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GPS is an acronym for Global Positioning System.

GPS is in fact a network of satellites continually transmitting coded information making it possible to precisely identify locations on earth by measuring the distance to the satellites.

The Global Positioning System (GPS) is a navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. Originally GPS intended for purely military applications however in the 1980’s the US Government decided to make the system available for civilian use. GPS works 24/7 anywhere in the world, in all weather conditions and there are no fees or charges for using the GPS system.

The vast majority of consumers are not aware that there are more GPS satellites in space than 24, and for some GPS receivers, particularly early models and this can create errors and problems with the units.

How It Works

GPS satellites circle the earth twice a day in a very precise orbit whilst transmitting 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 other satellites, the receiver can determine the user's position and display it on the unit's electronic map.



How accurate is GPS?

Modern GPS receivers are extremely accurate due to the use of parallel multi-channel design. When first turned on Multi parallel channel receivers are quick to lock onto satellites and maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. On average GPS receivers are accurate to within 15 meters.

The latest GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average.

The GPS Satellite System

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of approximately 43,000 Km an hour.

The vast majority of consumers are not aware that there are more GPS satellites in space than 24, and for some GPS receivers, particularly early models, this creates errors and problems with the units.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):

• 1978: The first GPS satellite launched.

• 1994: A full constellation of 24 satellites achieved.

• Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.

• A GPS satellite weighs approximately 900KG and is about 5.6 M across with the solar panels extended.

• Transmitter power is only 50 watts or less

What's the signal?

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.

A GPS signal contains three different bits of information — a pseudorandom
code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your GPS unit's satellite page, as it identifies which satellites it's receiving.

Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.

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.

A GPS signal contains three different bits of information — a pseudorandom
code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your GPS unit's satellite page, as it identifies which satellites it's receiving

Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.

Almanac data is constantly transmitted by each satellite and contains important information about the status of the satellite (healthy or unhealthy), current date and time as this part of the signal is essential for determining a position.

Sources of GPS signal errors

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.

• Signal multipath — 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.

• Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defence. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA
  in May 2000, which significantly improved the accuracy of civilian GPS receivers.