Archive for August 21st, 2010

APRS Beaconing: On Being Relatively Prime

I ran into an amusing situation on a recent family bike ride with our GPS-to-APRS trackers running: my ladies were transmitting a few seconds apart. As a result, I had to listen to a pair of very short data bursts in quick succession throughout the whole ride.

Under normal circumstances that doesn’t happen, because I set the TinyTrak3+ trackers to delay during and wait a second after a voice PTT that collides with an automatic beacon. Somehow they never managed to delay an APRS beacon to knock the synchronization off kilter.

So I tweaked the automatic transmission intervals to make us relatively prime: 179, 181, and 191 seconds. That’s close enough to the original 180 seconds as to make no difference, while now ensuring that we won’t collide with each other for very long even if we should get aligned.

An alternative is SmartBeaconing, which I’ll turn on in a while after I collect a bit more data.

A useful table of primes is there.

If you have some spare CPU and power, you can join the Great Internet Mersenne Prime Search and help find new primes, albeit ones much larger than I need…