The Satellite to Satellite Tracking Instrument (SSTI) consisted of an advanced dual-frequency, 12-channel GPS receiver and an L-band antenna. The SSTI receiver was capable of simultaneously acquiring signals broadcast from up to 12 satellites in the GPS constellation. The SSTI instrument delivered, at 1 Hz, pseudo-range and carrier-phase measurements on both GPS frequencies, as well as a real time orbit navigation solution.

SSTI was designed to operate in a low-Earth orbit environment with the objective to provide the SST-hl (Satellite-to-Satellite Tracking - high/low) contribution to the gravity field recovery, by the simultaneous tracking of up to 12 GPS satellite signals. In addition, SSTI provided data for precise orbit determination and was used for real-time on-board navigation and attitude-reference-frame determination.

In gradiometry, the difference in the acceleration measured by two accelerometers placed some fixed distance apart provides the basic observable, proportional to the gravity gradient in the direction joining the two sensors through a constant scale factor. In SST-hl, the positional data measured with respect to a constellation of reference satellites in known orbits were used to extract the gravity information through orbit perturbation analysis. In these techniques, knowledge of the orbit allowed for the retrieval of the underlying dynamic models that govern the satellite motion, including the Earth gravity field. The two techniques (with the EGG instrument) were complementary in that SST-hl worked best at providing the long and medium-wavelength part of the geopotential, while gradiometry is especially sensitive to the shortwavelength part.

The mass of a receiver unit was about 5.35 kg with a peak power demand of < 33 W. The GPS antenna had a mass of 0.49 kg. The receiver unit consisted of the following elements: RF/IF module, synch module, AGGA 2 module, processor module, power supply module and motherboard.

The SSTI instrument was based on the Lagrange architecture, a flight-proven device of Laben, Milan, Italy - a unit of Thales Alenia Space, Italy.