Yachting

GEAR TEST

MAY 2016 http://www.yachtingmonthly.com 67

PHOTOS: GRAHAM SNOOK/YM

is transmitted by a personal AIS
distress beacon, such as those
we tested in the September 2015
issue of YM.

Can I use my
existing GPS?
An AIS transponder determines
its position, speed and course
using its own GPS receiver.
This information can then be
combined with other navigation
data and automatically
communicated between AIS-
equipped vessels in range. By law
every AIS transponder must have
its own, dedicated GPS antenna,
to ensure it can’t be disconnected
by accident. Some have an
external antenna supplied with
the AIS unit, but it’s often an
optional extra, so when pricing
a system up, make sure to factor
it in. Some units have an internal
GPS antenna as well, although
you’ll need to consider where it
is installed to ensure that it gets
adequately good reception.
So, if you’re starting from
scratch with a new instrument

system, technically you could

feed the AIS’s GPS data into your

network or chartplotter via a

NMEA output from the AIS, but

it isn’t recommended. It’s better

to have a separate GPS receiver

for your plotter or network, for

security. What you’re not allowed

to do is feed your system’s

existing GPS signal into the AIS.

Some GPS antennae come

with a mini-FME plug on the

end; others have a full-size BNC

connector. If you want to keep

hole sizes to a minimum when

installing, or need to poke the

GPS cable down a narrow conduit,

then the smaller the connector the

better, as all GPS manufacturers

recommend not cutting the cable.

Unlike receive-only AIS devices

that only need a passive antenna

splitter, if you wish to use your

existing VHF radio antenna for

the AIS transponder as well as

the radio, you’ll need to install an

‘active’ antenna splitter. This is

not the same as a (much cheaper)

passive receiver-only splitter,

as it needs switch circuitry to

prevent the VHF radio and AIS

transponder from transmitting

simultaneously – and blowing

each other up in the process!

These will most often be an

optional extra (around £200),

as some people might prefer

to install a separate antenna

altogether for the AIS, so once

again you’ll need to budget for

one (or another antenna and

installation costs) if you need it.

All of the sets we tested have

at least one NMEA-0183 output

for direct connection to other

0183-compatible devices such

as a chart plotter or MFD. Many

now also have NMEA-2000 (N2K)

ports for connecting the device

to an N2K network. What we did

fi nd, however, is that very few of

the boxes contained an N2K drop

cable and T-connector, which are

essential for connecting it up.

These two items can cost £50 or

more, so yet another item you’ll

need to budget for if you want to

link the AIS to your N2K network.

Silent running

All of the units we tested offer a

silent mode, which halts all AIS

VHF transmissions. This is for

those who cruise in areas where

the risk of piracy might make

it sensible not to broadcast your

position and details of your boat

and crew to nearby vessels. This

mode can be activated either

in the software, if a laptop is

connected and the program

running, or usually there is a pair

of wires that can be connected to

a remote switch. Just one unit, the

Seapilot, had a ‘silent’ button on

the front panel.

Standard indication of silent

mode is a blue light on the

panel, but some did display a red

transmit LED instead, indicating

that the AIS was not

transmitting.

ÔWhat youÕre not allowed to do is feed your

systemÕs existing GPS signal into the AISÕ

PHOTOS: GRAHAM SNOOK/YM

ItÕs reassuring to

know that most

ships should be

able to see you in

poor visibility