Chapter 7 — COMMUNICATIONS EQUIPMENT AND OPERATIONAL PROCEDURES 



condensed form certain frequently sent orders, 

 instructions, requests, and the like, relating to 

 communications. Among the most important of 

 the prosigns are the precedence prosigns. (See 

 table 7-1.) 



Date-Time Groups 



All naval personnel whose work brings them 

 in contact with communications must have a 

 thorough knowledge of the communication's use 

 of time. 



The approved method of expressing time in 

 the 24-hour system is with hours and minutes 

 expressed as a four-digit group. The first two 

 figures of a group denote the hour; the second 

 two, minutes. Thus, 6:30 a.m. becomes 0630; 

 noon is 1200; and 6:30 p.m. is 1830. Midnight 

 is expressed as 0000 — never as 2400 — and 1 

 minute past midnight becomes 0001. The time 

 designation 1327Z shows that it is 27 minutes 

 past 1:00 p.m„ GMT. Numbers are prefixed to 

 the time to indicate the day of the month; in 

 other words, to form a date-time group (DTG). 

 The DTG 171 327 Z Nov 70 means the 17th day of 

 November 1970 plus the time in GMT. Dates 

 from the 1st to the 9th of the month are preceded 

 by the numeral 0. 



A date-time group is assigned to a message 

 by the message center at the time a message 

 is prepared for transmission. For standardiza- 

 tion, time expressed by a date-time group nor- 

 mally is GMT. The date-time group in a message 

 heading serves two purposes: It indicates the 

 time of origin of the message, and it pro- 

 vides an easy means of referring to the message. 



In addition to the external DTG, an encrypted 

 message has a DTG buried within the text. 

 This time is called the true date-time group 

 (TDTG) and it is inserted by the cryptocenter. 

 The TDTG is used when referring to a message 

 that has been encrypted. 



The DTG assigned to a general message 

 always has a slant sign (/) and additional digits 

 added to the DTG. Additional digits represent 

 the general message sequential serial number. 

 Example: 102347Z/35 Nov. 70. 



Time Conversion Table 



The time conversion table (Table 7-2) is 

 useful for converting time in one zone to time 



in any other zone. Vertical columns indicate 

 time zones. Zone Z is GMT. Time in each 

 successive zone to the right of zone Z is 1 

 hour later, and to the left of zone Z is 1 hour 

 earlier. Time in each successive shaded area 

 to the right is 1 day (24 hours) later; to the 

 left it is 1 day (24 hours) earlier. 



To calculate time in zone U when it is 0500 

 hours in zone I, for example, proceed as fol- 

 lows: find 0500 in column I and locate the time 

 (1200) in the corresponding line in column U. 

 Inasmuch as 1200 is not in the shaded area, 

 the time is 1200 hours yesterday. 



Weather Message Heading 



Most weather messages have a heading pre- 

 ceding the weather data enabling the Aerog- 

 rapher's Mate to identify the type and origin 

 of the data received at a glance. Normally, 

 the heading consists of four letters; for example, 

 SAUS. The first two letters identify the type 

 of weather data and the remaining two the 

 geographical area in which the data originated. 

 In the preceding example, the letters SA in- 

 dicate the message is a surface airways hourly 

 report and the letters US indicate it is from 

 a station in the United States. 



A complete discussion on the many different 

 types of weather message headings is covered 

 in chapter 10 of this manual. 



Standard Subject Identification 

 Code (SSIC) 



Computer scanning devices are now being used 

 by the Navy Communications System to increase 

 the speed of handling naval messages. With 

 the introduction of these scanning devices it 

 became necessary to devise a means of rapid 

 identification for message subject matter. The 

 result is that messages now contain a code which 

 is assigned by the originator and referred to 

 as the Standard Subject Identification Code or 

 more commonly the SSIC. 



The SSICs consist of two slants (//), the 

 letter "N", a five digit identification code, and 

 then two more slants. The SSIC is inserted into 

 the message following the security classification 

 as follows: 



FM NWSED MEMPHIS 



TO DIRNAVOCEANMET WASH DC 



UNCLAS //N03140// 



etc. 



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