6.3.5 GPS Observation Modelling




In these notes the term "double-difference" will refer to the observable which has been formed by differencing between satellites and between stations (see Figure below). (It is possible to generate other types of double-differences, involving different combinations of between satellite, between receiver, between epoch, and even between frequency, differencing strategies.) This receiver-satellite double-differenced phase may be created by forming the between-satellite difference phase and then differencing between stations, or by first forming the between-station difference and then differencing between satellites:


Assuming that the satellite clock phase errors cancel in a between-station difference, then it is possible to use eqn (6.3-5) to derive the double-difference observation equation:


The only clock biases remaining in this equation are the integer cycle ambiguities. The four individual cycle ambiguities n11, n12, n21 and n22 may be parameterised separately, or a new definition adopted for the ambiguity parameter consisting of the double-differenced cycle ambiguity K1212 (= n11 – n12 – n21 + n22).

The double-difference observable.

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© Chris Rizos, SNAP-UNSW, 1999