What Communications? |
From time to time, I am asked questions about what equipment it is sensible or what do I advise to have on board a leisure vessel in order to receive weather forecasts and warnings. Weather itself is complicated and so are the current systems for delivering weather information, As I see it the various possible systems are as follows -
VHF Radio |
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| Used for communication of weather and other Marine Safety Information from MRCCs and MRSCs. Range is limited by the nature of the signal, effectively, to line of sight. Prone to blocking of the signal by high ground or large structures. VHF weather broadcasts are normally of inshore waters forecasts but may also include some forecasts for open sea areas. |
Broadcasts are usually in the local language although some countries, notably Spain, do have English language versions as well. With European Latin based languages, there is normally little problem in learning the basic terms. These can be found in the RYA weather forecast booklet, G5. A comprehensive list of French terms is on one of my pages . I have not managed to cope with Slav languages, but Croatia for example, broadcasts in English as well as Croat. |
NAVTEX |
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| A long wave system intended for use from the Fairway buoy out to about 200 NM. It can often be received further out. The technology is old, late 1940s or 50s, I think. Speeds are slow being determined by teleprinter technology.. Stations broadcast, in turn, in 10-minute slots over a 4-hour cycle. All stations use a common international frequency of 518 kHz. There are reception problems for three main reasons. First, if one station overruns its allotted time and a vessel is within range of the next station, then the two signals will interfere with each other. Secondly, such interference can also occur when there is high pressure and a low level temperature inversion. The ground wave signal travels further than normal and two stations broadcasting at the same time, normally far enough away from each other, will cause interference. Thirdly, at night, depending upon the state of the ionosphere, signals can bounce back down to earth some 1500 miles or more away. |
This sky wave can cause interference with a much nearer station whose signal is being received by ground wave. Because of local interference and screening signals may not be received in harbour. NAVTEX provides, as a minimum, twice a day, 24 hour forecasts with a brief outlook for a further 24 hours. The areas covered are sea areas as used in the BBC shipping forecast. for example. The French term is "bulletins au large". Gale warnings and cancellations are also broadcast. NAVTEX broadcasts on the 518 kHz frequency are always available in English and are always for open sea areas ie not inshore waters. The UK, exceptionally at present, uses a national NAVTEX frequency of 490 kHz for inshore waters forecasts. |
INMARSAT-C |
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This is, effectively, the open ocean version of NAVTEX. Transmissions are from geostationary satellites. This, in effect, limit reception to latitudes equator-wards of about 60 – 65 degrees. Equipment can be expensive but it can also be used to send (slowly) text messages and emails. |
The weather information provided is a 24 hour forecast and a brief outlook for a further 24 – 36 hours. Areas covered are for the open ocean, eg the UK North Atlantic bulletin. The French term is "bulletins au grand large". |
Radio Teletype |
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This is a HF/SSB (Single Side Band) long range radio system. RTTY is used by the German weather service, DWD and the US Coast Guard in their SITOR (Simplex Teletype over Radio). Again, the technology is old and speeds are slow – similar to the football scores that appear on TV. RTTY can be received using a HF radio with the SSB capability plus a computer and some software. For the DWD service, there is also the NASA Weatherman which is, in fact, a HF/SSB radio pre-tuned to 4 of the six DWD frequencies. For more, see my Radio Teletype Page.
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24-hour forecasts are too general to be of much use. DWD schedules can be found from a link on my GMDSS page.
There are also broadcasts of weather reports from ships and land stations. These are in a numerical coded form but, with a little practice, can easily be interpreted. There is some clever software that will interpret the data and even draw a chart for you on your computer screen. For an example, click here. But, be careful because the data are not quality controlled. The garbage in garbage out principle holds good. SITOR has been called a long range form of NAVTEX. It is used by the USCG to broadcast navigation and weather information over the Atlantic and Pacific oceans. It is used over the polar regions which are not covered by INMARSAT-C. |
Radio Facsimile |
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| This is another old system that is used for broadcasting weather charts. Transmissions are very slow and it can take 20 minutes to receive a chart. There are moves to replace Radio Fax by a satellite based system which may or may not be sufficiently user friendly to attract leisure sailors. Radio fax schedules can be found from links on my GMDSS page. | Although it can be useful, and I have found it so, it has been largely superseded by alternative delivery systems and techniques. These include Internet web pages and the use of email and GRIB coded products. |
National and Local Radio |
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| Use of national radio very obviously depends upon ability to comprehend the language. The BBC shipping forecast is, arguably, one of the most useful simply because it is updated on a 6-hourly basis. Those sailing in the Northern North Sea and Southern Baltic have said that the German Public service radio broadcasts in German are exceptionally clear and easy to understand. | Local radio, in the UK at least, is a variable feast. Much depends on the interest of the local station controller and staff. Schedules and content can vary at no or short notice. Not to be relied upon. |
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| For most of us this means a mobile, terrestrial system but it must be likely that satellite phones will become cheaper to buy and use in time. For terrestrial telephone systems, a serious limitation is distance from an aerial. These are likely to be placed to optimise use over land. Use over sea may not be possible even 5 miles out. Use at anchor will depend, again on aerial location and local topography.
Using a mobile phone, access is possible to the Internet for browsing or email. However, GSM phone bandwidth is not great enough for economical browsing, but ii is useful if you know a specific page address. See my GMDSS page for some useful pages giving text. One problem, even for pages of text is that there may be advertisements and drop downs which slow the downloads. This puts up the cost. A typical effective download speed is about 1 kb per second. A text forecast that should be capable of being downloaded in a few seconds can take a minute from a page with advertisements and logos. If downloads from these pages are too slow, the try the Saildocs web page text service. A GSM phone can be cost effective for information received by email and email attachments. The GRIB code service available at no charge via, for example, Saildocs, is a good example. For example, a download of less than 10 Kb can give 8 charts of forecast wind vectors at 2 degree lat/lon spacing over an area 20 degree x 30 degrees lat/lon.. |
Faster transmission/reception is possible using GPRS and browsing is easier but not cheap. With a pay as you go mobile phone, with international roaming, costs can be up to £10 per Mb of data This is about 1p per Kb ie about the same as with a GSM phone given normal speeds. The gain with GPRS is that there is no cost penalty if the link fails. With a contract phone and GPRS roaming, costs are about £10 per Mb. Which is the better buy depends on your usage. To put the numbers in the last two paragraphs into perspective, a synoptic chart is likely to be between 50 and 80 Kb. Download speeds can be greatly increased using a data compression package such as
Onspeed or MailASail. This reduces the size of downloads by a factor of around five times. For advice on setting up and using mobile phone and laptop to connect to the Internet, see the Mobile Phone set up page. |
So, what do I need? |
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Each of the above has its own role and there is really very little duplication. VHF and NAVTEX should be regarded as necessary because these are the systems put in place under the GMDSS for the dissemination of MSI. If you are a blue water sailor, then INMARSAT-C is the NAVTEX equivalent. However some long distance sailors rely upon RTTY/SITOR. Perhaps what we need is a low cost, receive only INMARSAT-C terminal. In a sense, of course, weather forecasts on ocean crossings are not vital. If bad weather is coming there is little that the average cruising sailor can do. His yacht is too slow to be able to avoid major storm systems. All that is necessary is sufficient warning to prepare the boat and crew for the worst. This may seem like heresy, but it is a view put forward by blue water sailors. For European/Mediterranean sailors the DWD RTTY is an extremely useful service, especially for those all important 5 day forecasts. It is NOT a NAVTEX replacement, as so many seem to think.
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© Frank Singleton, October 2005, amended December 2005
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