Info

• Heritage

Subjective (Sec. 20.4) Measured in terms of previous spaceflight experience

In the "Other" category, heritage is important when the schedule and budget are tight and technology risk must be low. Typically, we measure heritage in terms of previous spaceflight experience. A lot of communication hardware meets all three selection criteria. A typical SGLS transponder (1) can close the links to the ground stations with an acceptable bit error rate, (2) is compatible with the ground stations in the SGLS, and (3) has flown before.

Figure 11-4 diagrams a generic TT&C subsystem. A typical subsystem contains two transponders (for redundancy). The transmitter (downlink) path is as follows. From the left side of Fig. 11-4, two digital bit streams enter the transponder. One enters either from data storage or real-time from the payload. The other stream comes from command and data handling with telemetry data on health and status. These two data streams are modulated onto subcamers, which are then modulated onto the carrier output The composite signal is then amplified and routed out of the transponder and through a low-pass filter. The filter reduces second and higher-order harmonics to decrease frequency spurs and intermodulation products from the spacecraft's receiver. From the filter, the composite signal travels through a band-reject filter, a double-pole, double-throw (transfer or 2P2T) RF switch; a diplexer, and finally to the antenna where it radiates to the ground station. The band-reject filter is a notch filter that attenuates frequencies coming from the transmitter and falling within the receiver's pass band. This filtering action further isolates the transmitter and receiver signals. The RF switch selects transmitter A or B and antenna A or B. The diplexer allows a transmitter and receiver to share the same antenna. It also isolates the transmitter from the receiver port at the receiver's center frequency, so the transmitter doesn't lock, jam, or damage the receiver.

Power

Storage Tim

C&DH

C&DH

Cmds

Transmitter Transponder A Receiver

C&DH

C&DH

Cmds

Power

Transmitter Transponder A Receiver

Low-Pass

0 0

Post a comment