Chapter 7 — COMMUNICATIONS EQUIPMENT AND- OPERATIONAL PROCEDURES 



FAA PROCEDURES 



Area circuit stations, from which weather 

 data is collected, communicate with the WMSC 

 (Weather Message Switching Center) over half- 

 duplex multipoint circuits. Data are collected 

 from stations on these circuits by use of a 

 polling procedure. The polling code is trans- 

 mitted by the WMSC activating only the polled 

 station's transmitter. If a reply is not received 

 within a specific time period, the next station 

 is polled. If a reply is received, the data is 

 stored for relay by the WMSC. To avoid ac- 

 cidental loss of the end of a message, the 

 WMSC waits a specified length of time after 

 the apparent completion of each station's trans- 

 mission before polling the next station. On 

 these polled circuits there are three collection 

 periods called scans. The first scan is the Al 

 Scan, which is polled at H+00 for scheduled data 



only. The only interruption to this scan would 

 be an urgent message in which the circuit is 

 manually broken into by the station transmitting 

 the urgent message. The next two scans are 

 the A 2 and A3 Scans which are for unscheduled 

 data at H+21 and H+39 respectively. 



All stations will be polled during the col- 

 lection periods. If a station does not respond 

 to a poll during a scheduled message collec- 

 tion period, the station is repolled at the end 

 of each polling cycle; if the station still does 

 not respond, a no-report message is generated 

 by the WMSC. 



Coding and tape preparation of the FAA cir- 

 cuits is too lengthy for discussion in this manual 

 and is not used by most military stations. For 

 further information on these procedures, refer 

 to the FAA Flight Services Manual 7110.10( ) 

 chapters 4 and 5. 



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