The Scientific Measurement System of the Gravity Recovery and Interior Laboratory (GRAIL) Mission

Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) mission to the Moon utilized an integrated scientific measurement system comprised of flight, ground, mission, and data system elements in order to meet the end-to-end performance required to achieve its scientific objectives. Modeling and simulation efforts were carried out early in the mission that influenced and optimized the design, implementation, and testing of these elements. Because the two prime scientific observables, range between the two spacecraft and range rates between each spacecraft and ground stations, can be affected by the performance of any element of the mission, we treated every element as part of an extended science instrument, a science system. All simulations and modeling took into account the design and configuration of each element to compute the expected performance and error budgets. In the process, scientific requirements were converted to engineering specifications that became the primary drivers for development and testing. Extensive simulations demonstrated that the scientific objectives could in most cases be met with significant margin. Errors are grouped into dynamic or kinematic sources and the largest source of non-gravitational error comes from spacecraft thermal radiation. With all error models included, the baseline solution shows that estimation of the lunar gravity field is robust against both dynamic and kinematic errors and a nominal field of degree 300 or better could be achieved according to the scaled Kaula rule for the Moon. The core signature is more sensitive to modeling errors and can be recovered with a small margin.

The JPL lunar gravity field to spherical harmonic degree 660 from the GRAIL Primary Mission

“…[14] The detection of the lunar gravity field is achieved by observing the effect of the gravity field on the motion of each GRAIL spacecraft as it orbits the Moon. The entire spacecraft, in essence, is the instrument used to measure the gravity field [Asmar et al, 2013] once accounting for all other nongravitational forces acting on the spacecraft. The three measurement types for the GRAIL mission are DSN twoway S-band Doppler, DSN one-way X-band Doppler, and instantaneous interspacecraft Ka-band range-rate (KBRR) data, which is derived from time differentiation of the biased-range (phase) Ka-band data.…”

“…Based upon the Earth Gravity Recovery and Climate Experiment (GRACE) mission [Tapley et al, 2004a], the GRAIL mission [Zuber et al, 2013a] uses two spacecraft flying in formation [Roncoli and Fujii, 2010] to provide the first highly accurate measurement of both the lunar nearside and farside gravity field. The prelaunch requirement for interspacecraft Ka-band measurement accuracy had red-noise characteristics varying between 0.4 μm/s (0.4×10 À6 m/s) for long wavelengths to 1.0 μm/s for the shorter wavelengths corresponding to 5 s data sample times, or a mean of 0.50 μm/s overall Asmar et al, 2013]. As we will display below, the realized GRAIL Ka-band measurement accuracy is more than 10 times better than these requirements, with noise of about 0.03 μm/s.…”

High‐resolution local gravity model of the south pole of the Moon from GRAIL extended mission data

“…The Gravity Recovery and Interior Laboratory (GRAIL) mission was designed to map the structure of the lunar interior through high-precision global gravity mapping [Zuber et al, 2013a]. The mission was composed of two spacecraft with Ka-band intersatellite tracking as the single science instrument, complemented by tracking from Earth using the Deep Space Network (DSN) [Asmar et al, 2013]. The mission consisted of two phases: a primary mission which lasted from 1 March 2012 to 29 May 2012, during which the spacecraft had a mean altitude of 55 km above lunar surface, and an extended mission which lasted from 30 August 2012 to 14 December 2012.…”

Determination and localized analysis of intersatellite line of sight gravity difference: Results from the GRAIL primary mission

“…[3] These L1B data sets are used by JPL and Goddard Space Flight Center project teams to determine the Level-2 (L2) global gravity field using the orbit and gravity analysis software system [Moyer, 2005;Pavlis et al, 2009;Asmar et al, 2013]. The iterative computation between the orbit and the gravity field solutions is required to improve the quality of L1B science measurements [Kruizinga et al, 2013].…”