Friday, March 2, 2012

GPS and SatNav - Removing Misconceptions

The Global Positioning System (GPS), originally designed for military use, has, as it has developed, revolutionised the art or skill of getting from one place to another among the general user population. Its development in coverage and reliability along with other technologies has enabled reasonably cheap SatNav systems for general usage, in our vehicles, mobile phones and other applications. Because of its reliability and growing popularity I believe SatNav is becoming widely accepted but without due consideration as to what it is really about. Many misconceptions have grown, some maybe not important, but in some cases having no awareness or the wrong ideas of what you have got or how it works could lead to problems of various severity.

The technology is fantastic but it is not without fault, just like any other technology it can go wrong. So let us remove these misconceptions and try to understand just where it can go wrong and how serious it may be.

First let us look at what SatNav is. We do know but do not really consider it in full. Now this is merely a loose use of words but many say they have GPS in their car. Well, yes they do, but they also have a full SatNav. This involves some method of obtaining position from orbiting satellites, some form of electronic map to put this position on, some software to navigate from here to there, and you may have engine sensor inputs to provide an inertial navigation back up when satellite signals are lost. That's essentially your SatNav system.

It is just a circumstance that the positioning system is mostly GPS. This is just one Global Navigation Satellite System (GNSS). Others are developed or in development. GPS is just the most commonly used. If you have car SatNav, this, with the inertial back up, if you have it, is what you are using. GPS is also included in mobile phones.

This next could be easy to fall for as the distinction may not be too obvious on your machine. I have heard some claim they can get SatNav on their mobile phone in the cupboard under the stairs. I am deeply curious of what advanced technology achieves this. The signal from GPS satellites is line of sight. An obstruction will block any satellite it is in the way of. A broom cupboard will block the lot of them. Now the mobile can also be located by the cell network. Although not as accurate, defaulting to the cell network when GPS is lost is handy. But, I do not think this can be classed as SatNav. No satellites are being used. That's the, er, 'advanced' technology out of the way with.

So far it is wordage and a bit of a lack in awareness. The end result is that by some method you can get a position. If you happen to be off road and lose GPS it may be important to realise you have lost some accuracy. GPS could be 10m or better. The cell being 50m at best in urban areas, no great problem, but worse in rural areas, maybe a problem.

Another one. There are many who say with pride that they know how GPS works. Triangulation. I believe they fall for this because it's a legacy from before GPS where control networks were surveyed in by measuring angles. A GPS receiver, specifically the antenna, which may or may not be attached to the box, is positioned by trilateration, using ranges. This was harder to achieve with accuracy in those older days.

Also, satellite pseudoranges are not measured directly, hence along with inaccuracies the term pseudo. They are determined from time differences and light speed, more correctly, the speed of electromagnetic waves as the GPS signal is in the radio or microwave bands, depending on who you listen to. The nearly 300 million m/sec assumed in range calculations is only valid in a total vacuum. Some of the GPS inaccuracies are due to varying light speed. Nanosecond accuracy is required. A thousandth of a second error (1millisec) equates to 200 miles or 300km. Not many realise the importance of such small times.

Now accuracy has been mentioned, this leads to my favourite but that is another story that can be found in 'GPS and SatNav. Pinpoint Accuracy Explained' by the same author.

John Maris is a professional navigator and surveyor, offshore and coastal within the Oil and Gas exploration industry. With the increasing use of SatNav among the general user population, in vehicles, mobile phones and other applications, he is attempting to increase the awareness of users of how GPS, navigation and mapping combine to give us these systems. It is a fascinating and fantastic technology but not without its problems. A higher understanding of how it works will enable you to get the best out of your system, avoiding confusion and being misled. If interested in more detailed information then please visit

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