allanhurst:
A field I kmow absolutely nothing about - but fascinating.For curiosity - why does ordinary GPS not work at very high altitudes? - It's closer to the satellites, so could get a (bit) better signal...?
regards
Allan.
Because they are poorly coded;
Note the important bit;
"In GPS technology, the term "COCOM Limits" also refers to a limit placed on GPS tracking devices that disables tracking when the device calculates that it is moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).[2] This was intended to prevent the use of GPS in intercontinental ballistic missile-like applications.
Some manufacturers apply this limit only when both speed and altitude limits are reached, while other manufacturers disable tracking when either limit is reached. In the latter case, this causes some devices to refuse to operate in very high altitude balloons."
So as long as the balloon is not travelling at more than 1,200mph it is legal for it to report position and altitude, which is what the inexpensive Ublox GPSs do.