[IGSMAIL-6613] Switch to daily terrestrial frame integrations in IGS Final products

[Jake Griffiths]

Authors: IGS Analysis Center & Reference Frame Coordinators

There has been much discussion about the effects of non-tidal loading
displacements on the IGS Final products. To facilitate further study of these
effects, the IGS will switch from products based on weekly terrestrial
reference frame (TRF) integrations to those based on daily integrations. The
switch will happen with products for GPS Wk 1702.

Of course, products based on daily TRFs could be up to sqrt(7) = 2.6 times
noisier. But our test combinations suggest that there is no noticeable impact
on the ERPs. Further, the RMS of GPS orbits will increase by ~0.5 mm; and the
RMS for the ensemble of station and satellite clocks will increase by about 2.5
ps. Meanwhile, kinematic PPP results indicate that there should be no noticeable
change in the orbit+clock quality. Thus, most users should not notice a change
with the new orbits & clocks. The RMS of station position residuals from the
SINEX combination increases by a factor of about 1.0 to 1.5, mostly in East &
Up, but much less than if the IGS errors were dominantly white noise. The North
residuals are generally less affected. The gain in temporal resolution should
be offset by little additional noise on station positions. Of course, the total
product errors, including systematic effects, are much larger than these small
changes.

The biggest change will be seen in the number, naming and content of files
from the Analysis Centers (ACs) and the SINEX combinations. Each AC will
contribute 7 daily SINEX files, which will be formed into 7 daily combined
files:

igsyyPwwww[0-6].[snx,ssc]

A weekly IGS SINEX solution will still be formed by henceforth stacking the 7
daily combined solutions:

igsyyPwwww.[snx,ssc]

The IGS cumulative solution will still be updated weekly:

IGSyyPww.[snx,ssc]

The ERP and SUM files from the SINEX combinations will continue to contain
weekly information, but the format of the SUM file will change to include a
complete summary of the added daily information:

igsyyPwwww.erp
igsyyPwwww.sum

Finally, the SINEX combination residual files will all become daily. Their
names will change to:

igsyyPwwww[0-6].res     (residuals between the daily AC solutions and
                         the daily combined solutions, ex-igsyyPwwww.res)
igsyyPwwww[0-6]_ITR.res (residuals between the daily AC solutions and
                         the current reference frame, ex-igsyyPwwww.itr)
igsyyPwwww[0-6]_IGS.res (residuals between the daily AC solutions and
                         the IGS cumulative solution, ex-IGSyyPww.res)

The orbit (SP3) and clock (CLK) files will not change in number or format. The
associated combination summary report, however, has been updated to include the
daily SINEX rotations applied to AC orbits and ERPs--a step that is made by the
Analysis Coordinator during combination.

Please feel free to examine the files from the test combinations (with prefix
igt, instead of igs) for wwww = 1686 thru 1691:

ftp://igs-rf.ensg.eu/1d_tests/wwww/

Additional test weeks will be added in time as to keep up with the operational
products. Hopefully these test files will assist you in making s/w changes.

Best regards,

Jake Griffiths (NOAA/NGS)
Paul Rebischung (IGN/LAREG)