Instrumentation for radio tracking of drifting buoys
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. A theoretical investigation has been undertaken into some of the problems associated with developing a hyperbolic direction-finding system for tracking multiple oceanographic buoys, and thereby inferring surface current patterns. A prototype system has been constructed embodying the results of this investigation. A modified hyperbolic navigational system is used. Each buoy carries a low-powered HF transmitter radiating a tone modulated double-sideband suppressed-carrier signal. Signals from each buoy, radiated in turn, are received at three shore-based stations. Their relative phase differences are used to determine the transmitter position. A large number of buoys (e.g. 20) can be simultaneously and continuously tracked over an area of several hundred square km for a period of several days, depending on the battery power available in each buoy. Special receivers using heterodyne phase-locked loop principles were evolved and tested. This design is shown to overcome phase errors introduced by transmitter frequency drifts. The equations describing the operation of the phase-locked loop system are derived and solved, and conditions for the required system accuracy are deduced. Several methods of demodulating the received signals are examined, including the quadratic envelope detector, the Costas loop and the squaring loop. It is shown that the equations describing the present system can be reduced to those describing the squaring loop. The behaviour of this loop in the presence of noise is analysed in detail. A precision phase-locked loop is employed to carry out phase estimation of the recovered modulation. A detailed analysis of loop performance in the presence of additive noise, and also phase noise on both received signal and internal VCO signals is carried out. A study of the lock acquisition behaviour is made for the expected operational situation, where the initial frequency error between the applied signal and the VCO quiescent frequency is less than the loop bandwidth.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1984