Ah, a trick question. You ask about a
remote valley surrounded by mountains with no
radar ADS-B radio station near.
Let's make it worse, the valley walls are high enough to also make communicating with the GNSS constellation difficult, further losing one's own location.
For those, you need Wide Area Multilateration (WAM).
A new surveillance system introduced, called multilateration or Wide Area Multilateration (WAM), is now allowing air traffic controllers to track aircraft along the difficult approach to Juneau, Alaska—a mountainous area where radar was not possible (FAA).
ADS-B is not a magic bullet for those very unique locations.
See: What is the purpose of wide area multilateration (WAM)?
Like Secondary Surveillance Radar (SSR) it is classified as a cooperative independent surveillance system; the aircraft has to cooperate (i.e. an active transponder) but position is determined independent from data sent from the aircraft (unlike e.g. ADS-B).