GPS
Global Positioning System, or GPS is a spaced based satellite navigation system that is used for a huge number of tasks. More recently, GPS receivers have become small enough that they can comfortably fit into phones and more importantly for runners, watches.
What exactly is GPS? How does it work? For the answer to these questions, plus a few more read on and find out about a product that has made a difference to the training of countless runners.
GPS started its life back in 1973, a US Department of Defence initiative in order to overcome some of the previous limitations of navigational methods. It is thanks to Ronald Reagan that we have access to GPS, as he signed a directive guaranteeing that GPS signals would be available at no charge to the world when the system became operational.
What is GPS?
The Global Positioning System is made up of three parts, the Space segment, the Control segment and the User segment.
Space Segment
The space segment consists of a constellation of satellites which transmit radio signals to users. There are currently 31 operational GPS satellites, plus 3 - 4 decommissioned satellites that can be reactivated if needed. Each satellite circles the earth twice a day. In 2011, The Air Force updated the GPS constellation so that it is at its most optimal geometry, which maximises GPS coverage for all worldwide.
The satellites being used are constantly being upgraded to provide better reliability, accuracy and longevity. There are a mixture of a number of generation of Satellites in the air at the moment.
Control Segment
The GPS Control Segment is a global network of ground facilities that track GPS satellites, monitor the transmissions and send commands to the constellation. The control segment has 12 command and control antennas, and 16 monitoring sites.
User Segment
The user segment is the GPS receiver that we use. It receives the signal from the GPS satellite and uses the transmitted information to calculate your position and time. This is what you pay your money for.
How the system works:
The GPS receiver calculates its position by precisely timing the signals sent by GPS satellites.
Each satellite constantly transmits messages that include
•Precise orbital information
•Time message transmitted
•General system health and rough orbits of all GPS satellites
To get an accurate location from GPS satellites, you will need four satellites.
To find your position, a mathematical principle called trilateration is used.
I will explain two dimensional trilateration first then try and explain how 3D trilateration works.
To find out where, you are some helpful person (or satellite) says that you are 120 miles from Birmingham.
You could then be any where in that circle (see diagram)
Now, a second helpful person (or satellite) says you are 50 miles from London.
You can now be anywhere that the two circles intersect.
However, that is not enough information, you could currently be in two locations, so a third helpful person (or satellite) says you are 80 miles from Cardiff.
The intersection of the 3 circles is your current location. In a basic way, this is how GPS works.
However, because we have satellites, the signal that the GPS receiver transmits goes out in a sphere and not a circle. It is a lot more difficult to visualise, so if you can't make head nor tail of what I am saying just move on.
So, the first GPS satellite (GPS1) lets you know you are 15 miles away from point A, so a sphere with 15miles radius is created.
GPS2 then tells you that you are 22 miles away from point B. It will then create another sphere with a radius of 22miles. Where GPS1 and GPS2 overlap, you get a perfect circle.
GPS3 is then needed to create another sphere that intersects that perfect circle at two different points.
The forth sphere is then the earth. How is this so? The two possible points that GPS3 gives us once it intersects the perfect circle, one is in space, the other rests on the surface on the planet, this makes the earth our fourth sphere.
Technically, with thee satellites you can get a positional fix, however, with more satellites you get a more accurate positional fix.
How does the GPS receiver create the sphere?
The GPS receiver uses the times sent and the satellite positions corresponding to these times sent and the time the time the signal was received. Using a fairly complex equation which involves the speed of light (the speed of the message) you can work out the pseudorange, the pseudo distance between a satellite and the receiver. Using the satellite position and the psuedorange of the satellite it can then create the sphere.
The theory of 4?
As suggested before, we could find your position using just three satellites but there are errors wiht GPS.
One of the biggest sources of errors is the GPS receiver's clock. As the vale of speed of light is very large, any error in the GPS receiver clock value will throw out the pseudorange, which is necessary to create our sphere. For example, an error of one microsecond (0.000001 second) will lead to an error of 300 metres.
It then seems that the clock used on your GPS receiver needs to be as accurate as possible. However, to keep the costs down of GPS receiver units, the clocks used are not atomic clocks.
Due to this clock 'inaccuracy' it is highly unlikely that the four GPS satellite sphere's will actually cross at one point, to overcome this failure of the GPS receiver, the receiver will reset its clock to the same time value as that of the atomic clocks which are onboard the GPS Satellites. By making these adjustments constantly on the move, the GPS receiver is able to make sure that the four spheres do intersect at one point.
As mentioned before, one other piece of information that the GPS satellite sends out is the rough location and health of the GPS system. This information is called an almanac -- the location of every satellite at any given time.
How accurate?
The accuracy of a GPS-determined position depends on the receiver. Most hand held GPS units will have an accuracy between 10 and 20 metres. By using differential GPS you can get a much more accurate location.
Most running GPS units will claim to have a 2.5% accuracy rate. So they expect their result to be out by 2.5% either side.
What causes inaccuracies?
The calculation of your position depends upon using the constant value of the speed of light, which is the speed the radio signal travels at as is comes from the satellite.
However, the radio signals are interrupted as they travel down from space to earth. A delay can be caused by the earth's atmosphere, also, radio signals can be bounced off large objects, such as skyscrapers and taller buildings.
If the almanac data is incorrect, this will also lead to inaccuracies in your location.
Speed and Distance
I have described how a GPS receiver can place you anywhere in the world at a given time. By staying in constant communication with the GPS satellites the receiver can see how your location is changing.
With that information and the onboard clock, it is possible to give you all sorts of information including
Distance
Time of travel
Speed
Average Speed
History of the system?
When the GPS system was created, the Defence Department inserted timing errors into its transmissions to limit the accuracy of nonmilitary GPS receivers to 100 meters. This “selective availability” was eliminated in May 2000. Bill Clinton ordering Selective Availability to be turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 meters (330 ft) to 20 meters (66 ft).
However, there is still a difference between civilian GPS and military. Civilian only broadcasts on one frequency, whereas the military is broadcast on two frequencies. Two frequencies means that military users are able to perform corrections on the signal by reducing the degradation of the roadie signal caused by the Earth's atmosphere. Thus there is better accuracy for military users.
Running inaccuracies
Have you ever noticed that the half marathon you ran never comes out at 13.1 miles on your GPS device?
I have already explained that a GPS device only places you to within 10 - 20 metres of your location because of the restrictions with the radio signals travelling down to earth.
However, you should also know that a running race route is marked in a straight line, which is often a chalk line on the road. If you see it in a race, follow it, as it is the shortest distance to the finish line.
It is very rare that you are able to follow this line because of other runners and the need to weave. Weaving is a phenomenon observed over the first 2 miles of any race which, as the race goes on and everyone gets tired, reduces. What it will do is increase the overall distance you run during a race.
As you can see from the illustration, the yellow line is the race distance line, whilst the red is the weaving runner (pretty crazy I know). As you can see the red line is much longer than the yellow line, so our runner has run a lot further than the measure of the course.
Corner like a champion
Corners also present a difficulty to a runner with a tracking system on them.
As shown here, race directors will usually measure the inside of the corner (the yellow line), however a runner is unlikely to be able to the the yellow route due to other runners or just a desire to take the corner a bit wider.
The same issues can arise at drinks stations.
What these rather ridiculous drawings are designed to illustrate is that it is rarely possible to run the exact course route. This is one reason that GPS is not 100% accurate on race day.
There will be a piece on Garmin results from the Virgin London Marathon 2011, comparing a number of peoples Garmin results.
That is why courses have mile markers on them, so you know how far you have actually gone.
The opening of the satellites spawned a multi billion dollar industry that we have today and it has allowed runners to train and measure their performance by just stepping out the door and running. Whilst becoming a mile junkie is never a good thing, it is always nice to see how far you are running and at what pace. Everything that we do today is measured and compared, and GPS allows us to compare ourselves after every session, to our own and others performance. It allows coaches to look at your training and see exactly what you have done.
However, you have to remember that GPS is not 100% accurate for civilian users. Due to this everything that is shown on your watch should be taken with a slight pinch of salt. It is a revolutionary service that I am not sure how I would train without.