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© Addition of modulo 2

Figure 65.- Command acquisition signal.

After having initially established the subcarrier and B/S references, the detector entered the data mode and was ready to detect command information and provide it to the CCS. In going from its initial state to the data mode, the detector went through 9 of its 10 possible modes of operation. The normal modes of CDU operation, indicated in the MDS status word via telemetry, were out of lock, subcarrier lock, and B/S lock. These operating modes were observed (in real time or recorded by the V0 DSS for transmission later) when an Earth to VO uplink existed and the uplink signal consisted of rf carrier only, command subcarrier modulated carrier (idle 1), and command subcarrier modulo 2/bit-sync modulated subcarrier (idle 2), respectively. (See fig. 66.) In addition to these status indications described, the CDU oscillator frequency and the command data signal-to-noise ratio can be checked via telemetry whenever desired. As shown in figure 67, the oscillator frequency was counted down inside the CDU, and the resultant frequency was then counted by the FDS with a seven-stage binary counter for a period of 71.68 sec. The counter overflowed 917 times leaving an average count of 64.51 at the end of the count period. The sensitivity of this measurement was approximately 11.16 Hz (at 1.3 MHz) per DN. The countdown chain in the CDU was affected by the subcarrier tracking process. In order to obtain a valid measurement of the CDU oscillator frequency, the command data bits could not be present on the uplink. In addition, the stability of the timing source in the FDS from which the 71.68-sec gate was derived also influenced the accuracy of the CDU oscillator frequency measurement. To calibrate this effect, a measurement of the CDU oscillator frequency was made when the

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