Bernese GPS Software Version 5.0 - Bernese GNSS Software

8. Initial Phase Ambiguities and Ambiguity Resolution
L1-ambiguity (cyc)
8.3.4.2 The Role of the Ionosphere
QIF: L1-L2-ambiguity space
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L2-ambiguity (cyc)
Figure 8.8: Search ranges in (n1,n2) space.
For baselines longer than about 10 km processing of the two frequencies L1 and L2 separately
does not give sufficiently good initial real valued estimates b1 and b2 due to the influence
of the ionospheric refraction. Two types of models to reduce the ionospheric biases are
considered (see also Chapter 12):
(1) Satellite and Epoch Specific Ionosphere Estimation: One ionospheric correction I i k (tj) for
satellite i, receiver k and epoch tj is estimated. Estimating these parameters without
any a priori constraints would be equivalent to processing the ionosphere-free linear
combination. If we want to resolve the integer ambiguities it is necessary to constrain
these parameters to within a few decimeters. This constraining may be achieved by
introducing an artificial observation
I i k (tj) − I i k,apr (tj) = 0 (8.25)
for each epoch with a non-zero a priori weight. The actual values I i k,apr (tj) may stem
from an ionosphere model, in many cases (baselines up to 500 km) even I i k,apr (tj) = 0
may be sufficient. It is of course necessary to pre-eliminate all epoch-specific ionosphere
parameters I i k,apr (tj), i = 1,2,... ,ns (ns is the number of satellites per epoch) after
having processed epoch tj, because a “terrible” number of parameters would have to
be handled in the normal equation system after ne epochs (see handling of epochparameters
in GPSEST in Section 7.5.3).
(2) Deterministic Model: single-layer models developing the electron content in a layer of
infinitesimal thickness in a height of about 350 km above the surface of the Earth into
a series of harmonical coefficients in latitude and hour angle of the Sun. Such a model
should be used if long baselines (500 km – 2000 km) are processed.
A combination of the two types of models may be used.
Page 178 AIUB