@VinRogue
It's a shame the pinger does not work for say 10 days, then switch off until it receives an activation ping from a search asset, say submersible. The activation ping could be coded and limited to a set frequency range.
Increased complexity, yes, but problems with pinger battery life would be resolved.
Unfortunately this won't help a whole lot. The ULB is a very simple, low power piece of equipment. The key to the low power consumption is due to its simplicity. To incorporate a transpond functionality requires a detector to run on the ULB. So lets crunch some theoretical numbers.
The ULB pingers output at a level of 160 dB re 1uPa. That is equivalent acoustic energy of 0.075W, or 75mW. Assuming inefficiencies in amplification require a 10x amount of electrical power to produce that amount of acoustic power that gives us 0.75W.
The ping is 10ms long, thus the ENERGY per ping is 0.75*0.01 = 0.0075joules or 7.5mJ.
At a rate of 1 per second, gives an effective average power requirement (ignoring circuitry – which should be pretty simple for a dumb pinger) of 7.5mW.
Now, the company I work for makes underwater acoustic communications equipment. An example of a simple, high efficiency FSK detector uses approximately 77 milliwatts when running in continuous detect mode. That is 10x my estimated consumption for the simple pinger. An that is without the power requirement to generate an acoustic signal in response.
So not a practical option with the current ULB form factor.