er to reduce the accuracy for the Russians, once considered our enemy. This was called "Selective Availability," and it was turned off in 2000.
If you're interested in reading more about the end of Selective Availability, see GPS.gov.
Though the GPS system was developed for the military, it now obviously has many civilian uses. And now that Selective Availability has been turned off, the system is much more accurate for you and me.
When using a GPS in the field, it's important to remember that, except for the trip odometer function like that in a vehicle, distances are "as the crow flies" -- in a direct line from point A to point B. They don't take into account necessary turns in your route by trail or road or in order to bypass obstacles.
The U.S. NAVSTAR GPS System
NAVSTAR stands for NAVigation Satellite Timing And Ranging, which was conceived in the 1960s, begun in 1978, and went operational in 1995.
NAVSTAR is made up of a Space Segment, a Control Segment and a User Segment. The Space Segment is the satellites. The Control Segment is comprised of the receivers or dishes on the ground. And the User Segment is you, me and anyone else who holds a GPS unit in his or her hand.
The Space Segment of NAVSTAR is composed of 24 satellites and 2 spares, which orbit 12,000 miles above the Earth. There are six orbital paths, with each orbit taking 12 hours. At any one time, any spot on Earth should have a line of sight to a minimum of 6 satellites in the absence of impenetrable obstacles.
How To Receive A Good Signal
There's more to it than just turning it on....
In order to accurately navigate with a GPS, you need a good line of sight to the satellites so your unit can receive their signals. These signals don't pass through most solid objects, such as trees, canyon walls, bridges or metal roofs. Even your body can block a satellite's signal.
A GPS will work well outdoors in the open and in vehicles, boats and planes as long as there's a good sky view. It should also work well in a tent and under canvas or nylon as long as, again, there's a good sky view on the other side.
A GPS may work okay in a wooden structure on an upper level and near windows, in open forest, a canyon or even in a city between tall buildings, but the accuracy will be compromised.
And the GPS won't work well in thick forest and not at all in metal buildings away from windows, in tunnels, caves, under water, enclosed vehicles or in parking garages, for example.
For the best accuracy, your GPS should receive signals from six satellites but no less than four. Your unit will have a screen that displays the number of satellites it's making a connection with at any time.
How A GPS Works
Here are the basics:
Your GPS unit communicates with satellites, which send out radio signals. Since the speed of those radio signals is a known factor, the receiver is able to use that speed to calculate its distance--and therefore the distance of the one holding it--from a satellite. This calculation puts you and the GPS in an imaginary sphere.
So imagine yourself holding your GPS and standing in a big circle (See: "A" in the image below) on the ground. This reflects the information from a single satellite.
The GPS will do the same thing with the other satellites with which it can communicate via a clear line-of-sight. So for the second satellite, imagine another circle which intersects with the first. (See: "B" below) This second calculation will put your near the intersection of the two circles (See: "A^B").
Then Add a Third satellite
and another intersecting circle.....
This third calculation will place you in the intersection of the three circles. This is considered a "2D" (or two-dimensional) mode, figuring your location horizontally only. In this case, you can input your elevation, if you know it, for more accuracy with the GPS unit. Keep in mind that 2D navigation can be unreliable.
If your GPS can communicate with 4 or more satellites, your location will be more accurate and you'll be in "3D" mode, which will include an elevation calculation. This is what you're aiming fo