[motohagiography]

Not a pilot, but played with ADS-B tracking several years ago, and my impression was below something like 4-5000' the transponder wasn't required to transmit location. This meant that surveillance aircraft (usually single engine propeller craft and helicopters) didn't send GPS, only the fact that it was in the air.

[jhugo]

Yeah. It's a requirement in a lot of US airspace, but not all airspace. Even at low altitudes it's required near major airports though.

Surveillance aircraft, if operated by a government entity, would not be required to transmit anyway. (Though they may choose to for safety purposes if it doesn't compromise the mission, just as the military often do in busy airspace.)