Differential Code Biases (dcb) in gnss analysis Stefan Schaer


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Differential Code Biases (DCB) in GNSS Analysis

  • Stefan Schaer


GPS P1-P2 DCB Values, Computed at CODE



GLONASS P1-P2 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



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:



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 (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



GPS C2 Tracking Data (2/2)



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



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



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://igscb.jpl.nasa.org/projects/bcwg/ (or simply 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



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://www.aiub.unibe.ch/download/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/ 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



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 (…)




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