Sonardyne provide a navigation system called “HydroPos”, and in its early years we suggested that Group Interrogation was the better option. However, we recommend (and implement) Unique Interrogation mode now.

The original idea was that the vessel could sail between two seismic lines, and interrogate multiple beacons during its transit.

This is true – and it worked – but the problem is that, to target multiple beacons, the Range Window needs to be quite large. The result is that the single interrogation from the Transceiver instigates lots of beacon replies, but many of them are too far away to provide useful range data.

Also, the associated Range Wait in the topside equipment means that the most important beacons (which are nearest the vessel) only provide a relatively small number of ranges (because the Transceiver is waiting for all the other replies to arrive before the next acoustic cycle).

The best approach is to use “Unique Interrogate” mode.

Set the Range Gate to a modest value so that you focus on only 1 to 3 beacons at a time. The associated Range Wait time will be much smaller, and the result will be a much larger, and more useful set of range data.

In the example below, Address 11329 means this:

1 = “Nodal” architecture (always 1)

13 = Group Interrogation Address

56 = Group Reply Channel

However, when used in “Unique Interrogate” mode, this is what happens:

Transceiver sends Unique Interrogation Channel 1356.

SST 6 receives this, then waits 320ms (Turn-Around-Time).

SST 6 transmits Group Reply Channel 56.