Peter Reilley suggests a backup to GPS using terrestrial transmitters. This
idea has been around since the early days of GPS. The terrestrial
transmitters were called "pseudo-satellites", or "pseudolites" for short.
The big problem with this idea is that the GPS signal format has a narrow
dynamic range. The signal strength from a terrestrial transmitter varies
widely (inverse square law) from positions near the transmitter to
positions far away. The variation in any practical system is larger than
the GPS signal format can handle. This is called the "near-far problem".
For an extensive discussion of the pseudolite concept, including the
near-far problem, see my dissertation. You can find it with a web search
for my full name and the word "pseudolites".
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