Digital Telemetry Transmitter in Crew Escape System

This article presents an efficient digital telemetry transmitter with Offset Quadrature Phase Shift Keying (OQPSK) modulation for launch vehicle applications with direct modulation in RF carrier. Analog transmitters with phase modulation are widely used for data transmission in launch vehicles. To gain the advantage of digital modulation like bandwidth efficiency and power efficiency, a digital telemetry transmitter with OQPSK modulation and half rate Turbo coding was flown in ISRO’s Human Space Programme Pad Abort Test Crew Escape System. In OQPSK, compared to conventional QPSK, the quadrature channel bits lag the in-phase channel bits by one bit period. This feature is an advantage of the OQPSK modulation technique as it overcomes the 180° phase transitions, eliminating the possibility of crossing through the origin. Thus it provides a narrow power spectrum with less inter-symbol interference. This transmitter is employed with direct modulation in S-band RF carrier as it provides a less complex and simple architecture compared to the conventional IF modulation and up-conversion techniques.