A comparison of theoretical estimates of the errors associated with ultrasonic tracking using a fixed hydrophone array and field measurements

Consideration of position fixing errors associated with fixed array acoustic tracking techniques has generally concentrated upon the consequences of inaccuracies in the timing of the arrival of the ultrasonic pulse at each hydrophone, the overall accuracy of the system being determined by combining timing errors with information on the position of the transmitter in the array. This paper presents a mathematical treatment which develops previous studies, allowing the distribution of position errors to be predicted from this same information and also making it possible to quantify multi-path effects and the resulting errors in position fixing. These theoretical considerations are compared with the distributions of 1000 position fixes generated from each of five stationary transmitters using a fixed hydrophone array. In each case the transmitters were on the sea-bed within the boundary of the hydrophone array. Four of them were monitored in what would generally be considered unfavourable tracking conditions on an underwater reef characterised by deep crevices and a dense covering of Laminaria sp. Although the total area encompassing all estimated positions obtained during the field measurements varied considerably between tags, all exceeded 150 m^sup 2^. The position errors of the four tags associated with the reef were large compared to theoretical estimates based upon expected errors in time measurement. It is concluded that multi-path effects and random noise can make a substantial contribution to the errors associated with fixing the positions of tagged individuals, particularly when tracking conditions are difficult and the signal level reaching any given hydrophone is low relative to the background noise. Thus timing errors and the general position within the hydrophone array are only contributory factors in the determination of the total error, which may be more strongly influenced by the micro topography in the vicinity of the tag.[PUBLICATION ABSTRACT]