E-SAR – The Airborne SAR System of DLR

E-SAR identifies the DLR airborne experimental synthetic aperture radar system which is operated by the Microwaves and Radar Institute in cooperation with the DLR flight facilities onboard their Dornier DO228-212 aircraft . Being developed in the Institute E-SAR delivered first images in 1988 in its basic system configuration. Since then the system has been continuously upgraded to become what it is today: a versatile and reliable workhorse in airborne Earth observation with applications worldwide.

E-SAR onboard DLR’s Dornier DO228-212 aircraft touching down after a successful measurement flight.
© Renato Burkhart, AIRLINERS.NET
A view inside the cabin of the DO228 aircraft. The racks are approx. 1.2m high
Repeat-Pass SAR Interferometry baselines achieved during INDREX 2004 campaign in Indonesia.
Deviations are less than 1m (rms) for each of the 3 passes with a nominal horizontal baseline of 5m
Airborne platform

The DO228 is a twin-engined short take-off and landing aircraft. The landing strip may have a minimum length of about 900m only. The maximum operating altitude of the Do-228 aircraft with E-SAR onboard is about 6000m above sea level. For SAR operation the ground speed ranges from 140kt to 200kt. Depending on the SAR configuration the endurance varies between 2.5 and 4 hours.

Radar system

E-SAR operates in 4 frequency bands, X-, C-, L- and P-band, hence it covers a range of wavelengths from 3 to 85 cm. The polarisation of the radar signal is selectable, horizontal as well as vertical. In polarimetric mode the polarisation is switched from pulse to pulse in (hh-hv-vv-vh)-sequence.

E-SAR Technical parameters

. X C L P
RF[MHZ] 9600 5300 1300 350
BW[MHZ] 100 or 50 (selectable)
Sampling 6 or 8 bits, complex (I and Q)
Data rates 8 or 16MBytes/s (selectable)
Rg res.[m] 2 or 4 (selectable)
Az res.[m] 0,25 0,3 0,4 1,5
Rg cov.[km] 3 or 5 (selectable)

E-SAR offers high operational flexibility. The measurement modes include:

  • single channel operation, i.e. one wavelength and polarisation at a time
  • single-pass SAR Interferometry in X-band (along or cross-track)
  • SAR Polarimetry . The system is polarimetrically calibrated in L- and P-band.
  • Repeat-Pass SAR Interferometry in combination with polarimetry
    (operational in L- and P-band; experimental in C- and X-band)

Navigation accuracy

A modern realtime DGPS/INS System (IGI CCNS4/Aerocontrol IId) combined with a FUGRO OmniStar 3000L DGPS receiver allows most precise navigation and positioning. E-SAR is hence able to generate geocoded image products of very high geographical precision. Repeat-Pass SAR Interferometry at baselines of less than 10m is possible, allowing the realisation of advanced and innovative techniques like Pol-InSAR and tomography as well as coherent change detection. An example for the precision in flight track maintenance is shown below. Three consecutive passes of about 6 minutes duration each were flown with a nominal baseline of 5m.

Ground segment

Part of the sensor system is an operational E-SAR ground segment. After transcription from HDDC (SONY SD-1) to hard disk drive the E-SAR Extended Chirp Scaling (ECS) processor converts the SAR data to calibrated image data products (refer to SAR Processing for details). To increase the product quality level to CEOS level 1b radiometric and polarimetric calibration, DEM generation and image geocoding are operationally implemented.
For calibration trihedral radar corner reflectors are set up on the Oberpfaffenhofen airfield, the premises of DLR and in the neigborhood. Their size varies between 0.9 and 3m leg length. The geographical positions are precisely known.
Finally, the ground segment is completed by DLR’s DIMS archiving system and the EOWEB internet portal , which is providing a user’s access point.

P-Band Antenne mit Radon 200x372
New P-band antenna including wind shield
E-SAR P-band image (300 to 400MHz) of a calibration test site. Scene size is about 3,2 km x 3.5 km, illumination direction from the bottom
The Oberpfaffenhofen airfield and DLR research center – X-band step frequency image at 170MHz total range bandwidth (20% spectrum overlap)
Improved range resolution demonstrated by a cut through the impulse response of a radar reflector. Dashed line: original impulse response function (IRF), continuous line: step frequency IRF
Recent E-SAR ystem upgrades

New P-band (300 to 400MHz) – Wideband low frequency SAR is highly susceptible to radio frequency interference. This experience was made in P-band in the range from 400 to 500MHz, which in Europe is fairly crowded with TV broadcasting stations. To achieve better image quality the centre frequency was shifted to 350MHz.
A new P-band subsystem was built, including antenna, IF converter and frontend sections. In 2003 the first flight tests were executed. The image quality proved to be very good in terms of geometric and radiometric resolution as well as SNR and RF interference level. An example is also shown with a radar single-look resolution of about 2.2m x 2m (range x azimuth).

Step Frequency Mode – Technology still poses some limits on radar chirp bandwidth, mainly if a reconfigurable signal generation is necessary as for the E-SAR system. To meet user requirements for very high resolution, alternative methods to synthetize bandwidths up to 1GHz are needed. One attractive method is the Step Frequency approach that was adopted for the E-SAR system. For experimental purposes a step frequency converter unit was developed and tested with E-SAR.
The unit provides 200MHz maximum bandwidth and spectrum overlaps ranging from 0 to 100% alternating the normal 100MHz-chirp pulse-to-pulse. Flight tests were conducted with different degrees of overlap (10%, 20% and 50%).
For processing of the data a dedicated software had to be developed. The work was completed successfully and the improvement in range resolution could be demonstrated.

Since many years E-SAR is being heavily used for SAR experiments and measurement campaigns. It has evolved to an important tool for SAR research and application in Europe. A number of important scientific and technical SAR missions and projects were executed in recent years with great success. However, while many questions could be answered in the past, new problems have come up requiring new solutions. The Institute is building up a new airborne SAR facility identified as F-SAR. While F-SAR is in the building phase E-SAR is still maintained and used for campaigns.

Ralf Horn
German Aerospace Center

Microwaves and Radar Institute
, SAR Technology
Tel: +49 8153 28-2384

Fax: +49 8153 28-1449

E-Mail: Ralf.Horn@dlr.de
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