Bernese GPS Software Version 5.0 - Bernese GNSS Software

12. Ionosphere Modeling and Estimation
12.1 Motivation and Introductory Remarks
You have to deal with ionospheric refraction, to be more specific, with the term I i k
observation equation (Eqn. (2.34)), in the following processing steps:
of the
(1) single-point positioning (program CODSPP), if you do not use the ionosphere-free (L3)
linear combination,
(2) pre-processing (program MAUPRP),
(3) ambiguity resolution (program GPSEST), if you do not make use of the Melbourne-
Wübbena (L6) linear combination (see Section 2.3.6.4),
(4) parameter estimation (program GPSEST), if you do not use the ionosphere-free (L3)
linear combination, and
(5) ionosphere mapping (programs GPSEST and IONEST).
12.1.1 Choice of the Linear Combination
You have to be aware that the choice of the linear combination to be analyzed in your final
GPSEST analysis sets the course for your final results! This choice cannot be undone any
more at the stage of ADDNEQ2 analyses. The same is by the way also valid in terms of
elevation cut-off angle, weighting of observations, tropospheric mapping function, antenna
phase center corrections, and so on.
If you have solely single-frequency (L1) data at your disposal, there is nothing to choose
at all. However, if you consider dual-frequency (L1/L2) data you cannot avoid making a
decision. On one hand, in case of long baselines, you will certainly use the ionosphere-free
(L3) linear combination. On the other hand, it is known that on very short baselines, in
particular in the extreme case of so-called “zero-baselines,” L1/L2-based solutions perform
significantly better. As a consequence of this, there must be a trade-off between L3 and
L1/L2 solutions. Unfortunately, because of the pronounced variability of the ionosphere,
it is quite impossible to generally give the baseline length where both kind of solutions
perform similarly well. A “critical” length of, let us say, 1–10 kilometers is likely applicable
– or may give at least an idea of the order of magnitude. This length, however, is transferable
exclusively to mid-latitude regions. Finally, if you decide to use the basic carriers in their
original form, it is better from our experience to ignore L2 and to use L1 only (see also
Section 12.1.3).
Bernese GPS Software Version 5.0 Page 253