The GAMMA L-Band SAR

 

The GAMMA L-band SAR is a new compact and versatile system, which supports interferometric (repeat-pass and single-pass), polarimetric, and polarimetric-interferometric SAR data acquisitions from agile sensor platforms. Using two repeat-pass SAR images – or a longer time series of repeat-pass measurements – surface displacements in the direction of the line-of- sight (LOS) of the radar can be derived.

 

The L-band radar has 4 low-noise receiver channels that can operate simultaneously. A custom designed FPGA 14-bit/channel digitizer records from all four channels simultaneously and streams these data to an SSD for storage. The radar instrument is controlled by a Linux-based computer. Transmitter max. output is 10W (5W used in our experiments) and includes a transmitter output switch (2 alternating channels) that permits fully polarimetric data acquisitions, single-pass cross-track and along-track interferometric data acquisitions, as well as combination of these modes up to a total of 4 simultaneous receive channels. Total power consumption is less than 60W. The compact radar hardware implementation is specifically aimed at UAV-based and ground-based operation.

 


A CUDA/ANSI C implementation of a time-domain back-projection (TDBP) approach adapted to FMCW systems is used to focus the airborne or car-borne L-band SAR images directly in map coordinates. The TDBP processing approach allows to handle deviations from linear flight trajectories, and in general, non-linear sensor trajectories in a stringent way given that the sensor trajectory and the topography (the SwissAlti3D model in this specific case) of the illuminated area are well-known. A nearly ideal repeatability of the sensor trajectory can be obtained by operating the L-band radar on a rail. An additional advantage of a rail-based setup is that long time-series with frequent measurements at short intervals can be obtained in an automated way. Instead, the trajectory on dynamic platforms (such as car or UAVs) is retrieved with INS/GNSS system (Honeywell HGUIDE n580), providing high resolution information on the position and orientation of the system. GNSS data is post-processed in kinematic mode relative to an ad-hoc local GNSS reference station installed in the field followed by a loosely-coupled GNSS/INS integration. In this way, focused single-look complex SAR images can be obtained directly in map coordinates where the geometric phase component is already removed, which allows to directly compute differential interferograms.

 

The GAMMA L-Band SAR has been used for repeat-pass interferometric measurements on the following airborne and terrestrial platforms :

 

 

Technical Characteristics

 

  • Frequency: 1.2 - 1.4 GHz (wavelength: 0.226 m)

  • Chirp bandwidth: 50-200 MHz

  • Range resolution: 1.5 m @ 100 MHz

  • Azimuth resolution: <0.5 m @ full SA < 0.5 m; ~2.3 m @ SA = 250m & Range = 5 km 

  • Transmit power: max 10 W (used 5 W)

  • Dimensions: L= 406 mm, W= 330 mm, H = 174 mm

  • Weight: 7.65 kg

UAV-borne L-Band SAR
Car-borne L-Band SAR
L-Band SAR on a linear rail

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