Differential GPS and GLONASS P1-P2 Code Biases
Differential GPS P1-C1 Code Biases
Differential GPS P2-C2 Code Biases

Differential GPS and GLONASS P1-P2 Code Biases

Our IONEX files contain in the header part information concerning the so-called differential (P1-P2) code biases (DCBs) for all active GPS and GLONASS satellites (see also IONEX example). A brief DCB description can be found in the IONEX manual. A combined set of DCB values for the GPS satellites and the IGS stations processed at CODE - taking into account the last 30 daily sets (PDF file) - is regularly computed. The daily repeatability of the satellite-specific biases is currently 0.033 nanoseconds rms for the GPS constellation and 0.057 nanoseconds rms for the GLONASS constellation.
(Note that the rms specification may be falsified, associated with a DCB jump concerning
PRN G11 on day 256, 2001,
PRN G17 on day 055, 2003,
PRN R07 on day 121, 2003,
PRN R23 on day 266/267, 2003,
PRN R05 on day 274, 2003,
PRN R24 on day 328, 2003,
PRN R24 on day 358, 2003,
PRN R21 on day 019, 2004,
PRN R18 on day 036, 2004,
PRN R24 on day 139, 2004,
PRN R23 on day 241, 2004,
PRN R06 on day 265, 2004,
PRN R04 on day 291, 2004,
PRN R06 on day 345/346, 2004,
PRN G14 on day 355/356, 2004,
PRN R02 on day 047, 2005,
PRN R05 on day 073, 2005,
PRN R06 on day 078/079, 2005,
PRN R23 on day 059, 2006,
PRN G17 on day 256, 2006, or
PRN R08 on day 257, 2006


The present CODE DCB list contains estimates for 32 GPS plus, 24 GLONASS satellites, 56 GNSS satellites and 287 / 173 tracking stations of the GPS/GLONASS system.

GLONASS P1-P2 DCB results are plotted here (PDF). The differing DCB value belongs to R05 (105), which is a GLONASS satellite of the GLONASS-M modernization program.

Monthly P1-P2 DCB solutions are available as of October 1997. The corresponding files are called P1P2yymm.DCB.Z, or P1P2yymm_ALL.DCB.Z, if station-specific bias values are included. They are archived here. The latest two monthly DCB solutions are compared here.

A comparison of current CODE 30-day GPS P1-P2 DCB averages with GPS broadcast group delay (GD) values may be found here (PDF). The CODE computed DCB values are indicated by encircled dots. Note that the GPS broadcast GD values were converted into DCB values and realigned following a zero-mean condition. The GPS DCB offset is 5.252 nanoseconds, corresponding to a mean GD value of -8.118 nanoseconds divided by -1.55.

Differential GPS P1-C1 Code Biases

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, 2744, 2827, 2879, and 3160.

CODE is accounting for this type of code bias as from GPS week 1057 (April 9, 2000) by solving for satellite-specific differential (P1-C1) 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 about 80 stations in all used for the clock estimation may be related to a cross-correlation style receiver providing C1 and X2 code measurements. The improvement of our clock estimates due to the mentioned measure is clearly detectable.

• p1c1.dcb - gives the resulting DCB values of a moving 30-day combination (considering the bias estimates of the latest 30 daily solutions). Please note that we are no longer computing 7-day combinations.
• p1c1.f - may be used to easily update the RINEX converter utility, CC2NONCC, developed by Jim Ray.
• p1c1.ps/pdf - visualizes the result of the moving 30-day combination. Red dots indicate the daily estimates considered; the green circles indicate the combined values.
• p1c1_ar.ps/pdf - shows ambiguity-fixed P1-C1 bias retrievals obtained from a double-difference (DD) analysis step. The day-to-day bias repeatability is, with currently !! picoseconds rms, !!3% smaller than that resulting from the zero-difference analysis (see below). Details on the ambiguity-fixed P1-C1 DCB product will be published soon.
• p1c1_lst.ps/pdf - compares the latest two monthly P1-C1 DCB solutions computed by CODE. Red dots indicate the monthly values; the green circles indicate averaged values. Systematic differences related to particular PRNs are entirely possible. They may be usually attributed to a significant temporal variation of individual bias values during the recent past (or to problem satellites, like PRNs 15 and 17).
• p1c1_ref.ps/pdf - compares the moving 30-day P1-C1 DCB average with the set of DCB values currently adopted by the IGS.

The day-to-day reproducibility of our daily P1-C1 DCB estimates is of the order of 0.05 nanoseconds, or 15 millimeters (currently 0.018 nanoseconds rms). (Note that this rms specification may be falsified, associated with a DCB jump concerning PRN 11 on day 256, 2001, or PRN 17 on day 055, 2003.) Following the square-root-of-N law, our 30-day combined estimates should show an uncertainty of approximately 0.01 nanoseconds, or 3 millimeters.

The straightforward method to derive P1-C1 code biases is to analyze P1-C1 code differences based on GPS tracking data from non-CC receivers, like Ashtech Z12, AOA Benchmark, or AOA Turborogue upgraded with ACT, etc. Depending on the receiver type considered, slightly different results are obtained, however. This reveals that a receiver-specific component may be present with respect to P1-C1 biases (specific to the satellites).

Remark: P1-C1 DCBs specific to receivers may be expected to be existing for both non-CC and CC receivers. In principle, there is only one case where the user may not simply connive at receiver-specific P1-C1 DCBs, namely if tracking data of a CC receiver has been recorded in both AS-on and AS-off mode (depending on the PRN observed).

CODE uses a fundamentally different, more sophisticated approach:

• The bias values are estimated directly from the data sets for which they will be applied.
• We do not make use of C1 code measurements from non-CC receivers (providing C1/P1/P2).
• Instead of these measurements, C1/X2 code measurements from CC receivers are considered. In other words, our P1-C1 DCB estimates directly reflect the code bias differences between non-CC and CC receivers as seen by an analysis center in its clock estimation procedure (when processing C1/X2 and P1/P2, respectively).
• Our analysis includes a large number of receivers, usually a superset of those used by other analysis centers, and does not explicitly rely on any particular receiver models.

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. Another motivation for us to continue with this service is certainly the circumstance that our P1-C1 bias values are recommended to be adopted for use with the IGS official products from GPS week 1097 onwards (see IGS Mail 3160). The history of the values adopted by the IGS may be traced in the file p1c1bias.hist maintained by the IGS Central Bureau. The interested reader is also referred to the IGS/BIPM web site.

Our continuously updated P1-C1 and P1-P2 DCB sets are posted to the ftp directories BSWUSER50/ORB and CODE. At the beginning of each month, monthly DCB solutions with respect to the preceding month are computed and automatically archived in the subdirectory CODE/yyyy, where yyyy is the 4-digit year. Corresponding files are addressed with 2-digit year and month (e.g., P1C10012.DCB.Z or P1C10012.F.Z). Monthly P1-C1 DCB solutions are available as of May 2000. Daily P1-C1 bias estimates are available on special request.

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-correlation 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. It is worth mentioning that in addition to P1/P2 and C1/X2 receivers a third receiver class must be recognized: C1/P2 receivers (see IGS Mail 3737 for more details). Our receiver table includes the receivers commonly used within IGS and EUREF.

CC2NONCC is an easy-to-use tool to handle P1-C1 biases. This tool works properly on the condition that (a) receiver names are used following the IGS naming conventions and (b) current RINEX data is converted. In the case the user wishes to convert older data, he is advised to revert to our DCB data archive and to adjust CC2NONCC accordingly. The same is valid if somebody is interested in updating CC2NONCC more frequently.

Differential GPS P2-C2 Code Biases

Publication of corresponding results (concerning PRN G17) is intended within the next months.