Redundant telemetry transmission path

1. A spacecraft comprising:

a signal source;

a commandable switch

a computer;

a TLM encoder coupled with the computer;

a telemetry (TLM) transmitter having a modulation input coupled by the commandable switch with the signal source and having a communicative coupling with the TLM encoder that excludes the commandable switch; and

in the absence of a failure of the TLM encoder, the TLM encoder encodes telemetry data received from the computer into a bitstream that is forwarded over the communicative coupling to the TLM transmitter for transmission over a downlink to a ground station;

the TLM transmitter is configured to receive an output from the signal source when the commandable switch is closed; and

the computer is configured to detect a failure of the TLM encoder, and, upon detecting the failure of the TLM encoder, encode TLM data bits onto a TLM downlink signal by actuating the commandable switch between a closed position and an open position, such that a signal from the signal source received at the modulation input by the TLM transmitter is temporally varied between an on condition and an off condition.

2. The spacecraft of claim 1 , wherein the signal source is a command (CMD) receiver, the CMD receiver having an output coupled by the commandable switch with the modulation input of the TLM transmitter.

3. The spacecraft of claim 2 , wherein the output from the CMD receiver results from demodulation of a modulated uplink carrier signal.

4. The spacecraft of claim 3 , wherein the modulated uplink carrier signal is a ranging signal.

5. The spacecraft of claim 2 , wherein the output from the CMD receiver received by the TLM transmitter is noise.

6. The spacecraft of claim 1 , wherein the commandable switch is solid state.

7. The spacecraft of claim 1 , wherein the computer includes at least a portion of the TLM encoder.

8. The spacecraft of claim 1 , wherein the TLM data bits are encoded at a rate between 8 and 200 bits per second.

9. The spacecraft of claim 1 , wherein the TLM transmitter modulates the output from the signal source received by the TLM transmitter onto an RF carrier using phase modulation or frequency modulation.

10. A method comprising:

encoding, with a computer on board a spacecraft, telemetry (TLM) data bits onto a TLM downlink signal by actuating a commandable switch, the commandable switch being disposed between and communicatively coupled with a signal source and a telemetry (TLM) transmitter of the spacecraft; and

transmitting, with the TLM transmitter, the encoded TLM downlink from the spacecraft to a ground station, wherein:

the TLM transmitter is configured to receive an output from the signal source when the commandable switch is closed;

the computer is configured to actuate the commandable switch between a closed position and an open position, such that a signal received by the TLM transmitter is temporally varied between an on condition and an off condition;

a TLM encoder is coupled with the computer and has a communicative coupling with the TLM transmitter that excludes the commandable switch, the TLM encoder being configured to encode telemetry data received from the computer into a bitstream that is forwarded to the TLM transmitter for transmission over a downlink to the ground station; and

the encoding the TLM data bits onto the TLM downlink signal by actuating the commandable switch is performed only after detecting a failure of the TLM encoder.

11. The method of claim 10 , wherein the signal source is a command (CMD) receiver, the CMD receiver having an output coupled by the commandable switch with the modulation input of the TLM transmitter.

12. The method of claim 11 , wherein the output from the CMD receiver results from demodulation of a modulated uplink carrier signal.

13. The method of claim 12 , wherein the modulated uplink carrier signal is a ranging signal.

14. The method of claim 11 , wherein the output from the CMD receiver received by the TLM transmitter is noise.

15. The method of claim 10 , further comprising modulating the output from the signal source received by the TLM transmitter onto an RF carrier using phase modulation or frequency modulation.

16. A non-transitory computer readable medium having software stored thereon, the software including instructions for causing a computer on board a spacecraft to:

encode telemetry (TLM) data bits onto a TLM downlink signal by actuating a commandable switch, the commandable switch being disposed between and communicatively coupled with a signal source and a TLM transmitter of the spacecraft; wherein

the TLM transmitter is configured to receive an output from the signal source when the commandable switch is closed and to transmit the encoded TLM downlink from the spacecraft to a ground station;

the computer is configured to actuate the commandable switch between a closed position and an open position, such that a signal received by the TLM transmitter is temporally varied between an on condition and an off condition;

a TLM encoder is coupled with the computer and has a communicative coupling with the TLM transmitter that excludes the commandable switch, the TLM encoder being configured to encode telemetry data received from the computer into a bitstream that is forwarded to the TLM transmitter for transmission over a downlink to the ground station; and

the encoding the TLM data bits onto the TLM downlink signal by actuating the commandable switch is performed only after detecting a failure of the TLM encoder.

17. The computer readable medium of claim 16 , wherein the signal source is a command (CMD) receiver, the CMD receiver having an output coupled by the commandable switch with the modulation input of the TLM transmitter.

18. The method of claim 17 , wherein the output from the command receiver results from demodulation of a modulated uplink carrier signal.

19. The method of claim 18 wherein the modulated uplink carrier signal is a ranging signal.