ABSTRACT
Both the telemetry and the telecommand link require channels to carry their respective data transmissions. It is typical for these links to be on entirely separate channels with different data rates to match the needs of the respective sources. The most commonly used modulation formats for transmitting telemetry and telecommand data are Frequency Modulation (FM) and Phase Modulation (PM). When the modulator sends analog data using FM, the modulation format is not qualified further. This chapter covers FM and PM systems for transmitting analog data and Phase Shift Keying (PSK) and Frequency Shift Keying (FSK) systems for transmitting digital data. It also examines the trade-offs involved in choosing a modulation approach. The Signal-to-Noise Ratio (SNR) is an important criterion engineers use to determine the quality and performance of analog transmission systems. A normal way to express the metric is the ratio of the SNR on the demodulator's output to the SNR on the demodulator's input.
