I don't understand why the pingers are on continuously until battery depletion. Assuming we can't have them be triggered by incoming sound (making them essentially sonar transponders), why couldn't they be on for e.g. 15 days, then off for a month, on for 7 days, off for two more months and on for 7 days? The power requirements of the timing device are minuscule (my watch needs a new battery every few years).