DLR SAR Calibration Center

DLR Calibration Site
The DLR calibration site consists of 37 target positions with 20 permanently installed corner reflectors. The six newly installed remote-controlled targets are shown in blue (Map: (c) OpenStreetMap).

The DLR SAR calibration center is a facility for efficient and robust SAR system calibration – enabling extended field campaigns – for multimode SAR missions as successfully executed for TerraSAR-X and TanDEM-X as well as for Sentinel-1A and Sentinel-1B.

It is well equipped with a large number of accurate passive and active reference targets (see below) and includes several analysis and evaluation tools (see below) based on precise algorithms. Within an area of 120 km x 40 km, 37 targets are deployed and maintained in South Germany at different sites. Within this DLR calibration site (see Figure 4), six of these test sites are remotely controlled and operated from Oberpfaffenhofen.

Analysis and Evaluation Tools

Several software tools have been developed and established for analyzing and evaluating the different measurements executed for the calibration and the verification of spaceborne SAR systems. This comprise both: the derivation of various calibration parameters like the absolute calibration factor, antenna patterns or instrument drift parameters, and the determination of several image parameters like the resolution, PSLR or ISLR for calibrating and assessing the quality of the SAR data products respectively.

The multitude of analysis tools are summarized by:

  • the Internal Calibration Module (ICM), for drift monitoring of the instrument, replica generation and RF characterization,
  • the Antenna Characterization Module (ACM), for determining the antenna pointing and verifying the antenna model,
  • the Antenna Model (AM) for deriving the reference pattern and optimizing the excitation coefficients
  • CALIX, the tool for point target analysis and consequently well suitable for geometric and radiometric calibration as well as for product quality verification
  • a Calibration Budget Tool for deriving the performance of SAR system calibration
  • an Alignment Tool for deriving the alignment data for pointing the reference targets towards the satellite
  • a Campaign Planning Tool for estimating satellite overpasses and preparing an efficient in-orbit calibration plan.,
The DLR calibration site consists of 37 target positions with 20 permanently installed corner reflectors. The six newly installed remote-controlled targets are shown in blue (Map: (c) OpenStreetMap).

One example of these tools is given in the picture above, a screenshot of CALIX showing the impulse response function of a point target within a SAR image and extracted in range and in azimuth. Based on this point target analysis measurements of several impulse response function parameters can be derived, like the integrated point target energy (for deriving the absolute calibration factors), target/clutter ratios (to weight calibration factor estimates), as well as peak amplitude and phase estimates in the case of multi-channel (e.g. quad-pol) data. Beyond that CALIX offers tools for distributed target analysis and for geometric calibration (internal delay and pixel localization accuracy derived from accurately surveyed targets).

New Generation of Passive and Active Reference Targets

DLR corner reflector
Remote controlled DLR corner reflector with a leg length of 2.8 meter.

Man-made point targets are an essential part of external calibration and serve as absolute radiometric and geometric reference. They are mainly deployed during the commissioning phase for the proper calibration and verification activities. Strong requirement for the SAR system on radiometric and geometric accuracy can only be achieved with highly accurate and stable point targets, and their position on Earth surface have to be precisely surveyed.

Point targets are generally distinguished in passive and active targets depending on the manipulation of the received signal: passive targets like corner reflectors only reflect the received signal while active targets, so called transponders are able to modify the retransmitted signal (e.g. by amplification, adding a delay to it or changing the polarization).