What do you have for receiving gear ? So far as I can find radiosondes output pressure, temp and humidity … with an option for GPS data. The newer ones are digital, tx’ing at 1680 MHz* (settable via dip switches), although I’m less certain that the GPS data is in that stream. A high altitude balloon project (amateur) was using an LMS-6, using it’s GPS as a backup for the APRS tracking. The tx freq was listed as 400+ MHz. I saw other sites listing that as 403 MHz.
http://patkilroy.com/simsat/3/
My point is you may be able to figure out the simple PTH data by examination w/your existing gear. Moreover if your plan is to recover the probe, perhaps a datalogger is all you need, vs a telemetry system ?
OTOH see this link re: US radiosondes and the RS92SGP digital protocol …
Seems odd to me that the US would have 2 incompatible systems (LMS-6 and Vaisala RS92-NGP) to replace the older MarkIIA’s. So does the LMS-6 also follow the RS92SGP digital protocol ?
http://www.rtl-sdr.com/tag/rs92sgp/
*Perhaps it’s a case of all the older MarkIIA’s (and ???) using 400+ MHz for downlink.