Processing of ultra-wideband low-frequency signals, for application in Foliage Penetration (FOPEN) Synthetic Aperture Radar (SAR) systems

Abstract

Foliage Penetration (FOPEN) radar systems were introduced in 1960, and have been constantly
improved by several organizations since that time. The use of Synthetic Aperture Radar (SAR)
approaches for this application has important advantages, due to the need for high resolution in two
dimensions. The design of this type of systems, however, includes some complications that are not
present in standard SAR systems.
FOPEN SAR systems need to operate with a low central frequency (VHF or UHF bands) in order
to be able to penetrate the foliage. High bandwidth is also required to obtain high resolution. Due to the
low central frequency, large integration angles are required during SAR image formation, and therefore
the Range Migration Algorithm (RMA) is used. This project thesis identifies the three main complications
that arise due to these requirements. First, a high fractional bandwidth makes narrowband propagation
models no longer valid. Second, the VHF and UHF bands are used by many communications systems.
The transmitted signal spectrum needs to be notched to avoid interfering them. Third, those
communications systems cause Radio Frequency Interference (RFI) on the received signal.
The thesis carries out a thorough analysis of the three problems, their degrading effects and
possible solutions to compensate them. The UWB model is applied to the SAR signal, and the
degradation induced by it is derived. The result is tested through simulation of both a single pulse stretch
processor and the complete RMA image formation. Both methods show that the degradation is negligible,
and therefore the UWB propagation effect does not need compensation.
A technique is derived to design a notched transmitted signal. Then, its effect on the SAR image
formation is evaluated analytically. It is shown that the stretch processor introduces a processing gain that
reduces the degrading effects of the notches. The remaining degrading effect after processing gain is
assessed through simulation, and an experimental graph of degradation as a function of percentage of
nulled frequencies is obtained.
The RFI is characterized and its effect on the SAR processor is derived. Once again, a processing
gain is found to be introduced by the receiver. As the RFI power can be much higher than that of the
desired signal, an algorithm is proposed to remove the RFI from the received signal before RMA
processing. This algorithm is a modification of the Chirp Least Squares Algorithm (CLSA) explained in
[4], which adapts it to deramped signals. The algorithm is derived analytically and then its performance is
evaluated through simulation, showing that it is effective in removing the RFI and reducing the
degradation caused by both RFI and notching. Finally, conclusions are drawn as to the importance of each
one of the problems in SAR system design.