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Bernese GPS Software Version 5.0 - Bernese GNSS Software

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14.5 Generation of High Rate Clocks Using Program CLKEST<br />

14.5 Generation of High Rate Clocks Using Program CLKEST<br />

Program CLKEST allows to generate high rate clocks in an efficient way. It is based on known<br />

orbit, station coordinate, and station troposphere information that is, e.g., obtained from<br />

a global network double-difference solution (troposphere delays may also be obtained from<br />

a model). Only clock parameters remain as unknowns in the observation equations of the<br />

ionosphere free linear combination with this information introduced as known. The program<br />

uses pseudorange and carrier phase observations from a (preferentially global) network of<br />

tracking stations.<br />

In a first step pseudorange observations are screened and receiver clocks synchronized. Clock<br />

parameters are then computed from the pseudorange observations for each satellite clock and<br />

each involved station clock for each observation epoch. In a second step carrier phase epochdifferences<br />

are formed for each clock and each observation epoch-difference, screened, and<br />

used to compute clock epoch differences for each clock and each observation epoch-difference.<br />

This second step allows to get rid of the phase ambiguities. Cycle slips are identified as<br />

outliers in the phase epoch differences. In the third and last step pseudorange derived clock<br />

values and carrier phase derived clock epoch-differences are combined independently for<br />

each clock using an efficient algorithm (option TRIDIAG). The singularity in the system is<br />

avoided by either fixing the values of one reference clock or the mean of an ensemble of<br />

clocks.<br />

The procedure is very fast and allows the processing of data at high rate for a large network<br />

of stations. Drawbacks are, on one hand, missing covariance information (not provided by<br />

the efficient algorithm), on the other hand, and more importantly, neglected correlations<br />

between successive clock epoch-differences. These missing correlations are equivalent to<br />

the estimation of individual ambiguities for each epoch difference and are the cause for a<br />

variation of the estimated clocks in a band of about ±1 ns around the true values. This<br />

pseudo-random walk may be reduced by lowering the relative weight of the pseudorange observation<br />

with respect to the carrier phase observations. Alternatively only a time-averaged<br />

pseudorange clock value may be used to define the datum of the carrier phase clock time<br />

series obtained by integrating the clock epoch-differences (option SUM CONDITION). Finally,<br />

it is also possible to constrain the carrier phase clock time series at specific epochs to precise<br />

clocks that are available (e.g., every 5 minutes) from another source (option “Fix on a-priori<br />

clocks”). This later procedure is used at CODE to densify the 5 minutes precise clocks to<br />

30 seconds high rate clocks [Bock et al., 2000].<br />

The program may only be started using the RUN<strong>GPS</strong> command (see Section 18.8).<br />

<strong>Bernese</strong> <strong>GPS</strong> <strong>Software</strong> <strong>Version</strong> <strong>5.0</strong> Page 307

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