Radar and Sonar Processing Expertise

NA Software (NAS) has a long experience of developing both airborne and spaceborne radar surveillance systems for government and defence organisations. This includes both Synthetic Aperture Radar (SAR) high-resolution image processing systems and Ground Moving Target Indication (GMTI) radar systems. In more recent years we have also carried out a number of sonar projects for leading defence contractors in Naval systems.

Modular SAR Processors

In Long-range, high resolution, SAR systems we encounter problems with geometric linearity and focus quality caused by unknown sensor motions or atmospheric variation. We have incorporated autofocus and phase correction into our modular SAR processors that rectify both these problems.

Both satellite and airborne SAR data is subject to a number of perturbations that stem from various causes and lead to unknown phase changes in the raw data and subsequent imagery which may be defocussed or non-linear. As part of their SAR processing capability, NAS has implemented offer a SAR Processor capability that allows unknown sensor motions to be derived from the received SAR signal. The derived motions can then be applied in a motion compensation process to correct the sensor track to a straight line.

We provide an advanced implementation of these techniques in our Modular SAR Processor. This processor is designed to provide SAR development groups with the ideal tool to study the effects of a variety of perturbations or of new processing techniques, on SAR image quality. It comes complete with our SAR data simulator, enabling the construction of realistic simulated data of arbitrary ground scenes. 

GMTI Processors

NAS has developed a range of Ground Moving Target Indication (GMTI) processors for traditional MTI radar systems that operate with a low ground resolution and for systems such as SAR surveillance systems that operate at a high resolution. A new radar processing technique has been developed in-house that detects ground moving targets from high-resolution SAR data. We have called this technique SARMTI, which offers a unique capability in SAR imaging of moving targets. The technique provides full resolution SAR images of moving targets, detected and correctly positioned against the SAR background image with measured values of the target motions. This is an enormous advance on conventional methods, as discussed below.

Many applications need to identify moving targets. The standard approach (usually given the acronym MTI or GMTI) utilises a separate mode of data collection from SAR: at any one time, you collect SAR data OR MTI data. MTI is not an ideal methodology:

Standard GMTI techniques

• MTI is effective for ground-based sensors viewing the sky when there is negligible clutter.
• For an airborne sensor viewing the ground, GMTI has a significant clutter contribution within the received signal.
• Target and clutter returns may arise from anywhere within the radar beam.
• The signal to clutter ratio depends on the integrated target and clutter returns within the beam.
• Targets are detected with a resolution much poorer than that of the corresponding SAR data. The poor resolution means that clutter often dominates target returns.
• For effective GMTI it is essential that the contribution from ground clutter be removed.
• The Displaced Phase Centre Antenna (DPCA) method reduces the contribution considerably, but a minimum of three antennae, instead of the single SAR antenna, are required.
• Since the essence of the technique is to remove clutter, there is no ground referencing capability in GMTI comparable with SAR.
• The position accuracy is determined by the real beamwidth.

SARMTI

NAS has developed novel algorithms and technology for extracting MTI information directly from SAR data. We call the techniques SARMTI.

• SARMTI exploits the fact that the return from a moving target is made up of a linear frequency ramp, generated by the imaging process, combined with a Doppler frequency offset, arising from target motion.
• Thus one could derive a high-resolution SAR image of the moving target by applying a SAR filter at the correct offset frequency.
• The crucial elements of SARMTI are making use of the known relationships of target and image position, motion and target detection.
• Images from different centre-frequency SAR filters can be combined so that moving target images can be inserted onto the clutter background.
• Note that the identical data is used for all processes so that registration is exact.

SARMTI provides:

• High-resolution imaging of stationary and moving objects This means that target recognition based on the spatial distribution is possible.
• Estimation of across-track velocity and acceleration and along-track velocity.
• Accurate motion estimation both within and outside the clutter band, unlike conventional MTI.
• Automatic registration with the accuracy of about SAR resolution.
• Detection of moving targets.

For more information on SARMTI please Contact Us.