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Briefing
European
inland radio rules (RAINWAT)
FMCW
Radar
NMEA
0183
Official
text of Collision Regulations
Official text of CEVNI Rules
Official text of RAINWAT
Regulations
European inland radio rules (RAINWAT)
Discrepancies between UK and
European rules about the use of VHF radio threaten to make life a
little more difficult and expensive for any UK boat-owners who intends
to venture into the European inland waterways.At the heart of the problem is a document known as the Regional Arrangement concerning the Radiotelephone Service on Inland Waterways – bizarrely abbreviated to RAINWAT – which was agreed by an international Quango almost ten years ago.
Like almost all such documents, the introduction to RAINWAT uses benevolent-sounding words like “harmonisation” and “contributing to safer navigation”. But – also like almost all such documents –it has nothing to do with harmonisation or safety. It boils down to nothing more than a host of rules and regulations that pander to the universal bureaucratic urge to monitor and control.
RAINWAT rules
The RAINWAT rules aren’t terribly onerous if you look at them in isolation: like the IMO and ITU rules that govern radio procedure world-wide, they require operators and radios to be licensed, they designate specific frequencies for specific purposes, and they impose limits on the power that can be transmitted.
The trouble is that the RAINWAT rules don’t quite match their IMO and ITU counterparts.
• DSC is virtually compulsory under EU type-approval rules but is illegal under RAINWAT
• The inter-ship channels (6,8, 72, and 77) are limited to 25 watts under ITU rules, but are restricted to 1 watt under RAINWAT
• Dual watch functions are illegal under RAINWAT
• Portable radios are limited to Channels 15 and 17
• And – presumably to make up for the ban on DSC – a feature known as “Automatic Transmitter Identification System” (ATIS) is compulsory. It’s a bit like DSC, in that ATIS automatically transmits a digitised identity signal. The big difference is that the ATIS squawk is transmitted every time you release the transmit button, and that it doesn’t actually achieve any of the potentially useful selective calling functions of DSC.
The bit about limiting hand-helds to two of the least useful channels is a bit of a pain, but technically, it’s very easy to conform to the RAINWAT rules.
Pretty well all radios produced this century already include software that allows them to switch between EU and US channel usage (such as Channel 80 being a single-frequency channel in the USA, but duplex in the rest of the world), so a software upgrade should be enough to allow them to switch to RAINWAT .
The vast majority can store and transmit digital identity signals, as part of their DSC facility. And a growing number already include an ATIS option, simply because it is easier and cheaper for the manufacturer to include it in every radio than to produce umpteen different radios to cope with the quirks and foibles of local regulations.
Ofcom's "solution"
But even if you go out and buy a shiny new radio, you still have to contend with Ofcom.
Ofcom has just announced its intention of joining the list of RAINWAT signatories. But it hasn’t done anything to smooth out the wrinkles where its rules conflict with RAINWAT.
In particular, there’s the glaring anomaly that ATIS radios are compulsory in European inland waterways, but are illegal on UK vessels in UK territorial waters.
Ofcom’s solution to the problem is:-
• If you don’t already have an MMSI, apply for one through the Ofcom Website .
• Write to Ofcom, giving the licence number, vessel name, the call sign and MMSI, formally requesting that they vary the Ship Radio Licence to allow ATIS and supply an ATIS number.
• Ensure that your letter specifically authorizes Ofcom to disclose licence details to the Belgisch Instituut voor postdiensten en telecommunicatie (BIPT, Ofcom’s Belgian counterpart) “for the purposes of safety and ATIS administration, including making the details available to RAINWAT member states”. (If you don’t, they will invoke the Data Protection Act to say that they can’t register your ATIS number, so they won’t vary the licence!)
• Ofcom will allocate an ATIS number (by putting an extra “9” on the front of your MMSI), will notify BIPT of the vessel details, and will issue you with a Notice of Variation.
• Print a copy of the RAINWAT Arrangement from http://www.rainwat.bipt.be and keep it and the Notice of Variation with the ship radio licence documents.
• Enter your new ATIS number into your radio.
Quite why all this palaver should be thought necessary, when it would be so easy to add a sentence to the existing online radio licence is anybody’s guess. I can’t help wondering, though, whether it is all part of the growing pressure to double the time, price, and complexity of our present SRC courses.
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FMCW Radar
The initials FMCW stand for
Frequency Modulated Continuous
Wave. They refer to a completely new kind of radar, introduced by
Navico in
November 2008.
Like
conventional marine radar, FMCW
radar transmits
microwave energy, and listens for the echoes that bounce back to its
antenna.
But instead of transmitting pulses that last less than 1 microsecond
each (1 millionth
of
a second) it transmits “sweeps” that last a millisecond each (one thousandth of
a second).
In a
conventional pulsed radar,
switching from short pulse
(about a tenth of a microsecond) to long pulse (about a microsecond)
packs more
energy into each pulse, so it significantly increases the range at
which any
given target will produce a discernible echo. The ultra long pulses of
FMCW
have a similar effect, except that instead of increasing the range, it is used to reduce the transmitter power. Navico claim that their 100mW (one tenth of a
Watt) FMCW radar has a nominal range of 24 miles – comparable with a
2kW
(2000watts) conventional radar.
In a
conventional radar, increasing
the pulse length from
short pulse to long pulse reduces the radar’s range
discrimination. On short pulse, it can separate objects that are about
15
metres apart, but on long pulse that increases to about 150metres.
On
that basis, the ultra-long pulses
of FMCW would reduce
its range discrimination to about 80 miles – which would make it almost
completely useless.
This
problem is overcome by
modulating the transmitter frequency, increasing it progressively (by
about 65MHz) throughout
each sweep, before dropping back to its starting frequency for the
beginning of
the next sweep.
This means that by the time
the echo arrives back at the
radar, the transmitted frequency has increased – so the received
frequency is
always lower than the transmitted frequency.
If the
echo has come from a target at
short range, the
transmitted frequency will not have increased very much, so the
difference will
be small. If the echo has come from a more distant target, the
difference will
be larger. In other words, it is frequency
difference (rather than time) that is used to measure of the range of
the
target.
Bearing
measurement is achieved in
exactly the same way as a
conventional pulsed radar – by using a directional antenna to measure
the
direction from which the echo returns.
- no minimum range
- clearer picture – improved range discrimination
- clearer picture – virtually immune to rain clutter and less susceptible to sea clutter
- instantly available – no warm-up
- user friendly – no tuning required
- low radiation
the
explosive burst of power from a pulsed radar transmitter would
quickly destroy its sensitive receiver. To prevent this, the receiver
of a
pulsed radar is disconnected from the antenna while each pulse is being
transmitted. This means that targets at close range (typically about 30
metres
on short range scales, up to several hundred metres at long range
scales) cannot be received. The
much lower transmitted power of an FMCW radar means that the receiver
needs no
such protection: instead, it has a separate antenna, and is always able
to
receive echoes. clearer picture (better discrimination)
a conventional radar’s range discrimination (its ability to separate two objects on the same bearing but at slightly different ranges) is limited by the length of its pulse. If the two targets are closer together than half the pulse length, the echoes that they produce will merge, and they will appear as one. To an FMCW radar, the beginning and end of each pulse/sweep is irrelevant: each target will produce an echo, and they can be distinguished from each other because they will be at different frequencies
clearer picture (less clutter)
clutter is caused by tiny echoes (from rain drops or ripples) merging together (as above) to create echoes that are strong enough to be detected. In FMCW radar, this cannot happen – though it may still detect larger waves – so it is virtually immune to rain clutter and less susceptible to sea clutter
instantly available – no warm-up
in a pulsed radar, a vacuum tube called a magnetron is required to create the microwaves. As part of the process, it has to be hot – and in order to prolong its life, it has to warm up gradually – usually over two or three minutes. The much lower transmitter power required for FMCW radar can be achieved by solid-state electronics, which require no warm-up.
user friendly
Navico claim that “no tuning is required”. As most current production radars have automatic tuning, this is hardly relevant – but the lack of clutter etc mean that the use of other user set-up controls is very much simpler
low power consumption
reducing the transmitted power by 99.95% should reduce the total power consumption! But an FMCW is transmitting for 20% of the time, compared with less than 0.1% for a pulsed radar, so the difference is not as great as one might expect. And the power consumption of the display is not affected. In total, Navico’s FMCW radar uses about 8watts less than a 2kW pulsed radar radar
low radiation
lower power means a reduced health hazard from the transmitted radiation. But small pulsed radars don’t present a significant health hazard either, unless you spend hours with you head virtually touching the radar. So this is a bit of a red herring.
NMEA 0183
Back in 1983, we had Decca
(well, some people did) but
for most of us, navigation still involved hand bearing compasses and
drawing
lines on charts. And as for on-board computing – well, even ashore, if
you
wanted a letter typed, you did it on a typewriter.
But over in America, the
National Marine Electronics
Association decided that it would be a neat idea if different
instruments could
communicate with each other. And what they came
up with was so good that a
quarter of a century later, we are still using it.
We’re still having problems with
it, too.
The main problem, nowadays, is
simply that there has
never been a standard colour coding to show which wire should connect
to what.
So although wiring a NMEA 0183 interface is less complicated than
fitting a
domestic plug, it is almost essential to look at the manual. It’s
certainly far
quicker, easier and cheaper than guesswork or trial and error.
Unfortunately the terminology
used by different
manufacturers varies!
Output may be labelled Out, Transmit or
TX
Input may be labelled In,
Receive, or RX
Positive may be labelled Positive, Signal, or +
Negative may be labelled Negative, Ground (or GRD) or -
The key points to remember are
that you must
·
connect output to input
·
connect positive to
positive and negative to negative
This (above) will achieve one-way communication between two instruments, But to achieve two way communication you have to connect a second pair of wires, following exactly the same rules but with the roles of talker and listener reversed (below):-
To save cabling however, some manufacturers make one wire
serve as a "common ground" (below).
The next complication is that
one talker can supply information
to several listeners (below). The basics of the connections are exactly
as they are for a
single
talker and listener, but it's important to make sure that there is only
one
talker in the system. If two devices are trying to talk simultaneously,
the
listeners will receive only a meaningless jumble.
There are two other requirements
for a standard
NMEA0183 interface. The first is to make sure the talker is actually
“talking”
– i.e. that it is transmitting NMEA 0183 data. Many GPS sets, for
instance,
leave the factory with their output ports switched off, so you have to
get into
the "setup" menu to switch them on.
The second is to check that each
listener is are
expecting the data that it is being sent. Some DSC radios, for
instance, look
for position and time data in a sentence that begins with the letters
GPRMC. The
GPS set, however, may be transmitting the same data in a sentence that
begins
with the letters GPGLL. If the radio’s little electronic brain hasn’t
been told
that it can pick up time and position data from GPGLL, it will listen
in vain.
The Five Rules of NMEA 0183
Interfacing
Only one talker per circuit
Output to Input
Positive to Positive
Ports open (i.e. switched on,
and set to transmit or receive NMEA0183)
Check (in their manuals) that the
listener can accept the sentences that
the talker is supplying.
The NMEA0183 code consists of groups of low-voltage electrical pulses, delivered in groups of eight, with each group representing a particular character – rather like a more sophisticated version of Morse Code. All the clever stuff is handled within the instruments themselves: all we have to do is ensure that there is a decent electrical connection between the equipment that is transmitting the information (called the "talker") and the one that is receiving it (the "listener”). The connection itself is just a pair of wires, so it’s just as robust and nearly as straightforward as the connection between a battery and a navigation light.
AIS and NMEA
Ordinary NMEA
0183 is
painfully slow, transmitting and receiving at just 4800 pulses per
second. For
the job for which it was originally intended, this is fast enough, even
though it
seems painfully slow, by modern standards.
But one
particular type of
instrument – AIS – usually uses a high-speed version of NMEA0183.
This means that
if you want to
connect an AIS receiver to something such as a chart plotter, the
receiving
instrument must be capable of handling the AIS sentence (called AIVDM)
and of
doing so at a “baud rate” of 38000 instead of 4800. Again, the key is
to read
the manual.