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Launched and operated by the European Space Agency, the ERS satellites are the first missions acquiring commercially available microwave radar data, offering new opportunities for all-weather remote sensing applications. The radar images are independent of lighting or weather conditions, while other instruments measure wave height and frequency, wind speed and direction. ERS 1 operated regularly from 25/7/1991 to 10/3/2000, ERS 2 started regular acquisitions in May 1995 and it is still operational.
ESA plans to launch ENVISAT in 2002, carrying advanced versions of the SAR and ATSR instruments, plus several new sensors.
Both ERS satellites were launched into a sun-synchronous orbit at an inclination of 98° 52’ and an altitude between 782 and 785 km. During the tandem phase (from 21/4/1995 to 10/3/2000) ERS–2 was flown 1 day behind ERS–1, and in an orbit offset a few hundred metres, to allow similar acquisition conditions for stereo imaging and interferometry.
The ERS-SAR Reference System is based on a regular grid of tracks (501) and frames. Successive tracks are counted in a continuous non-repetitive way to give a unique value for the orbit which identifies both a track and the relative acquisition date.
Low Bit-Rate (LBR) data are typically global products, and are provided on a yearly (or a 35 day cycle) basis.
The AMI comprises two separate radars: the Synthetic Aperture Radar (SAR), operated in Image or in Wave Mode, and a Wind Scatterometer.
In Image Mode, the SAR obtains strips of high-resolution (30 m) imagery, 100 km wide, to the right of the satellite track. The 10 m antenna, aligned parallel to the flight track, directs a narrow radar beam onto the Earth’s surface over the swath. Imagery is built up from the time delay and strength of the return signals, which depend primarily on the roughness and dielectric properties of the surface and the range from the satellite.
Wave Mode operation of the SAR measures the changes in radar reflectivity of the sea surface due to surface waves, and provides 5 x 5 km images of 10 meter resolution at intervals of 200 km along track. These ‘imagettes’ are transformed into spectra providing information about the wavelength and the direction of wave systems. Series of power spectra can be used to determine the evolution of swell wave systems.
The Wind Scatterometer uses three sideways-looking antennae illuminating a swath 500 km wide, giving three independent backscatter measurements in three different directions, separated by a very short delay, from which the surface wind vector can be calculated.
The Wind Scatterometer cannot be operated in parallel with the SAR in Image Mode but parallel operation of the wind and wave modes is possible.
The ESA ground stations directly acquire SAR data over the European area. International Ground Stations (IGS) also receive ERS data. Full-resolution SAR data cover almost all of the land surface, and much of the oceans. Low Bit Rate (LBR) data from the other instruments are stored on board the satellite for transmission to ground stations, providing complete worldwide coverage.
The ESA acquisition strategy to-date is to acquire, where possible, data of all land areas using SAR Image Mode, within the constraints of power limitations. To be certain of any future acquisition, programming requests should be placed in advance. The AMI is routinely switched to Wind/Wave Mode over oceanic areas. Orders for SAR Image data of oceanic areas (e.g. for ship and oil monitoring) should be made in advance.
ERS data are subject to speckle, SAR antenna pattern loss and range-spreading loss, and are acquired in slant range. There may also be terrain-induced radiometric effects and terrain distortion. Various levels of correction are available:
•. Speckle reduction by multi-look processing
•. SAR antenna pattern loss and range-spreading loss corrected by calibration
•. Slant range corrected to ground range
•. Terrain-induced effects corrected by applying a Digital Elevation Model
•. Data georeferenced to a map projection
ESA produces a variety of standard ERS products from data acquired at the Earthnet stations (Kiruna and Matera). ESA also produces standard products from data acquired at the International Ground Stations. Formats from IGS may differ slightly from ESA standards.
Produced by the SAR operating in Image Mode. All products are images in digital format:
Raw SAR telemetry data corresponding to one frame of data acquisition (16 seconds) with all auxiliary data necessary for processing.
This is pre-processed but retains the data in complex (i.e. phase-preserving) form. The product is single-look (no speckle reduction) and in slant range.
There are two forms of the SLC product:
1 Full frame (SLCI) format.
2 Quarter frame (SLCn)
Where ‘n’ is the quadrant number.
Other SAR Image products are processed to increasing levels of radiometric and geometric accuracy. All products are multi-look (to reduce speckle), ground range and system corrected, calibrated to correct SAR antenna pattern and range spreading loss.
This is the standard product for most imaging-radar applications. It is the first product from which radar backscatter can be derived for geophysical modelling. No correction is applied for terrain-induced radiometric effects, the image is not geocoded, and terrain distortion (foreshortening and layover) has not been removed.
This high level product precisely locates and rectifies the SAR image on a map projection. Radar backscatter can be derived for geophysical modelling. The scene measures 100 x 100 km and is rotated according to the map grid. There is no correction applied for terrain distortion or radiometry.
Contact Eurimage Customer Service for details and prices of ATSR and other LBR product.
See ERS images from the Image Gallery
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