Deep space transponder

A transponder is a communication system placed on board a relay satellite, which receives and re-transmits the signal, often on different downlink and uplink frequencies. This system is activated when the ground stations are out of sight from the orbiting satellite or from the rover and the direct communication link is lost.

General Dynamics has worked with JPL to provide the spacecraft terminal for X- and Ka-band telecommunications with DSN. Making extensive use of the MMIC technology (for multipliers and amplifiers), multi-chip modules and a signal-processing ASIC, the SDST's flexible design provides the capability to meet the telecommunications needs of nearly every deep-space mission in a smaller, lighter and cheaper package compared to previous designs.

The transponder shows the next performances:

- X-band uplink: 7.145-7.235 GHz

- X-band downlink: 8.400-8.500 GHz

- X-band TX/RX ratio: 880/749

- Ka-band downlink: 31.800-32.300 GHz

- Ka-band TX/X-band RX Ratio: 3360/749

The X-band receiver shows the next performances:

- Noise figure: < 2.5 dB at 25°C

- Carrier tracking signal range: -70 to -156 dBm

- Carrier loop bandwidth (2-sided): 20 Hz nominal at threshold (expands to 200 Hz, strong signal). 

- Carrier loop damping factor: 0.5 at 0 dB loop S/N (Type 1, 2nd order loop) 

- Tracking range: > 200 kHz.

- Ranging filter type: 3-pole Chebyshev, other options being available

- Ranging filter bandwidth (3 dB) : 1700 kHz nominal, other options being available

- Temperature stability: about 3 ppm (-20°C to +60°C)