WHAT GALILEO CAN LEARN FROM THE SCIENTIFIC INVESTIGATIONS AND SOME OF THE CONTROVERSIES ON GPS AND RELATIVITY
Spallicci, A.
Universite d'Orleans

The GPS has represented an outstanding achievement of relativity since relativistic effects have been incorporated in the engineering design of the system. In other words, relativity was not under scientific test, but conversely a tool for a space project. Time dilation, red shift and Sagnac effects were important enough to be taken into account in the design and in the navigation message, but their measurements were not accurate enough for representing a scientific step forward. This peculiar situation created a distance between most of the scientific community in relativity and GPS, with few notable exceptions.
Furthermore, the GPS has been characterised by endless controversies and discussions on the validity of the relativistic corrections, few times within the scientific community but more often the arguments were generated outside, in the GPS community at large.
In this contribution we review, in light of the development of satellite navigation systems, the measurements that the scientific community proposed in the perspective of an improved GPS system. Among these: perigee advance, Shapiro time delay, preferred frames, Lense-Thirring and light bending. One of the most ambitious propositions has been the conception of the GPS constellation for detection of gravitational waves. The 42 (L1 and L2) space to ground links of the 21 satellites would provide continuous coverage and capability of source location and of polarisation identification. The limiting factors, clock stability, ionosphere deviations, ephemeris errors, Earth gravity model inaccuracies were too heavy for a concrete perspective. Conversely, the difficulty of conceiving a sensitive detector in the 1-10 Hz gravitational wave spectrum suggests pursuing analysis of close to Earth satellite constellation systems.
Finally, we address the necessary relativistic corrections to be adopted by navigation receivers in satellites.
A large bibliography of about thirty references is given.

- Spallicci A., Jimenez C., Prisco G., Ashby N., 1992. Relativistic effects for low earth orbit satellites using GPS, 6th European Frequency and Time forum, 17-19 March 1992 Noordwijk, J.J. Hunt Ed., ESA-SP-340, 61.