Sistema de sincronía PRF para TerraSAR-X

Synchronism Recovery strategy for a TerraSAR-X Bistatic SAR receiver

José A. García, J.C. Merlano-Duncan, Mario Fortes, Paco López-Dekker, Toni Broquetas and Jordi J. Mallorquí

Universitat Politècnica de Catalunya

Introduction

Bistatic and multistatic Synthetic Aperture Radar (SAR) configurations are opening new research lines, exploring the potential benefits of new geometries and exposing different scattering mechanisms, using a variety of orbital SAR transmitters as opportunity transmitters. The most advanced of these orbital SAR systems, like TerraSAR-X and RADARSAT, achieve a big improvement in the image resolution, that can also be exploited by bistatic systems. As an instance, the TerraSAR-X monostatic system provides resolutions up to 3 meters with StripMap mode and up to 16 meters with ScanSAR mode. In this context, the Remote Sensing Laboratory (RSLab) at the Universitat Politècnica de Catalunya, has developed a dual channel X-band bistatic receiver that is capable of use the SAR systems onboard the TerraSAR-X satellite as source of opportunity, named SABRINA-X (SAR Bistatic Receiver for INterferometric Applications). This receiver was designed beginning from SABRINA-C, that is a C-Band version of the bistatic SAR receiver .

The ground based system down converts the X-band signal to baseband using a I/Q home-grown RF front end and samples the signal using an off-the-shelf PXI-based high speed digitizer. The received signal from TerraSAR-X have a bandwidth of 150MHz, requiring sampling rates above 150MS/s per each I/Q channel, resulting in a constant trough-put of 300MSP/s per channel that, restricted by the on-board memory of the digitizer, limits the acquisition length only to approximately 1 second, which is enough only to store a little portion of the main lobe of the signal transmitted by the satellite. This is for the best case, given that the receiver is synchronized and is able to find the mainlobe. For this reason a PRF synchronism recovery mechanism and power detector is implemented to reduce the memory space used to store the received signal. This is performed saving the data obtained selecting the intervals of time where the information of the scattering of the target area appears, storing then the valid signal in an efficient way and allowing an increase of the acquisition length.

This paper presents the design and implementation of the PRF synchronism recovery mechanism and power detector used for the Terra-SAR Bistatic SAR receiver. Synchronism is recovered by matched-filtering the direct received signal, which is performed efficiently in the frequency domain using the FFT algorithm.

System description

The TerraSAR-X bistatic SAR receiver is composed by the RF front-ends, the synchronization recovery block and the data storage block. The RF front-ends down convert the direct and scattered X-band signals to band base, getting the I/Q components after filtering and amplifying. The synchronization recovery block is used to synchronize the receiver with the SAR transmitted signals, predicting and selecting the sampled signal that contains valid data, i.e. signal scattered by the target area. The valid data is stored in the data storage block. Figure 1 shows functional diagram of the receiver.

Figure 1. Functional diagram of the bistatic receiver

The RF Front-ends blocks down convert direct and scattered X-band signals toband base with commercial I/Q Mixers. As the usual received X-band signals of TerraSAR-X are about -50dBm in the direct path and -70/-90dBm in the scattered path, a chain of RF-amplification is required also. This RF-amplification chain must be adjusted to be above the Quantification Noise of the Adquisition Target, otherwise the Quantification Noise would determine the system noise. This block also filters the received RF X-band signal in both channels to avoid undesired signals. The X-band OL signal is generated by a 2,4-2,5GHz PLL after a frequency multiplier x4 allowing the conversion to band base to the I/Q mixer. Finally a band base filtering and amplification step is done in both channels to obtain the desired level of Gain.

The synchronization recovery block is implemented applying a matched filter in real time to the direct path and detecting the correlation peaks in its output. The matched filter is implemented efficiently in the frequency domain with the FFT algorithm. The autocorrelation peak detection is done applying an adaptive threshold and a DLL (Delay-Locked Loop) is used to predict the time intervals at which scattered signal is going to be received. The direct signal used to extract the synchrony information is filtered to reduce the bandwidth, allowing to reduce the I/Q sample rate to 40 MS/s. This is equivalent to reduce the length of the direct pulsed chirp signal and allows decreasing the recovery block process velocity. This is not a limitation to recover the synchrony since the filtered signal is enough to recover the PRF synchrony, due to the correlation properties of the pulsed chirp received.

The system validation is shown by means of experimental results in which real bistatic data is acquired using the TerraSAR-X satellite.