POLDER/PARASOL
POLarization and Directionality of the Earth’s Reflectances / Polarisation et Anisotropie des Réflectances au sommet de l’Atmosphère, couplées avec un Satellite d’Observation emportant un LidarPOLDER (POLarization and Directionality of the Earth’s Reflectances) instrument analyses the intensity and direction of light reflected by the Earth and its atmosphere, as well as its polarization, a physical characteristic describing how waves propagate. Such measurements reveal some interesting properties of clouds and aerosols, thereby telling us more about how they affect climate.
Designed and developed by CNES, in close collaboration with the LOA atmospheric optics laboratory in Lille, the POLDER instrument is mainly dedicated to the study the of the clouds and aerosols properties, but it also provides valuable information on the directionality of terrestrial reflectances and on the ocean colour. It has been the first space instrument to provide global and systematic spectral, directional and polarization measurements of radiation reflected from the Earth and atmosphere.
There have been three successive generations of instruments. POLDER 1 and POLDER 2 were launched on the Japanese ADEOS and ADEOS 2 satellites on 17 August 1996 and 14 December 2002. Unfortunately, both satellites encountered serious technical hitches that brought their mission to a premature end. With POLDER 3 on the other hand, which was launched on 18 December 2004 on the French PARASOL microsatellite, the scientists were able to exploit the full potential of the instrument. The PARASOL mission continued to operate up to the end of 2013, exceeding its planned design life by more than 5 years.
Instrument
The POLDER instrument consists basically of a digital camera composed of a 284×364 pixels CCD detection matrix, a wide field of view telecentric optics (± 51° along the great axis) and of a rotating wheel carrying spectral and polarized filters.
There are 15 filters (plus a dark current measurement) which could cover a spectral domain ranging from 443 to 1020 nm. For some wavelength, 3 filters measure the linear polarization of the incoming light in three directions separated by 120°, thus allowing to supply the Stokes parameters and the total radiance. Moreover, the acquisition of a sequence of images every 20 seconds makes it possible to observe the same target on ground from several directions (12 for POLDER 1 and 2, up to 16 for POLDER 3).
Main characteristics of the instrument:
- Total mass about 30 kg
- Volume 0,8 x 0,5 x 0,3 m3
- Field of view ±43° x ±51
- Swath width 2200 km (POLDER1 & 2) – 1600 km (POLDER 3)
- Resolution about 6 km
- Encoding 12 bits
- Average power about 30 W
- Telemetry rate 880 bits/s
The POLDER 1 and POLDER 2 instruments are strictly identical. For POLDER 3, the matrix orientation of the telecentric lens has been modified. It has been turned 90 degrees reducing the swath but increasing the number of acquisitions of the same target on ground to favour multidirectional viewing over daily global coverage. Also, some modifications of spectral bands have been made to better harmonize with companion instruments in the A-Train in which the PARASOL satellite has been placed.
Spectral bands:
| POLDER 1&2 channels | PARASOL channels | Bandwidth | Rationale |
| 443P | / | 20 nm | Clouds and radiation balance |
| 443 | 443 | 20 nm | Cloud detection, ocean color |
| 490 | 490P | 20 nm | Aerosol retrieval, cloud pressure |
| 565 | 565 | 20 nm | Aerosols and Calipso lidar at 532 nm |
| 670P | 670P | 20 nm | Aerosol retrieval, cloud properties |
| 865P | 865P | 40 nm | Aerosol retrieval, cloud properties |
| 763 765 | 763 765 | 10 nm 40 nm | Cloud oxygen pressure by differential absorption technique in oxygen A band |
| 910 | 910 | 40 nm | Water vapor retrieval |
| / | 1020 | 20 nm | Aerosols ans Calipso lidar at 1064nm |
Products
AERIS provides access to Level-1, 2 and 3 products. Level-1 and Level-2 products are daylight orbit passes whereas Level-3 products are global scale, monthly syntheses (averages and statistics).
All POLDER 1 and 2 data have been reprocessed with the improved algorithms developed for PARASOL.
Only “Atmosphere” products (Aerosols and Radiative Budget/Clouds) are accessible via the AERIS portal. The “Land” products are available at CNES and the “Ocean Color” products are no more supported.
Products available at AERIS
- Level 1 – Top-of-the-atmosphere normalized radiances
- Level 2 – Daily Aerosol Product over Ocean
- Level 2 – Daily Aerosol Product over Land
- Level 2 – Daily Cloud & Radiative Budget Product
- Level 3 – Monthly Aerosol Product over Ocean
- Level 3 – Monthly Aerosol Product over Land
- Level 3 – Monthly Cloud & Radiative Budget Product
Daily products and monthly syntheses are produced at 20 km resolution (after cloud detection performed at full resolution, 6 km, and for every direction).
“Aerosol over Ocean” products provide the following set of parameters:
- total and small particles optical thickness (aerosol load)
- non sphericity index
- angström coefficient
- effective radius and refractive index of the small particle mode
- refractive index of large spherical particles
“Aerosol over Land” products provide the following set of parameters:
- optical thickness of the small particles
- angström coefficient of the small mode
“Earth Radiation & Cloud” products provide the following set of parameters:
- cloud cover
- cloud phase
- cloud optical thickness
- spectral albedo
- shortwave albedo
- 2 cloud pressures: from oxygen absorption and from Rayleigh scattering
- water vapor integrated content