TWSTT Range Measurement precision

A by-product of the measurement of the very precise time using the TWSTT method is that one can measure distances to communications satellites very precisely and accurately. The travel time of the Ku-band radio signals is the speed of light. For a first approximation, we could assume the speed of light is that in a vacuum, i.e. 299792458 meters/second. However, At Ku-band frequencies, the ionosphere is a significant propagation medium that the signal must pass through. This has a small but measurable effect. A small refraction (effectively increasing the path length of the signal) takes place while the signal passes through the Earth's ionosphere. The ionosphere can introduce one-way path delay errors of from several tens of picoseconds to over 800 picoseconds, equivalent to ~0.003 meter and ~0.25 meter respectively in distance traversed by the light.

Figure 1. This plot shows a measured example of the precision reached in the range measurements using the TWSTT method during low ionospheric electron column densities. The y-axis is in kilometers (km) and the x-axis is in seconds. The RMS of these 5-minute one-way range data is 0.000023-km (2.3 cm). The true accuracy of the method in a triangulating scheme should be of the order of 1 meter or better.