Nate Harvey

High‐resolution lunar gravity fields from the GRAIL Primary and Extended Missions

The resolution and accuracy of the lunar spherical harmonic gravity field have been dramatically improved as a result of the Gravity Recovery and Interior Laboratory (GRAIL) mission. From the Primary Mission, previous harmonic gravity fields resulted in an average n = 420 surface resolution and a Bouguer spectrum to n = 330. The GRAIL Extended Mission improves the resolution due to a lower average 23 km altitude orbit. As a result, new harmonic degree 900 gravity fields (GL0900C and GL0900D) show nearly a factor of 2 improvement with an average surface resolution n = 870 and the Bouguer spectrum extended to n = 550. Since the minimum spacecraft altitude varies spatially between 3 km and 23 km, the surface resolution is variable from near n = 680 for the central farside to near n = 900 for the polar regions. These gravity fields with 0.8 million parameters are by far the highest-degree fields of any planet ever estimated with a fully dynamic least squares technique using spacecraft tracking data.

Sub-Centimeter Precision Orbit Determination with GPS for Ocean Altimetry

We assess the accuracy of JPLs estimated OSTM/Jason-2 Global Positioning System (GPS)-determined orbits based on residuals to independent satellite laser ranging (SLR) data, compared with orbits produced by different software from different data (SLR/DORIS), Geophysical Data Record version C (GDR-C) orbits, and altimeter crossover tests. All of these tests are consistent with sub-cm radial accuracy: high elevation SLR residual standard deviation lies at 6.8 mm, RMS differences from GDR-C in the radial component typically fall below a cm, and altimeter crossovers from JPL orbits have a variance 89 mm2 smaller than altimeter crossovers from GDR-C orbits. Although RMS differences between radial components of different orbit solutions typically lie below a cm, we observe systematic dependences on both time and geography. The improved precision and accuracy of JPLs OSTM/Jason-2 orbit solutions rely on a new algorithm for applying constraints to integer carrier phase ambiguities. This algorithm is sufficiently robust to improve solutions despite half-cycle carrier phase identification issues in OSTM/Jason-2s BlackJack receiver. Although Jason-1 receiver performance differs, our algorithm should extend to Jason-1 processing (during the time span of nominal GPS receiver operations).

An innovative direct measurement of the GRAIL absolute timing of Science Data

The Gravity Recovery and Interior Laboratory (GRAIL), a NASA Discovery mission, twin spacecraft were launched on 10 September 2012 and were inserted into lunar orbit on 31 December 2011 and 01 January 2012. The objective of the mission was to measure a high-resolution lunar gravity field using inter-spacecraft range measurements in order to investigate the interior structure of the Moon from crust to core. The first step in the lunar gravity field determination process involved correcting for general relativity, measurement noise, biases and relative & absolute timing. Three independent clocks participated in the process and needed to be correlated after the fact. Measuring the absolute time tags for the GRAIL mission data turned out to be a challenging task primarily because of limited periods when such measurements could be conducted. Unlike the Gravity Recovery and Climate Experiment (GRACE), where absolute timing measurements are available using the GPS system, no absolute timing measurements were available on the far side of the Moon or when there were no DSN coverage periods. During the early cruise phase, it was determined that a direct absolute timing measurement of each spacecraft Lunar Gravity Ranging System (LGRS) clock could be directly observed by using a DSN station to eavesdrop on the Time Transfer System (TTS) S-band inter-satellite ranging signal. By detecting the TTS system directly on earth, the LGRS clock can be correlated directly to Universal Time Coordinated (UTC) because the TTS and LGRS use the same clock to time-tag their measurements. This paper describes the end-to-end preparation process by building and installing a dedicated hardware at Goldstone station DSS-24, selecting favorable lunar orbit geometries, real time signal detection and post processing, and finally how the absolute timing is used in the overall construction of lunar gravity fields.