Interference suppression for 121.5/243 MHz SARSAT signals

Abstract

<p>When an aircraft or marine vessel is in distress, the emergency unit is activated either automatically or by manually setting a switch. The signal is then received by an orbiting satellite as it sweeps out a path over the search and rescue satellite aided tracking (SARSAT) region of interest. Due to the relative motion between the satellite and the Emergency Locator Transmitter (ELT) or the Emergency Position Indicating Radio Beacon (EPIRB) unit, the signal received at the spacecraft is Doppler shifted. New processing techniques for detecting ELT signals in an interfering background are investigated, based on not only computer simulations but also experimental data. The new methods involve the use of the Geometrically Averaged Spectrum (GAS) and the Harmonically Averaged Spectrum (HAS) in addition to the Arithmetically Averaged Spectrum (AAS) adopted by the existing system. Specifically, this thesis investigates the theoretical performance of AAS, GAS and HAS averaging in practical circumstances involving signals, noise and interference. It is shown that the carrier peaks of good ELT signals can be substantially enhanced in an interfering background. The treatise identifies certain types of interference such as Carrier interference with strong or weak AM, On-Off Modulation interference, Swept Carrier interference, and Ringing Response interference. Using computer simulation, it is demonstrated that under certain conditions the effects of background interference can be greatly reduced and the detection of ELT carrier peaks can be improved by employing a combination of AAS, GAS and HAS averaging. Real SARSAT pass data signals are fed into the computer and processed using AAS, GAS and HAS averaging. The background interference postulated in this thesis is found to exist and real ELT signals are identified. ELT signals representing the NIC model and a new model, called the Frequency Modulated ELT (FME) model, are identified along with a new ELT signal for which a model cannot presently be envisaged. The results of letting AAS, GAS, and HAS work together are compared to that of using only AAS. It is seen that the new strategy shows superior performance over the original method in a congested signal environment even though interference power exceeds ELT signal power. (Abstract shortened by UMI.)</p>