Modelling and resolving of the ambiguous angle of arrival measurements of the SANAE IV SuperDARN Radar

Master Thesis


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University of Cape Town

The Super Dual Aural Radar Network (SuperDARN) forms a multinational collaboration consisting of over 30 HF radars used to monitor charged particle convections in the high latitudes of the ionosphere. This research is inspired by the antenna arrangement of one of these HF radars situated at the South African National Antarctic Expedition (SANAE) IV base. This radar consists of a steered 16-element primary array, and a second 4-element interferometer array. Phase wrapping is introduced into echoed returns due to the two arrays being separated by 100 m, several wavelengths of the carrier frequency. As a result, the radar system is only able to unambiguously determine the Angle of Arrival (AOA) for a given target provided it resides below 33ᵒ in elevation when observing along boresight. This dissertation models a subcomponent of this phenomenon and develops an entirely separate pulsed-Doppler radar system on an FPGA platform known as the Red Pitaya. Operating parameters are specifically chosen to be consistent with the SANAE IV radar. An antenna arrangement consisting of two receivers and a transmitter is constructed to replicate the AOA issue experienced by the SANAE radar. Instead of focusing on detecting targets in elevation, as done with the SuperDARN radars, targets are detected in azimuth. Three algorithms are developed to autonomously measure the AOA and to counteract the effects of 2π phase wrapping. The naïve measurement applies no corrections and is only able to detect targets at angles within ±17.10ᵒ of boresight. This is the theoretical maximum measurable AOA when operating at 17 MHz. A second algorithm involving a Standard Deviation (STDEV) search process is used to successfully measure angles up to ±70ᵒ for the same arrangement. It does this by varying the carrier frequency and using numerous received aliased AOAs to correct and expand for a true AOA. Empirical results yield an average Mean Square Error (MSE) of 0.29 between the true and measured target angles. A third algorithm involving the Chinese Remainder Theorem (CRT) was shown to work in theory (albeit only up to ±66ᵒ), but failed to perform in practice. Results presented in this dissertation indicate that the maximum measurable AOA is expanded by a factor of 4 by using the STDEV algorithm. It is postulated that the same result can be achieved using the actual SuperDARN radar. Theory relevant to generating the RF Linear Frequency Modulated (LFM) chirp and its use for transmitting, receiving and processing is covered. Topics of pulse compression, range resolution, environment modelling, reconstruction and matched filtering, coherent integration, sidelobe reduction and demodulation are detailed.