IGSMAIL-6355/6374 - Upcoming switch to IGS08 reference frame and the consequences to the GPS users (II)
April 12, 2011 By: Ralf SchmidAs announced in IGSMAIL-6354, the IGS will adopt a new reference frame called IGS08 that should be used together with igs08.atx, an updated set of receiver and satellite antenna phase center corrections. Those will replace the absolute phase center correction model igs05.atx adopted in November 2006 (GPS week 1400) together with the IGS reference frame IGS05.
Since then, it was only possible to add correction values for new satellite or receiver antennas. In general, updates of existing values were not possible in order not to jeopardize the consistency of IGS products. So, for a lot of IGS stations converted field calibrations were still applied, although azimuthal phase center corrections down to the horizon from robot calibrations would have been available.
Due to a strong correlation between the satellite antenna phase center offsets in the Earth radial direction (z-PCOs) and the scale of the terrestrial reference frame, the scale difference between ITRF2005 and ITRF2008 is also relevant for the antenna model. If satellite z-PCOs were not adapted before the switch to IGS08, the scales of the terrestrial frame solutions of the IGS analysis centers (ACs) would no longer be close to the ITRF scale.
Due to these reasons a general update of the IGS antenna phase center correction model became necessary. As coordinate jumps are unavoidable when adopting a new reference frame, receiver antenna calibrations are ideally updated at the same time. So, besides replacing converted field calibrations, more or less all type-specific correction values were updated with results from recent calibrations of further individual antennas.
All these changes will have an impact on the users. However, this impact will be much smaller than was the case with the switch from relative to absolute phase center corrections in 2006 (see [IGSMAIL-5189]). Details on the various model improvements will be given in the following. If you have any further comments or questions, please send those to schmid@bv.tum.de.
The new model, including GLONASS satellite antenna corrections, is available at ftp://igs.org/igscb/station/general/igs08_1629.atx
or
ftp://igs.org/igscb/station/general/igs08.atx
Summary of major changes of igs08.atx w.r.t. igs05.atx
Satellite antenna phase center corrections
- The satellite z-PCOs from igs08.atx are consistent with the new IGS reference frame IGS08, whereas those from igs05.atx were approximately consistent with IGS05.
- The z-PCO estimates are based on the results of more ACs. The GPS values in igs08.atx were derived from the weekly SINEX files of five ACs (CODE, ESA, GFZ, MIT, NRCan), whereas igs05.atx was based on two ACs only. The GLONASS corrections were estimated by CODE and ESA, whereas igs05.atx contained a CODE-only solution.
- All preliminary z-PCOs for satellites launched in recent years could be replaced by actual estimates. It is the intention for igs08.atx that preliminary values should be replaced within few months after the launch.
- Due to an improved quality of the ITRF vertical rates (ITRF2008 compared to ITRF2000), the z-PCOs are no longer trend-corrected.
- igs08.atx also contains information on historical satellites. All GPS Block I satellites were added with block mean values. The same is true for all GLONASS satellites back to the IGEX-98 campaign.
Receiver antenna calibrations (refer to http://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006347.html for more details)
- Robot calibrations for 15 additional antenna types were added.
- For 9 additional antenna types, robot calibration results were copied from antenna types that should be identical in construction.
- Existing type-specific phase center corrections from 46 robot calibrations were updated with results from recent individual antenna calibrations, if available. However, the correction values for 41 antenna types with robot-based values remained unchanged.
- GLONASS-specific corrections from robot calibrations were added, if available. Those will be used to generate the GLONASS products of the IGS. In case that GLONASS-specific values are not available for a certain antenna type, the correction values for the GPS frequencies will be used instead.
- As the correction values of the IGS reference antenna AOAD/M_T got updated, all converted calibrations slightly changed. This concerns 87 antenna calibrations converted from NGS field results and 14 types converted from igs_01.pcv.
- To improve internal consistency the correction values of 3 antenna types are now converted from NGS field calibrations, rather than from igs_01.pcv.
- The calibrations of 2 antenna types were removed due to naming inconsistencies between IGS and NGS.
- The calibrations of 2 antenna types remained untouched due to problems with unmodeled subtypes having the same name. The Antenna Working Group will try to approach this problem in the near future.
Some remarks on the magnitude and impact of the changes
"AOAD/M_T NONE" as an example
Although the Dorne Margolin antenna AOAD/M_T of Allen Osborne Associates, Inc. was the reference antenna of the IGS, as long as relative phase center corrections were used, its "type mean values" contained in igs05.atx were based on the calibration of one single antenna only, namely SN 404 (serial number). However, the igs05.atx correction values were a mean of 42 individual calibration runs. For igs08.atx, additional 20 calibration runs of a second antenna (SN 393) were considered.
As the number of individually calibrated antennas is small, the phase center corrections change significantly, if additional calibrations are taken into account. The differences between calibration results for antennas of the same type contain the errors of the calibration procedure, but also the effect of deviations of the individual antenna from the ideally manufactured one. The latter effect could only be compensated by using individual rather than type mean calibrations. However, individual calibrations are not considered within the IGS so far.
If the number of individual calibrations to derive the type mean values was bigger, the transition from one phase center model to the next would be smoother. On the one hand, this would have advantages for the maintenance of the reference frame, but on the other hand, the stability of the phase center corrections would somehow be illusive, as actual deviations from the type mean hardly showed up anymore.
In the case of AOAD/M_T, the phase center correction changes due to the update from igs05.atx to igs08.atx are on the sub-mm level. The biggest effect shows up in the up component of the L1 PCO (0.6 mm) as well as in the phase center variations (PCVs) for L1 (up to 0.6 mm below an elevation of 10°). If these corrections are applied to derive the ionosphere-free linear combination, the effect is amplified. Thus, IGN could find coordinate changes for stations equipped with AOAD/M_T of about 2-3 mm (see coefficient "a" of the model for up position shifts) that were too small to be considered for IGS08: ftp://igs-rf.ensg.eu/pub/IGS08/new_calib/lat_models.txt
As the AOAD/M_T antenna is used to convert relative field calibrations, also all those correction values change in the same manner. In the future, coordinate changes due to calibration updates will get smaller and smaller, whereas the problem of individual antenna deviations persists. Unfortunately, their impact on station coordinates could easily reach the 1 mm level.
Biggest phase center correction changes
By checking the coefficients "a" of IGN's latitude-dependent correction models (ftp://igs-rf.ensg.eu/pub/IGS08/new_calib/lat_models.txt), one can easily detect those antenna types that suffer the biggest changes (differences of more than 1 cm in the station coordinates). For example:
ASH700228C NONE
Whereas the former relative IGS model igs_01.pcv only contained one set of phase center corrections for all antennas of the type ASH700228x, NGS provides three different sets (ASH700228A/B, ASH700228C, ASH700228D/E). Therefore, igs08.atx contains values converted from relative NGS field calibrations that differ significantly from those converted from igs_01.pcv (contained in igs05.atx).
Biggest impact of robot-to-robot updates
In most cases the coordinate changes caused by robot calibration updates are small. The "a" coefficients from ftp://igs-rf.ensg.eu/pub/IGS08/new_calib/lat_models.txt are below 2 mm for most antenna types that already had robot-based correction values in igs05.atx. The calibration differences are smaller, but amplified by the ionosphere-free linear combination.
The biggest coordinate changes can mainly be explained by significant changes in the number of individually calibrated antennas. Detailed information on the number of individual antennas and calibration runs is given in the antenna-specific comments contained in igs08.atx:
- ASH700936E NONE (up component) 1 -> 2 individual antennas
- TRM29659.00 NONE (east component) 12 -> 18 individual antennas
- ASH700936D_M NONE (up component) 1 -> 4 individual antennas
- LEIAT504GG NONE (up component) 5 -> 25 individual antennas
Full text of IGSMAIL-6355 can be found at: http://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006347.html
igs08.atx including GLONASS satellite antenna corrections:
http://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006366.html
Ralf Schmid
Technische Universität München
schmid@BV.TU-MUENCHEN.DE
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