GLOBAL POSITIONING SYSTEM

INTRODUCTION

With several comments regarding some of the technical aspects of GPS recently 
appearing on the RoADA 'chat line', there are evidently some misconceptions 
on how GPS works and the sort of performance and accuracy we, as users, can 
expect from different systems. I thought I would crave your indulgences and 
write a small article scoping the history of GPS, a brief description of how 
it works, some of its inherent inaccuracies both as a system and when used in 
conjunction with mapping systems.

Having been a Fellow of the Royal Institute of Navigation since 1982, a 
Member of the Royal Aeronautical Society since 1990, written a post-graduate 
Aerosystems thesis on the subject of GPS and used it for the past 20 years as 
both a pilot and navigator, I consider myself reasonably qualified in the 
subject, as well as having a more than a passing interest in the subject!

HISTORY 

In the late 1960s and early 1970s, the United States military (Department of 
Defence) had a requirement for an accurate world-wide navigation system that 
could be used readily and easily by all arms of its armed forces and one 
which involved little training. This was a time when the United States was 
beginning to increasingly explore and exploit the use of space, for both 
military as well as commercial purposes. This led to the early 
communications satellites - for those of you who are, like me, middle-aged, I 
am sure you will remember the exciting BBC broadcasts with Richard Dimbelby 
from Goonhilly when Telstar made its first Transatlantic transmission - and 
eventually on to the early experiments in navigational positioning.

By the time Space Shuttle appeared in the early 1980s, a small constellation 
of up to 18 satellites had been 'parked' in orbit around the earth. The 
Shuttle allowed both the placing and servicing of these satellites and by the 
late 80s, the system became operational. All this was funded by the United 
States Department of Defence and they recognised that it would not be too 
long before many civilian applications of the system would be exploited to 
the full - which, of course, as we all know, has now come to fruition and 
impacts all forms of transportation systems. Hence they imposed two forms of 
accuracy into GPS - military; known as the 'P code', and civilian 'mode' 
which had inaccuracies built into it. It was as a direct consequence of the 
Gulf War in 1991 which led the United States to remove many of the 'P codes' 
so that the thousands of GPS receivers being used by the coalition forces 
would be working from the same referenced data base. 

The full constellation of 24 satellites came into service in the early 90s, 
replacing some of the older systems. To take into account the fact that some 
of these satellites had been in orbit for several years, in May 2000, the 
United States reconfigured its GPS data base to a common transmitted accuracy 
for all civilian receivers. The military still can access a data base which 
has a far greater positional accuracy.

HOW DOES GPS WORK

GPS is wonderful and can lead you to anywhere you want to go - or even places 
you do not want to visit! It can be a great friend or an embarrassing foe 
because, like all computer-based systems, it is only as good as the 
information you feed it.

GPS provides a radio navigation system using 24 satellites in a constellation 
about 11,000 nautical miles above the earth or approximately 20,000km, as 
David Wilkinson so rightly states. Each satellite is fixed in space at a 
known declination and local hour angle (the spatial equivalent to the earth's 
latitude and longitude). Thus each satellite is fixed in relative terms to 
every other satellite in the constellation - in the same way as the planets 
in our constellation are related to each other. As the earth rotates, each 
satellite will form its own orbit in a similar way as the earth and other 
planets in our constellation revolve around the sun. So, again David 
Wilkinson is correct in that some of the satellites orbit the earth every 11 
hours and 58 minutes, but others are geostationary and some orbit at a 
different rate, depending on their declination and in relation to the earth's 
graticule.

Each satellite contains an atomic clock for precise timing and broadcasts 
'position' messages. Part of this data gives the GPS receiver the orbital 
details of each satellite. The receiver measures the time taken for the 
signal to arrive from the satellites in its view above the observer's 
horizon, from which it calculates the position of the receiver. The rate of 
change of the receiver's position is its velocity.

Three satellites and an estimated height are needed to determine a two 
dimensional position (latitude and longitude) and a fourth for three 
dimensions - giving a display of height. Typical two-dimensional accuracy is 
100 metres or better 95% of the time and 300m or better 99.99% of the time. 
Vertical accuracy is about three times better than this, but the United 
States still imposes a degree of uncertainty into the system, the signals are 
modified and a reduced accuracy results.

Positional accuracy can be improved by Differential GPS. This is a system 
now very much in service in the aviation world. Differential GPS works on 
the principal that a signal from an additional ground-based, and thus fixed, 
site is transmitted together with those from the satellites. Thus, because 
the position of this extra transmitter is known very precisely, many of the 
random inaccuracies, such as satellite 'wobble', are removed. 

Other terms that you may come across when looking for a GPS system are: 
Parallel GPS - this is usually a set of 12 receivers within the system, all 
listening to one satellite each. Another is: Multiplex GPS - which is a 
receiver listening to one satellite for a fraction of a second before 
switching to the next and will take longer to 'find itself'. So, you can see 
some systems are more inherently accurate than others - you pays your money, 
you takes your choice. But it is not necessarily the most expensive one 
which is the best for you - it all depends on what your individual needs are. 
In my book, the best piece of advice I can give is based on the 'KISS' 
principal - Keep it Simple Stupid!! - especially when it comes to integrated 
systems. The less operator input, the better. You don't want to be spending 
all your time paging through a series of buttons just to find basic 
information - especially when you are on the move.

GPS is not, on its own, suitable as a sole means of navigation as it can 
fail, be electronically jammed, switched off by the 'owner' - the United 
States Department of Defence and the subject of solar aberrations (sun 
spots/storms etc). That is why the Civil Aviation Authority, along with 
other international regulatory bodies have not sanctioned GPS as the sole 
method of safe aerial navigation.

INACCURACIES

I have already tried to show the inherent inaccuracies caused by the system 
both within the broadcast from the satellites and in the receivers. These 
inaccuracies can be further compounded when transposed to maps and charts, 
whether they are paper based or electronically displayed. Disregarding human 
error in manual transposition of positional information, all maps and charts 
have different grid reference points. For example, Ordnance Survey 
(Landranger) grid is based on OSGB36 geodetic datum. The conventional (and 
default) datum is WGS for most GPS systems. Thus, unless the two geodetic 
reference points are EXACTLY the same, positional error will be significant. 
However, the majority of map and chart agencies are beginning to change their 
reference points to WGS standard (if they have not done so already). 
Ordnance Survey maps now show the corrections that need to be applied from 
WGS 84 to OSGB36.

CONCLUSION

Now that I have explained some of the principals and errors in GPS, perhaps 
you can begin to see why GPS is God's gift to navigation when it is ONLY used 
in conjunction and comparison with other systems. There have been many cases 
of aviators, mariners, mountaineers and hill walkers, some of which have been 
tragically fatal, because the individuals concerned have not understood the 
basic principals of GPS and its limitations or have solely relied on the 
quoted accuracy of the receiver manufacture. 

Well, folks, that was a lengthy diatribe, but one close to my heart and I 
hope it now gives you all a better understanding of GPS and its capabilities.

Safe driving (and navigating!)

Bob Wilkey

http://autos.groups.yahoo.com/group/roada/message/2355