# Presentation: 376459

## Differential Code Biases (DCB) in GNSS Analysis

- Stefan Schaer

## GPS P1-P2 DCB Values, Computed at CODE

**GPS P1-P2 DCB Values, Computed at CODE**

## GLONASS P1-P2 DCB Values, Computed at CODE

**GLONASS P1-P2 DCB Values, Computed at CODE**

## GPS P1-C1 DCB Values, Computed at CODE

**GPS P1-C1 DCB Values, Computed at CODE**

## Introduction (1/2)

- The following code (pseudorange) observables are available from GPS: C/A (=C1), P1(Y1), P2(Y2), and C2 (for the Block IIR-M constellation).
- In the presence of *Anti-Spoofing (AS),* P-codes P1, P2 are encrypted for non-authorized users to Y1, Y2.
- We have to distinguish between three receiver classes (in terms of the code tracking technology):
- P1/P2: C1, P1, P2
- C1/X2: C1, X2=C1+(P2-P1)
- C1/P2: C1, P2
- C1/X2 receiver models are identified as cross-correlation (CC) receivers. Prominent examples are Rogue and Trimble 4000 models.
- The operation of CC receivers depends on the AS state. It is assumed here that *AS* is always *on* (!).
- The latest generations of Leica, Novatel, Trimble receivers belong to the C1/P2 receiver class.
**	****Note:** This is true for GPS, but *not* for GLONASS observations (P1/P2).
- We have a method at CODE for verification of the receiver class.

## Number of P1/P2, C1/X2, C1/P2 IGS Receivers Used in Potsdam-Dresden-Reprocessing (PDR)

## Introduction (2/2)

- Instrumental biases, BC1, BP1, BP2, BC2, are present with respect to C1, P1, P2, C2. These biases are not accessible (in the absolute sense).
- It is common to address the following differences of code biases:
- P1-P2,
- P1-C1, and
- P2-C2 differential code bias (=DCB).
- By convention, precise satellite clock corrections contain a specific linear combination of P1 and P2 satellite biases, specifically the ionosphere-free LC: 2.55·BP1-1.55·BP2(+B0).
- It is obvious that code tracking data from both the C1/X2 and the C1/P2 receiver class must be corrected in order to achieve full consistency with P1/P2 data, or precise satellite clock information.
- Jim Ray’s cc2noncc.f is a RINEX converter utility program to make given code measurements consistent with P1/P2 data by applying satellite-dependent P1-C1 bias corrections (e.g. produced by CODE).

## How to Use P1-P2 and P1-C1 Satellite DCB Information

- The following table gives the corrections due to satellite-specific P1-P2 and P1-C1 DCB values for the most important linear combinations derived from various combinations of code observable types:

**Determination methods:**
- P1-P2: _Iono._ / Abs. receiver cal.
- P1-C1: Diff. / _Clock_ / _Amb. res._

## Time Series of Daily GPS P1-P2 DCB Values From Potsdam-Dresden-Reprocessing (PDR)

## Analysis of Realigned DCB Time Series 

- Realignment: G01, G04, G05, G09, G25, G26 selected as reference for DCB datum definition.
- The long GPS satellites lifetime (> 14 years) is remarkable.
- FODITS (Find Outliers and Discontinuities In Time Series), a new component of the Bernese SW for “automated” analysis of GNSS station coordinate time series (developed by Luca Ostini) was used for the first time for this specific application.
- Visual verification of realigned P1-P2 and P1-C1 DCB time series on the basis of:
- (stable) receiver DCB results and
- satellite DCB results for PRN23/SVN23 and PRN32, the reactivated SVN23.

## Time Series of Daily GPS P1-P2 DCB Values From PDR and CODE Analysis (Realigned)

## Time Series of Daily GPS P1-P2 DCB Values From PDR and CODE Analysis (Realigned)  SVN23

## Time Series of Daily GPS P1-C1 DCB Values From PDR and CODE Analysis (Realigned)

## Time Series of Daily GPS P1-C1 DCB Values From PDR and CODE Analysis (Realigned)  SVN23

**Time Series of Daily GPS P1-C1 DCB Values From PDR and CODE Analysis (Realigned) ******** SVN23**

## Time Series of Daily GPS P1-C1 DCB Values From PDR and CODE (Realigned)  Direct P1-C1 Results

**Time Series of Daily GPS P1-C1 DCB Values From PDR and CODE (Realigned) ******** Direct P1-C1 Results**

## GPS C2 Tracking Data (1/2)

-                                Current IGS GNSS L2C Data Holdings for 2008 (as of 30-May-08 09:40)
- Mon.                                    Receiver       Start   Start    End     End    No.
- Name    Site Name      Lat.   Long.       Type         Date     Day    Date     Day    Days
- ---- ---------------- ------ ------- --------------- --------- ----- --------- ----- --------
- ALIC Alice Springs    -23 40  133 53 LEICA GRX1200GG 30-Apr-08 08121 29-May-08 08150       14
- UNAC Boulder           40 00  254 44 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      147
- CCJ2 Chichi Jima       27 21  142 13 Trimble NETRS   09-Mar-08 08069 29-Mar-08 08089       16
- BAKO Cibinong         -06 29  106 51 LEICA GRX1200GG 09-May-08 08130 26-May-08 08147       14
- DARW Darwin           -12 51  131 08 LEICA GRX1200GG 25-Feb-08 08056 29-May-08 08150       78
-                                      LEICA GRX12000G 03-Mar-08 08063 03-Mar-08 08063        1
- DAVR Davis            -68 35   77 58 Trimble NETR5   01-Jan-08 08001 29-May-08 08150      128
- FAIC Fairbanks         64 59  212 30 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      150
- UNB3 Fredericton       45 57  293 21 Trimble NETR5   01-Jan-08 08001 29-May-08 08150      145
- GANP Ganovce           49 01   20 19 Trimble NETR5   22-Feb-08 08053 29-May-08 08150       90
- HRAC Hartebeesthoek   -25 53   27 41 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      150
- HERT Herstmonceux      50 52    0 20 LEICA GRX1200GG 09-May-08 08130 29-May-08 08150       20
- RECT Ile-Ife           07 20   04 20 LEICA GRX1200GG 01-May-08 08122 22-May-08 08143       15
- KOKC Kokee Park        22 08  200 20 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      150
- MAW1 Mawson           -67 36   62 52 LEICA GRX1200   05-May-08 08126 26-May-08 08147       21
- MCMC McMurdo          -77 50  166 40 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      143
- MOBS Melbourne        -37 50  144 59 LEICA GRX1200PR 29-Apr-08 08120 29-May-08 08150       31
- MCIL Minami Tori Shim  24 17  153 59 Trimble 5700    09-Jan-08 08009 27-Mar-08 08087       78
- STR2 Mount Stromlo    -35 19  149 01 Trimble NETR5   15-Feb-08 08046 29-May-08 08150       95
- STR1 Mount Stromlo    -35 19  149 01 LEICA GRX1200   30-Apr-08 08121 29-May-08 08150       26
- NYAC Ny Alesund        78 56   11 52 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      148
- OURI Ourinhos         -22 57  -49 54 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      125
- ROSA Rosana           -22 31  -52 57 Trimble NETRS   01-Jan-08 08001 23-Jan-08 08023       22
-                                      Trimble NETR5   31-Jan-08 08031 29-May-08 08150       94
- SFER San Fernando      36 28  353 48 Trimble NETRS   21-May-08 08142 29-May-08 08150        9
- SJRP Sao Jose Do Rio  -20 47  -49 21 Trimble NETRS   01-Jan-08 08001 29-May-08 08150      127
- STK2 Shintotsukawa     43 32  141 51 Trimble 5700    01-Jan-08 08001 01-Jan-08 08001        1
- NTUS Singapore          1 21  103 41 LEICA GRX1200GG 01-Jan-08 08001 03-May-08 08124      123
- AMU2 South Pole       -90 00  -00 00 Trimble NETRS   10-Apr-08 08101 29-May-08 08150       50
- TSK2 Tsukuba           36 06  140 05 Trimble 5700    23-Jan-08 08023 23-Jan-08 08023        1
- YEBE Yebes             40 31  356 55 Trimble NETRS   05-Mar-08 08065 02-Apr-08 08093       29
- ZIM2 Zimmerwald        46 53    7 28 Trimble NETR5   01-Jan-08 08001 29-May-08 08150      150
- **** **************** ****** *******                                                 --------
-                                                                                         2,391
- 32 sites archived

- _[ftp://cddis.gsfc.nasa.gov/reports/gps/daily/l2c_summary.current](ftp://cddis.gsfc.nasa.gov/reports/gps/daily/l2c_summary.current)_:

-   IGS Mail 5746

## GPS C2 Tracking Data (2/2)

- AMU2  TRIMBLE NETRS        C2 only (no diff.)          Receiver firmware downgraded from 1.20 to 1.15
- BAKO  LEICA GRX1200GGPRO   No C2
- BNDY  TRIMBLE NETRS        No C2
- BRAZ  TRIMBLE NETRS        C2 only (no diff.)
- CCJM  TRIMBLE 5700         No C2
- DAEJ  TRIMBLE NETRS        No C2
- DARW  LEICA GRX1200GGPRO   C2 only (no diff.)
- DAVR  TRIMBLE NETR5        C2 only (no diff.)
- GANP  TRIMBLE NETR5        P2 and C2
- HERT  TRIMBLE NETR5        C2 only (no diff.)
- LEIJ  JPS LEGACY           No C2
- MAT1  ASHTECH Z18          No C2
- MAW1  LEICA GRX1200        No C2
- MCIL  TRIMBLE 5700         No C2
- MOBS  LEICA GRX1200PRO     No C2
- NTUS  LEICA GRX1200GGPRO   No C2
- PDEL  LEICA GRX1200GGPRO   No C2
- PENC  LEICA GRX1200GGPRO   No C2
- PGC5  TRIMBLE NETRS        C2 only (no diff.)          l2ctest at CDDIS
- SASS  JPS LEGACY           No C2
- SFER  TRIMBLE NETRS        No C2
- SOFI  TPS E_GGD            No C2
- STR1  LEICA GRX1200        No C2
- STR2  TRIMBLE NETR5        C2 only (no diff.)
- SUWN  TRIMBLE NETRS        No C2
- TITZ  JPS LEGACY           No C2
- UNB3  TRIMBLE NETR5        P2 and C2
- UNBN  NOV OEMV3            P2 and C2
- UNBT  TPS NETG3            P2 and C2
- WARN  JPS LEGACY           No C2
- WTZJ  JPS LEGACY           No C2
- WTZR  LEICA GRX1200GGPRO   No C2
- YAR3  LEICA GRX1200GGPRO   C2 only (no diff.)
- ZIM2  TRIMBLE NETR5        P2 and C2

**GPS C2 Tracking Data (2/2)**

- Current situation concerning 2008 data archived at CODE:

## GPS P2-C2 DCB Retrievals for 2008 (1/2)

**GPS P2-C2 DCB Retrievals for 2008 (1/2)**

-            IIR-M (6):  G07       G12  G15 G17                           G29 G31

## GPS P2-C2 DCB Retrievals for 2008 (2/2)

**GPS P2-C2 DCB Retrievals for 2008 (2/2)**

## Additional Bias Issues

**GLONASS ambiguity resolution:**
- We already started to perform GLONASS ambigity resolution at CODE (and at swisstopo), specifically in
- our EPN (EUREF) GNSS regional analysis,
- swisstopo’s AGNES (Automated GNSS Network Switzerland) analysis, including the NRT processing of the AGNES network.
- CODE’s GNSS global analysis scheme will be upgraded in this sense in the near future. At present, GLONASS AR is performed just for shortest baselines using the direct L1/L2 resolution method.
- _GLONASS single-difference ambiguity initialization bias_ is crucial in particular for narrow-lane ambiguity fixing for baselines with different GNSS receiver models.
- Related problems are addressed on the poster by Heinz Habrich et al.
- _GLONASS code biases_ affect:
- wide-lane AR using the pseudorange method  (practically) impossible
- GPS/GLONASS clock estimation  more demanding (compared to GPS)
**GPS L2 ambiguity resolution:**
- We already got confronted with _0.25-cycle phase biases_ between L2P and L2C, revealed by UNBN NOV OEMV3 observation data.
- More details will be given in the presentation prepared by Werner Gurtner on “The RINEX format: Current status, future developments.”

## New DCB File Format for the Bernese SW

- (see poster by Michael Meindl et al.)

- See also: _[http://www.aiub.unibe.ch/download/bcwg/cc2noncc/](http://www.aiub.unibe.ch/download/bcwg/cc2noncc/)_

## Summary

- Steadily increasing number of types of biases to be dealt with (...).
- The knowledge of GPS P1-C1 biases is a must for successful ambiguity resolution (on long baselines) and for precise clock estimation when relying on a mixed receiver network.
- ESA/ESOC's latest version of cc2noncc (capable to cope with RINEX 2.11) was recommended in IGS Mail 5778/5781 (by Nacho Romero).
- Regularly updated P1-C1 bias table: p1c1bias.2000(p). See:
- 	_[http://www.aiub.unibe.ch/download/bcwg/cc2noncc2/](http://www.aiub.unibe.ch/download/bcwg/cc2noncc2/)_
- 	_[http://](http://igscb.jpl.nasa.org/projects/bcwg/)__[igscb.jpl.nasa.org/projects/bcwg](http://igscb.jpl.nasa.org/projects/bcwg/)__[/](http://igscb.jpl.nasa.org/projects/bcwg/)_ (or simply _[http://](http://igs.org/projects/bcwg/)__[igs.org/projects/bcwg](http://igs.org/projects/bcwg/)__[/](http://igs.org/projects/bcwg/)_)
- Problems related to datum definition, detection of jump discontinuities, drifts, outliers, anomalies are very similar to those of other time series, such as station coordinates.
- Time series results show
- commonly very stable satellite DCB retrievals,
- a considerably increased scattering of P1-C1 DCB estimates as long as the number of P1/P2 receivers is small (close to 1),
- only few jump discontinuities (for the GPS constellation)’
- Generation of a mean set of P1-P2 and P1-C1 DCB values (consistent to one common DCB reference, eventually aligned in the absolute sense) may be seen as a mid-term goal.

## Recommendations / Discussion 

- Continuous monitoring of differential code bias (DCB) values with respect to P1-C1 and P1-P2 must be continued.
- Time lab stations available in the IGS network should not only be considered for clock analysis, but also for ionosphere analysis (for monitoring of the corresponding receiver P1-P2 code biases).
- GLONASS satellite and receiver DCB results should be included in IGSG and IGRG IONEX products.
- Absolute receiver calibration with respect to C1 (and also C2) would be desirable.
- IGS C2 data availability has to be improved.
- cc2noncc is capable to correct RINEX observation data for GPS P1-C1 bias values only. IGS users (and in particular the ACs) should be prepared to apply corresponding code bias corrections in their own GNSS analysis software (in order to cope with P2-C2, other GNSS, RINEX 3, etc.).
- For the IGS reprocessing, AS-free periods in -1994, 1995 (3), 1997 (1) must be considered (applying *no* P1-C1 bias corrections there).
- File format for DCB data exchange.
- Use of RINEX 3 (?)
- Quarter-cycle phase bias issue (!)
- Provision of (mean) GLONASS single-difference ambiguity initialization bias values for the various types of IGS GNSS receivers (?)
- DCB quantities conform with “undifferenced ambiguity resolution” (...)

## Verification of Receiver Tracking Technology (Using the Bernese Software Version 5.0)

- ================================================================================
- PART 6: VERIFICATION OF RECEIVER TRACKING TECHNOLOGY
- ================================================================================
-  STATION NAME           MULTIPLIER   RMS ERROR    SUGGESTED RECEIVER TYPE    GIVEN RECEIVER NAME / TYPE     MATCH
-  ----------------------------------------------------------------------------------------------------------------
-  BRUS 13101M004  -G        -0.224        0.043    P1/P2   5.242  28.666      ASHTECH Z-XII3T       P1/P2    OK
-  FFMJ 14279M001  -G        -0.105        0.051    P1/P2   2.045  21.578      JPS LEGACY            P1/P2    OK
-  MATE 12734M008  -G         0.874        0.048    C1/X2   2.637  18.312      TRIMBLE 4000SSI       C1/X2    OK
-  ONSA 10402M004  -G        -0.104        0.041    P1/P2   2.536  26.943      ASHTECH Z-XII3        P1/P2    OK
-  PTBB 14234M001  -G        -0.124        0.048    P1/P2   2.579  23.352      ASHTECH Z-XII3T       P1/P2    OK
-  VILL 13406M001  -G        -0.062        0.044    P1/P2   1.404  24.048      ASHTECH Z-XII3        P1/P2    OK
-  ZIMJ 14001M006  -G         0.057        0.048    P1/P2   1.195  19.598      JPS LEGACY            P1/P2    OK
-  ZIMM 14001M004  -G         1.131        0.047    C1/X2   2.788  24.083      TRIMBLE 4000SSI       C1/X2    OK

- Excerpt of a PPP BPE processing summary file (PPP03139.PRC):

## BCWG: Status (1)

- Draft version of BCWG charter in form of a Word document (in preparation)
- Member list not yet complete (list includes currently principally “IGS external” persons)
- At present, proper handling of GPS P1-C1 bias corrections (relevant to GPS clock estimation and GPS ambiguity resolution when relying on the pseudorange method) is the primary issue:
- Daily GPS P1-C1 bias monitoring at CODE
- Sliding 30-day combination of bias values (archived at the end of each month)
- Possibility to verify the receiver tracking technology using Bernese PPP (on the basis of a RINEX data set) in order to assign to C1/P1/P2, C1/P2’, or C1/P2 class

## BCWG: Status (2)

- URL address for BCWG-related data established at the ftp server of the University of Berne:_[http://](http://www.aiub.unibe.ch/download/bcwg/)__[www.aiub.unibe.ch/download/bcwg](http://www.aiub.unibe.ch/download/bcwg/)__[/](http://www.aiub.unibe.ch/download/bcwg/)_ _[ftp://](ftp://ftp.unibe.ch/aiub/bcwg/)__[ftp.unibe.ch/aiub/bcwg](ftp://ftp.unibe.ch/aiub/bcwg/)__[/](ftp://ftp.unibe.ch/aiub/bcwg/)_
- Latest version of p1c1bias.hist (and cc2noncc.f) may be found at:_[http://www.aiub.unibe.ch/download/bcwg/cc2noncc/](http://www.aiub.unibe.ch/download/bcwg/cc2noncc/)_ _[ftp://ftp.unibe.ch/aiub/bcwg/cc2noncc/](ftp://ftp.unibe.ch/aiub/bcwg/cc2noncc/)_
- Bias file containing CODE monthly values (already made available, but not yet announced):
- p1c1bias.2000
- p1c1bias.2000p (p stands for plus) includes contains in addition a set of bias values for the current month. These extra values are averages coming from a sliding 30-day combination. Consequently, the last set of bias values may marginally change from one day to the next.

## BCWG: Next Steps, Plans

- Bias file containing PDR monthly values (PDR monthly sets created)  IGS reprocessing (!)
- Consideration of *AS-free* periods (in cc2noncc and AC SW)
- Bias file containing PDR monthly values plus direct P1-C1 retrievals before 1996/1997
- Bias file containing bias values properly aligned to one common GPS P1-C1 reference
- Old (pre-2000) ASHTECH Z-XII3 RINEX data online at CODE’s database (for direct P1-C1 bias retrieval)
- First GPS P2-C2 bias retrieval (from CODE side)
- Re-aligned GPS and GLONASS P1-P2 satellite and receiver bias time series  comparisons with absolute receiver bias calibrations (provided by BIPM)
- Collecting bias results from other groups at AIUB

## BCWG: List of Issues (1)

- Format(s):
- p1c1bias.hist (for GPS only)
- Bernese (V5.0) “.DCB”
- CODE’s “.F” files to be included in p1c1bias.hist
- Inclusion of various DCB parameters in SINEX (?)
- Receiver model list:
- p1c1bias.hist
- Bernese (V5.0) “RECEIVER.” file
- Should receiver class be specified in rcvr_ant.tab?
- P1-C1 bias correction at which stage?
- at RINEX level (using the cc2noncc utility)
- by the AC’s GNSS analysis software
- RINEX V3
- New GPS signals
- GLONASS and in particular Galileo

## BCWG: List of Issues (2)

- Update procedure with respect to
- bias values
- new receiver models
- Combination of various pseudorange bias products
- Comparison and validation with “external” data sources
- absolute receiver calibrations concerning P1-P2
- Time series analysis
- Identification of jump discontinuities
- Re-alignment of entire time series to one common P1-C1, or P1-P2 bias reference
- Interaction with Iono-WG in terms of GPS P1-P2 bias results

## BCWG: List of Issues (3)

- New types of pseudorange (or phase or ...) biases:
- P2-C2 for GPS (and GLONASS?)
- GLONASS: pseudorange biases (specific to satellite *and* receiver), “single-difference initialisation” bias (relevant to phase-based GLONASS ambiguity resolution)
- Multi-GNSS: intersystem phase biases
**Galileo (...)**