Commission 4: Ephemerides

Abstract

Jet Propulsion Laboratory, Caltech, USAJPL planetary ephemeris development has been very active assimilating measurements from current planetary missions and supporting future missions. The NASA Mars Science Laboratory (MSL) mission with launch in 2009 requires knowledge of the Earth and Mars ephemerides with 30 m accuracy. By comparison, the accuracy of the Mars ephemeris in the widely used DE405 ephemeris was about 3 km. Meeting the MSL needs requires an ongoing program of range and very-long baseline interferometry measurements of Mars orbiting spacecraft. The JPL ephemeris DE421 was released three months before the landing of the Phoenix mission on Mars, and has met the 300 m requirement. Continued measurements are planned to support the MSL landing.Measurements are now routinely made using several NASA and ESA spacecraft in orbit about Mars. VLBI measurements using DSN and ESA tracking stations have been supplemented by use of the Very Long Baseline Array for specific measurement campaigns. Currently the planetary ephemeris is aligned to the ICRF with an accuracy of 0.25 mas based on the VLBI measurements. VLBI and ranging measurements to the ESA Venus Express spacecraft have resulted in a significant improvement in the Venus ephemeris. Measurements of the Cassini spacecraft since 2005 have resulted in a significant improvement in the Saturn ephemeris. Measurements from the MESSENGER spacecraft encounters with Mercury are expected in the next several years. After a lapse of several years, lunar laser ranging are being included again in the JPL ephemeris development, to support planned lunar missions.The JPL ephemeris coordinates time is not the TCB time scale recently adopted by the IAU. Instead the time scale is consistent with the earlier JPL coordinate time, and is nominally the same as TDB as it has now been redefined by the IAU as a linear function of TCB. Because of potential harm to NASA spacecraft, there is no plan to change the coordinate time scale for ephemerides to be used with NASA missions.