We tested PLBs in a deep narrow canyon and they worked. The signal will also bounce off the rock walls as well. You may or may not get to the GEO Sat, but the LEO Sats will pick you up. Sure as heck a lot more likely than hoping an aircraft conveniently overflies your location at just the time you need it and you can hit it and get their attention with the laser flare in the brief moments it is visible from the bottom of the canyon.