AIRBORNE BISTATIC SAR RECEIVER WITH THE CAPABILITY OF USE DIFFERENT
OPPORTUNITY TRANSMITTERS.
J.C. Merlano-Duncan, Paco López-Dekker, Jordi J. Mallorqui and Sergi Duque
Universitat Politècnica de Catalunya
Email: {juan.merlano, paco.dekker, mallorqui, sergi.duque}@tsc.upc.edu
ABSTRACT
This paper describes the design and construction of a
bistatic SAR receiver suitable for airborne applications,
using orbital SAR systems (ENVISAT, ERS-2,
RADARSAT, TerraSAR-X among others) as opportunity
transmitters. The challenge of this design is to reduce the
required data throughput of the recorded data. This is
achieved storing data only in the time intervals when
scattered signal from the target area appears. The task of
detecting this time intervals is performed in real time
using a matched filter of the signal received directly from
the SAR transmitter.
Index Terms—SAR, bistatic radar, Programmable
Logic Devices.
1. INTRODUCTION
In the last decades, Synthetic Aperture Radar (SAR) has
developed into a standard tool for Earth Observation.
While monostatic SAR can be considered a mature field,
bistatic and multistatic configurations are opening new
research lines, exploring the potential benefits of new
geometries and exposing different scattering mechanisms.
In this context, the Remote Sensing Laboratory (RSLab)
at the Universitat Politècnica de Catalunya has developed
a dual channel C-band bistatic receiver that uses the SAR
systems onboard the ENVISAT and ERS-2 satellites as
sources of opportunity, named SABRINA (SAR Bistatic
Receiver for INterferometric Applications)[1]. This
ground based system down converts the C-band signal to
video band using a home-grown RF front end and
samples the signal using an off-the-shelf PXI-based high
speed digitizer.
Bistatic systems that use sources of opportunity have to
overcome a series of synchronization challenges. One of
these is the lack of an explicit PRF synchronism
mechanism. The current SABRINA prototype addresses
this lack of synchronism by acquiring continuously
during a temporal window around the expected overpass
time of the transmitter (which for ERS-2 and ENVISAT
can be determined with less than 0.5 second error). PRF
synchronism is recovered during post processing from
one of the two channels, which is used to acquire a direct,
line of sight, signal. For ERS-2 and ENVISAT's 16 MHz
bandwidth signals this results in a sustained throughput of
40MS/s per channel that, restricted by the on-board
memory of the digitizer, limits the acquisition length to
approximately 8 seconds, which is more than required.
However, new systems like RADARSAT-2, TerraSAR-X
or the future Sentinel-1 will transmit signals with much
larger bandwidth, requiring sampling rates above 200
MS/s, in which case the current acquisition strategy will
not be feasible. Moreover, the development of the next
SABRINA prototype aims at an Unmanned Airborne
Vehicle (UAV) based system that will eliminate some of
the geometric limitations associated to a ground based
systems. Thus a new compact and light weight receiver
needs to be developed that can sample and store high
bandwidth bistatic signals in an efficient way.
This paper presents the design and construction of a
bistatic SAR receiver suitable for UAV and able to meet
the throughput requirements previously discussed. This is
achieved saving the data obtained selecting the intervals
of time where the information of the scattering of the
target area appears. To accomplish this task it is necessary
to implement a system which continuously synchronizes
the receiver with the PRF of the SAR transmitted signal.
Synchronism is recovered by matched-filtering the
acquired signal in real-time, using a high density
Programmable Logic Device (PLD) to parallelize the
required signal processing.
Section 2 of this paper describes the architecture of the
proposed SAR bistatic receiver, explaining each of its
constitutive components. Then, Sections 3 and 4 describe
the main subsystems, the RF front End and the acquisition
system. Finally, conclusions are exposed in section 5.
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