Accuracy of GPS data
The accuracy of GPS data depends on many factors. For example, the quality of the GPS receiver, the position of the GPS satellites at the time the data was recorded, the characteristics of the surroundings (buildings, tree cover, valleys, etc.) and even the weather. This page gives a basic introduction as to how GPS works and describes some of the key issues related to accuracy
How GPS works
The Global Positioning System (GPS) is a satellite navigation system that provides location information anywhere on or near the Earth’s surface. It comprises several satellites in orbit above Earth. Each satellite continually transmits messages that include the time the message was transmitted, and the satellite position. On the ground the GPS unit receives these messages and, by comparing the time at which the message was received (on its internal clock) against the time which the message was transmitted, it works out how far away it is from each satellite.
To calculate its location, the GPS unit must receive messages (signals) from a minimum of four satellites. Consider the following:
|A GPS unit receives signals from several satellites. Let’s call them “Green”, “Red” and “Purple”. On receiving each signal, it calculates its distance from each satellite.
If the GPS unit only receives a signal from the “Green” satellite, then it can only determine that its location lies somewhere on the sphere of all locations that are the same distance from the “Green” satellite (as shown as the green sphere in the diagram above).
Now consider the case when the GPS unit receives signals from both the “Green” and “Red” satellite. As before it determines its distance from each satellite. As we have received two signals we can narrow the location down to those points where the two individual distance spheres intersect. This means the location must be somewhere on the blue circle as shown in the diagram.
|By introducing a third satellite we can further narrow the location down to two points (as shown as yellow dots). Only one of these points will be on the Earth’s surface and therefore we can discard the other. With just three satellites we have trilaterated (like triangulation) our location. In practice a fourth satellite is needed to improve accuracy (particularly altitude accuracy) due to errors in measuring the precise time at which each signal was received.|
Factors affecting accuracy
Given a basic understanding of how GPS works, this section describes some of the key issues effecting the accuracy of GPS. These include:
• The GPS receiver unit (quality);
• the position of the satellites at the time the recording was made; and
• the characteristics of the surrounding landscape.
There are many GPS devices that you can use to record track logs. This includes dedicated GPS loggers, to smartphones with built in GPS, and everything in between. As you might expect, the quality of the GPS receiver can greatly affect the accuracy of your recorded track logs. The following areas are of particular importance.
Most obviously, a good antenna (also known as aerial) is required to detect the message signals coming from the GPS satellites. The strength of a GPS signal is often expressed in decibels referenced to one milliwatt (dBm). By the time the signals have covered the 22,200km from satellite to Earth’s surface, the signal is typically as weak as -125dBm to -130dBm, even in clear open sky. In built up urban environments or under tree cover the signal can drop to as low as -150dBm (the larger the negative value, the weaker the signal). At this level, some GPS devices would struggle to acquire a signal (but may be able to continue tracking if a signal was first acquired in the open air). A good high sensitivity GPS receiver can acquire signals down to −155 dBm and tracking can be continued down to levels approaching −165 dBm
- Number of channels
As described in the #How GPS works section above, a 3-satellite system, in theory provides all the data you need to calculate a reasonably accurate location. However, clock inaccuracies mean that this theory resides to the textbooks. In practice signals must be received from a minimum of four satellites to correct for errors. The more the better. Although early GPS receiver were limited to the number of satellites they could track at any one time, modern GPS receivers have enough “tracking channels” to follow all satellites in view. More channels are however still helpful to reduce the time it takes to get an initial fix (cold start) and to reduce power consumption.
- Position algorithms
To calculate the distance the GPS receiver is from each satellite, the receiver first calculates the time that this signal has taken to arrive. It does this by taking the difference between the time at which the signal was transmitted (this time is included in the signal message) and the time the signal was received (by using an internal clock). As the signals travel at the speed of light, even a 0.001 second error equates to a 300km inaccuracy of the calculated distance! To reduce this error level to the order of meters would require an atomic clock. However, not only is this impracticable for consumer GPS devices, the GPS satellites are only accurate to about 10 nano seconds (in which time a signal would travel 3m). It is for precisely this reason why a minimum of four satellites is required. The extra satellite(s) is used to help correct for the error. Although rarely publicised, it is therefore important that your GPS receiver includes good error correction algorithms.
Position of satellites
Signals from a varying number of “in view” satellites to determine your position on the earth
As noted above, generally the more satellites used in calculating your position the greater the level of accuracy. As the GPS satellites orbit around Earth, the number of satellites in view (under optimal conditions) naturally fluctuates. This can be seen in the animation on the right. Obviously, the position of the satellites is completely out of our hands, however it is worth recognizing this as a factor influencing accuracy. For example, this is one of the many reasons two GPS tracks recorded on separate days will differ. If you have time, it may be worth recording a track twice (or more) and averaging the results.
Some GPS receivers can display the number of satellites currently in view and their positions on a radar type diagram. On some receivers, this can be prominently found in the within the standard menus, however on others it may be within a “hidden” or “debug” menu. Unfortunately, with hundreds of GPS receivers available, it is impossible to provide documentation for all devices – please refer to the manual that came with your device or try searching online. Smartphone apps with this “satellite view” feature are shown in the monitoring features table for both iOS and Android based phones.
Note: – See also the section on #Enclosed spaces below.
Reflections signal weakening
Error caused by reflections and shading under tree cover.
GPS requires a direct line of sight between the receiver and the satellite. When an object lies within the direct path, accuracy suffers due to reflections and weakening of signals. This is particularly problematic in urban environments, within valleys and on mountain slopes. In all three situations, the objects (buildings and the Earth itself) are substantial enough to completely block the GPS signals. When weak signals are received, they may have been reflected off buildings and the surrounding landscape. Reflections generate multi-path signals arriving with a small-time delay at the receiver. This results in inaccurately calculated position.
Even when the object is less substantial (tree cover, car roof, your body), reflection and weakening of signals may still occur. This can sometimes be observed when viewing your recorded GPS track logs on top of aerial imagery. In the image on the left, the true position of the footpath follows the shadowy area in the forest. However, as the GPS receiver enters the forest (walking from east to west), it can be observed that reflections cause the recorded track to incorrectly shift slightly to the south.
When carrying a GPS device, generally, the higher the antenna is fixed, the better the reception. Good positions include the shoulder strap or the top pocket of a backpack, mounted on top of a cycle helmet, or a roof antenna on a car.
More Issues with Accuracy:
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create
an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere. Obviously, mountains and clouds cannot be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. The net result can be best described by a study by Michael D. Londe PHD summary below.
What other factors affect measurements using a GPS
Not only is the GPS not accurate enough for a true reading but runners on a course are
not able to run the exact shortest distance due to a number of factors including:
- Courses with lots of turns often create longer GPS readings. Runners in a race can try to run the tangent or the shortest possible line on a corner, but often other runners are in the way, or traffic prohibits them from doing this safely.
- Water stops and other excursions from the course will make your GPS report a longer difference
- Not starting the GPS at the start line, often the runner starts the GPS before the start in the corral
- Inexperience with the course. If you are running a course for the very first time, you are not able to pick the best tangent
- shortest line since you have limited knowledge of the course
- Runners often choose to run on the softer side of the road, or on the cant of the road that feels best
- Runners almost never run in a straight line, they make hundreds of small adjustments in a race left and right. Try this test, run 10 miles hard on the roads, now go to a track and try to run following exactly on the white line for 3 miles. You will find yourself wandering ever so slightly.
- Runners are more focused on “running the race” and over time become tired and more focused on finishing. Professional certifiers ride smoothly along on a bike with little or no discomfort to distract them.