The GPS system consists of three main parts. The first part of the GPS system is a constellation of GPS satellites, which was implemented by the U.S. Military. They call the system NAVSTAR (Navigation Satellite Timing and Ranging) and in the 1980s U.S. Congress opened the system up for public use. The second part is a series of ground based stations which are used to tell the satellites if they are in their perfect orbit. The third part is the GPS receivers used by GPS users. While many think that a GPS ‘talks’ to satellites and the satellites ‘tell’ the receiver where it is, this is not really how it works. Another misconception is that the satellites can be used to track a GPS receiver. This is also a myth. Here we will learn exactly what GPS is and how it works.

GPS Satellites

These satellites orbit the earth at a distance of about 19,300 kilometres moving so fast that they circle the earth twice every day. They are spaced out so that from almost any point on earth, there should be at least 4 satellites in the sky somewhere above. Of the 24 GPS satellites, 21 are in full time use and 3 are operating spares, and each of these solar/battery powered satellites is equipped with a super accurate atomic clock. They have small rockets which are used to keep the satellite in its correct orbit, and the satellites only last about 10 years, so they are constantly being replaced. Civilian GPS uses a frequency of 1575.42 MHz in the UHF band. The signals don’t go through solids very well, so GPS doesn’t work inside buildings or anywhere you can’t see a large portion of sky. A GPS signal contains the following information:

1. An ID code (called pseudorandom code) which tells the receiver which satellite the signal is coming from.
2. Ephemeris data, which is a continuous signal containing the exact UTC date and time, and some information which indicates if the satellite is OK or not. The time data in this signal is the critical data which is used to measure the time it took the signal to travel from the satellite to the receiver.
3. The almanac data tells the GPS receiver where all the satellites in the GPS constellation should be at any given time.

GPS Receivers

A GPS receiver has an antenna which it uses to collect signals from the satellites. Some GPS receivers are hand held devices, which show your longitude and latitude on a screen, some even may overlay this data onto a map so you can see where you are. But how does it work out your position in the first place? The receiver uses the GPS signals to measure it’s distance from each of those satellites and mathematically calculates it’s own position on the earth.

Basically, if you know how far you are from several known points, then you can work out your own position. The further you are from a satellite, the longer it will take its signal to reach you. So if the signal says its 12:05pm and 13 seconds, but it reached your receiver at 12:05pm and 13.07 seconds, then the signal took 0.07 seconds to go from the satellite to you. Since we know how far the radio waves can travel in that time (at the speed of light, which is 299792km per second) we now can work out how far we are from the satellite by calculating 0.07 * 299792 = 20985 km away.

Clever clocks

Now a GPS satellite has an atomic clock on it, but the GPS receiver does not. We wouldn’t want to carry around a heavy, $100,000 atomic clock when we go bushwalking, would we? So somehow the GPS receiver needs to have a clock in close to perfect sync with the satellite clocks. To achieve this, the receiver uses an ordinary quartz clock, and when it gets a four-satellite lock it pulls itself into near perfect sync. We’ll find out more about how it does this soon. So every $100 GPS receiver out there has a near perfect time, extremely close to a $100,000 atomic clock! Amazing, huh?

So once you can accurately know the time difference between when the signal was sent and when it arrived, and you can calculate the distance, you then need to know the position of the satellite you are measuring. You see, we need to know our distance from known positions, not just any old point in space. So mixed in with the GPS signal is the coordinates of the satellite sending the signal. That’s the Almanac data. The GPS receiver stores the Almanac data in memory so it remembers which satellites to look for when it is first switched on. So now we can measure our distance from known points in space, but how do we use that information to find out where we are?

Finding our place in the world

The mathematical principle used by the GPS receiver is called Trilateration, also known as Triangulation. Trilateration is easy to try yourself on a map. Take this example on a 2 dimensional map:

Your location is where the three circles intersect.

Now this system works great when you can figure out the straight line distance from 3 towns as in the above example, but how will you do that out in the bush or out on a highway? For starters, you can’t send a signal in a straight line to 3 towns from your car; too many things would get in the way. That’s why GPS uses satellites. We can send a signal in a straight line from the satellite in space to most points on the ground (or on the ocean, or in the air for that matter) without stuff getting in the way (remember, GPS can’t work inside, the roof is in the way!) but now we have to work in 3D instead of 2D. For trilateration to work in 2D you need 3 known reference points, with circles around them. In a 3D space, you need 4 reference points, with a sphere around them! 3D trilateration using 4 spheres not only shows us our position extremely accurately, but it has another benefit. It simply won’t work if your measurements aren’t spot on. If the fourth sphere doesn’t intersect the first three at the place that the first three intersect each other, then you know that your clock might be slightly out. This is a great way to reset the clock. If we try the formula again but this time we assume that our clock was a few milliseconds slow, and our fourth sphere now intersects perfectly, then not only have we got a perfect position, but we can adjust the clock forward to be in perfect sync, too. If this adjustment makes our error worse, then we try assuming that our clock was fast. After a bit of trial and error, we can expect to reduce our clock error to near zero. So the next position might be right first time!

Apart from clock errors, other things affect the accuracy of GPS.

1. Radio waves don’t travel at the speed of light the entire distance from the satellite to you. The earth’s atmosphere, particularly the ionosphere and troposphere, slow the waves down. Since this is a varying amount, we use an average figure to correct it, but it still won’t be perfect.
2. Sometimes the Almanac data the GPS satellite sends is inaccurate, so one of our ‘known’ positions might be a little off.
3. Signal which is blocked by a building may bounce off other buildings and reach your antenna ‘the long way round’. This would make that satellite seem further away than it really is.

What about speed?

Now that we can accurately calculate our position, we can do it over and over again every second and measure how far we moved. The receiver uses mathematics to calculate the distance between the co-ordinates (latitude, longitude and altitude) of our position one second ago and our position right now. If we moved 3 meters in that second, then our speed was 3 meters per second, which is 10.8 kph. It’s that easy.

What can we do with this?

Positional data from GPS can be used to tell you where you are. You might want to use this when you go bushwalking so you can find your way home if you get lost. If your GPS receiver says you are 2km west of your car, you need to walk east to get home. By overlaying the position on a map, you could use it to find your way around a city. Alternatively, this data can be used to have an expensive asset constantly report it’s own position to you, so you know where it is. That’s what Fleet Logistics does. Our Automatic Vehicle Locators (AVL’s) record GPS data, as well as other telemetry data such as ignition, sensors such as temperature, so you can see a full status on your vehicle. They then send this data in real-time to our servers using the mobile data network, and make it available to the owner or manager of the vehicles via an intuitive web interface.

For more information on tracking assets using GPS enabled AVL systems, click here.