Abstract
Current civil spectrum monitoring systems are largely terrestrial-based and only cover a localised area limited to within the radio horizon of the measurement sites. Such systems typically use fixed frequency tuning receivers and can only acquire one specific frequency at a time. The aim of this research is to develop a system that is capable of performing global radio spectrum surveillance by mapping what frequencies are being transmitted and from which locations on the surface of the earth. The research comprise two areas, namely; 1) the use of orbiting satellites to host specialised radio frequency acquisition receivers; 2) the development of novel signal processing algorithms to extract the geolocation information and analyse the emission spectrum of the individual transmitting sources. The system configuration was designed based on a form of two-dimensional correlation function (otherwise known as the Ambiguity Function) to geolocate and map the source signals. It uses the principle of time correlation to identify the along-track location of a target, and Doppler difference frequency tuning to select the across-track range at which the correlation functions are to be evaluated. In this manner, a geolocation map can then be generated by stepping through and evaluating the individual correlation function at the various across-track ranges. In the proposed system, a constellation of two satellites will be used to form a measurement baseline. Each satellite will carry a specialised 'Microscan' receiver payload to scan through the entire monitoring bandwidth repetitively in fractional time, sampling the radio environment as it orbits Earth. The acquired data will then be down-linked during the next pass over the ground station for further post-processing. Spectrum analysis and transmitter geolocation will be achieved by selectively processing the relevant frequency bins of interest. To enhance the system's ability to detect weak radio sources, the Post-Detection Integration technique will be used to achieve the required processing Signal-to-Noise Ratio (SNR). The principles of Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA) will be applied to resolve targets located within the receiving swath. A novel variable slip technique was also conceived which will enable the entire receiving swath to be mapped from just one second worth of complex sampled data obtained by each of the receiving satellite. Simulation results confirmed that the proposed system has an along-track range resolution to the target which is inversely proportional to the signal bandwidth, typically a few hundred meters for CDMA mobiles. Across track resolution is variable and can always achieve this resolution or better. The processed SNR is proportional to the signal integration time so that given enough integration time, any radio sources can be detected however small the received SNR. The proposed system will make comprehensive global surveying of radio spectrum usage possible for the first time and will be useful for civil radio governing agencies. With some adaptation, such a system can also potentially function as a low cost electronic intelligence (ELINT) gathering asset suitable for use by military establishments.