Subject : Satellite Communication and Networks

Topic : Earth Station

Marks : 5M

Use of multi-tone frequency in tracking system

Distance (Range) measurement is performed by means of specific sub-carriers which modulate the telecommand carrier, are coherently demodulated in the receiver and are then used to modulate the telemetry carrier The tone system is currently used. In this case, the sub-carrier is a sinusoidal wave of fixed frequency f. Measurement of the phase shift between the transmitted and received tones, which is a function of the distance R from the station to the satellite (a round-trip trajectory of 2R), enables this distance to be determined:

$$ \Delta \phi = 2 \pi f (2R) / c \space \text{ --- where c is the velocity of light} $$

A high frequency (100 kHz) is necessary to ensure accuracy of phase measurement; conversely the frequency must be low enough for the wavelength to be long enough with respect to the distance to be measured for there to be no ambiguity. The difficulty is avoided by transmitting two tones simultaneously; these are a major tone at 100 kHz which permits good measurement accuracy and a minor tone, obtained by division of the major tone (and hence in phase with it), which enables the ambiguity to be resolved.