Precise GPS Orbital Data from IGS: Current Status
January 14, 2010
By: Jake Griffiths,Jim Ray
Report from the International GNSS Service
http://www.gpsworld.com/gnss-system/...s-9389?print=1

By Jim Ray and Jake Griffiths,
IGS Analysis Center Coordinators
NOAA/National Geodetic Survey

The International GNSS Service (IGS) has further improved the quality of
the products made available to the GNSS community.

The accuracy of the Final GPS Orbits (available weekly with a delay of
about two weeks) from IGS is now about 2 centimeters, based on an
analysis of discontinuities between separate daily products and
independent laser-tracking data. Errors in the along- and cross-track
orbital directions are about 75% larger than radially, but correlations
between components are significant. The predominant errors are near the
semi-annual (and/or second GPS draconitic harmonic with a period of
175.6 days), the 4th draconitic harmonic (at 87.8 days), and 14-day
bands due to modeling deficiencies. The draconitic period is the time
for the satellite constellation to complete a full solar revolution in
inertial space and the effects seen are probably related to modeling
solar radiation pressure. The higher-frequency precision at a few days
and shorter is around 7 millimeters due mostly to rotational scatter
followed by quasi-random subdaily variations. The rotational errors are
caused by orbit mismodeling and reference-frame effects. The performance
of the IGS Rapid Orbits (available daily with a delay of about 17 hours)
is very similar, including sharing the common long-period errors.

The high-frequency precision of the IGS near-real-time Ultra-rapid
Observed Orbits (updated four times daily with 3-hour delay) is only
about 40% poorer than the later Rapids and Finals. The Ultra-rapid
real-time predictions over the first 6 hours are about 3.5 times less
precise than the Rapids. This factor doubles if the predictions are
extended to 24 hours. Rotational scatter also dominates the Ultra-rapid
precision, but much more so for the axial than the equatorial
components. This is caused by prediction errors for polar motion and
especially UT1-UTC variations.

The high accuracies of the IGS orbit products benefit many GPS
applications but in particular they aid scientific and engineering
applications including various geophysical investigations and real-time
assessment of deformations of built structures such as dams and bridges
and the stability of rock ledges at open-pit mines.

The IGS is a voluntary federation consisting of over 200 actively
contributing organizations in more than 80 countries and a global
network of over 400 stations. In addition to providing GPS and GLONASS
raw measurements, the IGS contributes to the maintenance and improvement
of the International Terrestrial Reference Frame, produces high accuracy
GPS and GLONASS satellite orbit and clock data, and monitors the Earth's
rotation and the state of its ionized and neutral atmospheres.

Additional information on IGS products can be found at http://igs.org/
and http://acc.igs.org/. A full version of the report on which this
summary is based is also posted.