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Multiplexed scheme for Real Time Telemetry with Dwell and Playback in Small Satellites

CCSDS (Consultative Committee for Space Data Systems) Telemetry(TM)/ Tele-command(TC) standards have been widely used in the On-Board Computers (OBC) in order to standardize the interfaces and thus enabling the cross support for space missions. It is very important to know the health of the spacecraft after launch. The spacecraft is not visible continuously in IRS missions. Hence storage logic is implemented to store the telemetry data and stored data will be played back at a higher data rate on a different subcarrier during visible periods. Dwell data (high sampling) & Satellite position data also downloaded through this second subcarrier. As there are no subcarriers for telemetry in small satellites, downloading the stored data is not possible with single downlink. This single link is used for transmitting real time telemetry. Hence a novel scheme is adopted for downloading the real time telemetry along with playback data and dwell data by time division multiplexing at a higher data rate. The design also implements CCSDS format which is industry standard. A configurable design is provided to meet the different projects downlink data rate requirements. The challenges in this design were Synchronization between software and hardware for writing the telemetry data by processor and reading by hardware, FPGA design, and handshake between read and write operations of memory for simultaneous storage and playback, configurable download data rates. This paper addresses the need, design and implementation of multiplexed telemetry in small satellites where only one downlink channel is available.