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Do Not Believe Your SatNav Full Stop

Author : John Maris

These beliefs in SatNav reliability and accuracy that exist among the general user, the car SatNav, the mobile and other devices, are well founded, for they are mostly reliable and provide a very reasonable accuracy to allow safe travelling. This, I believe, leads to an over confidence and complacency such that there may be no realisation when things start to go wrong. And they can and do.

Taking the systems as granted we maybe do not fully consider that they are essentially three well integrated but separate parts each of which can be used independently for other applications. There is the positioning system, the digital map and some software that does sums to tell you how to get from where you are to your destination. Errors can occur in all three.

Looking first at the positioning system, GPS, it does not have the pinpoint accuracy anywhere in the world that many claim. This ‘many’ does not include manufacturers who state accuracy with the appropriate statistics. There are errors within the system and without, some of which can be corrected.

GPS works by a ranging system based on timing with nanosecond accuracy. Clock, orbital and noise errors exist within the system. At 300 000 000m/sec a millionth of a second gives 300m error, a thousandth of a second is 300km out. That’s how critical a bit of time is. Nowadays good receivers reduce much of the timing error to 1 or 2%, which is around 5m on the ground. On top of this other system errors creep in that are not always instantly corrected.

Externally there are atmospheric errors. Changes in conditions alter the travel times of the signal and hence the calculated ranges. Again, depending on the receiver, some can be allowed for. Added to this is multipath error, reflections from nearby objects which interfere with the direct signal or used itself.

The next is not an error as such but the satellite geometry, the number in view and their position in the sky relative to the receiver is a factor that amplifies the uncertainty in range errors. Generally there are sufficient satellites in view most times to provide the balance between angle of cut and travel path through the atmosphere. However, bear in mind that the best ones to use could be blocked by obstructions. It is a line of sight system.

Now the map. I guess we have all heard of some of the extreme and dafter stories regarding cliffs and oceans. In most parts of the world map standards are high. In some areas they may be made from original data that is not so good. Modern technologies aid a better updating or reconstruction but it takes time. Then there are the changes in road networks, new roads, new one way systems. Map makers do an excellent job of trying to keep up with a never ending task but perfection is not always achieved. Face it, some maps are better than others.

The sums. The algorithms that determine route directions may under circumstances slip up causing confusion. It is down to decision making processes on road values during map construction and programming to determine optimum routes. It is a detailed process. Glitches can be expected.

One useful aspect, and one that may increase the system accuracy, is a feature called Snap to Road. You probably will be on a road but GPS inaccuracy may put you off road. The position is forced onto the nearest road segment of the map. A ‘glitch’ to be aware of. If the GPS is particularly bad you may be snapped to the wrong road, the wrong A for instructions to get to B.

This is likely to be common where road junctions are close, a continually and random bouncing GPS position will cause continual redirections. Stick to your convictions, it should sort itself out sooner or later. If you have a built in SatNav you could have an inertial navigation input from engine sensors that take over in such situations. This should reduce this problem or at least correct it quicker.

Your GPS may go wandering. It may not be for an extensive period but may be enough to disrupt your guidance instructions. Do you need this distraction where you are not entirely sure of your route on a busy fast road in the wet. If aware that it is just one of those things that can happen you will be better armed to handle it.

Author's Resource Box

John Maris is a professional navigator and surveyor, offshore and coastal. With the increasing use of SatNav among the general user population he is attempting to increase the awareness of users of how GPS, navigation and mapping combine to give us these systems. If interested in more detailed information then please visit

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Tags:   GPS, SatNav, GPS misconceptions, SatNav systems, SatNav accuracy

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Submitted : 2012-03-07    Word Count : 752    Times Viewed: 730