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BUYER'S
GUIDE TO
GPS RECEIVER FEATURES
(This is a long printout)
Commonly Used Terms
Coordinate:
Set of numbers that describes your location on (or above) the earth.
These may be either latitude (N or S) and longitude (E
or W), which is the Universal Transverse Mercator system, or UTM
coordinates, which measures distance in meters from the equator
(N or S) and a prime meridian (E or W). UTM is based on a
grid-system that divides the Earth into 6 deg. longitudinal slices,
100,000 meters high. Another system, MGRS (Military Grid Reference
System), is based on UTM, but divides the UTM sections into even
smaller subsections. It will primarily be found on military GPS
receivers only.
2-D and 3-D Coordinates:
Your horizontal location, such as latitude and longitude, is called
a 2-D coordinate. It is available, by default, on some GPS
receivers, and requires a minimum of 3 satellites to get a 2-D
location. Sometimes, if satellites are obscured by trees, mountains,
or buildings, you may only get a 2-D location.
When you get latitude, longitude,
AND altitude, that is called a 3-D coordinate, and requires a
minimum of 4 satellites visible. Almost all GPS receivers provide
this as a standard.
Position Fix:
When your receiver obtains information from GPS satellites to
determine what coordinates you are at, it provides you with a
position fix. Each receiver has a minimum number of satellites it
must be able to "see" (usually 4 satellites minimum for
"3-D" coordinates), to give a good position fix. Most GPS
units allow you the option of marking and storing your current
position as a landmark or waypoint. Some even allow you to name your
position (CAR or DOCK) or attach an icon.
Landmark or Waypoint:
This is a position that is stored into the GPS unit memory. Your
position is not a WAYPOINT until you SAVE IT. It may be from
a position fix that you have taken, or you may input the coordinates
of other locations that may be intermediate or final destinations.
The GPS unit will either give the position a name, such as LMK02 or
LOC 01, or you may provide a name that you will easily recognize,
like CAMP. (NOTE: when you start your journey, it is usually
wise to store your starting position, especially if you are planning
to return!).
Newer mapping units allow you to look
up addresses, cities, points of interest and other places in
the mapping database that is built-in or that you may have
downloaded from a CD. You can save these as a waypoint that
you can easily refer to later. You can enter several addresses
of clients before you leave home, and then easily "find" them later.
There are software programs available
from the GPS manufacturers and from outside vendors, that let you
mark waypoints directly on a map on your PC, and then load them into
your GPS. Conversely, they usually also let you download
waypoints you have saved in your GPS to the program, and then let
you save them as a data file on your computer.
Route:
A route contains a starting and an ending position, as well as
intermediate locations along the way. Each segment between positions
is called a leg. Routes can be made up of one leg, or a
series of legs. If you are going on a hike, you might input a route
composed of the trailhead, planned rest stops or camp sites, and
your destination. Some units allow you to backtrack, or reverse your
route.
There are two basic ways to use a
route:
(1) If you are planning a hike, or
trip, you can extract the coordinates of your waypoints from a
highway map, topographic map, or map software.
This is extremely helpful, whether you are planning a Scouting
adventure, or visiting a business client or long-lost relative. Some
GPS receivers let you plan your trip on a computer, and upload the
route into the receiver.
(2) If you've taken your GPS
receiver on a hike or off-roading, etc., and you recorded your
waypoints as you traveled, you can come home and copy or download
your trip, and find out positions of that prize-winning vista, the
best fishing spot, that rare bird you saw, or the cave you got
trapped in during bad weather. If a member of your party is injured,
a volunteer can hike out and provide exact coordinates of that
location to rescuers. Search and Rescue personnel can download
completed search routes to record where all the teams have been.
Track, or BackTrack
When you walk or drive with your GPS on, most handheld receivers
will record the path you take as a dotted line, referred to as a
"bread-crumb" trail or "track". This lets you see where you
have been. Most receivers have an option that lets you save
the track to memory, and they will let you select the track as a
"backtrack" or route option. This way, if you get lost, or
just want to go back the way that you came, the GPS will direct you
where to go. When the GPS saves a track, it usually
consolidates the track to a fixed number of waypoints (ie 300), so
it will not duplicate the path exactly.
GPS receivers let track files save a
limited number of track points (1000-3000), and then usually
over-write the initial points when you reach the maximum, so that
the beginning of the journey disappears. Receivers record
track points at a default interval ("X" miles or "X" minutes), but
many let you change the interval, if you want to make the track last
longer.
Advice: If you are starting a journey
or a hike, it is wise to save or reset/empty the track file, so that
if you need to backtrack this journey, it is not contaminated by
previous trips.
Elevation:
GPS units can provide elevation information (altitude, usually above
sea level) if a minimum number of satellites are visible (at least
four). Due to the nature of GPS, this will be less accurate than
your horizontal position (See GPS Receiver Performance, below)
Heading:
This is the direction that the GPS receiver is moving over the
earth's surface, not necessarily the direction the unit is pointing.
This is best viewed while moving, because the value stops if you do.
Heading is a value in degrees in a clockwise direction, between 0
and 359 and it corresponds to compass values.
Bearing:
If you pick a landmark or waypoint, and you want to know which
direction it is from where you are now, you need to know its
bearing. This is a direction in degrees, in a clockwise direction
from north. It is the direction you need to head in order to reach
the selected point. If the bearing to your destination is, for
example, "270 deg." and you are on heading "240
deg", you are traveling 30 degrees away from your target. Now
this may be OK if it is the way the trail goes, but if it goes to
far off, you may be on the wrong path--Check your map!
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Operating Your GPS Receiver
Most GPS receivers are easy to
setup and operate. User guides usually have easy instructions that
will get you started. These instructions include how to install the
battery, how to determine your initial current position and how to
input a destination.
When you first turn on your GPS
unit at a totally new location, it helps to input an estimate of
your approximate location (like the coordinates of the nearest large
city). This helps your unit know where the satellites are supposed
to be, so that it takes less time to pinpoint your position. (This
is especially useful if you have traveled more than 300-400 miles
from your last GPS position)
If your
unit starts out "cold", or you are no where near your last
stored position, it may take up to 15 minutes (2 channel receivers) to find its place on
earth. However, if you give it a little hint, or you are near where
you were the last time the GPS unit was on, it usually takes no more
than 3-5 minutes to fix your position. Once again, please read your
user guide for actual instructions.
How to Input Data
There are different ways to input numbers and letters into your GPS
unit. Most units use arrows or buttons to move between functions and
screens. Some units use these arrows to select a number or letter
(for a location name or coordinate). The up arrow will advance thru
the alphabet and 10 digits. After you find the value you want, you
just enter it and proceed to the next letter. Other GPS units have
actual alphanumeric keys, like your phone, to make it easier to
enter names and numbers.
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Types of GPS Features
There are several buttons on the
face of most GPS units that, when pressed, route you to various
operating options. They may have different names, but they have
similar functions. They may allow you to view a navigation screen,
status satellite signal availability, review your positions list, or
determine your position relative to a selected destination. The
following explains what some of these options are:
Position:
This screen displays present
position, elevation, and time of day. If a destination or route is
active, it may also indicate direction toward the destination.
Pointer:
If there is an active
destination or route, this screen will point to that location and
provide bearing, distance and time to go to get there.
Navigation:
If you are moving, the current
heading and speed are displayed. If a destination or route is
active, this screen gives bearing and distance to that position. It
also provides an indicator if you are on or off course.
Menu:
This screen provides you access to the different listings of options
(like a pull-down menu) available for your computer. Usually you
move the cursor over the topic you want, and hit "enter"
to see what is available.
Plotter:
This screen provides a track
history of the route travelled, and the bearing and distance to the
listed destination.
Satellite Status:
Most GPS units have a feature to
display how many satellites it is tracking and what level of
satellite signal strength it is receiving. If more than 4 satellites
are visible, the receiver will choose the best 4 satellites, based
on signal strength and triangulation angle.
Landmark or Waypoint List or
Library/ Route List:
You usually can view the
positions you have stored in memory so that you can rename or delete
them, or plan a route. You can sometimes add icons (a FISH or CAR or
TREE, etc), to distinguish important waypoints. Usually, there also
is a route option, to review/edit the routes you have made, or to
prepare a new route.
Setup Options:
Most units allow you to choose
units, time system, the coordinate system, map datums, and true or
magnetic north. There may also be options to receive Differential
GPS signals, or download data.
Distance to (GO TO):
This feature allows you to
select a stored position, and make it active. Then it provides the
bearing and distance to that location, from where you are.
Cross-Track Error (XTE):
If you have selected a position
or waypoint that you wish to travel to, and make it a route or leg,
then as you move, the cross-tract error indicator will tell you if
you are on the direct line between your start and stop point, or if
you are left or right of that path, by 0.1 mile or 20 miles,
depending on the scale you choose.
Last Fix:
While your GPS unit is turned
on, it will continually update its location every second or two.
Your unit may regularly store this last fix location in memory every
10-30 minutes, and refer to it when you backtrack your route, or
restart your unit. If you are hiking, and turn it on only every half
mile, it will remember where you were last time, and easily relocate
the GPS satellites again. If you have traveled over 300-500 miles
from your last fix, you may have to re-initialize, or re-cold start
the unit.
Sunrise/Sunset Time:
Some GPS units can provide you
sunrise and sunset time for a given day at a given position. This
can help you plan to maximize activity in available daylight, or
just catch a great sunset shot.
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How to use GPS with Maps
Different maps have different
coordinate systems. The two most common systems, used by the US
Geological Service TOPO maps, are:
- Latitude (LAT) and longitude (LONG), with units of
degrees, minutes and seconds.
- The UTM (Universal Transverse Mercator) Grid system, with
units of meters.
LAT/LONG is system of
units most of us are familiar with. The distance of a location from
the equator to the poles is identified by latitude 0 to 90 degrees
north or south (horizontal lines on the globe). Going around the
earth east or west, the earth is divided into 360 degrees (vertical
lines on the globe). The line that passes thru Greenwich, England is
called the prime meridian, and longitudes are measured to 180
degrees east and west of this line. It is difficult to manually
measure distance between two points using this system because the
size of a degree varies with ones position on the round earth.
The UTM system is one
that is preferred by map users who need precise distance
information. This system selects a meridian and identifies how far
east or west of that meridian you are and how far north or south of
the equator you are (in meters). If you are unfamiliar with this
system, but have a UTM
map or need the accuracy that this system provides, please look
into it further.
Most GPS units allow you to choose
LAT/LONG or UTM, or other systems, as your coordinate system when
you use the setup mode. When you have the coordinates of your
position, you can look it up on a map and know just about where you
are (depending on the accuracy of your unit). When you look up your
destination on a map, you can input these coordinates and save the
position in your GPS unit for route planning. Transparent plastic
rulers to use with TOPO or UTM maps are available to
make it easier to estimate a coordinate from a map. Some GPS units
have a feature that allow you to identify coordinates of a position
on a map by just using the map name, and scale, and by measuring how
many inches the position is from the lower right hand corner.
Of course, the products to enhance
GPS receiver capabilities are becoming more available, and less
expensive. Keep an eye out for new software products
with exciting features, that seem to come out every month. There are
Atlas-type software, with streets, highways, restaurants and hotels,
that let you find almost any address in the continental US. There
are topographic-type map programs for several regions with real USGS
TOPO maps or hiking guides for popular mountainous areas (with more
coming soon). There are new PC Cable+software bundles being offered
by GPS manufacturers to upload/download GPS information. Some GPS
units can be connected to a portable computer, with mapping
software, and will actually track REAL-TIME, your location on the PC
screen. GPS application will continue to evolve and improve like
computers, but DON'T WAIT, if you need to use it NOW.
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GPS Receiver Performance:
(varies with model)
Satellite
Tracking:
There are 24 GPS
Satellites in six orbits (4 satellites each) around the earth.
Generally, no more than 12 satellites are on one side of the earth.
Most GPS receivers have the capability to track up to 8-12
satellites. Usually 3 satellites are needed to compute LAT/LONG
(2-D) with at least one more satellite needed to compute altitude
(3-D). For a given location, a GPS unit knows which are the
satellites that should be nearby at a given time, because it
receives this updated information in the signal from the satellites.
Serial and
Parallel Channel Receivers
A few older GPS consumers have 2-5 channels that receive data from
the satellites. Since there can be up to 12 satellites within view
on the horizon (average is 8), this means that the GPS receiver must
sequentially visit each satellite- in a kind of time-sharing
arrangement- in order to get the information from all the
satellites. This is type is either a SERIAL or MULTI-PLEXING (fast
sequencing) type of receiver.
However, most of
the newer GPS receivers on the market today are 12 parallel channel
receivers. This allows them to continually track each satellite and
eliminate sequencing requirements. The advantages of 12 channels
include faster cold-start and regular initialization (finding the
satellites), and much better reception in forested /wooded areas.
Usually 12 channel GPS receivers do not require an external antenna,
unless you are in a close-sided vehicle, like a step van, RV or boat
cabin.
Acquisition
Time:
This is the amount of time it takes for your GPS receiver to
determine its location when you turn it on. For 12 channel
receivers, it takes 3-5 minutes for a cold start, and 15-30 seconds
if you are near your last position. For 2 channel receivers, it
takes up to 15 minutes for a cold start, and 2-5 minutes if you are
near your last fix.
Accuracy:
Most basic consumer GPS receivers can get 20 to 30 meter accuracy
for HORIZONTAL positions, IF and ONLY IF, the government SELECTIVE
AVAILABILITY (SA) is NOT ON. You may find certain GPS
manufacturers claim this accuracy for their receivers, but look for
the small print that says "with SA off".
What you will find,
is that most manufacturers do not discuss VERTICAL accuracy, because
altitude readings are the least accurate of the GPS measurements. To
my best knowledge, all GPS units (except some designed for
ocean travel where height = 0 at sea level), will give altitude
information (3-D) if at least 4 satellites are visible and readable
in the hemisphere. However, because the triangulation angles ( and
time distances) are not as optimum as they are for horizontal
positioning, vertical readings can be up to 3 times less accurate.
What does 30 meter
accuracy mean? It means that, with SA OFF, statistically 95%
of your horizontal GPS position readings will be within 30 meters of
your true location. GPS receivers work based on the time it takes
for the satellite signal to reach the GPS receiver (time x light
speed = distance). Inaccuracies develop because of possible tiny
clock errors, or distortions in the atmosphere. For vertical
readings, this means your position is within about 45-100 meters,
95% of the time.
If the government
has SA ON (which is not true since May 2000),
horizontal accuracy changes to 95% within 100 meters (vertical
accuracy within 300 meters). Both are higher than consumers will
like, but it gets you to the right street corner, or stream branch,
or in sight of your car at the trailhead.
Better
Accuracy with Differential GPS:
To get around SA, there is a process called differential GPS (DGPS),
which minimizes the effects of SA and atmospheric distortions. A
stationary GPS receiver (set up at a surveyed location within a
100-200 mile radius), also receives the satellite's GPS signal. It
KNOWS exactly how much time it should take for the signal to be
received from the satellite, and compares it with the actual travel
time, to come up with a difference (aka. differential) that is very
close to that caused by SA and the atmosphere. It transmits this
information outward, and it is used by GPS receivers to get a much
better position reading (5-10 meters or less).
The US Coast Guard
has DGPS transmitters along the US coast, the Great Lakes, and the
Mississippi River. In addition, other land-based and satellite-based
transmitters are set up by private companies-and their signals are
sent to customers for a fee. To use DGPS, it requires a DGPS-capable
receiver, a DGPS Beacon Receiver (or comm device from private
company), and a DGPS signal source. So, if it is REAL important to
get a more accurate position reading, you need to get set up for
DGPS.
Better Accuracy with
WAAS:
In 2001, newer GPS
receivers were released to take advantage of the new WAAS system.
WAAS is Wide Area Augmentation System, and is a new correction
service available by satellite from the U.S. Government for users in
the U.S./ North America. It was developed for use by aviation, but
civilian users can now benefit.
Basically, GPS signal
corrections are calculated from stations throughout the U.S. and are
re-broadcast from two satellites over the equator. If you have
a clear view of the satellite (no trees or canyons), and a WAAS
capable GPS receiver, you should be able to get corrected accuracy
to within 3 meters 95% of the time. For more information visit
HERE.
Signal
Interference:
Your GPS unit needs
to be able to see at least 3-5 satellites to give you a good
position fix. If you are traveling in a canyon or the streets of
the urban jungle or in heavy tree cover, you may have difficulty
maintaining adequate satellite contact. You may lose your satellite
lock, or only be able to compute a 2-D position. Also, if you are
inside a building, you probably will not be able to update your
position.
Some GPS receivers
have detachable antennas that can be attached to your windshield, or
external antennas, that can be attached to the roof of your car, RV,
or boat. By bypassing the opaque blockage from your roof, this
enhances the receivers ability to see more available satellites.
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GPS Receiver Physical
Characteristics
Size:
Handheld GPS
receivers fit in the palm of your hand, and are similar in size to a
TV remote control or a thick calculator. Newer "handheld" units are
actually designed to be used for car navigation and are larger.
Most marine units are much larger, and designed to be mounted in
your boat
Weight:
Most typical handheld
GPS units weigh less than one pound, even with batteries installed.
GPS receivers with many added features, will be larger and heavier.
Display:
Size will vary with
manufacturer and model. Some users prefer larger screens to see maps
and position information better. Most units have liquid crystal
displays, with lighted display capability. Some units let you
customize one display screen. Others let you combine various types
of data onto one screen, in a windows format.
Some GPS units have
built-in databases, with general geographic details for a large
region of the world. Some also offer (optional) interchangeable
databases for very small regions in the US, that allow you to
display more detailed geographic information as background on your
display.
Housing:
The case that is the
outside of the GPS unit is usually fairly rugged and may be either
water resistant, or waterproof. (However, most GPS receivers are not
made to be thrown around, or submerged.) If you are using GPS during
mountain biking or off-roading, it is probably better to keep it in
a shock-absorbing location, like your body or backpack, than
attached to the frame.
Some units have the
receiver antenna integrated into the top area of the case, while
others have a detachable antenna that can be moved to a nearby
location for better satellite reception. (Put the GPS receiver on
your belt-loop or in your backpack, and run the detachable antenna
to a hat.) Some units offer options to add attachments to external
antennas, DGPS receivers, or computer.
Temperature
Range:
Be sure you are
familiar with the temperature limitations of your unit. Though units
will operate over a wide range of temperatures, be sure to keep your
unit warm enough in winter and cool enough in summer, even when in
storage. The displays are generally the most thermally sensitive
part of the receiver.
Power:
Most handheld GPS
units use AA batteries (2 or 4 or 6) as the primary power source. Battery
life varies with model type. It is dependent on unit power
requirements, on whether it is used continuously or intermittently,
and how long you use the display light. Most GPS models offer
accessories that allow you to plug into an alternate power source,
like your boat or car cigarette lighter. Few models are designed to
recharge batteries, but if you have
rechargeable batteries and rechargers available, then these alternate
sources are highly recommended if you will depend on your GPS and
use lots of batteries.
Note: You may find
battery life shorter than manufacturers claims, so bring lots of
batteries on a trip, just in cause. However, if the power fails
within a very short time, with new batteries (like 10-20 mins.) then
contact your manufacturer for warranty replacement.
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Basic Handheld GPS Receiver
Accessories
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Some accessories are
included with your GPS, but most are "optional" accessories.
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Mount
bracket: most handheld receivers have mounting brackets to
use in your car or boat
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Suction cup
and Magnetic mount: items used to mount detachable antennas
onto your car or boat
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Carrying
Case: keep your GPS receiver clean and secure when not in
use. Some offer holsters for your beltloop.
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Reference
guides: user instruction manual
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External
power units: accessory to hook up to cigarette lighter or
other external power source
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Lanyard: strap
to attach to your receiver so you can wear it around your neck
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External
antenna unit and adaptor: an active external antenna can be
hooked up to most receivers thru an adaptor. It usually requires
additional connecting cables and couplings.
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DGPS Cable: Used
to hook up to a DGPS Beacon Receiver for RCTM 104 input.
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Data
interface cable and software: some GPS receivers are capable
of downloading trip data or uploading planned routes. Most
mapping GPS receivers have more detailed map information available
from a CD from the manufacturer.
Need to know more about
GPS? We recommend several good books on GPS
HERE. You can find out
details about individual GPS receiver operations by downloading the
operating manuals from the manufacturers websites.
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