I was surprised that flight recorders only have enough battery to send out signals for 30 days. I'm guessing that it blindly sends out signals that help it to be located. But instead, why aren't they made to only send out a signal when they receive a ping? Surely, then, they could last for years?
Think about it a bit: you'd need a monstrous piece of gear to be able to receive a ping from a distant source on a search platform. Transmitters are not the same as receivers.
The primary design goal of the flight recorder assembly is crash survival. Further, after 30 days, there's pretty much no chance of finding survivors (with the notable exception of the Andes 1972 crash . The world's a big place. Aircraft occasionally disappear and are found years later pretty much by blind luck. For that matter, cargo ships sink all the time and even given their bulk and the theoretical ability to put a very large, long-lived beacon on them, they're not found either.
An ELT or Emergency Locator Transmitter is built to be a reliable emergency device to locate a wrecked aircraft relatively quickly by satellite and ground triangulation. The goal of locating the aircraft is to save human lives; so 30 days is usually sufficient. Finding remains and satisfying people's curiosity is not part of the current design criteria.
Transmitting on an interval is the most reliable method, as it starts transmitting on the emergency frequency as soon as a plane goes down; not when someone decides to start looking for it. Satellites and ground towers are always listening on this frequency and will triangulate the signal as quickly as possible. Think of it like your house is on fire and you are trapped inside. You would yell "help, help, help" loudly and continuously instead of listening at the door for bystanders before you yell.
To extend the amount of time an ELT signal lasts could be done by:
- Adding a larger battery (obviously)
- After the 30 days go into a "Marco Polo" mode, waiting to receive a signal before transmitting like you mentioned. Remember it still takes some power to listen.
- After the 30 days decrease how often the signal is transmitted logarithmically. If the ELT synchronized with the aircraft clock it could emit signals at predictable times so that search personnel would be ready to listen. This would allow for very very low power use during the waiting period.
- Adding water pressure sensing capability to the unit so that it only sends sonar pulses after being submerged to a certain depth.
- Adding multiple directional antennas instead of an omnidirectional antenna. Sensors in the unit could fire the antennas that are not pointed at the ground increasing signal strength and reducing power consumption.