GearReview.com's Guide to GPS Receivers.
By Jeff Porcaro
A GPS Receiver is a valuable tool that can help you find where you are regardless of weather
conditions or visibility. A GPS has many features and terms. We will talk about the
features that are best suited for Land navigation to help you make the best choice for a GPS Receiver that will
work for you.
A Waypoint/Landmark is a location that can be stored in the GPS Receiver. A Waypoint/Landmark can be saved
while at a location or it can be entered in from a position coordinate. The GPS uses the Waypoint/Landmark
to provide navigational information when you use the go to feature. The go to uses the Waypoint/Landmark and your current
position to provide the bearing and distance between them. You should always save the starting point of a trip as a
Waypoint/Landmark. If you get lost you will always be able to navigate back to the trailhead or car.
Some receivers use the term landmark while others use position or Waypoint. They are all referring to a
position that is stored in the receiver.
The bearing is the direction from your current position to the Waypoint/Landmark that you want to reach.
The bearing is the number of degrees that you set your compass to when you want to navigate to a Waypoint/Landmark.
The position is where you are in regards to the earth. This position can be saved as a Waypoint/Landmark.
The position can be shown in different formats on most GPS Receivers.
The GPS Receiver can show you your position in many different formats. The Most common is Latitude/Longitude.
For land Navigation in the USA the UTM format is great. All of the USGS 7.5-minute series maps have the UTM grid ticks.
This is the easiest method for land navigation and course plotting from a map.
A collection of Waypoint/Landmarks that take you from one to another is called a Route. Some receivers have features
that automatically change to the next Waypoint/Landmark in a route after you reach the current Waypoint/Landmark.
Some GPS Receivers can reverse the route and take you from the last Waypoint/Landmark to the first.
The distance from one Waypoint/Landmark to another in a route is called a leg. Some GPS receivers limit the
number of legs in a route.
This is the screen that shows a trail of where you have come from and gives you some visual indication of where
you need to go. Some GPS Receivers have included real maps on the plotting screen that show roads and waterways.
Some GPS Receivers include the bearing and distance to the listed Waypoint/Landmark.
This is the screen that shows the bearing and distance to the listed Waypoint/Landmark. If you are moving some
navigation screens will show you the direction you are moving as well as the ground speed. It is important to know
that the GPS will NOT show you the correct direction if you are not moving. A GPS cannot know the direction without
moving because it uses signals from the satellite to determine the direction you are moving. A GPS is NOT a compass.
The Sunrise/Sunset feature shows you when the sun will rise or set based on the current date or an entered date
for given position. This can be helpful for photographers who want to get the right light for their pictures.
The Satellite Status gives you information about the satellites and where they are in relationship to the
receiver as well as the signal strength.
Map Datum is the way the map creator uses information to create a map. Most maps will include the datum
used for creation. It is very important to have the right datum when you are adding Waypoints/Landmarks to
the GPS manually that you have plotted from a map. If you use a map to determine a position and then you enter
the position into the GPS. When you try to navigate to that position you may get the wrong bearing.
When using a GPS with a compass and map you must make sure the GPS and compass are both set to the same
North Reference. The key is to always make sure that if you have set the declination on the compass the GPS
MUST be set to the same declination or your navigation will be off.
A few things can influence the GPS Receiver's accuracy. The speed that you travel can cause the GPS to be
less accurate because of Selective Availability. Selective Availability is where the government sends random
signals to the GPS that are wrong. The signal causes the GPS to think it is within 100 meters of where the real
position is. When the GPS receives signals some are right and some are wrong. Some receivers will average the
signals to give you an approximate location. The faster you move the fewer random signals are received and the
accuracy is increased. Most GPS receivers are accurate to 100 meters 95% of the time and for elevation they are
accurate to 150 meters. Satellite geometry can also cause the GPS to be less accurate if you don't have the best
possible geometry for the triangulation of your position. For example; if you are in a deep narrow canyon you can
only see a few satellites. This will decrease the accuracy.
Acquisition Time is the time it takes for your GPS receiver to get its position when you turn it on.
This is faster for 12 channel receivers, which take 3-5 minutes for a cold start and 15-30 seconds if
you are within 300 miles of your last position. The 2 channel receivers are slower. It takes up to 15
minutes for a cold start and 2-5 minutes if you are near your last fix.
Ask yourself the following questions when you are about to purchase a GPS. Does the GPS have an
easy to use Menu system? Does the GPS have the memory to store all of my Waypoints/Landmarks? Will the
GPS function well in a canopy of trees? How long will the batteries last? A GPS is a tool that, when used
with a map and compass can help you always know where you are and where you need to go. This tool works 24
hours a day 7 days a week. It works when it is dark and it works in a whiteout.