(Apologies for the untimely reply) We used a UBlox5, which we purchased from diydrones.com. This was in 2009, so that particular model is now outdated. Both

(Apologies for the untimely reply) We used a UBlox5, which we purchased from diydrones.com. This was in 2009, so that particular model is now outdated. Both Adafruit and SparkFun now sell devices which are explicitly listed as being HAB-friendly. There's also good guide at http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm. That page states that SiRF-I and II are known to fail above 60k feet, and that some SiRF-III-based receives fail, while others don't. (If you happen to be in the Bay Area, I'd be happy to meet up and give you guys some pointers sometime; drop me a private email.)