GPS Basics
by Don Casey
If you are thinking about buying a GPS-or you just bought one-and operating
it seems uncomfortably complicated, my advice is relax. Today's GPS
units are easier to operate than a VCR.
Getting
Started
With a handheld GPS receiver, getting started is as simple as inserting
batteries and switching the unit on. Setting up a fixed mount unit is
a little more involved, but not that much. Mounting is simply a matter
of selecting a convenient location and screwing or bolting the bracket
in place. Position the receiver on the bracket and make the power connections-positive
(red wire) to positive and negative (yellow or black wire) to negative.
Be sure the positive side of the circuit includes a fuse of the specified
amperage.
If the receiver has a built-in antenna, it is ready to use once you
make the power connections. However, receivers with internal antennas
are intended for installation in an open boat or on a flybridge. If
you have a hardtop or the unit is installed below, you need a remote
antenna with a clear view of the sky. Since a GPS antenna is "looking"
up, there is no need to mount it high (nor is this desirable, since
a high mount location will cause course and speed variations when the
boat rolls). Typical locations are the stern rail, radar arch, or T-top
frame. Route the antenna cable to the receiver and plug it in. Now you
are ready to operate the receiver.
Initializing
When you turn on a GPS receiver, it begins to receive signals from all
satellites in its view. It takes the machine a few minutes to identify
and lock on to the satellites that will give the best fix for your location.
If you are turning the unit on for the first time, or if it has been
moved several hundred miles since it was last on, you can help the unit
to get its bearings by telling it approximately where you are. Different
units provide different methods for doing this, but essentially you
will select your general location-USA-NY, for example-from a scroll-down
menu on the display.
Often the display will show some kind of status screen while the receiver
is collecting data from the satellites. Each satellite has a 30-second
data transmission that must be collected by the receiver, so each time
you turn on the receiver, expect it to take at least that long to get
a fix. When it does, it will automatically display the latitude and
longitude for your current position. If you have purchased a cartographic
GPS, it will also show a chart with your position plotted-provided you
have the appropriate electronic chart cartridge for your location. Bear
in mind that GPS charts are only an aid to navigation. They are designed
to facilitate the use of authorized government charts, not replace them.
Press the POWER button and wait. That is all you have to do to get your
position (latitude and longitude) from virtually any GPS unit. And if
you are so inclined, you can stop right there and do the rest of your
navigation with a chart and a pencil. However, GPS receivers have an
additional ability, the ability to remember a specific location and
to tell you at any time how far you are from that point and what direction
you must travel to get there. Such "remembered" positions are known
as waypoints.
Waypoints
If you have ever used a Loran-C receiver, you already know about waypoints.
A fisherman finding a productive hole pushes the STORE button on the
Loran and the location is written into the unit's memory. Next week,
next month, or next year the machine will faithfully guide the fisherman
back to within a few feet of the same spot. GPS waypoints are the same,
except that they are recorded as latitude and longitude instead of TDs.
If you don't know what a TD is, that's OK; you don't need to.
Again, the mechanism is different for different receivers, but in general,
saving a waypoint is as easy as pushing the MARK button on the receiver.
Let's take a hypothetical trip to see how this works. Before we leave
the marina we turn on the GPS. When it has a fix, we press MARK to save
our "home" position. The receiver gives the waypoint a numerical designation-001
if this is our first waypoint-but we can use the receiver's keypad to
name it HOME if we want to. Underway, we follow a marked channel, and
at the final buoy we press MARK again, naming this waypoint BUOY1A.
We proceed to a position east of a shoal marker and enter this location
as SHOAL. We make three more course changes to bypass islands and shallows,
pressing the MARK button each time, and giving the waypoints names corresponding
to the chart-CAPE, NTIP, and OYSTER, let's say. We record one final
waypoint when we reach our destination.
Now, when we are ready to go home, we press the GOTO button on the receiver
and it displays a list of waypoints. We select OYSTER, and the display
shows us the course (bearing) and distance to this last waypoint on
our outward track. As we travel back toward that waypoint, the receiver
continually updates the bearing and distance, and typically indicates
what course correction is necessary to compensate for current or leeway.
When we reach OYSTER, the receiver notifies us with an audible alarm.
We select NTIP as our next destination and alter course accordingly.
However, we don't normally need to select each waypoint individually
when we are retracing an outward track. GPS receivers typically allow
waypoints to be saved in a specific sequence as a route, and when we
follow that route-or its inverse-as we reach each waypoint, the display
automatically brings up the next one on our route. Some units even record
temporary waypoints automatically, allowing you to follow the same route
home without the necessity of you saving waypoints at all.
Planning
a route
You don't have to be at a location to mark it as a waypoint. You can
plan out your course on a chart and, using the receiver's keypad, manually
input the latitude and longitude of each course change into the unit
as waypoints. The receiver will then not only tell you where you are
at any given moment, but it will tell you the course to steer, taking
you in sequence to the waypoints you have entered. Keep in mind, however,
that the GPS is unaware of any hazards on your course. If you enter
two sequential waypoints on opposite sides of an island, for example,
the course provided by the GPS will be directly across the island.
Accuracy
The accuracy of a GPS fix is usually pegged at around 100 meters. That
is plenty good enough to get you in sight of a particular marker, but
it isn't good enough to attempt to enter a narrow channel in fog or
dark based on a GPS position. Differential GPS is a technique that uses
a land-based receiver to measure the error and transmit corrections
to your DGPS-equipped receiver. A DGPS-equipped receiver has an accuracy
of around 10 meters-30 feet.
Turning on the unit and getting a fix is simple. Recording waypoints
while you are underway is also easy. Getting the receiver to tell you
the bearing and distance to any waypoint requires only that you push
the right button or two. And while entering latitudes and longitudes
manually and constructing routes is not particularly intuitive with
all GPS receivers, after you do it once you'll wonder what all the fuss
was about. At that point, you will also have a complete grasp of all
the functions you are likely to use regularly. You can experiment with
additional functions at your leisure-or not.
For more
information about making the electrical connections, consult Sailboat
Electrics Simplified by Don Casey.
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