8.4.1 Some Dual-Frequency Procedures

INTRODUCTION

There are a number of strategies for processing dual-frequency data. The ones that are most often used in the context of GPS surveying are:

• Processing L1 and L2 data separately.
• Processing the L3 ionosphere-free observable.
• Processing the L5 "wide-lane" observable, possibly in an iterative procedure with other types of observables.
• Using L3, L4, L5 and L6 in certain combinations to aid ambiguity resolution, or cycle slip detection and repair.

Before discussing these it should be noted that the processing procedures (1), (2), (3) attempt to either eliminate the ionospheric delay term from phase data, or at least reduce its impact on certain dual-frequency phase combinations. There are alternative approaches to dual-frequency processing that attempt to handle the ionospheric biases by modelling them in some way (generally with the aid of the L4 combination), or by estimating the ionospheric delay as some form of stochastic process within a Kalman filter. Whatever linear combination of phase (and pseudo-range) data is used, the following characteristics of the resulting linear combinations is desired (eqn (6.4-22)):

• The wavelength is reasonable, not too short, preferably long.
• The residual ionospheric effect is small.
• The measurement noise is kept small.
• The resulting ambiguity is an integer.

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© Chris Rizos, SNAP-UNSW, 1999