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Calibration Facility Oberpfaffenhofen (CALIF)

Figure 1: Examples of the Institute’s calibration ground equipment; left: corner reflector with 3 m leg length, middle: ground receiver for measuring transmit pulses, and right: transponder with adjustable RCS . Figure 2: Test site set up for Shuttle Radar Topography Mission (SRTM) calibration in 2000, indicating 26 ground target locations distributed over an area of almost 300km in South Germany . Figure 3: Test site set up for ENVISAT/ASAR ScanSAR calibration in 2002, indicating one ground target location the center of each of the five sub-swaths and in the overlapping regions between adjacent swaths in each center distributed over an area of almost 450km in South Germany . Figure 4: SAR image from the ASAR ScanSAR verification campaign in southern Germany showing the calibration sites and (inset) a close-up of a transponder response . Figure 5: Transmit pulses of the ENVISAT/ASAR instrument as function of time recorded by a deployed ground receiver. The reduced amplitudes indicate the switching of the instrument from beam to beam and the corresponding envelopes represents the one-way (transmit) azimuth antenna patterns of ASAR in ScanSAR operation

The Group is well equipped with calibration targets and operates and maintains a large calibration site in southern Germany. Three different types of calibration targets are used, as shown in Figure 1:

• Trihedral and dihedral corner reflectors as passive targets precisely surveyed (with differential GPS) and therefore well suited for geometric calibration
• Transponders with high radar cross-section providing accurately defined point targets within the SAR scene
• Ground receivers measuring the transmit pulses as function of time and consequently the 1-way antenna beam pattern of the SAR antenna

Six corner reflectors with a side length of 3 m are permanently installed around Oberpfaffenhofen. They have been regularly utilised for campaigns with different SAR missions (e.g. ERS-1/2, JERS-1, ASAR, ALOS, TerraSAR-X), as well as for E-SAR calibration.

In addition to CALIF, which is well-equipped with software tools and ground calibration hardware, the infrastructure for the preparation and execution of calibration campaigns over large test sites is an essential element to perform a successful commissioning phase. Several campaigns have been performed in the last years.

As shown in Figure 2, for the SRTM mission in February 2000 a large test site extending over almost 300 km in southern Germany was set up comprising 26 trihedral corner reflectors of different size and a number of ground receivers. All corner reflectors have been precisely surveyed using differential GPS serving not only as references for absolute radiometric calibration but also as absolute height reference points for DEM generation.

Under contract with ESA/ESTEC and as a contribution to the ASAR commissioning phase a ScanSAR verification campaign was performed in summer 2002. For that purpose, a calibration site stretching across the more than 400 km wide ScanSAR swath was prepared as shown in Figure 3, with targets in the center of each of the five sub-swaths and in the overlapping regions between adjacent swaths.

Point target analysis of the transponder responses in the ScanSAR image (see Figure 4) confirmed the beam-to-beam relative calibration as well as the absolute calibration of ASAR Wide-Swath products.

Furthermore, using the receiver unit of the transponder, the 1-way azimuth antenna patterns could be measured, as shown in Figure 5. The jumps in amplitude indicate the switching from sub-swath to sub-swath in ScanSAR operation.

Since 2003, the main efforts have been concentrated on developing the calibration concept for TerraSAR-X (Web page ‘ TerraSAR-X Cal’). New software tools have been implemented and existing algorithms were updated. Further calibration targets are being developed and purchased.