A radiosonde is an inexpensive sensor package that's intended to be used with a weather balloon for atmospheric measurements. The device transmits data in the 403 MHz band after being launched and they usually aren't recovered after use. You can pick them up for very little money on eBay so thought they might work well as a low cost GPS tracker. Unfortunately the Radiosonde doesn't transmit standard NMEA GPS data, but GPS doppler measurements. It's hard to determine what those are actually useful for. Nick did find one other paper documenting an unsuccessful attempt which he has posted to his site. So now Nick is looking for some help either making the data more useful or coming up with a functional device that's just as inexpensive.