Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 65

2.4.4 Satellite Gravimetry
The features of satellite gravimetry chiefly include the ground tracking satellite,
satellite-to-satellite tracking (SST), satellite gravity gradiometry (SGG), and satel-
lite altimetry (SA) (see, e.g., Torge and M¨ ller 2012). SA has been described in
Sect. 2.3 .
Determining the Earth's Gravity Field by Means of a Ground Tracking
Satellite
The Earth's gravity field can be determined by means of a ground tracking satellite,
using techniques such as satellite laser ranging (SLR), Doppler orbitography and
radiopositioning integrated by satellite (DORIS), and precise range and range-rate
equipment (PRARE).
The observed quantities involved with use of a ground tracking satellite include
primarily the direction, range, range rate, and phase from the ground tracking
station to the satellite being tracked. The geometric and physical functional rela-
tionships between the satellite orbit and the ground tracking station can be
established on the basis of these observational data. Since the satellite orbit is the
implicit function of the perturbation factors of the Earth's gravity field, the gravity
field of the Earth can be computed.
Determining the Earth's Gravity Field by Means of Satellite-to-Satellite
Tracking
The technologies of SST can be sorted into two modes: high-low satellite-to-
satellite tracking (SST-hl) and low-low satellite-to-satellite tracking (SST-ll).
SST-hl utilizes the space-borne GPS receiver and the GPS satellite constellation
(altitude about 21,000 km) on the low Earth orbit satellite (LEO, altitude 400 km),
forms the high-low satellite space tracking network, and estimates the three-
dimensional location, velocity, and acceleration of the low Earth orbit satellite,
namely the first derivative of gravitational potential (GPFD). SST-ll employs two
identical satellites in the same orbit with an inter-satellite distance of 200-400 km,
measures precisely the relative motion of the two satellites or the changes of inter-
satellite distance by using a microwave interferometer, and determines the coeffi-
cients of the Earth's gravity field based on the rate of change of the inter-satellite
distance (see Nin et al. 2006).
Germany's CHAMP (Challenging Mini-Satellite Payload for Geophysical
Research and Application) employs the SST-hl tracking mode, as illustrated in
Fig. 2.22 . CHAMP was successfully launched into an orbit of 418-470 km altitude
in July, 2000 on a 5-year mission. One of the scientific missions of CHAMP was to