[IGSMAIL-7361] igs08_1918.atx: Update considering change in the BeiDou constellation

[Ralf Schmid]

Dear IGS colleagues!

The absolute IGS antenna correction file (igs08_wwww.atx) has been 
updated. The new release is called 'igs08_1918.atx'. It contains the 
following changes:

1) Based on IGS/MGEX monitoring, BeiDou IGSO 6 (2016-021A) switched from 
PRN C15 to C13 on 11 October 2016. Therefore, the following satellite 
has been added with conventional MGEX phase center offset values:

C13 (C017)

2) Decommission dates for the following satellites have been added:

C13 (C014): 21 October 2014
C15 (C017): 11 October 2016


The updated antenna correction model is available at
ftp://ftp.igs.org/pub/station/general/igs08_1918.atx
or
ftp://ftp.igs.org/pub/station/general/igs08.atx

Please be aware that C15 (C017) and C13 (C017) are the first satellites 
contained in igs08.atx with start/end dates different from midnight:

C15 (C017):
>   2016    10    11     5     0    0.0000000                 VALID UNTIL
C13 (C017):
>   2016    10    11     7    30    0.0000000                 VALID FROM

More information on the BeiDou constellation can be found at
http://mgex.igs.org/IGS_MGEX_Status_BDS.html

Many thanks to Feipeng Zhang, Peter Steigenberger and Oliver Montenbruck 
for their help!

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