(1) Platform address identifier (or command word) which 

 initiates the appropriate response activity from 

 the platform to which it corresponds. 



(2) Coded time data, updated at 4-minute intervals, 

 referred to an NBS source. 



The CDA equipment which generates the Interrogate signal transmits 

 a different frequency for each spacecraft; a sensor platform receiver 

 must be equipped to receive the specific signal frequency trans- 

 mitted via the spacecraft serving its site. Currently, iDoth space- 

 craft transmit identical address identifiers. The Interrogate signal 

 modulation characteristics are those shown previously in Figure 6. 



The Interrogate message is formatted into blocks of 50 bits, as 

 follows : 



(1) 4 bits of time-code data. 



(2) 15-bit MLS for message sync; same as used 

 in platform reply (paragraph 2.2.2). 



(3) 31-bit address identifier. 



Message timing is derived from the CDA station atomic clocks which 

 are maintained by NBS to within a few microseconds of the master 

 NBS clock in Boulder, Colorado. As an approximate accommodation of 

 the satellite transmission path time delay, the time signals as 

 provided to the CDA Interrogate signal generating equipment are 

 advanced 260,000 microseconds. The leading edge of the first bit 

 of each message block coincides with the CDA station standard's 

 J-second mark. 



A complete time-of-year data record is transmitted piecemeal in 

 the time code bits every 30 seconds, beginning on the minute-and 

 4-minute marks. Figure 8 shows the format of the Interrogate 

 message block and the time code record. 



The serial bit transmission sequence is shown in real-time reading 

 from left to right. In all message segments the LSB is sent first. 



The format of the time-of-year data record is as follows: 



(1) Sync word, 40 bits in length, consisting of ten 4-bit 

 time code characters which designate whether the trans- 

 mission began on the minute-or |-minute mark. (1010 

 character for 1-minute records, 0101 for i-minute records; 

 LSB sent first). 



(2) Time-of-year word as eight 4-bit time code characters with 

 the hexadecimal value of each character representing a 

 decimal digit of the sequence; seconds (tens); minutes (units), 

 (tens); hours (units), (tens); days (units), (tens), (hundreds), 

 of Coordinated Universal Time (UTC). 



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