Design and Realization of Dual Frequency C-Band Telemetry Transmitter for Geostationary Spacecrafts

Criticality of Telemetry, Tracking and Command Subsystem in the space missions calls for dual redundancy of each of its elements at well separated spot frequencies. Increased number of geostationary missions along with dual redundancy philosophy resulted in scarcity of bandwidth for satellites to be co-located with existing ones in future. Some of the recent missions are allotted with the band of 4.53 - 4.55 GHz for on-orbit telemetry downlink where the regular slot of 4.18 - 4.20 GHz band is already in use. This paper describes the design of a telemetry transmitter package that contains both main and redundant units in a single mechanical housing. The design becomes challenging as the main and redundant frequencies are to be closely spaced within the allotted bandwidth. Each of them downlinks telemetry data to ground station on a phase modulated carrier signal. The carrier at 4.55 GHz is generated by a compact Step Recovery Diode based X13 frequency multiplier that obtains stable input from a Temperature Compensated Crystal Oscillator at 349 MHz. The design involves RF alignment and packaging strategies to ensure parallel operation of the two units without interference to each other and remaining subsystems of the spacecraft. The hardware test results analyzed at the end of the paper reflect the practical success of these strategies along with X13 frequency multiplication