Satellite Sensors

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Revision as of 10:26, 29 March 2012

Passive Imagers

SEVIRI
SEVIRI is a 50 cm-diameter aperture, line-by-line scanning radiometer, which provides image data in four Visible and Near-InfraRed (VNIR) channels and eight InfraRed (IR) channels. A key feature of this imaging instrument (Fig. 1) is its continuous imaging of the Earth in 12 spectral channels with a baseline repeat cycle of 15 min. The imaging sampling distance is 3 km at the sub-satellite point for standard channels, and down to 1 km for the High Resolution Visible (HRV) channel.
Data access for the CREW periods here: ./crew/observations/SEVIRI

MODIS
MODIS (Moderate-resolution Imaging Spectroradiometer) is a payload scientific instrument launched into Earth orbit by NASA in 1999 on board the Terra (EOS AM) Satellite, and in 2002 on board the Aqua (EOS PM) satellite. The instruments capture data in 36 spectral bands ranging in wavelength from 0.4 µm to 14.4 µm and at varying spatial resolutions (2 bands at 250 m, 5 bands at 500 m and 29 bands at 1 km). Together the instruments image the entire Earth every 1 to 2 days.
Data access for the CREW periods here: ./crew/observations/MODIS

AVHRR
The AVHRR instrument measure the reflectance of the Earth in 5 relatively wide (by today's standards) spectral bands. The first two are centered around the red (0.6 micrometer, 0.5 THz) and near-infrared (0.9 micrometer, 0.3 THz) regions, the third one is located around 3.5 micrometer, and the last two sample the thermal radiation emitted by the planet, around 11 and 12 micrometers, respectively.
Data access for the CREW periods here: ./crew/observations/AVHRR

POLDER
POLDER (POLarization and Directionality of the Earth's Reflectances) is a wide field of view imaging radiometer that has provided the first global, systematic measurements of spectral, directional and polarized characteristics of the solar radiation reflected by the Earth/atmosphere system. Its original observation capabilities have opened up new perspectives for discriminating the radiation scattered in the atmosphere from the radiation actually reflected by the surface.

MERIS
MERIS (The Medium Resolution Imaging Spectrometer Instrument) is a programmable, medium-spectral resolution, imaging spectrometer operating in the solar reflective spectral range. Fifteen spectral bands from 390 nm to 1040 nm can be selected by ground command. The instrument scans the Earth's surface by the so called "push-broom" method. Linear CCD arrays provide spatial sampling in the across-track direction, while the satellite's motion provides scanning in the along-track direction.

A-TRAIN reference instruments

CALIOP
The Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) is the primary instrument on the CALIPSO satellite, which was launched in 2005. CALIOP provides profiles of total backscatter at two wavelengths, from which aerosol and cloud profiles will be derived. The instrument also measures the linear depolarization of the backscattered return, allowing discrimination of cloud phase and the identification of the presence of non-spherical aerosols.
CALIOP observation and retrieval products are available on the ICARE FTP site: CALIOP.

CPR
The CPR (Cloud Profiling Radar) is a 94-GHz nadir-looking radar which measures the power backscattered by clouds as a function of distance from the radar (Nominal Frequency 94 GHz, Vertical Resolution 500 m, Cross-track Resolution 1.4 km, Along-track Resolution 1.7 km).
Cloudsat CPR observation and retrieval products are available on the ICARE FTP site: CLOUDSAT.

AMSR
The AMSR-E (Advanced Microwave Scanning Radiometer for EOS) is a twelve-channel, six-frequency, total power passive-microwave radiometer system. It measures brightness temperatures at 6.925, 10.65, 18.7, 23.8, 36.5, and 89.0 GHz. Vertically and horizontally polarized measurements are taken at all channels. The Earth-emitted microwave radiation is collected by an offset parabolic reflector 1.6 meters in diameter that scans across the Earth along an imaginary conical surface, maintaining a constant Earth incidence angle of 55° and providing a swath width array of six feedhorns which then carry the radiation to radiometers for measurement. Calibration is accomplished with observations of cosmic background radiation and an on-board warm target. Spatial resolution of the individual measurements varies from 5.4 km at 89.0 GHz to 56 km at 6.9 GHz.
Brightness temperatures of AMSR are available of the ICARE FTP site: AMSR_E.

Synthetic datasets

Instead of using real observations, it is possible to simulate the observations with a radiative transfer model. The advantage of these simulated satellites scenes is, that the state of the atmosphere and the clouds is known. In this way the results of the retrieval can be diretly compared to the 'known' atmospheric state. The disadvantage is, that assumptions and uncertainties may occur in the forward simulation, including the horizontal homogeneity, vertical structued clouds, three dimensional radiative transfer, and not perfectly known optical properties of the surface, gaseous absorption, and clouds. Noting deviations of the retrieval result and the 'known' atmospheric state, it is always nessesary to check weather the deviation is caused by an imprecise retrieval algorithm or an inaccuracy of the forward model. Using the same forward model and assumptions for the simulations of satellite observations and for training the retrieval algorithm may also lead to a situation, where inaccuracy of radiative transfer and retrieval cancel each other out. Nevertheless this approach is useful as plausibility check, to identify to rough assumptions in the retrieval algorithm, and to distinguish properties that may be retrieved and that are impossible to retrieve. Simulations may be performed with different radiative transfer models for different sensors. The most known radiative transfer models are libRadtran, S6, and RTTOV and more.