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Real Time Kinematic GPS in an Urban Canyon Environment GPS Measurement Models The observables considered in this paper consist of double differenced between two receivers. For two receiver stations (1 and 2) tracking simultaneously the same set of satellites (s and l), we have the code double difference equation for dual frequencies as 
Subscripts p, q are used to identify measurements with L1, L2 with frequency and respectively. 
The double difference code and carrier phase measurements can now be reduced to the form given in equation (5) 
where . Multipath f1 / f2 =154/120 = 1.2833333... Multipath signal propagation has remained a dominant cause of error in RTK GPS positioning. Multipath errors are due to reflected GPS signals from surfaces (such as buildings, metal surfaces etc) near the receiver, resulting in one or more secondary propagation paths. These secondary-paths signals, which are superimposed on the desired direct path signal, always have a longer propagation time and can significantly distort the amplitude and phase of the direct-path signal (Iyiade and Owusu-Nkasah, 2002, Cross et al, 2003). Multipath error is scaled according to wavelength and is generally therefore nearly 100 times lager for P-code pseudoranges than it is for carrier phase measurements. Instantaneous multipath error can be as large as a few meters for P-code and a few centimetres for carrier phase. Thus, multipath becomes a dominant source of error in the measurement, in a situation in which range and phase data are needed instantaneously. Wavelength Resolution In resolving for the ambiguity, the combinations of wide-lane and narrow-lane observables are mostly used. The wide-lane observable provides faster solution than using either L1 or L2 observables (Cardoza et al, 1994). The combinations are 
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