An AUV can be known to be lost or it can be missing. It may also have sunk in water depths exceeding its depth limit. In this case, there is likely little to be done. If the AUV is known to be stopped on the bottom or under the ice surface, it is probably disabled, and some form of intervention will be required.
As is the case in open water, AUVs can also become lost for periods of a few minutes up to a very long time. In cases where transits are involved, the vehicle will leave an area where positioning and communications are possible and then check-in at pre-planned points along the route. In this case, any loss will become apparent some time after it occurred.
From time to time, AUVs which have gone missing are found exactly where the mission plan took them and a careful review of the mission plan may unearth a simple mistake. Along the same line, a review of the fault table may provide some clue as to where to look for the vehicle.
As there is some uncertainty about the status of an AUV that has not been heard from at an expected time, it may not be prudent to alert higher authorities or the insuring authority immediately.
If the vehicle’s location is not known, a search may be organized. If the vehicle is on the bottom, it should be quite easy to track the emergency acoustic beacon when the search party is within range.
When the vehicle is located, an attempt is usually made to establish communications through the telemetry link. If this is operational, it should provide a synopsis of the vehicle's condition as well as reveal what the problem is. From there, it may be possible to "coax" the vehicle back to the surface, or to resume its mission. If not, it may become necessary to resort to the use of heavier, more sophisticated lift equipment such as ROVs, and at this point, the economics of a recovery should be re-examined.
As with mission operations, the best practice will take common sense, good judgement and an appreciation for local knowledge into account.
When a vehicle is missing, the mission plan and the fault management program should be examined carefully in an attempt to reconstruct where the vehicle is most likely to be. Common sense and judgement will determine what is the best time to alert authorities and seek outside help.
In the event that the vehicle’s position cannot be confirmed, a search should be organised using a receiver to listen for the emergency beacon of the vehicle. Generally, the search should start from the last known position and proceed along the planned track.
As a minimum, a search party should carry a surface acoustic telemetry transceiver and computer as well as the directional receiver. This will allow them to establish communications with the AUV when they find it.