Hence, the gradient can be measured by determining the difference in gravity at two close but distinct points. This principle is embodied in several recent moving-base instruments. The gravity gradient at a point is a tensor, since it is the derivative of each component of the gravity vector taken in each sensitive axis. Thus, the value of any component of the gravity vector can be known all along the path of the vehicle if gravity gradiometers are included in the system and their outputs are integrated by the system computer. An accurate gravity model will be computed in real-time and a continuous map of normal gravity, elevation, and anomalous gravity will be available. Example: GOCE Satellite-to-satellite tracking This technique uses satellites to track other satellites. There are a number of variations which may be used for specific purposes such as gravity field investigations and orbit improvement. A high altitude satellite may act as a relay from ground tracking stations to a low altitude satellite. In this way, low altitude satellites may be observed when they are not accessible to ground stations. In this type of tracking, a signal generated by a tracking station is received by the relay satellite and then retransmitted to a lower altitude satellite. This signal is then returned to the ground station by the same path. Two low altitude satellites can track one another observing mutual orbital variations caused by gravity field irregularities. A prime example of this is GRACE. Several high altitude satellites with accurately known orbits, such as GPS satellites, may be used to fix the position of a low altitude satellite. These examples present a few of the possibilities for the application of satellite-to-satellite tracking. Satellite-to-satellite tracking data was first collected and analyzed in a high-low configuration between ATS-6 and GEOS-3. The data was studied to evaluate its potential for both orbit and gravitational model refinement. Example: GRACE GNSS tracking Examples: CHAMP, GOCE List of geodetic satellites ANNA-1B Beidou BLITS CHAMP Diadème Echo Envisat ERS-1 ERS-2 Etalon Experimental Geodetic Payload "Ajisai" Explorer program Galileo Geo-IK-2 GEOS-3 Geosat Geosat Follow-On GFZ-1 GLONASS GRACE GOCE GPS ICESat-1 ICESat-2 LAGEOS LARES Larets H-IIA-LRE PAGEOS Seasat Starlette and Stella TOPEX/Poseidon TRANSIT WESTPAC See also Geodetic astronomy Satellite gravimetry References Attribution Further reading Smith, David E. and Turcotte, Donald L. (eds.) (1993). Contributions of Space Geodesy to Geodynamics: Crustal Dynamics Vol. 23, Earth Dynamics Vol. 24, Technology Vol.