character. If necessary binary data may be transmitted when formatted 

 into pseudo-ASCII characters, as shown in Figure 5. These special 

 requirements are needed to ensure that binary data are not mis- 

 interpreted as control characters, affecting communications link 

 operation . 



The sensor data must not contain certain ASCII characters that have 

 special control functions in the DCS dissemination system. These 

 prohibited characters are: DLE.NAK, SYN, ETB, CAN, GS , RS , SOH, STX, 

 ETX, ENQ, and ACK. EOT characters must appear only at the end of 

 transmission and will be deleted from the data prior to dissemina- 

 tion. Data characters containing parity errors will be replaced 

 with NUL or $, depending on the specified dissemination link. 



2.2.2 Sensor Platform Transmitter Response Format 



The platform response format satisfies the CDA requirements for 

 response signal acquisition, clock recovery, data bit synchroni- 

 zation, message synchronism, and optimum response channel 

 utilization. The data are Manchester encoded to provide a self- 

 clocking signal which is used to modulate the transmitted carrier 

 by phase shift keying (PSK). Figure 6 defines the coding and 

 modulation characteristics of the platform transmitter response 

 signal . 



A platform response transmission (Figure 7) begins with unmodulated 

 carrier (0° phase shift) for 5 seconds to allow the CDA data 

 demodulator to acquire the signal and establish a phase reference. 

 Next, the response signal is PSK'd with 2i seconds of alternate "one," 

 "zero" data (Manchester coded) so the data demodulator may obtain 

 the 100-Hz bit rate clock and data bit synchronization. Then the 

 46-bit preamble is sent, consisting of the 15-bit MLS message sync 

 word (100010011010111) followed by the 31-bit sensor platform address 

 identifier word with the most significant bit (MSB) of those 

 sequences transmitted first. The sensor data in serial ASCII 

 character (odd parity) format are sent immediately following the 

 preamble with the least-significant bit of each character transmitted 

 first. A postamble consisting of 3 8-bit EOT characters marks the 

 end of the response message. 



The CDF considers all the data in the platform response message 

 framed by the preamble and postamble to be sensor data characters in 

 ASCII. At the CDA, the received platform address identifier is 

 compared with the expected address identifier obtained from the CDF 

 computer, and the sensor data characters are tested for errors by 

 examining received character parity. Detected error conditions will 

 cause the appropriate error status to be transmitted along with the 

 sensor data to the CDF computer. If no response is received at the 

 CDA when one is expected, a "no message received" notice is sent 

 to the CDF computer and on to the user. 



2.2.3 Command/Interrogate Signal Format 



The signal transmitted from the CDA to the deployed sensor platform 

 field via the spacecraft transponder contains two information items: 



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