Automatic weather serviceJan 1, 2003
From Ocean Navigator #122 May/June 2002
Navtex receivers with LCD screens, like the ICS Nav6 from Northern Airborne Technology, display current messages and store previous messages in memory so they can be recalled if necessary.
Worldwide broadcasts on the main frequency are in English, while an alternative frequency allows for more localized, national forecasts and urgent marine safety warnings in local languages. And because the system is one of the primary components of the Global Maritime Distress and Safety System (GMDSS — certain types of commercial vessels are required to carry GMDSS gear), it is being supported by both new radio receiver products and new broadcast stations worldwide.
GMDSS requires an automatic, unattended system to receive and store navigational and meteorological warnings. Paper roll-type Navtex receivers can fulfill this requirement, although there are now Navtex units that store the messages in RAM as they are received. Users can then scroll the messages on the unit's LCD screen.
Navtex is limited in its coverage, however, and will not provide information for most ocean crossings, except when within approximately 400 miles of land. And many areas of the world have no Navtex broadcasts at all. Another disadvantage is that reception in port, when surrounded by blocking buildings and lots of metal objects, like other masts, is likely to be poor to non-existent. Most manufacturers claim a maximum effective range of around 200 to 400 nm.
Many yachts now sail with multiple weather-reception systems. It is not uncommon to see nav stations crowded with a VHF radio for near-shore (within 25 to 30 miles) weather broadcasts, an SSB radio for voice reception offshore, a PC hooked to the SSB to receive weatherfax charts, and possibly another software program hooked to a different receiver and antenna getting weather satellite broadcasts. Not to forget the option of getting weather via Inmarsat or other satellite communication systems. With all of this onboard complexity, the simplicity and low cost of Navtex makes an appealing backup system or a primary system for navigators staying within the Navtex coverage area.
Broadcasting primarily on medium frequency 518 kHz, Navtex has a range of approximately 400 nm offshore, according to the U.S. Coast Guard. In the United States, coverage is "reasonably continuous," again, according to the Coast Guard, along the East, West and Gulf coasts, as well as the area around Kodiak, Alaska; Guam; and Puerto Rico. The U.S. stations don't cover the Great Lakes, but there is Canadian Coast Guard coverage.
There are some coverage gaps in the United States, primarily in the Southeast, and the stations must broadcast on different schedules to avoid interference.
An alternative frequency of 490 kHz has also been approved for Navtex. It is used by countries to broadcast more localized forecasts in national languages (notably in Europe). A worldwide list of Navtex broadcasts is available at www.hffax.de.
In addition, broadcasts have been approved on 4209.5 kHz, which could provide greater range, but at press time, the U.S. Coast Guard had only begun to test this option on an experimental basis.Coastal and offshore forecasts
Within the United States the forecasts are for coastal and offshore waters, with the emphasis on the zone beyond 20 nm from shore. The Miami and Puerto Rico stations cover portions of the southwest North Atlantic and the Caribbean Sea, including the waters of the Bahamas. Navtex can be a real godsend in the Bahamas, where reliable VHF weather is hard to receive or non-existent in most areas, and infrequent SSB forecasts tend to occur at inconvenient times. To read examples of current Navtex broadcasts on the Internet, go to: http://18.104.22.168/om/marine/navprod.htm.
Navtex broadcasts vary from country to country, with some offering only minimal weather information, while others offer extensive broadcasts. In addition to weather, a wealth of navigational and other warnings will come over the airwaves. It may raise anxiety onboard to know that containers have been lost overboard or lights have gone out, but at least there is some hope of taking evasive or preventative action. And the new LCD screen units mean that paper isn't wasted on navigational warnings that may be of little significance to your vessel.
In use, receivers will begin reception with a start of message function, ZCZC, followed by four B characters. The B1 character identifies the station being received; the B2 subject indicator follows, and the B3 and B4 characters form the consecutive number of the message from that station. The message will follow, including the time of transmission, the area covered, the station broadcasting and the subject. The message will be ended with the NNNN characters.
You can program your receiver to accept messages from selected B1 identifiers only, and you can select which B2 subjects are included. However, you can't exclude critical subjects A, B and D.Equipment
One of the beauties of Navtex is that it requires relatively inexpensive and simple equipment. Dedicated receivers can be had for around $300 to $400, including compact antennas that can be mounted near deck level or on a stern pulpit. Power consumption can be down around the 40-mA range (12 volts), allowing continuous monitoring of all broadcasts.
The latest units utilize LCD displays, eliminating the need for special printing paper, which might be difficult to obtain. Receivers in this category include the Furuno NX300, the ICS Nav6 and Nav6plus, available in the United States from Northern Airborne Technology, and the NASA Clipper, Target and Target Pro-Plus from British. company NASA Marine. The Furuno has a 30,000-character memory for message storage, and stations can be selected automatically based upon the vessel's position when using a GPS/DGPS interface in NMEA 0183 format. Messages can be printed out using a PC interface.
The NASA cylindrical antenna is only about 12 inches tall and can be rail-mounted. Furuno offers two different antennas: the NX-3H, which looks like a 4.6-inch-high mushroom, and the NX-3E, which is only 3.9 inches high but can utilize an optional 47-inch whip antenna.
Furuno also makes a compact unit using a paper roll, the NX 500, and JRC makes the NCR-300A using paper rolls. These units are more expensive than the LCD models, costing around $1,000 to $1,400.
Those thinking of heading abroad might want to consider one of the European units from ICS and NASA that can receive on both 518 kHz and 490 kHz. The ICS Nav6plus can receive both frequencies simultaneously, improving the odds of receiving forecasts throughout Europe and the Mediterranean. ICS claims that 25 percent of the world's commercial ships use the company's Navtex units, and many countries use ICS systems for broadcasting Navtex messages.
Another way to go for those who already have a receiver capable of tuning in to the proper bands is to get a software package that can interpret the broadcasts, allowing the text to be displayed on a standard laptop or PC.
Software Systems Consulting offers weatherfax programs selling around the $200 mark, and they also have the capability to decode Navtex broadcasts. Transas Marine offers something called the Navtex Manager. This software package can display the text messages on a PC and extract latitude and longitude positions to plot them on an electronic chart of the area using special symbols. Naturally, this feature has to be used with one of Transas' electronic charting programs. The manager also sorts and stores the messages in a database. The software packages require an interface cable linking the PC to your receiving radio.Interpretation
Those used to deciphering the meaning of the U.S. SSB forecasts broadcast by the U.S. Coast Guard will have no difficulty using the Navtex version, which is very similar.
It is handy to have downloaded a weather chart showing the various areas described in the forecasts. Where's the Hague Line? What does the southwest North Atlantic include? The U.S. Coast Guard website has helpful maps showing the areas described in these forecasts: http://22.214.171.124/om/marine/zone/ wrdoffmz.htm.
Another invaluable tool to have is a very small-scale (large-area) chart of the whole region. Geographically large features, like cold fronts and high-pressure areas, can be plotted or at least noted. You might not want to mark up your navigation charts with this information, so it is helpful to carry copies of the government's hurricane tracking chart onboard for this purpose. The National Hurricane Center has downloadable tracking charts on its site: www.nhc.noaa.gov.
One advantage Navtex has over weatherfax is that the detailed interpretation of weather features is done for you by professional meteorologists. There is much use of and discussion surrounding weatherfax in today's voyaging community, but it is wise to also heed the advice of those who do this for a living. Obviously, the best system is to use text forecasts together with fax charts, but I prefer to defer to the experts when in doubt. Particularly when weather systems are progressing over land before heading out to sea, land-based meteorologists will have access to a lot of data on expected locations, strengths and potential dangers of major weather features that would not necessarily be apparent when using weatherfax maps alone.
Another great boon of Navtex is the ability to receive forecasts at frequent intervals (usually every four hours per station) and at all times of day. This can be a real plus when trying to update information from the last weatherfax chart, which may be as much as 12 hours old. The receivers don't take any handholding when all this is happening, so a sudden need to rush up on deck and reef the mainsail doesn't wipe out the hope of an updated forecast, as is so often the case with both voice and fax broadcasts.
Navtex is simple, reliable and a modest investment that creates redundant weather capabilities for contemporary voyagers.
Sailor and freelance writer John Kettlewell, along with his wife Leslie, authored the Intracoastal Waterway Chartbook. See his website at www.kettlewellpublishing.com.