Bernese GPS Software Version 5.0 - Bernese GNSS Software

15.4 Estimation of Earth Orientation and Geocenter Parameters
15.4.3 Estimation of Earth Orientation Parameters
Today, in view of the products made available through the IERS and the IGS it will in general
not be necessary to solve for Earth orientation parameters, but to use Earth orientation
parameters consistent with the orbits and coordinates used. For research type applications
based on data stemming from a global GNSS network it makes sense to set up Earth orientation
parameters. This section is important for advanced users of the Bernese GPS Software
who wish to gain insight into global applications of GNSS. Let us start by mentioning a few
basic facts:
• You will always estimate Earth orientation parameters on top of an a priori model.
These are characterized by the values in a pole file (default extension ERP). If you
want to estimate the Earth orientations from scratch you have to use an a priori file
with all parameters set to zero.
• The (empirical) model for the Earth orientation parameters is set up in program
GPSEST, it may be modified in program ADDNEQ2, where you have to be aware of
the fact that you may simplify but not generalize the model in ADDNEQ2.
• If you solve for Earth rotation parameters you will probably process exactly one session
of one day in GPSEST and produce longer series using ADDNEQ2. You should be aware
of the fact that for the Earth orientation parameter estimates you may divide the
time interval of the session into a number of subintervals of equal length. In program
GPSEST each Earth orientation parameter (improvement on top of the a priori model)
is represented by a polynomial within each subinterval while in ADDNEQ2 the Earth
orientation parameter improvements are represented by a piece-wise linear function
with nodes at the subinterval boundaries (see Section 7.5).
• For diurnal and semi-diurnal variations of the Earth orientation parameters, the IERS
2003 model [McCarthy and Petit, 2004] or the model from [Ray et al., 1994] may be
used. The model name is logged in the pole file header.
• UT1-UTC estimates are special in one respect: due to the correlations with the orbital
nodes mentioned above, it is not possible to solve for the zero degree coefficient of the
polynomial pertaining to the first sub-interval of the session (as it is used in program
GPSEST) or the first value in the piece-wise linear representation (as it is used in
program ADDNEQ2) if you introduce the orbital elements as unknowns in the same
adjustment. The same is the case for nutation parameters.
15.4.3.1 Options in GPSEST
Panel “GPSEST 5.2: Setup of Parameters and Pre-Elimination 2” handles many special requests
in connection with parameter estimation. You have to enable the parameter type “Earth
orientation parameters”. In this case the menu system gives you access to panel “GPSEST 6.13:
Earth Orientation Parameters” (see Figure 15.5), where you may first define the spacing of the
Earth orientation parameter sets resulting in the total number of polynomials for each of
the selected Earth orientation parameters. The start and end time of the parameter sets
are computed using the general parameter time offset defined in option “TIME OFFSET FOR
PARAMETER INTERVALS” in panel “GPSEST 5.2: Setup of Parameters and Pre-Elimination 2”.
Bernese GPS Software Version 5.0 Page 321