And of course GPS is
not affected by the weather, so even in the
poorest visibility your GPS will still give you an accurate
Not only has GPS has improved the efficiency of navigation but it has
improved safety at sea by making it possible to give a precise position
rescuers if, god forbid anything should go wrong.
GPS was first developed for the US Department of Defence.
The system uses between 24 and 32 satellites, each of which orbits the
twice every day.
were launched in 1978, and a second-generation of
satellites was launched beginning in 1989 until the system became fully
operational in 1995.
Each satellite constantly transmits data about its position
receiver can use to calculate the reciever's position in relation to
Once the receiver has data from three satellites it can calculate an
of its position in Latitude and Longitude.
Add data form a fourth satellite and it can also calculate its altitude.
By storing and comparing the incoming data, the receiver can then
calculate such things as the speed and the course made good, time
positions, estimated arrival time at the next waypoint, heading and a
other useful information for the navigator.
Setting up the
newer marine GPS units is
becoming less of a chore than it used to be.
operating manuals do still tend
to appear to be written by geeks and often they are over complicated,
confusing instructions on some of the bells and whistles, which are
ever going to be used.
The main thing
you should get right is setting up the correct reference coordinate
to be set to the same
horizontal datum showing on your nautical charts.
vertical datum should be
coordinated with yourcharts
You may also
need to feed in the height of
your antenna above sea level.
charts show the depth in feet
or meters, having everything set to the same reference will avoid
proper horizontal datum will
improve the accuracy of the data provided by your GPS.
charts use either the
’NADB3’ datum (North America) or the ‘W0S84’ datum
(World Geodetic Survey).
Set your GPS to
the one you most commonly
use, some minor corrections may be necessary when plotting on the other
datum, however, providing the charts are modern ones, this correction
Beware, on older
charts the difference can be quite large depending on your
and longitude position shift
can be as much as 500 yards/meters, quite a lot if you are trying to
narrow passage in fog.
The best bet
for anyone using GPS is to
replace all those old charts
with modern up to date
"Radar seems to be on the
don’t panic!! The GPS says
there is clear water ahead, the fish finder says we have plenty of
depth, the iPad Ap says we are on course……."
Nowadays it is rare to
find any boater who doesn’t
have a marine GPS on board.
However, it shouldn’t be
that using a marine GPS is the answer
all your navigation problems.
But then the same could be said about the echo sounder, which is a must
boats these days.
in my home sailing area there is often a lot of silt being stirred
the tidal stream, which often fools the depth sounder.
And an echo sounder won’t show you nature of the seabed the way the
of tallow in the bottom of an old fashioned lead could.
they are both such great aids to navigation it would be
ignore them but they should not be relied upon entirely.
To use your GPS effectively you should learn about the system’s
limitations and how to make simple checks to ensure the GPS information
leading you in to danger.
It should always be
used with reference to an up-to-date nautical paper chart.
Accidents still occur where a boat’s crew have blindly followed their
instructions with out regard for or understanding of the limitations or
of faulty input of information.
In practice the main danger in using Marine GPS to navigate, is
by operator error.
It should always be backed up with a set of checks making use of
and your senses.
As with any navigational error they are especially dangerous when close
At sea it always pays to be just a bit sceptical.
Fortunately, as sets become more robust, it is becoming increasingly
to ever find ourselves in a situation where the marine GPS has suffered
And I sincerely hope that the system never has to be again shut down
military operations, as it was during the Kosovo and Iraqi conflicts.
"Setting the course, keeping the dead
reckoning up to date, and fixing the position by observations of the
bodies, call for nothing more than simple arithmetic, a little
some dexterity in handling a sextant."
I am perfectly aware that the majority of Wooden Boat aficionados are sensible folk. However, I need to point out that I am an amateur wooden boat enthusiast simply writing in order to try to help other amateur wooden boat enthusiasts. And while I take every care to ensure that the information in DIY Wood Boat.com is correct, anyone acting on the information on this website does so at their own risk.