IGSMAIL-6356 - Upcoming switch to IGS08 reference frame and the consequences to the GPS users (III)
March 24, 2011 By: Jim RayAs announced in the previous post, the IGS will switch from the (IGS05 + igs05.atx) framework to (IGS08 + igs08.atx) in the coming weeks. Many users have accumulated results expressed in the old framework, either based on the regular IGS operational products since November 2006, or in concert with the IGS 1st reprocessing campaign (acc.igs.org/reprocess.html). This note provides advice to those users on transforming their old results to be consistent with the new frame, to high accuracy.
The most direct method to establish consistency with the (IGS08 + igs08.atx) system is to reprocess all old data in the new framework. The IGS will do that over the next few years, in its 2nd reprocessing campaign. But many users may not find this option to be timely or cost-efficient for the near term. An alternative strategy is outlined below that does not require data reprocessing.
We assume that a user has a long time series of data reduced using igs05.atx antenna calibrations and that the results have been stacked into a long-term linear frame aligned to the IGS05 reference frame (RF), or possibly some other well-defined RF. To generate an equivalent frame solution aligned to the IGS08 RF and consistent with igs08.atx antenna calibrations involves two main steps--application of position corrections to account for antenna calibration changes and a Helmert alignment to IGS08.SNX.
1. POSITION CORRECTIONS TO ACCOUNT FOR ATX CHANGES
A simple, direct Helmert transformation from the user igs05.atx-calibrated frame to a set of the included IGS08 stations is *not* recommended. For one, the IGS08 positions have been derived from ITRF2008 values by directly applying station-dependent corrections to account for the impact of changing from igs05.atx antenna calibrations (which were used to realize IGS05 and ITRF2008) to igs08.atx. Equivalent position corrections are needed for the user frame before attempting any Helmert alignment.
1.1 Station-dependent corrections
For the IGS08 RF stations contained in the user frame, the same corrections as used to create IGS08 can be applied, which are available at:
ftp://igs-rf.ensg.eu/pub/IGS08/ITRF2008_to_IGS08.txt
ftp://igs-rf.ign.fr/pub/IGS08/ITRF2008_to_IGS08.txt
For each antenna type with an antenna calibration update, sets of station positions were estimated using the Bernese GPS Software 5.0. A static PPP processing strategy was used with 24h of data every 30 days during the span for which the particular antenna (plus radome) was deployed at the station. For each span, one set of positions was estimated using the igs05.atx calibration of the antenna, the other using the igs08.atx calibration. Then, the two sets of positions were differenced (igs08 minus igs05) and the weighted-average taken over the differences to provide the corrections found in the above table.
These station-dependent corrections are best to use. However, since additional user corrections may also be needed for the non-IGS08 stations to ensure overall self-consistency, the procedure in the next section can be applied for non-IGS08 stations, and, if more convenient, also for IGS08 stations.
1.2 Latitude-dependent correction models by antenna type
The station-dependent corrections above have been calculated for each individual station separately, whereas antenna type correction models are fits over latitude. Here, for each antenna type with an updated calibration, two sets of station positions were estimated using the Bernese PPP engine and 24h of data for 82 well-distributed global stations. For each updated calibration, it was assumed that all 82 stations were equipped with the antenna of interest, which, of course, is not true. However, both sets of positions were then differenced, minimizing common errors, and latitude-dependent functions fitted to the global ensemble of East, North and Up position differences.
Discrepancies (de, dn, du) derived by subtracting the latitude-modeled from the directly calculated position corrections have been computed. Most de and dn discrepancies are smaller than 0.5 mm in magnitude, and most in du are smaller than 1 mm in magnitude. Though there are several that are larger (units below are mm):
| #CODE | PT | SOLN | ANTENNA | DOME | de | dn | du |
| CONZ | A | 2 | TPSCR3_GGD | CONE | 0.52 | 0.17 | -0.75 |
| CONZ | A | 3 | TPSCR3_GGD | CONE | 0.52 | 0.17 | -0.75 |
| JAB1 | A | 6 | LEIAT504 | NONE | -0.65 | -0.04 | -1.73 |
| EISL | A | 2 | ASH701945C_M | NONE | -0.76 | 0.21 | -1.57 |
| NKLG | A | 1 | TRM29659.00 | NONE | 0.03 | -0.02 | 1.09 |
| KERG | A | 3 | ASH701945E_M | SNOW | -0.03 | -0.25 | -1.04 |
| PIMO | A | 4 | ASH701945C_M | NONE | 0.28 | 0.16 | -1.53 |
| EISL | A | 2 | ASH701945C_M | NONE | -0.76 | 0.21 | -1.57 |
| JAB1 | A | 6 | LEIAT504 | NONE | -0.65 | -0.04 | -1.73 |
| HOFN | A | 4 | TPSCR3_GGD | CONE | 0.19 | 0.01 | -2.65 |
| SCOR | A | 1 | TPSCR3_GGD | SCIS | -0.16 | -0.38 | -3.38 |
The overall average ENU discrepancies and standard deviations are:
avg. de = -0.02 (+/- 0.19) mm
avg. dn = -0.02 (+/- 0.13) mm
avg. du = -0.13 (+/- 0.67) mm
Thus, on aggregate, the deficiencies of the modeled corrections are negligible compared to the sigmas in IGS08. This is confirmed by rigorously comparing the IGS08.SNX file and a pseudo SINEX file derived from applying the modeled corrections (see Table 1).
Table 1. Helmert parameters from a comparison of IGS08.snx and a pseudo IGS08. The parameters were derived using the 91 primary IGS08 core sites. Note that the sigmas were *not* rescaled to force chi2=1, though, they were reduced by a factor of sqrt(2).
| TX(mm) | TY(mm) | TZ(mm) | S(ppb) | RX(mas) | RY(mas) | RZ(mas) | |
| offsets | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 |
| ± | 0.06 | 0.06 | 0.06 | 0.010 | 0.002 | 0.002 | 0.002 |
| rates | 0.00 | 0.00 | 0.00 | 0.000 | 0.000 | 0.000 | 0.000 |
| ± | 0.00 | 0.00 | 0.00 | 0.001 | 0.000 | 0.000 | 0.000 |
Although the comparisons made for IGS08.SNX are promising, actual changes due to the igs05.atx->igs08.atx calibration updates may differ from those obtained by the latitude-dependent models, especially for non-IGS08 stations, for reasons such as:
- the models were obtained from simulations using data from stations not actually equipped with the antennas of interest and only 24h of data
- the precise impact of antenna calibration changes on station positions is a function of latitude *and* perhaps other parameters, such as station environment (observation masks and multipath) and elevation cut-off angle.
1.3 Utilities
A few Perl utilities, designed for applying position corrections with some level of modularity in mind, are available at:
ftp://igs-rf.ign.fr/pub/IGS08/scripts
ftp://igs-rf.ensg.eu/pub/IGS08/scripts
where,
snx2ant.pl - builds antenna information table from igs05.atx-calibrated SINEX
comp_corr.pl - evaluates latitude-dependent models for each antenna(+radome) in antenna information table to obtain position corrections
cmp2disc.pl - matches position corrections to solution numbers given in a table of discontinuities for the user's igs05.atx-calibrated SINEX
apply_shifts.pl - applies position corrections to solutions in igs05.atx-
calibrated SINEX
date_conv.pm - Perl module called by the above scripts
Running the scripts without command-line arguments will provide usage information and other information about the input files.
The scripts for computing, matching and applying the modeled position corrections assume that three files are provided by the user. Those files are:
- • a table of antenna information for sites in the igs05.atx-calibrated solution in a particular format (see usage info in comp_corr.pl) — if the SITE/ID and SITE/ANTENNA blocks in the SINEX file of the stacked frame are accurate, then snx2ant.pl can be used to create the ant.info table
- a list of sites and solution numbers appearing in the stacked frame, also in
a particular format (see usage info in cmp2disc.pl)
a definitive, SINEX-formatted discontinuity table (named soln.snx below) containing a record for each site in the igs05.atx-calibrated SINEX file — if a site has no discontinuities, we recommend inserting a record that corresponds to the solution number and data span
A fourth file containing a table of coefficients for the latitude-dependent models is also required. As stated in IGS Mail #6354, such a table is available containing models for each antenna type with *.atx calibrations. The file can be found in two places:
ftp://igs-rf.ign.fr/pub/IGS08/new_calib/lat_models.txt
ftp://igs-rf.ensg.eu/pub/IGS08/new_calib/lat_models.txt
All antenna types appearing in both igs05.atx and igs08.atx have latitude-dependent models in the lat_models.txt table. The models for antenna/radome combinations with unchanged calibration values have zeros for their coefficients, so a zero position correction is applied when the model is evaluated. In the special cases where the user’s stacked frame solution contains sites with uncalibrated equipment, no correction is available to make its position consistent with IGS05 or IGS08. Users should take care in how such stations are used in order to avoid introducing frame inconsistencies in downstream processing.
The sequence of Korn shell commands using the above scripts and tables could look something like:
snx2ant.pl igs08.atx igs05-atx.SNX > ant.info 2> log
comp_corr.pl ant.info lat_models.txt > ant.info_corr 2>> log
cmp2disc.pl pts.dat ant.info_corr soln.snx > shifts.dat 2>> log
apply_shifts.pl shifts.dat igs05-atx.SNX > igs08-atx.SNX 2>> log
where the igs05-atx.SNX and igs08-atx.SNX frames are consistent with usage of igs05.atx and igs08.atx antenna calibrations, respectively. The user should also note that the scripts provide diagnostic information to the log file that we hope can be helpful in computing, matching and applying position corrections to the data intervals in the user’s regional solution. Of course, one can imagine the case where a discontinuity due to an antenna change was omitted because no position jump was detected. Such cases leave the user to decide whether additional discontinuities are to be inserted, and the matching process be repeated, or to simply omit that particular station/solution from the final SINEX file.
It is important to note that the SOLUTION/EPOCHS block from the user's SINEX file is not used to match latitude-dependent models to SINEX solution numbers. By definition, the SOLUTION/EPOCHS block contains information about the span of data used in the estimation of the position and velocity. The span of data may not coincide with the discontinuity interval. Thus, the definitive beginning and ending epochs used for matching are assumed to be in the user's soln.snx discontinuity file.
2 - HELMERT ALIGNMENT TO IGS08.SNX
Once station-dependent corrections are computed and applied, then a Helmert alignment to IGS08 should be performed. However, one must take care to match discontinuity intervals between sites in IGS08 and the user’s solution.
Note that within the entire IGS contribution to ITRF2008 of about 558 station time series, which span up to about 12.5 yr, there is an overall average of 0.9 position discontinuities per station. The continuous span of data without any break averages about 8.7 yr with a standard deviation of 3.2 yr. Consequently, it is only practical to realize long-term GNSS RFs these days by including position discontinuities, which is the case with the IGS08 RF. Aligning a user frame to IGS08 requires that the same set of discontinuities applied in IGS08 be used in the user frame for the stations used in the alignment. The set of IGS08 discontinuities is available at:
ftp://igs-rf.ensg.eu/pub/IGS08/soln_IGS08.snx
ftp://igs-rf.ign.fr/pub/IGS08/soln_IGS08.snx
We recommend that, once all station-specific corrections have been applied to the user frame coordinates, the solution be aligned to IGS08. The set of sites used for the Helmert alignment should be based on the IGS08 core network (e.g., ftp://igs-rf.ensg.eu/pub/IGS08/IGS08_core.txt), while also taking care to match IGS08 discontinuities. Of course, the user can adopt their own set of discontinuities, but it may be most convenient to use the same as those used by the IGS for any common IGS08 stations.
Jake Griffiths
NOAA/National Geodetic Survey
Jake.Griffiths@noaa.gov
Jim Ray, NOAA/National Geodetic Survey
IGS Analysis Center Coordinator
jim.ray@noaa.gov
Paul Rebischung, IGN/LAREG
paul.rebischung@ign.fr
Ralf Schmid
Technical University of Munich
schmid@BV.TU-MUENCHEN.DE
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