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How GPS Works

Published: September 25, 2004

Updated: October 14, 2012

graphic representation of a satellite system

So, you want to know how GPS works? GPS, short for Global Positioning System, is a means for locating any point on the earth. It has many uses; navigation, surveying, vehicle tracking, hiking and outdoor recreation just to name a few.

In the 1970s the Department of Defense (DoD) conceived the idea of GPS. It was born from a need to accurately determine the position of ballistic missile submarines prior to launching missiles. All the old methods of determining position had their flaws. Those methods were affected by atmospheric conditions, limited in range, subject to enemy jamming, or degraded by interference.

GPS Satellites

The satellite network which makes GPS technology possible is maintained by the US Air Force. The GPS system is made of 24 NAVSTAR satellites (although there is in fact a total of 27 units in orbit - the extra three are a failsafe in case one of the primary 24 runs into difficulties or fails altogether) and five ground stations. Together, the satellites are referred to as a 'constellation'. The ground stations are responsible for keeping the satellites in precise orbit.

The DoD placed each of the 24 satellites in a precise orbit at an altitude of 10,900 miles. The orbits of the satellites have been carefully calculated so that no matter where you are on Earth, there will always be at least four satellites in the sky above you. If this were not the case, there would be certain times and places on Earth where GPS wouldn't work. Each of the satellites completes two revolutions of the Earth every day. They derive their power from the sun. Each satellite weighs two tons, is 18.5 feet long, and orbits the earth in a little less than 12 hours.

The Trick: Measuring Distance with Time

Each of the 24 satellites transmits its own unique signal. The GPS unit has stored in it those 24 separate "signatures" and therefore knows the position of each satellite. By measuring the distance to at least four satellites, each in its distinct orbit, location of the GPS receiver can be pinpointed down to as little as 3 meters. Distance to each satellite is measured simply by the time it takes for a radio wave to reach the GPS unit.

To be able to lock onto four signals, a GPS unit needs to have at least four channels. Most units have 12 channels. Calculations were made for the orbits necessary for each of the 24 satellites so that at least five are "visible" to any one point on the earth at one time.

GPS can be used in any type of weather, and is used on land, in the air, and for marine applications. Some conditions limit its usefulness. Heavy tree cover and cliffs, steep hills, or tall buildings can interfere, but often in those situations one can move to a better location and still not be too far off the intended route.

GPS Triangulation

Let's get into the real mechanics of how GPS works with a satellite to determine your position anywhere on the globe.

The process your GPS uses to determine your location is known as GPS triangulation, and if you know a bit of math you can probably already guess from the name how it works. Triangulation is a method for figuring out a position using a calculation based on angles and lines between several satellites at different locations in space. The known locations of GPS satellites in space are used to determine the distance between multiple satellites and your GPS receiver. From this data, the GPS uses calculations based on the geometric properties of triangles (hence the name triangulation) to determine your position.

Put in simple terms, your GPS device knows the location and distance of the satellites, based on the length of time it takes for radio waves to reach your GPS from the satellite as noted above. By using data from four separate satellites, it can narrow down your current location with an amazing degree of accuracy. By determining the distance between each satellite and your GPS receiver, the receiver is able to determine the size of an imaginary sphere around each satellite – the radius of the sphere being the distance between satellite and receiver. Just knowing the distance between yourself and one satellite tells you nothing about your location – but by overlapping four 'spheres' from four different satellites, you can get an accurate fix on your location and altitude.

Ephemeris Data

In order to work effectively, modern GPS units utilize what's known as 'ephemeris data.' There are two types of data GPS satellites send out to receiver units: ephemeris and almanac data. Ephemeris data is more precise than almanac data and is therefore used when you need to get an accurate fix on your position. Each satellite broadcasts it’s on ephemeris data every 30 seconds or so – this is why your unit will often take about 30 seconds to get a fix on your location when you first switch it on. The ephemeris data remains valid for roughly half an hour.

Click here for an excellent in-depth tutorial on how GPS works (Macromedia Shockwave Player is required to view the tutorial - it's free). Basic understanding of how GPS works ensures that you can make the most of this very useful tool.

Sources:

  • http://airandspace.si.edu/gps/work.html

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