[IGSMAIL-6355] Upcoming switch to IGS08/igs08.atx - Details on igs08.atx
schmid at bv.tu-muenchen.de
Mon Mar 7 04:00:04 PST 2011
As 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, in the near
future. 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
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 at bv.tum.de.
A preliminary version of the antenna model without GLONASS satellite
antenna corrections is currently available at
As soon as the GLONASS satellite antenna corrections are finalized, the
final model will be posted at ftp://igs.org/igscb/station/general/ and
announced via IGSMAIL.
Many thanks to
- all ACs providing sat. PCO estimates within their reprocessed SINEX files
- Xavier Collilieux (IGN) for analyzing those SINEX files
- CODE and ESA for providing GLONASS satellite antenna corrections
- Martin Schmitz (Geo++ GmbH) for providing updated robot calibrations
- Paul Rebischung (IGN) for analyzing the impact on station coordinates
With best regards,
on behalf of the IGS Antenna Working Group
Major changes of igs08.atx w.r.t. igs05.atx
* Satellite antenna phase center corrections (GLONASS still tbd.!)
+ 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 (still tbd.) 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 (still tbd.).
* Receiver antenna calibrations
+ Robot calibrations for 15 additional antenna types could be added. For
those types, converted field calibrations or copied values were applied
so far or they were not contained in the file:
+ For 9 additional antenna types, robot calibration results were copied
from antenna types that should be identical in construction. For those
types, converted field calibrations were applied so far or they were not
contained in the file:
+ Existing type-specific phase center corrections from robot
calibrations were updated with results from recent individual antenna
calibrations, if available. Therefore, 46 robot calibrations changed:
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
+ 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)
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:
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):
+ 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).
+ ASH700718A NONE
Both NGS and igs_01.pcv provide identical correction values for
ASH700718A and ASH700718B. Therefore, the robot-based values for
ASH700718B were copied. The big offset component in North direction only
shows up in the igs_01.pcv values (used for igs05.atx), but not in the
NGS or Geo++ results.
+ ASH701941.B SCIS
For that antenna/radome combination no calibration results were
available so far. Therefore, the correction values for the antenna
without radome (NONE) were applied within the IGS. So, the dramatic
coordinate change in vertical direction demonstrates the effect of
ignoring the radome.
+ LEIAT202-GP NONE
As both NGS and igs_01.pcv provide identical correction values for
LEIAT202-GP and LEIAT302-GP, robot-based values were copied from
LEIAT302-GP. The values contained in igs05.atx were converted from
+ LEISR299_INT NONE
According to NGS, LEISR299_INT should be identical to LEISR399_INTA, but
not to LEISR399_INT as was the case in igs_01.pcv. This might be related
to the confusion as regards these antenna names within rcvr_ant.tab.
+ TRM39105.00 NONE
In this case, converted field calibrations could be replaced by
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
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