[IGSMAIL-2827]: Monitoring (P1-C1) code biases

[Stefan Schaer]

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IGS Electronic Mail      09 May 09:38:40 PDT 2000      Message Number 2827
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Author: Stefan Schaer


Dear colleagues:

Starting with GPS week 1056, the IGS analysis centers have to take (P1-C1) code
biases into account in order to ensure that their precise clock information is 
fully consistent to P1/P2 code measurements. Background and details may be 
gathered from IGS Mails 2320 and 2744 at

  http://igscb.jpl.nasa.gov/mail/igsmail/1999/msg00564.html
  http://igscb.jpl.nasa.gov/mail/igsmail/2000/msg00084.html

CODE is accounting for this type of code biases as from GPS week 1057. It might
be interesting for some of you that CODE solves for satellite-specific 
differential code bias (DCB) parameters as part of the clock estimation 
procedure. Our approach works as long as a mixture of data of cross-correlation
style receivers and modern receivers is processed. At present, about 30-40 of 
80 stations in all used for the clock estimation may be related to a cross-
correlation style receiver providing C1 and P2' code measurements. The 
improvement of our clock estimates due to the mentioned measure is clearly 
detectable.

The day-to-day repeatability of our daily (P1-C1) DCB estimates is of the order
of 0.08 nanoseconds and 25 millimeters, respectively. Following the square-
root-of-N law, the weekly combined estimates should show an uncertainty of 
approximately 0.03 nanoseconds and 10 millimeters, respectively. A first 
comparison of three 7-day DCB combinations confirms this accuracy. We refer in
the following to our ionosphere-dedicated website 

  http:/www.aiub.unibe.ch/ionosphere.html

where we already put some (P1-P2) DCB results, a by-product of the TEC 
determination. Let us briefly highlight the most important (P1-C1) DCB files 
automatically updated:

  http://www.aiub.unibe.ch/ionosphere/lastweek_p1-c1.dcb - gives the 
    DCB values of a moving 7-day combination. Note that the rms values listed 
    correspond to the daily repeatabilities (and not to the rms errors of the 
    combined values).

  http://www.aiub.unibe.ch/ionosphere/dcb_p1-c1.ps - visualizes the moving
    7-day combination. Red dots indicate the daily estimates; the green circles 
    indicate the combined values.

  http://www.aiub.unibe.ch/ionosphere/dcb_p1-c1_ref.ps - shows our accumulated
    daily DCB solutions and compares these to the values provided by 
    Muellerschoen from JPL and adopted by the IGS (indicated by green circles).
    The agreement is pretty good. For some PRNs, however, significant 
    deviations may be seen. Let us mention PRN 08 as a prominent example. It
    remains questionable whether these differences may be explained with the 
    different observation epochs, remembering that Muellerschoen's results were
    derived at the beginning of this year.

  http://www.aiub.unibe.ch/ionosphere/dcb_p1-c1_wk1057ff.ps - this figure 
    demonstrates the internal consistency of our weekly solutions on a level
    of 30 picoseconds rms.

We will continue in monitoring these code biases since they are not as constant
as one might like - and since several launches of new GPS spacecrafts are 
announced for the near future. Our daily (P1-C1) DCB estimates are available 
on special request. The same is valid for long-time combinations which may be
generated very easily.

For the user of IGS precise clock information, it remains important to realize 
that the knowledge of (P1-C1) and/or (P1-P2) code biases allows them to 
transform the P1/P2-consistent clock estimates to clock estimates which are 
consistent to the ionosphere-free linear combination of code observations 
provided by cross-correlations style receivers, or, in the single-frequency 
case, to C/A code observations. It is obvious that corresponding clock
corrections may be also derived for other combinations, if desired.

Best regards,

The CODE AC Team