Wavecom W61PC V7.5.0 - Manual (Page 223)

WAVECOM Decoder W61PC/LAN Manual V7.5 Transmission Modes  213
Ships located outside a circular or rectangular area will not receive a message addressed to these areas,
provided the Inmarsat C satellite terminal is connected to a GPS or similar navigational receiver, or the
ship's position has been recently updated manually. If the terminal is not connected to a navigational re-
ceiver, then it will receive all such messages. Ships will receive all messages addressed to a NAVAREA if
the user has entered the NAVAREA number into the terminal.
The printing of messages already received correctly is automatically suppressed.
Safety NET messages include a special header consisting of five "C" codes:
 C1 - priority code - 1 digit - distress, urgency, safety, routine
 C2 - service code - 2 digits - type of message broadcast
 C3 - address code - 12 characters - area broadcast instructions
 C4 - repetition rate - 2 digits - number and frequency of broadcasts
 C5 - presentation code - 2 digits - type of alphabet used
The National Geospatial-Intelligence Agency (NGA) maintains the Safety NET Users Handbook on the In-
marsat Homepage, and is available at no charge (http://www.inmarsat.com/Maritimesafety/snet.pdf).
Receiving Broadcasts
Most Inmarsat C terminals will not receive a safety broadcast if it is transmitting a message, or if it is
tuned to an Inmarsat ocean region not used for safety broadcasts in the area travelled. Most Safety NET
messages are rebroadcast after 6 minutes, to give a transmitting terminal time to receive missed messag-
es. Lists of Safety NET broadcast schedules and areas have been published by the JOINT WMO/IOC COM-
MISSION FOR OCEANOGRAPHY AND MARINE METEOROLOGY to assist ship operators tune Inmarsat C
terminals to the proper Inmarsat ocean region.
Although reception of Safety NET traffic is automatic, the shipboard operator must set up the receiver
properly at the start of the voyage:
 Select the appropriate broadcast channel. This can often be accomplished by logging on to a land
earth station in the ocean region for which needed broadcasts are made.
 Select the NAVAREA identification code.
 If travelling near Australia, select the proper coastal area codes.
 Ensure the Inmarsat C station is connected to a working navigational receiver. If a connection
cannot be made, the ship's position must be manually updated every four hours during the ship's
voyage. Without these updates, reams of unnecessary broadcast messages will be received.
Broadcast Channel Information
The modulation rate of the broadcast channel is 1200 Baud. Forward error correction is applied to this,
creating an effective information transfer of 600 bits per second (rate 1/2 convolutional coding with inter-
leaving is used to disperse error bursts which arise when deep fades are present). This ensures that there
is a high probability of receiving a message correctly at the first transmission, irrespective of the atmos-
pheric conditions or the ship's position within the satellite coverage.
Inmarsat Utilities
A copy of the latest version of the “Safety NET Users Manual” is available from the Inmarsat web site,
www.inmarsat.com/Maritimesafety/snet.pdf. . The “Inmarsat Maritime Communications Handbook” is no
longer available from Inmarsat, but maz be donloaded from other Internet sites.
STATUS OF MARITIME SAFETY INFORMATION BROADCASTS
NAVAREA NAV MET SAR OCEAN REGION
METAREA WARNINGS FORECASTS & WARN-
INGS
ALERTS FOR SCHEDULED BROAD-
CASTS
I (UK) X X X AOR-E
II (France) X X X AOR-E
III (Spain/Greece) X X X AOR-E
IV (USA) X X X AOR-W