Command/Interrogate data are applied to the Interrogate Channel 

 Modulator which Manchester encodes the data and applies them to 

 a Phase Shift Keying (PSK) modulator. The modulator output 

 provides the input to the Interrogate Channel Frequency Control, 

 which supplies the CDA S-band transmitter with the Interrogate 

 Signal for transmission to the spacecraft transponder. The UHF 

 Command/ Interrogate signal is received at the CDA and applies to 

 the Interrogate Channel Data Demodulator which produces an error 

 signal for automatic frequency control and demodulates the 

 Interrogate signal for bit error rate performance testing. (See 

 Figure 3, page 19.) 



2.4.2 Time Data and Command/Interrogate Signal Format 



Details of the modulation and data contained within the Command/ 

 Interrogate signal are presented in paragraph 2.2.3. Since a 

 Command/ Interrogate signal is transmitted continuously, a 

 dummy address is utilized in those periods when the CDF is not 

 providing platform addresses for interrogation. In the absence 

 of any address identifiers from the CDF for transmission to the 

 sensor platform field, an idle pattern of 31 binary bits is 

 used (00110100100001011101100011111). These bits are Manchester 

 encoded and transmitted MSB first. 



2.4.3 Response Data Demodulation and Transmission to CDF 



After recovery of the response signals as described in paragraph 

 2.4.1, the response data are demodulated in the appropriate CDA 

 Channel Data Demodulator and held in buffer storage until the 

 entire response is received at the CDA. The response address 

 identifier is compared at the CDA with the expected platform 

 address, and the message characters are checked for parity errors, 

 The platform sensor response and any applicable error conditions 

 are then forwarded to the CDF for logging and dissemination, or 

 for special attention based on error conditions. 



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