Abstract
The Space Ground Link Subsystem provides the telemetry, tracking, and command (TT&C) functions compatible with the Air Force Satellite Control Network (AFSCN). The ranging signal is made up of a 1 mega-chip-per-second pseudo-random code, directly modulating the carrier signal using binary phase shift keyed (BPSK) modulation. The command signal is made up of three-tone frequency shift keyed signal amplitude modulation (AM). The telemetry signal is on a subcarrier, and all the three signals together phase modulate (PM) the carrier before transmission. Modulation indices in the phase modulator determine the distribution of total power among all the component signals, and it is very important to know the actual measured modulation indices to ensure link closure for the command, telemetry, and ranging signals. There are several methods used in practice but the measurement accuracy depends on the method used to measure them. In this paper, different methods used to determine the modulation indices — more specifically range modulation index (RMI) — in practice are discussed and various methods are compared. Sensitivity analysis of RMI measurement methods is also provided in the paper.
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