Precise GPS Orbital Data from IGS: Current Status

January 14, 2010 By: Jake Griffiths, Jim Ray

Report from the International GNSS Service

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 discontinuties 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.