The uncertainty in our knowledge of the Earth's changing orientation in space is a major source of error in tracking and navigating interplanetary spacecraft. Because the Earth's orientation changes rapidly and unpredictably, measurements must be acquired frequently and processed rapidly in order to meet the near-real-time Earth orientation requirements of the interplanetary spacecraft navigation teams.

These requirements are currently met at JPL by using the global positioning system (GPS) to provide daily determinations of polar motion and length-of-day (LOD) within 24 hours of acquisition. Single baseline very long baseline interferometry (VLBI) measurements are taken twice-per-month by the Time and Earth Motion Precision Observations (TEMPO) project in order to provide the benchmark Universal Time (UT) measurements between which the GPS LOD measurements are integrated.

The Kalman Earth Orientation Filter (KEOF) is then used to combine the GPS polar motion and LOD measurements with the TEMPO VLBI variation-of-latitude and UT0 measurements, along with other publicly available Earth orientation measurements including proxy measurements such as atmospheric angular momentum (AAM), in order to generate and deliver the required polar motion and UT1 Earth orientation parameters to the spacecraft navigation teams.

Combinations:

Reference series of Earth orientation parameters are generated annually at JPL. Generally, three such reference series are generated and are consistent with the IERS reference frames that were in effect at the time of generation:

• SPACE: consisting of values and rates for polar motion and UT1 starting at September 28, 1976 at daily intervals. Generated by combining Earth orientation measurements taken by the space-geodetic techniques of lunar and satellite laser ranging (LLR & SLR), VLBI, and GPS
• COMB: consisting of values and rates for polar motion and UT1 starting at January 20, 1962 at daily intervals. Generated by additionally including the BIH optical astrometric measurements with the space-geodetic measurements used to generate SPACE
• POLE: consisting of values and rates for polar motion only starting at January 20, 1900 at monthly intervals. Generated by additionally including the ILS optical astrometric measurements with the other optical astrometric and space-geodetic measurements used to generate COMB.

The latest reference series, a report describing their generation and archives of previous series can be obtained via:

• • Latest POLE, COMB, SPACE series (HTTP)
• • Latest POLE, COMB, SPACE report (PDF, ~2MB)
• • Latest POLE, COMB, SPACE _README_ file (HTTP, ~2KB)
• • POLE, COMB, SPACE Archives by year (HTTP)

Predictions:

The near-real-time Earth orientation requirements of the interplanetary spacecraft navigation teams are met by daily updating of the annually generated reference series. The updated Earth orientation series are generated by additionally incorporating measurements that are rapidly available such as the GPS measurements from the JPL Analysis Center of the IGS and the AAM measurements and forecasts from NCEP that are used as proxy LOD measurements.

In addition, short-term predictions of the EOPs are produced in order to provide the navigation teams with an uninterrupted series of Earth orientation parameters. These predicted series, by design, are consistent with the IERS reference frames that were in effect at the time that the series were generated.

The updated EOP series with predictions as well as EOP archives are available:

• • Latest EOP Estimate & Predictions (consistent with the current reference frame): Noon time-tags (HTTP, ~400KB)
• • Latest EOP Estimate & Predictions (consistent with the current reference frame): Midnight time-tags (HTTP, ~400KB)
• • Latest EOP Estimate & Predictions with non-tidal UT1 (consistent with the current reference frame): Noon time-tags (HTTP, ~400KB)
• • Latest EOP Estimate & Predictions with non-tidal UT1 (consistent with the current reference frame): Midnight time-tags (HTTP, ~400KB)
• • EOP Archives by year (HTTP)