Memo: Baseline dependent clocks

Some geodetic VLBI sessions show substantial baseline dependent clock offsets. Four sources of baseline dependent clock offsets are possible:
  1. Unmodeled source structure effects;
  2. Systematic clock misclosure introduced by correlation and post-correlation procedures;
  3. Presence of strong outliers;
  4. Errors during resolving group delay ambiguities what may result in appearance of permanent clock misclosures at X or S band to be a multiple of group delay ambiguity spacings.
It is questionable whether we should always estimate baseline dependent clocks due to the points 1,2,3; John Gipson got evidence that it may even slightly degrade performance in some specific cases. These baseline dependent clocks adjustments do not exceed 100 psec. At the same time we HAVE TO ESTIMATE baseline dependent clocks when a mistake has been made in group delay ambiguity resolution and some baselines have clock misclosures to be a multiple of group delay ambiguity spacings. If the database has a clock misclosure at the X band one group delay ambiguity spacing it propagates to the ionosphere free linear combination multiplied by the factor 1.08. If the database has a clock misclosure at the S band one group delay ambiguity spacing it propagates to ionosphere free linear combination multiplied by the factor 0.08 (or divided by 12). Baseline dependent clocks due to the errors in resolving group delay ambiguities are of order 1-200 nsec and totally degrades fit when not applied. There several ways to overcome this problem in SOLVE:
  1. Resolve group delay ambiguities once more and correct databases. Unfortunately S band data are lost for some databases and it is not always possible to do.
  2. Estimate baseline dependent clocks for all sessions. There is a keyword BASELINE_CLOCKS in $FLAGS section of control file. It may have three values: YES or NO or IN. "YES" means to estimate baseline dependent clocks for the maximal number of baselines which still doesn't lead to a normal matrix singularity. "IN" means to estimate baseline dependent clocks for the baselines which are specified in the database itself. "NO" means not to estimate baseline dependent clocks.
  3. Estimate baseline dependent clocks for the specific sessions only. $FLAGS section of the control file should contain the line BASELINE_CLOCKS NO and the lines in arc-file for the problematic sessions should be updated. A new option of the format of arc-file was added to SOLVE 04-JAN-99: a line can have a token BASDEP_CLO with value YES NO IN. Valve has the same meaning as in FLAGS section, but it overrides global flag specified in FLAGS for this session only. Example: $96NOV04XA 4 FREE BASDEP_CLO YES !

Some lists.


Back to Leonid Petrov's discussion page.

This memo was prepared by Leonid Petrov.

Last update: 04-JAN-99 14:55:16