NEW PRODUCT - ITRF POSITIONS AND VELOCITIES

The National Geodetic Survey has embarked on a program to compute and publish positions and velocities for its most accurate geodetic control stations in the IERS Terrestrial Reference Frame (ITRF).

The International Earth Rotation Service(IERS) computes ITRF positions and velocities for a global set of fundamental stations from data obtained via highly precise space geodetic techniques, such as GPS, VLBI, and satellite laser ranging. Geodetic research agencies in many countries send these data to the IERS, which merges the data to produce a combined global solution. The IERS typically generates a new ITRF solution for the fundamental stations each year.

ITRF positions are generally determined with an accuracy of a few centimeters or better, even over continental or global distances. At this level of accuracy one must account for ongoing tectonic plate motion (continental drift), as well as other forms of crustal motion. For this reason ITRF positional coordinates are considered valid for a specified epoch date, and one must apply appropriate velocities to estimate positional coordinates for any other date. Relative to the ITRF, even points located on the stable part of the North American plate move continuously at a rate of about 2.5 cm/yr.

NGS plans to compute ITRF positions for those stations that have been occupied for one or more highly accurate GPS survey. Over 2000 such stations now exist, and the number may eventually grow to 7000. NGS will initially predict ITRF velocities, except at a few stations where sufficient observations now exist to compute such velocities.

The use of ITRF positional coordinates is appropriate for those applications demanding the highest accuracy obtainable. In a differential GPS survey, the base station should be set up over a monumented point whose ITRF coordinates are known. Because GPS orbits are given in the ITRF coordinate system, processing differential surveys using the ITRF system will ensure consistency.

NGS will provide ITRF coordinates in addition to the normal NAD 83 coordinates. Where appropriate, the statement "ITRF positions and velocities available for this station" will appear on the NGS data sheet. For data sheets provided over the NGS Web Site, this statement will provide a hot link to the actual ITRF positions and velocities. For CD-ROM products, the ITRF positions and velocities are in a separate directory. ITRF coordinates are now available for all CORS stations on Web based data sheets and on CD-ROM products produced after November 1996.

Watch this space for further developments.