Modern tracking tools are using wifi, A-GPS, and satelite
repeater....where tracking via sats get assisted via GSM beacon,
internet or simply extended satelite signals. Jargons and
technicalities aside, it is already possible for vehicular
tracking device to go underground and waterways.
Also, outdoor and mil spec GPS without the assisted function can
already track without direct exposure to clear sky. Those green
'bricks' used in SAF training are 10 years old so line-of-sight is
still needed to get a fix.