Hi all

I've recently learned about PLBs and EPIRB, which basically send distress signals with position information from the device built-in GPS. They can be relatively expensive, so I'm wondering if anyone has made any work towards a home-brew device?

Legal Disclaimer: haha, homebrew is a poor term. These devices need to be registered etc., so it's not recommended (and actually unsafe) to build your own device. What I'm looking for is education on how these things work and general pointers on how to approach the designing of such a device.

Thanks!

 

NOTE: I don't know the specifics and am just speaking in terms of what makes sense based on what they need to do.

Basically you have a pretty simple system with four parts: GPS receiver, transmitter, processor, and activation circuitry.

The activation circuitry is designed to activate the transmitter once certain conditions are met depending on the specific intended application. This might be immersion in salt water, for instance. There is probably a manual activation mode as well and some might only have a manual mode.

Once activated, the processor takes a GPS reading (and it might periodically take a reading when not activated so that it has a relatively recent reading in case the GPS signals are not available, for whatever reason, once activated.

The processor then formats the message data and sends it to the transmitter, which transmits it continuously (or according to some schedule so as to prolong battery life).

Nothing that is technically very challenging, in fact it is simpler than most radios since it doesn't have to receive. I think the expense is because they have to be rugged and reliable under extreme conditions (namely the conditions that prompted their activation).

If you wanted to design one, the first thing you would have to understand is the waveform and message protocol they use since, if you don't meet those expectations the receivers will ignore the signal.

 

This might be a good educational starting point. http://www.cospas-sarsat.org/

 

If you want to play with parallel technologies, there is the Garmin Rino. This is a handheld GPS which also incorporates a UHF transceiver. It works under FRS as opposed to PMR446 so generally only suitable for North America.

Another location aware tracking system although again, fully transponding, is the APRS system which is designed for amateur radio use and again usually requires the appropriate licence.