Query: The latest  measurements involve the use of satellite gravimetry, estimating  the mass of terrain beneath by detecting slight changes in gravity as a  satellite passes overhead.

Relevant passages:
GRACE and GRACE-FO <=SEP=> The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center (DLR). Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its science mission in October 2017.  The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) is a continuation of the mission on near-identical hardware, launched in May 2018. By measuring gravity anomalies, GRACE showed how mass is distributed around the planet and how it varies over time. Data from the GRACE satellites is an important tool for studying Earth's ocean, geology, and climate. GRACE was a collaborative endeavor involving the Center for Space Research at the University of Texas at Austin, NASA's Jet Propulsion Laboratory, the German Aerospace Center and Germany's National Research Center for Geosciences, Potsdam. The Jet Propulsion Laboratory was responsible for the overall mission management under the NASA ESSP (Earth System Science Pathfinder) program. The principal investigator is Byron Tapley of the University of Texas Center for Space Research, and the co-principal investigator is Christoph Reigber of the GeoForschungsZentrum (GFZ) Potsdam.The two GRACE satellites (GRACE-1 and GRACE-2) were launched from Plesetsk Cosmodrome, Russia, on a Rockot (SS-19 + Breeze upper stage) launch vehicle on 17 March 2002.  The spacecraft were launched to an initial altitude of approximately 500 km at a near-polar inclination of 89°.  During normal operations, the satellites were separated by 220 km along their orbit track. This system was able to gather global coverage every 30 days. GRACE far exceeded its 5-year design lifespan, operating for 15 years until the decommissioning of GRACE-2 on 27 October 2017. Its successor, GRACE-FO, was successfully launched on 22 May 2018. In 2019, a glacier in West Antarctica was named after the GRACE mission.

Gravity of Mars <=SEP=> The gravity of Mars is a natural phenomenon, due to the law of gravity, or gravitation, by which all things with mass around the planet Mars are brought towards it. It is weaker than Earth's gravity due to the planet's smaller mass. The average gravitational acceleration on Mars is 3.72076 ms−2 (about 38% of that of Earth) and it varies laterally. In general, topography-controlled isostasy drives the short wavelength free-air gravity anomalies. At the same time, convective flow and finite strength of the mantle lead to long-wavelength planetary-scale free-air gravity anomalies over the entire planet. Variation in crustal thickness, magmatic and volcanic activities, impact-induced Moho-uplift, seasonal variation of polar ice caps, atmospheric mass variation and variation of porosity of the crust could also correlate to the lateral variations. Over the years models consisting of an increasing but limited number of spherical harmonics have been produced. Maps produced have included free-air gravity anomaly, Bouguer gravity anomaly, and crustal thickness. In some areas of Mars there is a correlation between gravity anomalies and topography.  Given the known topography, higher resolution gravity field can be inferred. Tidal deformation of Mars by the Sun or Phobos can be measured by its gravity. This reveals how stiff the interior is, and shows that the core is partially liquid. The study of surface gravity of Mars can therefore yield information about different features and provide beneficial information for future landing projects.

Gravimeter <=SEP=> A gravimeter is an instrument used in gravimetry for measuring the local gravitational field of the Earth . A gravimeter is a type of accelerometer , specialized for measuring the constant downward acceleration of gravity , which varies by about 0.5 % over the surface of the Earth . Though the essential principle of design is the same as in other accelerometers , gravimeters are typically designed to be much more sensitive in order to measure very tiny fractional changes within the Earth 's gravity of 1 g , caused by nearby geologic structures or the shape of the Earth and by temporal tidal variations . This sensitivity means that gravimeters are susceptible to extraneous vibrations including noise that tend to cause oscillatory accelerations . In practice this is counteracted by integral vibration isolation and signal processing . The constraints on temporal resolution are usually less for gravimeters , so that resolution can be increased by processing the output with a longer time constant . Gravimeters display their measurements in units of gals ( cm/s2 ) , instead of more common units of acceleration .   Gravimeters are used for petroleum and mineral prospecting , seismology , geodesy , geophysical surveys and other geophysical research , and for metrology .   There are two types of gravimeters : relative and absolute . Absolute gravimeters measure the local gravity in absolute units , gals . Relative gravimeters compare the value of gravity at one point with another . They must be calibrated at a location where the gravity is known accurately , and then transported to the location where the gravity is to be measured . They measure the ratio of the gravity at the two points .

Gravity Probe B <=SEP=> Gravity Probe B ( GP-B ) was a satellite-based mission which launched on 20 April 2004 on a Delta II rocket . The spaceflight phase lasted until 2005 ; its aim was to measure spacetime curvature near Earth , and thereby the stress -- energy tensor ( which is related to the distribution and the motion of matter in space ) in and near Earth . This provided a test of general relativity , gravitomagnetism and related models . The principal investigator was Francis Everitt .   Initial results confirmed the expected geodetic effect to an accuracy of about 1 % . The expected frame-dragging effect was similar in magnitude to the current noise level ( the noise being dominated by initially unmodeled effects due to nonuniform coatings on the gyroscopes ) . Work continued to model and account for these sources of error , thus permitting extraction of the frame-dragging signal . By August 2008 , the frame-dragging effect had been confirmed to within 15 % of the expected result , and the December 2008 NASA report indicated that the geodetic effect was confirmed to better than 0.5 % .   In an article published in the journal Physical Review Letters in 2011 , the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of and a frame-dragging drift rate of , in good agreement with the general relativity predictions of and , respectively .