Overview

Group 3D Localiser

Overview

The Group 3D Localiser is designed for 2D (latitude and longitude or x and y) and 3D (latitude, longitude and depth or x, y and depth) localisation using wide aperture arrays.

With a wide aperture array, there is an implicit assumption that a detection on one hydrophone is no guarantee of detections on other hydrophones, either because the sound source was too far from some of the hydrophones, the sound source was too directional, or noise local to one or more hydrophones was too high for detection to be possible.

Furthermore, by their nature, wide aperture arrays are large compared to the duration of a typical sound being detected (this is true whatever the species, for example with harbour porpoise a ‘wide’ aperture array may only be a few metres across, which equates to a few milliseconds travel time, but porpoise clicks are less than 100µs long; similarly baleen whale sounds may be a couple of seconds long, but arrays for baleen whales are likely to be spread over many km).

The Group 3D localiser is therefore used with detectors configured to detect independently on individual hydrophones or compact hydrophone clusters. It then looks for possible matches of detections between available hydrophones, based on time of arrival differences, and forms groups of detections which are then passed to the localisation algorithm. If an array is large, the possible time of arrival of a sound on multiple hydrophones may be great enough that there are multiple possible matches of detected sounds between the various hydrophones. This problem is known as spatial aliasing and may make localisation of some sounds impossible.

Careful array design, considering animal vocalisation rate, overall call rate if many animals are likely to be present, detection range, sound directionality, the extent of the study area, and number of hydrophones to use is therefore required if this localiser is to be successfully employed.

Device Synchronisation

Most of the algorithms rely on Time Difference of Arrival of signals at multiple hydrophones. If the hydrophones are autonomous recorders deployed some distance apart, then it is likely that their clocks will be running at slightly different rates and methods must be employed to accurately measure these clock offsets and to account for them when merging files from separate recorders into multi channel files for analysis. How accurately clocks need to be synchronised depends on the size of the array. As a rule of thumb, clock synchronisation should aim to be better than 1% of the time separation of the hydrophones. So if you’re hydrophones are several km apart, then a few milliseconds time offset may be acceptable. A smaller array will require more accurate synchronisation.

Simulating Success

When considering using the Group 3D Localiser a great way of testing a configuration is to set up a simulated sound source. This allows you to test out configurations with high signal to noise ratio sounds at known locations. While the simulation is somewhat idealised and a combination of detectors and localisers which work with simulated data is no guarantee that the configuration will work with real data, if a configuration doesn’t work with simulated data, the chances of it working with real data are going to be impossibly low.

A series of demonstration configurations, all using simulated data, are being developed and made available through the PAMGuard website. You may want to take a look at these before developing your own configuration.

Next: Configuration