Abstract
A new system’s geo-referencing from space is entirely free from any GNSS (GPS or equivalent) systems. The system addresses to various strategic and economic applications such as in remote clock synchronism, aircraft and balloon navigation, missile and smart bombs tracking, satellite orbital determination and remote target geo-positioning. The new geometry concept corresponds to an “inverted GPS” configuration, utilizing four ground-based reference stations, synchronized in time, installed at well known geodesic coordinates and a repeater in space, carried by an aircraft, balloon, satellite, etc. Signal transmitted by one of the reference bases is retransmitted by the transponder, received back by the four bases, producing four ranging measurements which are corrected for the time delays undergone in every retransmission. A minimization function was derived to compare the repeater’s positions referred to at least two groups of three reference bases, to correct for the signal transit time at the repeater and propagation delays, and consequently to provide the accurate repeater position for each time interaction. Once the repeater’s coordinates are known, the other determinations and applications become straightforward. The system solving algorithm and process performance has been demonstrated by simulations adopting a practical example with the transponder carried by an aircraft moving over bases and a target on the ground. Effects produced by reference clock synchronism uncertainties at the four bases on the measurements are reviewed.
Highlights
A very well known principle to determine indirectly the distance of a remote object utilizes the echo of a transmitted signal, which propagation speed is known
In practical applications, when applied to time reference marks transported by electromagnetic waves, the feasibility of the concept becomes critically dependent on the knowledge of temporal effects due to four principal causes: (a) the signal speed propagation the medium causing path length variations; (b) propagation time at instruments, cables and connectors at the transmission module; (c) propagation time at instruments, cables and connectors at the final reception modules; and (d) time of signal transit at the remote transponder, which distance is to be determined
To calculate the system errors on the remote clock synchronism, on the repeater’s position and on the target location caused by propagation, we adopted a Gaussian distribution of uncertainties set in the r.m.s. range of ±0.5 ns, which is attainable when the bases and target clocks are periodically synchronized, as for example every month
Summary
A very well known principle to determine indirectly the distance of a remote object utilizes the echo of a transmitted signal, which propagation speed is known. Doppler path change caused by frequency shifts in the direction of the repeater [8] [9] and of relativistic effects relative to the reference system containing the sites to which the distances are to be determined, which become further accentuated when the satellites move over distinct gravity potentials relative to the geoid [10]. They may be neglected for velocities the speed of light, because they produce effects much smaller in comparison to propagation and delays at the repeater. For accurate ranging measurements, it becomes essential requirements to know the precise knowledge of the path length changes due to propagation as well as time changes for each coded time signal interaction at the transponder, for applications in time synchronization, navigation and remote positioning
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