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GPS Navigation basics

by Alain Hoffmann.

First let's see how GPS systems work. One of the best pages I stumbled on is this one by Trimble. They explain it way better than I can. But basically it's a simple system. It relies on 3 different components:.

1. Very, VERY precise clocks. This is achieved by atomic clocks. However your GPS doesn't have one of these - stop dreaming. They cost between 50.000$ and 500.000+$. But it has a good quality clock built in which is constantly corrected by the receiving signals. This is why you don't have to tell your unit the time. You just have to put in man-made corrections like daylight saving time etc. Why must it be precise? The GPS measures the time a signal travels from a satellite to your handheld unit. As the signal travels at 300.000Km/s or 180.000miles/s a mismeasurement of 1/1000th of a second means an error of 300 KM!.



2. A network of 24 satellites that orbit the earth on exactly defined tracks. Every GPS unit does have an almanach built in with all the positions of those 24 satellites at all time..

3. Your GPS. The rest is basic mathematics. The unit measures the distance from different satellites. Imagine you have 3 ropes tied to different trees. There are only 2 possibilities where all the 3 lenghts of rope meet at the same point - one of these 2 points normally doesn't make sense as it lies below the surface. But if you use an 4th rope you can determinate a SINGLE point.

So why do good receivers have the ability to track 12 satellites simultaneously? Why not only 4? Or all 24? Simple. You can't have more than 12 satellites visible at any time, the other 12 are under the horizon. And the more satellites you have the better the precision. If you have 2 satellites standing close together you will have a much greater error than if those 2 satellites were standing in opposite directions. Good units choose the best (e.g. the furthest apart) signals to get the best possible accuracy. In real world you will constantly be losing and reacquiring signals when you move as the satellites are obstructed by trees or buildings. So the more satellites your unit tracks the better its accuracy. Below you see an example. If the unit can get a fix on satellite 28 and 06 precision will go up to less than 4 metres.

(Left: Signals acquired after about a minute. Full bars show stable and identified signals, white bars not yet confirmed signals. The round display shows their geographical positions).

What's the best accuracy I can obtain? Well, THEORHETICALLY you can PINPOINT your position on earth to 4mm. In practice, with your ordinary GPS alone you will come to around 4 metres or 12 feet in good real world conditions. If your unit can receive differential signals (ground stations that emit correction signals) you have an accuracy of about one finger wide. Not bad for a system which main parts are orbiting 11000 km out in space. Survey systems are capable of doing better than that and 4mm is good enough for almost all work on the ground.

Why the error at all? Until 2000 the DoD (the US Department of Defence) fed purposely an error into the signals. One of the last acts of President Clinton was to abolish this error. Now the only factors are variations in athmosphere and in orbit position. By using some tricks they can be eliminated almost completely. The GPS shows those EPE (Estimated Position Error) and I've had already readings below 1m/3 feet.

What's WAAS? You will stumble about this term sooner or later. This was born out of the "I'm in trouble" problem the satellite gets from time to time. As anything manmade (as all Land Rover drivers can testify) satellites too have malfunctions from time to time (no, they don't run Windows). As soon as this is detected a signal is emitted that tells all GPS to disregard that signal until further notice. But this takes time. So why not put up a geostationary satellite which just does this monitoring? But wait, when we have it up there why not use it as a differential GPS unit? One of which the position is always stable and over the same place?.

This system is in place over the US as if they were the only ones to position a satellite. The efficiency of this system is good enough to allow category 1 landings of aircraft (almost no visibility). Many airports have added their own differential GPS and this brings accuracy up to level 3 landings (zero visibility).

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