6.3.3 GPS Observation Modelling



Rece iver clock phase errors may be eliminated by forming the difference between simultaneous observations by one GPS receiver to two satellites (see Figure below). The operator indicates a between-satellite difference. The differenced observable can be written as:


where it is assumed that the two observations from receiver j to satellites 1 and 2 have been made at the same time Tj. The new differenced observation is (based on eqn (6.1-13), with the inclusion of an orbit error term ejor that represents the mapping of orbit error along the line-of-sight between receiver and satellite):


Note that the receiver clock phase error has been eliminated and the clock bias terms that remain are the between-satellite clock phase error and the between-satellite cycle ambiguity. As far as setting up the design matrix for a Least Squares adjustment based on between-satellite differences is concerned, there is a choice in the cycle ambiguity modelling. Either use individual ambiguity modelling for nj1 and nj2, or adopt a new definition for the ambiguity parameter consisting of the between-satellite cycle ambiguity kj12 (= nj1 – nj2).


Between-satellite differencing eliminates receiver dependent biases.

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© Chris Rizos, SNAP-UNSW, 1999