Accuracy of GPS data

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GPS Satellite NASA art-iif.jpg
A good way to get involved in the OpenStreetMap project is to upload GNSS (GPS, Galileo, GLONASS, BeiDou/COMPASS, etc.) traces. Recorded by your satellite receiver or mobile phone, the typical trace is a record of your location every second, or every meter (“tracelog”). Convert it to GPX format if it wasn't done for you automatically. The collected data can be displayed as a background of thin lines or little dots within the map editor. These lines and dots can then be used to help you add map features (such as roads and footpaths), similar to tracing from aerial imagery.
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The accuracy of GNSS data depends on many factors. For example, the quality of the GNSS receiver, the position of the GNSS 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 GNSS works and describes some of the key issues related to accuracy.

How GNSS works

In years past, the terms GNSS (Global Navigation Satellite System) and GPS (Global Positioning System) were essentially interchangeable in a discussion of "GPS accuracy". The United States GPS system was the first GNSS system available to consumers. In recent years, many additional GNSS systems have gone live globally (GLONASS, QZSS, Beidou, Galileo, and yet more others), and many devices can use multiple GNSS systems at the same time to improve coverage and accuracy. With that said, it's important in any modern discussion of accuracy of these devices to talk about overall GNSS accuracy.

GPS

Global Positioning System (GPS) is a satellite navigation system that provides location information anywhere on or near the Earth's surface. It comprises a number of 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.

In order to calculate its location the GPS unit must receive messages (signals) from a minimum of four satellites. Consider the following:

Sphere2-intersect.svg A GPS unit receives signals from a number of satellites. Lets 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.

Sphere3-intersect.svg 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 (similar to 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.
For more information see this video or animation.


Factors affecting accuracy

Given a basic understanding of how GNSS works, this section describes some of the key issues effecting the accuracy of GNSS fixes. These include:

  • The feature set of a GNSS device
  • The position of the satellites at the time the recording was made
  • The characteristics of the surrounding landscape.

GNSS receiver

There are many GNSS devices that you can use to record track logs. This includes dedicated GPS loggers, to smartphones with built in GNSS (many phones simply call this "GPS"), and everything in between. As you might expect, the quality and feature set of the GNSS receiver you use can greatly effect the accuracy of your recorded track logs. The following areas are of particular importance.

  1. GNSS systems the device can receive
    Many modern devices are capable of receiving numerous GNSS systems at the same time. GPS was the first system available, but today, many systems are. This includes GPS, Galileo, QZSS, Beidou, and so on. The more systems a device is capable of receiving, the more resilient it will be when recording position and traces.
  2. GNSS frequency bands the device can receive
    Historically, consumer GNSS devices were only able to receive in what is known as the Upper L-Band, in the 1500 MHz range.[1][2] In the Upper-L Band, GPS has the L1 signal, Galileo has E1, and GLONASS has G1. Most GNSS receivers that can receive only Upper-L Band will generally discuss maximum accuracy of about 3 meters. However, some new to market GNSS devices can leverage the newest Lower-L Band GNSS signals, represented by L5 in GPS, G3 in GLONASS, and E5a and E5b in Galileo. These newer signals are broadcast in the 1100-1200 MHz range; they penetrate structures more easily, and are less prone to reflections. Being able to receive both bands in a GNSS device is a huge advantage, and devices that do so advertise accuracies as high as 30 centimeters (rather than the legacy 3 meters). GNSS devices that do this are almost always referred to as "Dual band GPS" or "Dual band GNSS". If you plan to use a device to do GNSS/GPX traces, buying a dual band device if possible will provide significant opportunity for higher accuracy.
  3. Antenna
    Most obviously, a good antenna (also known as aerial) is required in order to detect the message signals coming from GNSS satellites. The strength of a GNSS signal is often expressed in decibels referenced to one milliwatt (dBm). By the time a GNSS signal has covered the distance from a satellite in space 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 GNSS devices will struggle to make an initial signal acquisition/fix (but may be able to continue tracking if a signal was first acquired in the open air). A good high sensitivity GNSS receiver can acquire signals down to −155 dBm and tracking can be continued down to levels approaching −165 dBm.
  4. Number of simultaneous receive GNSS receive channels
    As described in the #GPS section above, 3 visible GPS satellites, in theory provide all the data you need to calculate a reasonably accurate location. In practice, however, signals must be received from a minimum of four GPS satellites in order to correct for errors: the more the better. Modern GNSS receivers have enough "tracking channels" to follow many satellites at once, and can typically do so across multiple GNSS providers. More simultaneous receive channels are useful for overall accuracy, to reduce the time it takes to get an initial fix (cold start) and to reduce power consumption. For more reading see here.
  5. 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 GNSS devices, the GPS satellites themselves, in particular, 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 discussed at the consumer level, it is therefore important that your GNSS 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 GNSS system 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 GNSS 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 GNSS receivers can display the number of satellites currently in view, the GNSS system a given satellite is a part of, and satellites' 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 GNSS 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.

See also the section on #Enclosed spaces below.

Your location

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.

Enclosed spaces

Highly clustered satellites can give large errors.
A disperse set of satellites improve accuracy.

Being in an enclosed space, such as a steep sided valley or a high rise urban environment, reduces the area of sky visible to the GPS receiver. This causes two problems. Firstly, it reduces the number of satellites that are in direct line of site of the receiver, therefore breaking the "the more the better" rule described above. Secondly, it prevents the GPS device from receiving GPS signals from a disperse set of satellites - that is, the satellites used to calculate your location are clustered within a small area of the sky.

Highly clustered satellites can result in large positional errors, up to several hundred meters. Although there is little that can be done to improve the situation in enclosed spaces, it is worth keeping an eye on your GPS device so that you are aware of when the signal quality drops. Look for a "satellite view" diagram (as shown in the images on the right) on your device.

For more information, or if your device also reports a "DOP Value", you may wish to read wikipedia:PDOP.

Troubleshooting GPS reception

In vehicles

If you plan to record a track from a vehicle, get a very good fix before you enter it. This is especially true for newer trains, where you might well never get one otherwise.

When do you know reception is good?

A 3D fix is not a sufficient criterion of quality. The PDOP is an indicator of the precision of the GPS measure (Position Dilution of Precision). If it is higher than 6, you can consider that you do not have a good fix. Under 4, it is good enough for OSM tracking. Less than 2 means you have a very good fix. The quality of the DOP depends on the GPS capacity of correcting the satellite's signal. You can have a good DOP with only a 2D fix.

See also

External links