More on GPS autoland
The way model aircraft use GPS to allow autoland takes advantage of the fact that their flight durations are short. If you measure your GPS location at a fixed location, then measure it again a few minutes later, it will be different, but not that different. The location wobbles about over time, partly as a function of weather in the ionosphere making signal path lengths from the GPS satellites vary, partly due to the very, very, slight irregularity in the clocks in the satellites (and for many other reasons). What you will find that over a 24 hour period, if you plot all your position fixes, 95% of them will be within a circle of small radius that will very likely also have your real position within that circle. So any particular location fix is likely to be wrong, as is the next one, but they will be wrong by roughly the same amount as the wobble or drift usually takes place over relatively long time periods. In the link here: ( http://gpsworld.com/gpsgnss-accuracy-lies-damn-lies-and-statistics-1134/ ) Figure 4 shows a plot of 10,000 GPS fixes taken over 3 hours as a scatter plot, and you can see how successive plots don't differ much from each other, but over time, they wander about quite a lot.
This means a model aircraft can take a GPS reading at take-off and 10 minutes later, when it is landing, the wobble or drift won't have been that much, so you can rely upon the GPS position you are currently measuring to be 'near-enough' the take-off location for landing purposes.
Large military UAVs will typically have flight durations that are greater than 10 minutes. According to Wikipedia, the General Atomics MQ-9 Reaper can have 14 hours endurance, and that is long enough for GPS position errors to add up, so knowing what the GPS position of your take-off point was 14 hours ago doesn't help much, because in the 'average' worst case, the current fix could be 40 metres off vertically from the one at take-off; and with 14 hours endurance, there is no guarantee the mission requires you to land where you took off, either.
The FAA are looking at using GPS for autoland on commercial airliners.
http://aviationweek.com/commercial-aviation/faa-targets-2018-gps-based-autoland-capability
However, that uses ground-based transmissions to augment the basic GPS signals. This uses ground-based GPS receivers that have well-defined known fixed locations so that the current GPS errors can be determined and sent to the aircraft to correct its own GPS reading.
I imagine that military UAVs might have a requirement to be able to operate in 'radio silence', as you don't really want a SAM locking onto its radar altimeter transmissions; or indeed a missile locking onto ground transmissions at its landing point. That does make operating in IMC difficult.
I agree with other posters that a strain sensor rather than a WoW switch makes sense.