[IGSMAIL-6786] igs08_1745.atx: Update including GPS satellite antenna PCV extension

[Robert Khachikyan]

Dear IGS colleagues!

The absolute IGS antenna correction file (igs08_wwww.atx) has been
updated. The new release is called 'igs08_1745.atx'. According to an IGS
2012 Workshop recommendation, the satellite antenna phase center
variations (PCVs) of the following GPS block types were extended with
correction values [mm] for nadir angles between 15 and 17 degrees
derived from the analysis of low Earth orbiter (LEO) data performed by CODE:

|-------------------|--------|--------|--------|--------|
|       Nadir angle |        |        |        |        |
|         [degrees] |     14 |     15 |     16 |     17 |
|                   |        |        |        |        |
| Block             |        |        |        |        |
|-------------------|--------|--------|--------|--------|
| I                 |  -0.30 |   --   |   --   |   --   |
|-------------------|--------|--------|--------|--------|
| II/IIA            |  -0.90 |  -0.90 |  -0.90 |  -0.90 |
|-------------------|--------|--------|--------|--------|
| IIR-A             |  -1.40 |  -0.80 |  +2.00 |  +6.40 |
|-------------------|--------|--------|--------|--------|
| IIR-B/M           | +12.10 | +22.00 | +30.40 | +40.60 |
|-------------------|--------|--------|--------|--------|
| IIF               |  +5.70 | +12.40 | +18.20 | +23.50 |
|-------------------|--------|--------|--------|--------|

The results are based on ionosphere-free GPS data of the LEO missions
Jason-2, GRACE-A, GRACE-B, GOCE, and MetOp-A from 2009. In addition,
Jason-2 data from the second half of 2011 were analyzed to derive
extended PCV estimates for Block IIF. Block I satellites were only
active before the first launch of a GPS-based LEO mission. Therefore,
Block I PCVs remain unchanged.

The PCVs of Block IIA satellites were extended with constant values
beyond 14 degrees and constrained when estimating PCV values for the
remaining satellite block types (IIR-A, IIR-B/M, IIF). Such a constraint
for one of the satellite blocks is necessary, as PCVs for the
transmitter and the receiver antennas on board the LEOs have to be
estimated simultaneously. In fact, the analysis is only sensitive to PCV
differences between different satellite blocks.

More details can be found in the following IGS 2012 Workshop presentation:
Jäggi A, Dilssner F, Schmid R, Dach R, Springer T, Bock H, Steigenberger
P, Lutz S: Extension of the GPS satellite antenna patterns to nadir
angles beyond 14°.
http://www.igs.org/assets/pdf/Poland%202012%20-%20P09%20Dach%20PR14.pdf

The PCV extension is mainly relevant for the analysis of LEO data.
Solutions using terrestrial data will be hardly affected. If any, there
might be a tiny benefit for terrestrial observations at very low
elevations that slightly exceed a nadir angle of 14 degrees. LEO users
should be aware that an update of the LEO antenna PCVs might be
necessary when implementing the PCV extension for the transmitter antennas.

Many thanks to Adrian Jäggi and the CODE group for providing the final
PCV estimates as well as to Florian Dilssner and the ESOC group who
supported this effort by independent PCV solutions for the purpose of
comparison.

The updated antenna correction model is available at
ftp://igs.org/pub/station/general/igs08_1745.atx
or
ftp://igs.org/pub/station/general/igs08.atx

Those users who don't want to implement the new model including the PCV
extension immediately are referred to
ftp://igs.org/pub/station/general/igs08_1745_woPCVextension.atx
that is identical with the previous release (igs08_1740.atx). In order
to allow a smooth transition, parallel model updates with and without
the PCV extension will be released until about end of July 2013.

With best regards,
Ralf Schmid and Robert Khachikyan
on behalf of the IGS Antenna Working Group