Bernese GPS Software Version 5.0 - Bernese GNSS Software

14.5 Generation of High Rate Clocks Using Program CLKEST
14.5 Generation of High Rate Clocks Using Program CLKEST
Program CLKEST allows to generate high rate clocks in an efficient way. It is based on known
orbit, station coordinate, and station troposphere information that is, e.g., obtained from
a global network double-difference solution (troposphere delays may also be obtained from
a model). Only clock parameters remain as unknowns in the observation equations of the
ionosphere free linear combination with this information introduced as known. The program
uses pseudorange and carrier phase observations from a (preferentially global) network of
tracking stations.
In a first step pseudorange observations are screened and receiver clocks synchronized. Clock
parameters are then computed from the pseudorange observations for each satellite clock and
each involved station clock for each observation epoch. In a second step carrier phase epochdifferences
are formed for each clock and each observation epoch-difference, screened, and
used to compute clock epoch differences for each clock and each observation epoch-difference.
This second step allows to get rid of the phase ambiguities. Cycle slips are identified as
outliers in the phase epoch differences. In the third and last step pseudorange derived clock
values and carrier phase derived clock epoch-differences are combined independently for
each clock using an efficient algorithm (option TRIDIAG). The singularity in the system is
avoided by either fixing the values of one reference clock or the mean of an ensemble of
clocks.
The procedure is very fast and allows the processing of data at high rate for a large network
of stations. Drawbacks are, on one hand, missing covariance information (not provided by
the efficient algorithm), on the other hand, and more importantly, neglected correlations
between successive clock epoch-differences. These missing correlations are equivalent to
the estimation of individual ambiguities for each epoch difference and are the cause for a
variation of the estimated clocks in a band of about ±1 ns around the true values. This
pseudo-random walk may be reduced by lowering the relative weight of the pseudorange observation
with respect to the carrier phase observations. Alternatively only a time-averaged
pseudorange clock value may be used to define the datum of the carrier phase clock time
series obtained by integrating the clock epoch-differences (option SUM CONDITION). Finally,
it is also possible to constrain the carrier phase clock time series at specific epochs to precise
clocks that are available (e.g., every 5 minutes) from another source (option “Fix on a-priori
clocks”). This later procedure is used at CODE to densify the 5 minutes precise clocks to
30 seconds high rate clocks [Bock et al., 2000].
The program may only be started using the RUNGPS command (see Section 18.8).
Bernese GPS Software Version 5.0 Page 307