The principle argument supporting satellite-based GPS transmitters seems to be the line-of-sight argument. Specifically, 2fst4u is stating that in a valley, one would only have line-of-sight with a high altitude satellite and therefore satellites must be the source of the GPS signal. This is not accurate though. GPS operates in the range of 1.1 ~ 1.5GHz. The 23cm amateur band operates in the 1.2GHz range. If you ask a radio amateur if (s)he must maintain line-of-sight with another station or a repeater in order to communicate, they will report that they do not. (The 23cm band is favored by some for this very reason.) Therefore, the argument is insufficient to prove the existence of satellites and certainly insufficient to prove RET.
Incorrect on several levels.
GPS operates at 575.42 MHz -1176.45 MHz placing it in UHF not 1.1 ~ 1.5GHz like you said. Similar, but still shifted too high for the purpose of explanation (infact, the figures you gave would back me up more but lets not go there)
The 23cm band is slightly lower frequency. Still in UHF and therefore, if you ask a radio amateur using UHF (A rare occasion in itself. Most will use VHF as UHF equipment is expensive. VHF will diffract over hills but only slightly) if they need to use a repeater station to communicate, they will say "yes. Yes I do". An amateur working on lower frequencies will not.
Radio communication can also rely on bouncing off the ionosphere at low frequencies and so they can be used in isolated places such as valleys. No higher frequency communication devices will work in such a place. However, GPS is not any kind of communication, it relies on the fact that one must be in a straight line distance to the object it is being measured from. No frequency (bar X-rays or something intense like that) can travel through the earth to a station in a straight line. Radio communication
does infact therefore rely on either low frequency, or line of sight. Either way, a GPS device will work in a valley and
must be in a straight line from what ever it is measuring it's distance from. A wave diffracting around a ridge line will take longer to reach you and will therefore bear inaccurate results.
A few more examples of UHF use is cellphones, cordless phones bluetooth an TV. All of which we know aren't exactly good in isolated positions (valleys for TV and cellphones, behind a few walls in a house for bluetooth and cordless phones). The UHF band relies on line-of-sight no matter what. And even after a repeater station or reflection off the atmosphere for a lower frequency, that distance has become longer and rendered a GPS device's accuracy useless. This does not happen in a valley and as such we only deduce that all communication between device and what-ever-the-hell-the-other-thing-is [read, satellites] is in a straight line.