Modernised GPS: Phase-only Integer Ambiguity Estimability in the Presence of Ionospheric Delays

In GPS data processing it is not uncommon to eliminate the presence of ionospheric delays by taking certain linear combinations of the carrier phase data. This approach is in fact equivalent with a processing of the original, not combined, L1 and L2 phase observations in which the ionospheric delays are modelled as unknown parameters. When using phase data only, the presence of the unknown ionospheric delays produces a rank defect in the model of observation equations. Eliminating this rank defect leads to the conclusion that not all of the original L1 and L2 ambiguities can be estimated as integers, but only a certain linear combination of them. In this contribution it is investigated how this situation changes when triple-frequency, phase-only data of modernised GPS are used. We identify the integer rank defects, show which integer ambiguity combinations are estimable and determine their corresponding ambiguity success-rates.