Several factors play a part in determining the geographical coverage of ERS products, including instrument, orbital configuration, ground station coverage and operational constraints.
The ESA ground stations directly acquire SAR data over the European area, while the global dataset for the LBR instruments is obtained using the on-board recorders. Other stations around the world also receive ERS data thus extending the coverage of the SAR Image Mode.
ERS-1 and 2 - Geometric & Radiometric Characteristics
ERS data are subject to speckle, SAR antenna pattern loss and range-spreading loss, and are acquired in slant range. Further, there may be terrain-induced radiometric effects and terrain distortion (foreshortening and layover). Various levels of processing are available to correct these effects:• Speckle reduction is achieved by multi-look processing
• SAR antenna pattern loss and range-spreading loss can be corrected by calibrating the image
• Slant range can be corrected to ground range
• Terrain-induced effects can be corrected by the application of a Digital Elevation Model
• Data can be georeferenced to a map projection
Full specifications of all aspects of any of the instruments can be supplied, direct from the ESA documentation library. A summary of the most relevant information is supplied here for each sensor, with the emphasis placed on the Active Microwave Instrument in SAR Image Mode, the data which are expected to be of most interest to most customers.
Active Microwave Instrument (AMI)
The AMI comprises two separate radars: the Synthetic Aperture Radar (SAR), which can be operated in Image or in Wave Modes, and a Wind Scatterometer.
SAR Image Mode
In Image Mode, the SAR obtains strips of high-resolution (10-30 m) imagery, 100 km in width, to the right of the satellite track. The 10 m long 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 its range from the satellite. The SAR is a C-Band radar (5.66 cm, 5.3 GHz) with a bandwidth of 15.55 MHz, and operates at an incidence angle of 23°, utilising LV polarisation. The radiometric resolution of the SAR in Image Mode is 2.5 dB at sigma 0 = -18 dB, and the quantisation of each signal component is to 5 or 6 bits depending on the applied range compression. The noise equivalent is sigma 0 = -25 dB.
Operation in Image Mode excludes the operation of the other AMI modes, and power considerations limit operating times to a maximum of 12 minutes per orbit. The data rate of 105 Mbit/s is too high to allow on-board storage, and so images are only acquired within the reception zones of suitably equipped ground stations.
SAR Wave Mode
Wave Mode operation of the SAR measures the changes in radar reflectivity of the sea surface due to the surface waves, and provides 5 km x 5 km images (`imagettes») 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.
Wind Scatterometer
The Wind Scatterometer uses three sideways-looking antennae, one pointing normal to the satellite flight path, one pointing 45° forward and the third pointing 45° backwards. These antenna beams illuminate a swath 500 km wide as the satellite advances along its orbit and each provides measurements of radar backscatter from the sea surface for overlapping 50 km resolution cells using a 25 km grid spacing. The result is three independent backscatter measurements relating to cell centre nodes on a 25 km grid, which have been obtained using the three different viewing directions and are separated by only a very short time delay. Calculation of the surface wind vector in terms of speed and direction takes place using these so called `triplets» within a mathematical model which defines the relationship between backscatter, wind speed, wind direction and incidence angle of the observation.
The Wind Scatterometer cannot be operated in parallel with the SAR in Image Mode; however, parallel operation of the wind and wave modes is possible (Wind/Wave Mode).
Radar Altimeter (RA)
The Radar Altimeter is a nadir-pointing pulse radar designed to measure the echoes from ocean and ice surfaces. It operates in the K-band at 13.8 GHz, and has a maximum 10 cm vertical resolution with a swath width of 1.3°. It has two measurement modes, optimised for measurements over ocean and ice, respectively.
Ocean Mode
In ocean mode, the RA is used to measure wave height, surface wind speed and sea-surface elevation, the last of which is appropriate to the study of ocean currents, the tides and the global geoid.
Ice Mode
In ice mode, the instrument, operating with a coarser resolution, provides information on ice sheet surface topography, ice types and sea/ice boundaries. The operation of the two modes is pre-planned.
Along-Track Scanning Radiometer (ATSR)
The ATSR is a four-channel infrared radiometer, of which three channels (1.6, 10.8 and 12 µm) are still operating, providing measurements of sea-surface and cloud-top temperatures with higher accuracies than similar instruments flown on previous satellites. The scanning technique enables the Earth»s surface to be viewed at two different angles (0° and 52°) in two curved swathes 500 km wide and separated, along track, by about 700 km. Data from the two swathes are then combined to eliminate atmospheric influence in the calculation of sea-surface temperature. The instrument has been designed to provide an absolute accuracy in sea-surface temperature of better than 0.5° K when averaged over areas of 50 km x 50 km and in conditions of up to 80% cloud cover. For cloud-free pixels, of 1 km x 1 km, the relative accuracy is about 0.1° K.
Microwave Radiometer (MWR)
Attached to the ATSR is the Microwave Radiometer, which is a nadir-viewing passive radiometer operating in two bands (23.8 and 36.5 GHz) over the same swath to provide measurements of the total water content of the atmosphere within a 20 km pixel. This is used to improve the accuracy of sea-surface temperature measurements and also to provide accurate tropospheric range correction for the Radar Altimeter. The altimeter height correction accuracy is in the order of 2 cms
Global Ozone measuring Experiment (GOME)
The Global Ozone Measuring Experiment is a new sensor flown on ERS-2 which provides a complete world Ozone map every three days. Thanks to its differential optical absorption spectroscopy technique and broad spectral range, GOME gives greatly enhanced accuracy in ozone measurement and in the detection of other trace gases as well as the determination of aerosol and micro-particle pollution in the lower atmosphere.
Precise Orbit Determination Instrumentation
There are no products from the final two instruments, which are used to measure the position and orbit of the satellite.
Precise Range and Range-rate Equipment
The PRARE is an all-weather system designed to perform high-precision two-way microwave range and range-rate measurements, through ground-based transponder stations, to be used for orbit determination and for geodetic applications. Unfortunately, the PRARE of ERS-1 suffered fatal damage to the Random Access Memory due to radiation after a few hours of nominal operations. An improved version of PRARE is operating to nominal specification on ERS-2.
Laser Retroreflector
The Laser Retroreflector is a passive optical device operating in the infrared which is used as a target by ground-based laser ranging stations and thus enables the accurate determination of the satellite»s height.
| AMI (Active Microwave Instrument) | ||||||
|---|---|---|---|---|---|---|
| Sensor | Spectral Bands | Frequency | Polarisation | Incidence Angle | Spatial Resolution | Swath (Km) |
| SAR Image Mode | C-band | 5.3 GHz | LV | 23° at mid swath | 10 - 30 m | 100 |
| SAR Wave Mode | C-band | 5.3 GHz | LV | 23° + 0.5° | 30 m | 5 X 5 |
| Wind Scatterometer | C-band | 5.3 GHz | LV | 25° - 59° at Fore/aft 18° - 47° at Mid |
50 Km | 500 |
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