Correlation and Prediction of Propagation Time-Delays Along Earth-Space Links

摘要

High precision in radar detection, in earth-satellite orbit determination, and in satellite navigation necessitates that the signal data used be corrected for the errors imposed by the ionosphere. Signal time-delays, or equivalently range errors, are always encountered in transionospheric measurements because the electromagnetic propagation velocity in the medium is slightly less than the free-space velocity. For frequencies at VHF and above, an excess time delay is inversely proportional to the square of the frequency and is directly proportional to the integrated electron density along the propagation path (i.e., total electron content (TEC) measured in units of e1/m squared). Thus, if TEC is known, or is measured in real time, a perfect correction to ranging can be performed. The TEC may be measured in real time, provided the user has dual-frequency capabilities. Since the ionosphere is a dispersive medium, the relative time delays (or phase differences) between the two frequencies may be used to eliminate the error introduced by the TEC.