2 years in orbit: where are we?," Proceedings of the 4S Symposium: `Small Satellite Systems and Services,' Chia Laguna Sardinia, Italy, Sept. 25-29, 2006, ESA SP-618

5) "PARASOL - Parasol effect and greenhouse effect," CNES Dossier, Nov. 2004, pp. 20, http://smsc.cnes.fr/PARASOL/dossier_presse_parasol.pdf

6) "Building on the heritage of POLDER and Demeter," http://www.cnes.fr/automne_modules_files/standard/public/p2885_2a1e6c9205cf3ae25c9994e6635d6f6edossier4.pdf

7) F. Bermudo, B. Fougnie, T. Bret Dibat, "POLDER-2 In-Flight Results and PARASOL Perspectives," Proceedings of the 5th International Conference on Space Optics, March 30- April 2, 2004, Toulouse, France, ESA SP-554

8) http://www-icare.univ-lille1.fr/parasol/?rubrique=mission_parasol

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

10) http://www-icare.univ-lille1.fr/