Abstract
Subcarrier modulation, along with direct modulation, is typically used in telemetry, tracking, and command (TT&C) systems. The Space Ground Link Subsystem is an example of the TT&C system using a subcarrier and direct modulation together in a phase modulator. In this system, telemetry is placed on a 1.7 MHz subcarrier, whereas ranging and commanding are modulated onto a carrier without a subcarrier. The ranging signal is made up of a 1 megachip-per-second pseudorandom code, directly modulating the carrier signal using binary phase shift keyed modulation (BPSK). The command signal is made up of amplitude modulated (AM) 3-tone frequency shift keyed signal. All three signals together phase-modulate (PM) the carrier before transmission. In a phase-modulated system, power in each of the services-command, telemetry and ranging-is apportioned from the total available power based on the modulation indices used. The performance of each of the services is dependent on the amount of power in those services. It is important to measure the modulation indices accurately so that the required level of performance can be achieved. In this paper the issues related to the measurement of command and telemetry modulation indices are addressed and the link sensitivity to the measurement errors in modulation indices is also discussed.
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