Black boxes put out a sonar ping at about 32kHz. From what I've read, depending on sea conditions the ping will be audible at the surface out to about 16 km, leaving a search area of around 1024 square km. By adding a little complexity to the ping circuit, you could encode within it the last received GPS position of the box, before it hits the water. With average sea depth and average sink angles of less than 45° that information would cut the search area down to about 10 square km. Is there some reason this tech is not used, or is it still in the rigorous testing pipeline these devices must go through?
Current underwater locator beacons (ULBs) have a detection range of only a few km. 16 km would be under exceptionally favourable conditions. So in most cases the GPS data would not be of any additional help. It might tell you where the aircraft was before it hit the water, not exactly where the debris ended up after falling through several Km of water.
An advantage of current ULBs is their extreme simplicity and small size. There's no need for external data-feed and power connections. There are no issues of openings in the casing for waterproof connectors that also need to withstand the g-forces and pressures expected to be endured by a ULB. They are easy to test regularly and relatively easy to maintain.
From what I have read, current proposals for improvements to ULBs mostly focus on
- increase duration from 30 days to 90 days
- increase range by changing frequency from 37.5 kHz to ~10KHz
There are trade-offs here - longer duration means bigger batteries. changing the frequency may make it harder to distinguish from other sources of sound in the ocean.
An interesting 2011 dissertation describes the difficulties involved and discusses modulating the beacon signal to include data.
Even if one started a search with many vessels whose equipment can detect nothing more than that a beacon exists somewhere within 16km, a ship that detected a beacon's signal could summon one or more ships with fancier equipment which would be able to home in on the signal much more accurately. I don't think that encoding a plane's last above-ground location in the ping data would assist significantly with locating the beacon.
Having the black box "blindly" encode any significant amount of information in its pings would add greatly to the power requirements; there might be some benefit to having a black listen occasionally (the pings would indicate when) for an outside request for data, and then transmit some of its data in response to such requests. This could be helpful in cases where the box could be located, but was in a place that salvage equipment could not reach. Only a limited amount of data could be transmitted, however; depending upon how far away it needed to be received, transmitting even a minute's worth of audio might use more energy than a month's worth of pings. I don't know that the number of cases where a box is detected, and holds data, but cannot be physically retrieved, would be sufficient to justify the extra cost and complexity of ultrasonic data communication. It would probably cheaper to add redundant recorders to the aircraft in places that would likely end up in different parts of the debris field, so as to maximize the likelihood that at least one would be reachable by salvage craft.
Main reason those signals can be distinguished for what they are is because they have a specific signature. If the signals were to change all the time because you mix in extra information, they'd be impossible to recognise and thus useless.
Combined with the short range of the signal (inevitable consequence of the need to be low power, anything more would mean heavier equipment and/or shorter lifespan), there's just no benefit.