Space-borne satellite gravimetry started in the early eighties when the radar altimeter on board the SEASAT satellite (NASA), was used to measure the marine geoid and its variation globally. Consequently over the last thirty years, a number of dedicated satellite missions were carried out by various space organizations for measurement of the gravity field and its variations (on both continental and oceanic regions). Initially, altimetry data was from space-borne altimeters like TOPEX/Poseidon (NASA, CNES) was used to derive marine geoid and consequently derive gravity anomaly. In the last decade, Challenging Mini-satellite Payload (CHAMP) and Gravity Recovery and Climate Change Experiment (GRACE) gravity missions started measuring gravity data field directly by methods other than satellite altimetry (Tapley et al., 2004). These satellites used the idea of satellite to satellite tracking (SST) for measuring changes of acceleration due to gravity (Rummel et al., 2002) and its variations due to changes in lithospheric density. Complete and uniform global coverage was achieved with high accuracies, which enabled creation of gravity models with higher degrees of spherical harmonics (Reigber et al., 2005a). In addition to this, GRACE, owing to its lower repeat cycle, can provide information on temporal variation of earth's gravity field (Tiwari et al., 2009).
2. GOCE: An Overview
The Gravity field and steady state Ocean Circulation Explorer (here after referred to as GOCE) was launched in March, 2009, is till date the most sophisticated gravity mission. Unlike its predecessors, GOCE uses the concept of satellite gradiometry (Rummel et al., 2002) for the measurement of gravity field and its gradients. The GOCE mission was specified to measure the variations of earth's gravity field up to accuracies of 1 to 2 mgal and variations of geoid in order of 1 to 2 cm (Drinkwater et al., 2006). Owing to the low orbital height of 250km (with near sun synchronous orbit), GOCE is capable of measurement of gravity variations with spatial resolutions in order of about 100km (Drinkwater et al., 2006). Due to the higher spatial and sensor accuracy, data from GOCE has been successfully used to create gravity models with spherical harmonic degrees and order of around 200 or more (Pail et al., 2011).
The details of the principle of gravity gradiometry (as well as satellite to satellite tracking) in space-borne gravity sensors are mentioned in Rummel et al. (2002) and elaborated in Eshagh (2009). Herein we present a short summary of the principle satellite gravity gradiometry as employed in the GOCE. Satellite gravity gradiometry measures the differences in accelerations between pairs of test masses, in all the 3 spatial directions. This is done by the help of a configuration of 6 highly sensitive accelerometers named as Electrostatic Gravity Gradiometer (EGG) on board the space-craft. The measured signal, arising from the changes in acceleration between the...