technical article from Lowrance
What Is GPS?
The Global Positioning System (GPS) is a space age navigational
system that can pinpoint your position anywhere on the globe,
usually within a few yards or meters. This amazing technology is
available to everyone, everywhere, day and night, and best of
all, at no cost for use of the navigational data. GPS uses a
constellation of 24 satellites in precise orbits approximately
11,000 miles above the earth. The satellites transmit data via
high frequency radio waves back to Earth and, by locking onto
these signals, a GPS receiver can process this data to
triangulate its precise location on the globe.
GPS operates 24 hours a day, in all weather conditions, and can
be used worldwide for precise navigation on land, on water and
even in the air. Some of its many current applications include:
boating, fishing, hunting, scouting on land or from the air,
hiking, camping, biking, rafting, pack trips by horseback, hot
air ballooning, general aviation, snowmobiling and skiing, search
and rescue, emergency vehicle tracking, 4 wheeling, highway
driving and a host of other outdoor activities where accurate
positioning is required.
How GPS Determines Your Position
satellite ranging to triangulate your position. In other words,
the GPS unit simply measures the travel time of the signals
transmitted from the satellites, then multiplies them by the
speed of light to determine exactly how far the unit is from
every satellite it's sampling.
By locking onto the signals from a minimum of three different
satellites, a GPS receiver can calculate a 2D (two-dimensional)
positional fix, consisting of your latitude and longitude. By
locking onto a fourth satellite, the GPS can compute a 3D
(three-dimensional) fix, calculating your altitude as well as
your latitude/longitude position.
In order to do this Lowrance uses a 12 parallel-channel receiver
in all of its current products. Three of the channels lock on to
satellites for triangulation. Another channel locks on to a
fourth satellite for 3D navigation, which lets the unit calculate
altitude in addition to latitude and longitude. These four
channels continuously and simultaneously track the four
satellites in the best geometrical positions relative to you. The
additional eight channels track all other visible satellites,
then add this data to the data from the original four satellites.
The unit then over-resolves a solution, creating an
accuracy-enhanced reading. The additional channels also ensure
reliable, continuous and uninterrupted navigation, even in
adverse conditions such as valleys or dense woods.
GPS was conceived in the 1970s, and is controlled by the United
States Department of Defense. Although GPS was initially
envisioned for military use, the Government realized early on
that there would be numerous civilian applications as well.
Subsequently, the Department of Defense (DOD) created two
transmission codes; the P code (Precision code) for military use,
and the C/A code (Civilian Access code) for civilian use.
The highest accuracy levels were to be reserved for the military
so as to prevent hostile enemy attacks against the U.S. using our
own navigational system. However, once in operation, the civilian
GPS receivers using the C/A code proved to be more accurate than
the DOD had intended. Consequently, the military developed a
system for randomly degrading the accuracy of the signals being
transmitted to civilian GPS receivers. This intentional
degradation in accuracy is called Selective Availability or S/A.
This reduced the civilian GPS accuracy levels to being within 100
meters or less, 95% of the time. However, typical accuracy for
most users averaged between 20 and 50 meters the majority of the
time. You could easily see the effects of S/A on a GPS receiver
when you were not moving. Typically, there would be random
movements in speed, altitude and position readings, along with
slow position "wandering" on the plotter trail. This was easily
seen when you were on a .1 or .2 mile zoom range and not moving.
For example, while parked at the dock in your boat, you would see
unexplainable changes in your digital speed readings up to a few
miles per hour, even though you were not moving.
Plot of position accuracy using standard Lowrance GPS receiver
(stationary). Note the differences in scale. 5.5 hour period
immediately prior to shutoff of selective availability 8 hour
period immediately after shutoff of selective availability
longitude meters | longitude meters
Effective May 2, 2000 selective availability (S/A) has been
eliminated. The United States Department of Defense now has the
technology to localize the control system to deny GPS signals to
selected areas. It is not often that your electronics products
increase in value after you've purchased them. Now boaters,
aviators, drivers, hikers, hunters, and outdoor enthusiasts of
all types can locate their position up to ten times more
precisely (within 10 to 20 meters) and navigate their way through
unfamiliar terrain. Anglers can now return to their favorite spot
on a lake or river instead of just their favorite area. A
Lowrance GPS receiver in combination with advanced technology of
today's GPS management will take you anywhere you want to go.
The decision to allow civilians so much accuracy in location
information was finally made because GPS is continually playing a
more important role in the lives of people around the world -
it's becoming a national utility. GPS is the global standard in
navigation because it is completely free of charge to the
Differential GPS (DGPS)
Differential GPS, or DGPS, has been developed to improve GPS
accuracy to within a few meters. DGPS was originally initiated by
the U.S. Coast Guard to counter the accuracy degradation caused
by Selective Availability. Even with S/A now eliminated, DGPS
continues to be a key tool for highly precise navigation on land
and sea. DGPS technology adds a land-based reference receiver
– located at an accurately surveyed site – to the
other GPS components. This non-moving DGPS reference station
knows where the satellites are located in space at any given
moment, as well as its own exact location. This allows the
station to compute theoretical distance and signal travel times
between itself and each satellite. When those theoretical
measurements are compared to actual satellite transmissions, any
differences represent the error in the satellite's signal. All
the DGPS reference station has to do is transmit the error
factors to your DGPS receiver, which gives the information to the
GPS receiver so it can use the data to correct its own
measurements and calculations.
The two most common sources of corrective DGPS signals
currently are: (1) Coast Guard, land-based beacon transmitters,
broadcasting the data at no charge to the public, covering all
coastal areas and much of the inland USA as well; and (2) FM
radio sub carrier transmissions available both in coastal and
inland areas, but limited to paid subscribers. In order to
receive DGPS correction data from Coast Guard beacon
transmitters, a mobile GPS unit requires a separate beacon
receiver. And to receive FM sub carrier DGPS signals from local
subscriber radio stations, the GPS unit requires a separate FM
receiver, normally the size of a pager. Naturally, your GPS unit
must have the capability to both receive and process DGPS
Wide Area Augmentation System (WAAS)
GPS is plenty accurate for route navigation, but the U.S. Federal
Aviation Administration has special need for aircraft traffic
control that go beyond basic GPS. The FAA has a plan under way to
boost GPS performance even further with its Wide Area Augmentation System, or WAAS. This GPS add-on will include a time control element that will help airliners fly closer together while avoiding collisions. In addition to carefully spacing airplanes along travel corridors, WAAS will eventually make instrument landings and takeoffs more accurate as it replaces existing aviation navigation systems.
Non aviators can use WAAS signals to make their GPS navigation
even more accurate. However, WAAS has some limits you should know
First, the U.S. government has not completed construction of
the WAAS system, so it is not yet fully operational. The ground
stations are in place, but only a few of the needed WAAS
satellites have been launched.
WAAS can boost the accuracy of land GPS navigation, but the
system is designed for aircraft. The satellites are in a fixed
orbit around the Equator, so they appear very low in the sky to
someone on the ground in North America. Aircraft and vessels on
open water can get consistently good WAAS reception, but terrain,
foliage or even large man-made structures frequently block the
WAAS signal from ground receivers.
You'll find that using your GPS receiver without WAAS is both
easy and amazingly accurate. It's easily the most accurate method
of electronic navigation available to the general public today.
Remember, however, that this receiver is only a tool. Always have
another method of navigation available, such as a map or chart
and a compass.
GPS Navigation 101 Power
To turn on your
Lowrance GPS unit, press the PWR key. Read the message which
appears on the screen, then press the EXIT key to erase it. Your
Lowrance GPS unit is now ready for use. To turn your Lowrance GPS
unit off, press and hold the PWR key for three seconds. A
countdown until shutoff will appear on the screen.
New GPS Receiver Initialization
your unit is turned on for the first time, it does not know where
it is, the date or the time. To initialize a GPS unit is to
basically tell the receiver where it is, what the date is, and
what the time is. This allows it to know which satellites should
be overhead, so it can start searching for them to lock onto
them. When a new GPS receiver is first powered-up, even if it is
not initialized, it can still determine its position after a few
minutes. It will however, achieve a much faster satellite lock-on
if it is initialized.
To lock onto satellites, a GPS receiver must first find them. If
you simply turn on the GPS receiver and wait, it may take more
than two minutes to find and lock onto the appropriate
satellites. That's referred to as a cold start. In contrast, when
initialized by the user, the GPS receiver typically takes only a
few seconds to lock onto the satellites. The GPS receiver should
have a clear view of the sky during initialization.
Initialization requires that you provide the GPS receiver up to 3
pieces of information:
- Your approximate present position in
- Your approximate elevation, or altitude: and
- The current local time and date. Normally, initialization
is necessary only once, provided each subsequent time the GPS
receiver is turned on it's within approximately 300 miles of
where it was last turned off. Regardless of which start up
method is chosen, initialization or cold start, once the GPS
receiver has achieved satellite lock on, it will typically
begin tracking much faster the next time it's turned on, often
Satellite Information Screen
A simplified screen
displays this satellite information by putting it into a
graphical format (see screen). For each of the 12 channels a SAT
number (satellite number) is shown along with a bar graph showing
the relative strength of the signal. A circular overhead view of
the satellite position in the sky is above this information. The
center of the circle corresponds to a satellite position directly
overhead. The edges of the circle are at the horizon. The top of
the circle is North. If the satellite number is highlighted, it
is being tracked and data is being measured from it. The display
also shows the EPE (estimated position error) in feet or meters.
This will be in the upper right hand corner of the screen once it
locks on. This is an estimate of the accuracy of your position.
It depends on the geometry of the location of the satellites
tracked, and other factors.
The vertical bar on the bottom of the screen is the battery life
left (on portable models only).
Saving Waypoints in Memory
A waypoint is
a position you wish to save and return to later. GPS receivers
typically offer two methods to store waypoints in memory: 1. The
Quick Save method, which uses the coordinates from either your
present position, or those from the cursor position in the
plotter mode. In this method, the waypoint is automatically
identified with the next available waypoint number in the list;
2. The View & Save method, which lets you pick the specific
waypoint number under which you want to store the new waypoint.
You can also name the waypoint during the same procedure.
Using GPS to Navigate to a Waypoint
There are three basic methods you can use to navigate to a
waypoint: 1. If it's already stored in memory, the waypoint can
simply be recalled and the unit instructed to navigate to the
waypoint; or 2. If it's determined from a navigational chart or
communicated by some other means, the waypoint can be entered
using the unit's keypad, then navigated to; and 3. On the
plotter, the cursor can be used to pinpoint the location of a
waypoint, then the unit instructed to navigate to the cursor
position. All three techniques employ easy-to-understand,
on-screen menus, guiding the user through every step.
Straight Line Navigation
GPS products use what is called "straight line" navigation. The
units, when commanded to navigate to a waypoint, draw a straight
line from their present position to the destination waypoint. The
straight line represents the shortest, most direct route to the
One very important point must be made about "straight
line" navigation: It does not take into account any
obstacles in the path (on land, in the air or in the water).
Consequently, it may be necessary in some situations to record
interim waypoints that alter the course to navigate around
obstacles. These additional minisegments of the journey will each
represent straight line routes. New GPS users should be cautioned
to take these considerations seriously, and to never rely solely
on a single navigation aid.Using a Route to Bypass
Since GPS products use straight line
navigation, it is necessary to use a waypoint at each place you
need to turn when you are navigating around an obstacle such as a
cliff, or navigating down a highway or river channel. By
connecting each of these waypoints in a chain, you form a
"Route". This provides the automatic capability to navigate
through several waypoints in order, without having to manually
recall another waypoint in the unit. Once programmed into a GPS
unit, a route provides the option of navigating forward through
the waypoints, or navigating in reverse in order to go either
direction through the route.
Using Plot Trails to Find Your Way Back
One of the most important features in a Lowrance GPS unit is the
ability to display, save and navigate plot trails. This is the
feature that allows you to retrace your steps or repeat a journey
at a later date, and it's especially useful when navigating in
roadless areas. A plot trail is a line plotted or drawn on the
screen tracing the path you've taken, from your starting point to
your present position.
A plot trail appears on your GPS screen as a line that flashes
once per second, for easy viewing. Your GPS plots a trail by
placing a position marker dot on the screen every three seconds
as you travel. (This can be adjusted from one dot per second to
one dot per 30 minutes, or you can update your trail by distance
instead of time.) With menu commands, you can save, recall and
navigate a trail (forward or backward) just as you would a route.
Navigation steering information is provided in either a compass
rose display (with an arrow pointing out the correct travel
direction) or as a dotted line on a unit's map display. Or, you
can skip the navigation commands and simply retrace your path
following the flashing plot trail on the map display.
GPS NMEA Interface with Other Electronics
NMEA is an abbreviation for the National
Marine Electronics Association, the group that establishes the
data protocol and wiring standards for the marine electronics
industry. As previously discussed, some GPS units can receive
DGPS data from beacon and FM receivers. GPS receivers must also
be able to send standard positioning and navigational information
to a variety of listener devices such as charting instruments,
autopilots and others. Most quality built GPS products permit
their users to select from two different NMEA data protocols that
transmit data output sentences. The first protocol is NMEA 0180,
which is reserved strictly for sending steering information,
primarily to marine auto pilots. The second protocol, NMEA 0183,
sends latitude/longitude position, steering, speed and other
navigational data. Depending on the specific GPS product, these
NMEA protocols are in code versions 1.5 and/or 2.0.