GPS, or global positioning system, is a tool that is used to measure distance and to help pinpoint location any place in the world. The fact is that it is an excellent tool that can do so much more than just tell you where you are. And, it is being used in everything from heart rate monitors to automobiles. If you haven
Tags: global positioning system, GPS, gps review, gps system, GPS TRACKING
In this second of a three part series we’re going to go over how GPS receivers actually measure distance. A lot of math and science goes into this stuff so sit back and get ready to strain your brain.
At any given time, let’s choose midnight for this example, the satellite begins transmitting a digital pattern called a pseudo-random code. At that same time the GPS receiver begins running that same random pattern. When the satellite’s signal reaches the receiver the pattern transmission will lag a little behind the receivers playing of the same pattern. The length of the delay is equal to the travel time of the signal. The receiver multiplies this time by the speed of light to measure how far the signal actually travelled. We assume the signal travelled in a straight line and this therefore is the distance from receiver to satellite.
In order for this measurement to be able to be made the receiver and the satellite both need clocks that can be synchronized to the nanosecond. In order to make this possible you need atomic clocks, not only in the receiver but in every satellite as well. Atomic clocks cost between $50,000 and $100,000. That makes them a little too expensive for everyday use.
To overcome this cost problem the GPS system has a very clever solution. Every satellite contains an expensive atomic clock but the receiver contains a regular quartz clock which the receiver itself constantly resets. The receiver looks at the incoming signals from four or more of the satellites and compensates for it’s own inaccuracy. Once it calculates the correct time value this will cause all the signals that the receiver is getting from the satellites to align at a single point in space. That is the time value held by the atomic clocks in the satellites themselves. So the receiver sets it’s clock to that time value and therefore has the same time value as all the satellites. Atomic clock accuracy for quartz clock prices. You can’t beat that.
When you measure the distance to four satellites you can draw four spheres that all intersect at one point. Three spheres will intersect even if you’re way off but four spheres will only intersect if you are exactly right. The receiver can calculate the time needed for the spheres to intersect at one point. Based on this it resets it’s clock to match the atomic clocks of the satellites. The receiver does this constantly as long as it is on, which gives it the same accuracy as the atomic clocks in the satellites.
In order for this info to be of any use, the receiver also has to know where the satellites actually are. To do this, the receiver stores an almanac that tells where each satellite is at a given time. Any adjustments that need to be made because of gravitational pull are transmitted to the receivers by the department of the military.
In the last instalment of this series we’ll go over problems with the system, how they can be compensated for and how to use the data itself.
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Michael Russell
Your Independent guide to Global Positioning Systems
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Tags: global positioning system, GPS
GPS, or Global Positioning System, is the latest thing in keeping from getting lost when trekking out to visit your Aunt Louise. Years ago you had to rely on landmarks, your handy compass or the guy at the corner gas station, assuming he knew where everything was. Today for about a hundred bucks you can get a little gadget that will tell you exactly where you are, any place on Earth. By using some basic mathematical calculations and a rather complex system, you can be in the middle of the desert, lost as can be, push a button and have your GPS tell you exactly where you are so that you can hopefully find your way back home.
So what exactly is GPS and how does it work?
GPS is actually a very complex and expensive system but the fundamental concepts of the system are very simple.
The GPS system is a group of 27 Earth orbiting satellites. Twenty-four of them are in operation and 3 are backups in case one fails. The system was originally created for the US military but was eventually opened up to everyone else.
Each one of these satellites, weighing in at about 4,000 pounds, circles the Earth at about 12,000 miles per hour and makes 2 complete rotations everyday. The orbits are arranged so that at any one point in the day 4 of these satellites are visible from any place on Earth.
The complimenting receiver that was created to receive signals from these satellites has the job of locating 4 or more of these satellites, figure out the distance to each and from that information determine where on the planet the receiver itself is located. It then transmits this location back to the person who happens to be in possession of the receiver. The operation is based on a simple mathematical principal called trilateration. To simply explain how this works, let’s say someone tells you that you are 10 miles from Newark, NJ, another person tells you that you are 15 miles from Maplewood, NJ and a third person tells you that you are 20 miles from Woodbridge, NJ. By taking those three facts you can figure out exactly where you are because each radius of each distance when linked together will intersect at only one point. That point is where you are located.
In order to actually do the calculation from space the GPS receiver needs to know 2 things. The location of at least 3 satellites above you and the distance between you and each of those satellites. The reason that only 3 satellites or spheres are needed is because the Earth itself can act as a fourth sphere. The GPS receiver itself works by radio waves which are a kind of electromagnetic energy and travel at the speed of light The receiver can actually figure out how far the signal has travelled by timing how long it took for the signal to arrive from each of the satellites.
In the next article on GPS we’ll go over exactly how this system works to measure distance.
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Michael Russell
Your Independent guide to Global Positioning Systems
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Tags: global positioning system, GPS