GEOG 862
GPS and GNSS for Geospatial Professionals

A Different Strategy

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One-Way Ranging: GPS Satellite to GPS Receiver, length rho
One-Way Ranging
Source: GPS for Land Surveyors
Two-Way Ranging: EDM to Retroprism and Retroprism to EDM, length rho
Two-Way Ranging
Source: GPS for Land Surveyors

Such a method works in the EDM’s two-way ranging system, but the GPS one-way ranging makes the use of the same strategy impossible. GPS ranging must use an entirely different strategy for solving the cycle ambiguity problem, because the satellites broadcast only three carriers, currently. The venerable L1 and L2 are now joined by L5 from some of the satellites. Those carriers have constant wavelengths, and they only propagate from the satellites to the receivers, one direction. Therefore, unlike an EDM measurement, the wavelengths of these carriers in GPS cannot be periodically changed to resolve the cycle ambiguity problem. Still, the carrier phase measurements remain an important observable in GPS ranging.

Using exactly the same sort of phase comparison that is convenient for the EDM two-way system is impossible in the ranging of the GPS system. GPS ranging has to use a different strategy to solve this cycle ambiguity or integer ambiguity problem. Because the satellite broadcast have only three carriers and they have constant wavelengths, and because they propagate in one direction from the satellite to the receiver, the integer ambiguity remains.