Abstract
The RF spectrum is a limited resource which cannot easily be globally monitored. RF emissions are regulated by diverse bodies in different regions of the world. Regulations are not globally homogeneous and in some cases not strictly observed. Satellite operations using the crowded VHF and UHF bands at Surrey Space Centre have highlighted the usefulness of LEO RF signal analysis. SSC satellites have been used to make global measurements of RF signal levels in VHF uplink channels. This has provided information on specific occurrences of RFI and general channel usage statistics. This sort of information can be useful for regulation against and mitigation against sources of RFI. It can also be used by regulatory bodies when considering frequency allocations, both for spacecraft operations and other applications. It would be desirable to have a payload in LEO capable of making similar measurements to those already taken but over a wide frequency range. A LEO system which can perform this task is not currently available in the civilian arena. This study proposes a novel system for RF monitoring which could be implemented as a payload on board a small LEO satellite. For the purposes of this study we are interested in the VHF and UHF bands. Signals from ground based sources will only be detectable by a satellite in LEO for a few minutes per orbit. In addition, we are assuming that the majority of signals we are likely to detect will be either continuous or ‘bursty’ in nature. Therefore, we must be able to carry out spectral analysis over a wide frequency range in a few seconds to detect the majority of signals. This requirement is beyond the capability of conventional spectrum analysers. This study proposes the use of a system based upon a type of spectrum analyser which can be called a chirp spectrum analyser. It can be thought of as enhancing a standard spectrum analyser by the addition of chirp filter processing. All analogue implementations are limited to detection of pulsed signals by available analogue chirp filters. An all digital implementation offers an alternative to the discrete Fourier transform but with little advantage over efficient implementations such as a fast Fourier transform. This study proposes a hybrid analogue/digital implementation which can be used as a more general purpose instrument than either an analogue or a digital system. The implementation of chirp filter processing has also been investigated. It was found that an analogue chirp filter, often called a dispersive delay line, produces a different output to a chirp filter which is a matched filter to a chirp signal. The novel achievements presented in this thesis can be summarised as: a proposal for a novel system to be used as a LEO payload for general purpose RF monitoring. the analysis and design of an efficient sweeping signal capture system. This includes a comparison with a more conventional channelised or stepped frequency system. the incorporation into the proposed system of chirp spectral analysis using an analogue sweeping section and digital chirp filter processing. This technique can be applied to a general purpose instrument and is not limited to the application presented here. the use of a software chirp filter implemented as an expression for a dispersive delay line. retrieval of individual signals, for the purposes of further analysis, from data that relates to a continuous sweep across a frequency range.