GPS FAQ

Q. What is GPS?

A. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense.  GPS was originally intended for military applications, but in the 1980’s, the government made the system available for civilian use.  GPS works in any weather conditions, anywhere in the world, 24 hours a day.

Q. How does GPS work?

A. GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth.  GPS receivers take this information and use triangulation to calculate the user’s exact location.  Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received.  The time difference tells the GPS receiver how far away the satellite is.  Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement.  With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude).  Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset and sunset time and more.

Q. How accurate is GPS?

A. Many things can affect how accurate your GPS receiver works.  The atmosphere, the ionosphere and the position of your receiver could all affect the GPS accuracy.  Also any buildings, natural structures or heavy foliage that obstructs the GPS view (line of sight) of the sky may decrease the position accuracy.

In general, civilian (not military) GPS can provide position information with an error of less than 25 meters, and velocity information with an error of less then 5 meters per second.

Q. What can cause a bad/inaccurate GPS signal?

A. Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays – The satellite signal slows as it passes through the atmosphere.  The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

Signalmultipath – This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver.  This increases the travel time of the signal, thereby causing errors.

Receiver clock errors – A receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites.  Therefore, it may have very slight timing errors.

Orbital errors – Also known as ephemeris errors, these are inaccuracies of the satellite’s reported location.

Number of satellites visible – The more satellites a GPS receiver can “see” the better the accuracy.  Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

Satellite geometry/shading – This refers to the relative position of the satellites at any given time.  Ideal satellite geometry exists when the satellites are located at wide angles relative to each other.  Poor geometry results when the satellites are located in a line or in a tight grouping.

Q. What exactly is a GPS signal?

A. GPS satellites transmit two low power radio signals, designated L1 and L2.  Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band.  The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

Q. What can block a GPS signal?

A. Because GPS is a relatively weak signal (1575.4MHz), it cannot travel through metal, concrete, or other like materials.

This means that if the GPS antenna on a tracking unit is blocked by any metal of a vehicle, or if the vehicle is parked in a parking garage or a busy downtown area near tall buildings, you may have problems getting a location for the vehicle at that time.