Navigation David Burch
The Real Value of GPS
GPS tells us where we are – our latitude and
longitude – but that is not always the most important
contribution it makes to our navigation.
This is especially obvious when we In other words, it is not the position where you are now, and it has a
happen to be in our home waters, in you get from the GPS that makes the computer processor in it with a clock
sight of our marina. In cases like this, device so valuable, or that makes it, and memory, so it knows where you
we can tell where we are by just looking let us say, uniquely valuable. What were a few seconds ago as well. Then
around, and if we want to know more, makes it uniquely valuable, essentially all it has to do is compute the direction
chances are the radar will tell us at a indispensable for sensible navigation, from where you were to where you are
glance more of what we want to now and that is your course over
know than the GPS will. ground. That is the true direction
Even when the GPS position you are moving, regardless of
is plotted on an electronic chart what the compass says. This is the
(using some form of ECS, electronic most important piece of navigation
charting system), the radar information you can have.
image can be more valuable. Our The GPS can also figure the
heading line on the radar screen SOG (speed over ground) by
tells us which way we are pointed dividing the distance between
and our relative position to the where you are now and where
shoreline is obvious at a glance. If you were a few seconds ago by the
there is any traffic in the area, then time between these two positions,
the radar shows right where they but strange as it may seem, the
are relative to our heading line. SOG is not nearly so crucial to safe
In the fog or at night, many navigation as the COG is.
navigators feel much more Knowing your COG is
comfortable from the radar image everything when it comes to good
than they do from a boat icon on an navigation. All good navigation
echart. The radar image is actually focuses on waypoints—even if
real data. You are seeing the results you do not have a single piece of
of real radar pulses bounding electronics on the boat! We design
off of real objects, whereas ECS a route as a sequence of judiciously
navigation takes a lot of faith that chosen waypoints, then we sail
the signals you are getting are from one waypoint to the next.
indeed coordinated properly with If you do not do more or less
the echarts being displayed. When just that, then you are not really
operating in tight quarters, you navigating. Following precisely
can be rather seriously in error, along your selected route is called
for example, if you do not have the good navigation; wandering off of it
chart datum in the GPS set to match the is the COG (course over ground) it is called poor navigation.
chart datum from the echart display. provides. We have lots of ways to know More specifically, as you pass one
Or what about sailing into a small where we are, but the GPS is the only waypoint you turn to the course that
bay to anchor that is not even shown on way to get instantaneous knowledge of your planned route tells you will take
the largest scale chart you have for the which way you are moving. you to the next way point. Then you
area. Radar position is then what you The GPS can figure this accurately steer that course as carefully as you can
go by. because it knows very accurately for a while and watch the COG on the
48° No r t h , Ja N u a r y 2010 Pa g e 66
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