Bernese GPS Software Version 5.0 - Bernese GNSS Software

14. Clock Estimation
RINEX files the offset cannot be computed. All clock values of the epoch are then copied
into the output clock array without alignment to a common reference. If only a subset of
input clock RINEX files has common clocks to compute the offset only the clock values
from those files are considered for the output clock array.
After combining the clock values from the input files and inserting the results to the output
clock array the reference clock selection, the clock jump detection, and/or the clock
extrapolation are done according to the selected program functions in panel “CCRNXC 4:
Select Program Functions, Program Output”.
14.3.4 Selection and Alignment of the Reference Clock
Because GPS is only able to determine the differences between clocks one degree of freedom
remains: For each epoch one of the clocks can be set to an arbitrary (reasonable) value.
Therefore the selection or the change of the reference clock does not affect the estimated
clock solution. The only requirement is that the reference clock must be available for each
epoch where clock values are computed.
Nevertheless it is preferable (e.g., for the extrapolation, see Section 14.3.6) if the time series
of the clock values can be easily modeled (e.g., by an offset and a drift as it is expected for
a perfect clock). A reference clock is selected and modeled as a perfect clock. A polynomial
is fitted to the combined values for the clock. In this way the time scale represented by
the reference clock (and therefore for the entire solution) does not change. If the clock was
synchronized, e.g., to the GPS broadcast time during the processing the aligned reference
clock will refer to the same time scale.
The behaviour of the selected reference clock has to correspond to the model used for the
alignment. All deviations of the real behaviour of the reference clock from the assumed
model (e.g., noise or jumps) are reflected in all other clocks of the solution. Therefore, the
reference clock must be selected carefully. A receiver clock driven by a H-Maser is preferable.
The program CCRNXC supports two cases:
• The reference clock is retained from one of the input clock RINEX files (option “Retain
the reference clock from an input file” in panel “CCRNXC 4: Select Program Functions, Program
Output”). This requires that the reference clock in this input file covers all epochs of
the output file. This option is preferable, e.g., when merging clock RINEX files. With
this option the reference clocks from all input clock RINEX files cannot be deleted
with the options “DEFINE A LIST OF CLOCKS TO BE PROCESSED” (see Figure 14.5).
• The program selects a new reference clock if you choose the option “Select a new reference
clock for the output file”. This becomes necessary
– if the input clock RINEX files contain clocks from different time intervals,
– if the reference clocks in the input clock RINEX files are not aligned (e.g., when
the zero-mean condition for the reference clock realization was used in GPSEST),
or
– if one of the input clock RINEX files does not contain a reference clock, because
CCRNXC writes output clock files only if a reference clock is defined 2 .
2 The reference clock records are mandatory according to the clock RINEX format description.
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