Here, below: Introduction, Mission objectives, Heritage, Instrument description, Atmospheric composition products, More information?. This WebPage is mostly based on Dubovik et al., (2019) and other additional contributions.
Introduction
POLarization and Directionality of the Earth’s Reflectance (POLDER) instrument observations have spanned about 10 years and represent the first and most extensive record of polarimetric, multi-angular & multi-spectral imagery of the Earth atmosphere from space.
Routine orbital polarimetric observations of the terrestrial atmosphere started in 1996 with the launch of the POLDER-1 instrument (Deschamps et al., 1994) on the ADEOS-1 platform (17th August 1996 – June 1997). This observational record was continued by two subsequent POLDER instruments launched on the ADEOS-2 (14th December 2002 – October 2003) and PARASOL (18th December 2004 – December 2013) satellite platforms. The PARASOL satellite was part of the A-Train and enables researchers to take advantage of the presence of other instruments in the constellation (Tanre et al., 2011).
Mission objectives
Aerosol particles constitute a highly variable atmospheric component characterized by a large number of parameters describing particle sizes, morphologies (including shape and internal structure), absorption and scattering properties, amounts, horizontal and vertical distribution, etc… Reliable monitoring of all these parameters is very challenging, and therefore the aerosol effects on climate and environment are considered to be among the most uncertain factors in climate and environmental research.
Heritage
The critical need to use space-borne polarimetry for global accurate monitoring of detailed aerosol properties was first articulated in the late 1980s and early 1990s (Travis, 1992; 1993) based on previous tremendous successes of planetary polarimetry (Lyot, 1929; Dollfus & Coffeen, 1970; Hansen & Hovenier, 1974; Kawabata et al., 1980; West & Smith, 1991). As a consequence, the EOSP was included in the NASA EOS payload. Unfortunately, this instrument was later descoped because of budget constraints and expectations that radiometers like MODIS and MISR would provide the requisite aerosol information.
By now, several orbital instruments have already provided polarization observations from space, and a number of advanced missions are scheduled for launch in the coming years by international and national space agencies. Mishchenko and Travis (1997a, 1997b), Mishchenko et al. (1997), and Hasekamp and Landgraf (2005, 2007) were among the first to suggest that aerosol amount, type, and other detailed properties such as the ability to absorb solar radiation can be derived from polarimetry with an accuracy sufficient for the requisite reduction of the uncertainty in aerosol climate forcing (Mishchenko et al., 2004; Kokhanovsky et al., 2010, Knobelspiesse K., et al., 2012).
Many airborne versions of orbital polarimeters have been developed and deployed during field campaigns to test and improve the concept of polarimetric remote sensing. Polarimetric observations of aerosol properties have also been implemented by ground-based radiometer networks. In addition, several in situ and laboratory polar-nephelometer systems have been designed for accurate measurements of spectral, angular, and polarimetric characteristics of light singly scattered by aerosol particles.
In addition to remote-sensing observations, polarimetric characteristics of aerosol scattering have been measured in situ as well as in the laboratory using polar nephelometers. Such measurements constitute direct observations of single scattering with no contributions from multiple scattering effects and therefore provide unique data for the validation of aerosol optical models and retrieval concepts.
Instrument description
The POLDER instruments (Deschamps et al., 1994) consist of a digital camera with a 274 × 242-pixel CCD detector, wide-field telecentric optics, and a rotating filter wheel enabling measurements in 9 spectral channels with bandwidths between 20 and 40 nm. Because it acquires a sequence of images every 20 seconds, the instrument can observe ground targets from different viewing directions. The two instruments onboard ADEOS-1 and -2 are identical, while the instrument on the PARASOL platform (Tanre et al., 2011) was rotated by 90° to favour multidirectional viewing (a maximum of 16 directions compared to 14) over daily global coverage. Determined by the altitude of the corresponding orbits, the size of the images varies from 2400 × 1800 km2 to 1600 × 2100 km2 (across/along track) with the respective ground resolutions of 7 × 6 and 5.3 × 6.2 km2 at nadir. The spectral coverage of the three instruments ranges from blue (443 nm) through near-infrared (910 nm) with three polarized spectral bands. For POLDER-3, the “bluest” polarized channel was moved from 443 to 490 nm, and a 1020 nm channel was added. Innovative techniques [86], [87], [88], [101] have been developed to calibrate the POLDER instruments in flight.
POLDER instruments, like essentially all previous and current aerosol–cloud polarimeters, were designed to measure only the first three Stokes parameters (I, Q, and U) describing the intensity and linear polarization state of the diffusely reflected sunlight reaching the orbital instrument.
Atmospheric composition products
A dedicated aerosol polarimeter, the NASA APS [23], was lost during unsuccessful launch in 2011. Quite recently, several satellite instruments with polarimetric capabilities have been deployed by national space agencies. A number of future satellite polarimetric instruments and missions are planned and scheduled for launch in the coming decade.
Homepage Julien Chimot: a journey in Earth observation satellite science 