Accuracy of GPS Observations
Product specifications for many GPS receivers sold in the market indicate that their accuracy is within 3 to 15 meters 95 percent of the time. This assumes the receiver has a clear view of the sky and has acquiring a sufficient number of satellites. Many receivers are WAAS enabled, which can enhance accuracy in many parts of North America and Hawaii. The WAAS or Wide Area Augmentation System is an air navigation aid meant to augment the accuracy, integrity and availability of the Global Positioning System. This system was developed by the Federal Aviation Administration.
But if you are working in areas outside the North American continent and Hawaii, your receiver will not use WAAS. However, other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Indian is also putting in place its own system the GPS Aided Geo Augmented Navigation (GAGAN). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems. All things considered, you can usually expect to be within about 20 to 30 feet of the mark with most consumer grade receivers.
Factors affecting accuracy
Given a basic understanding of how GPS works, let us examine some of the key issues effecting the accuracy of GPS. These include:
1. The quality of GPS receiver unit
2. Position of GPS satellites at the time of fixing position
3. Characteristics of the surrounding landscape.
1. GPS RECEIVER
There are many GPS devices that can be used to record track logs. These include dedicated GPS loggers, to smartphones with built in GPS, and everything in between. As might be expected, the quality of the GPS receiver can greatly affect the accuracy of recorded track logs. The following areas are of particular importance.
A good antenna is required to detect the 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 travelled from satellite to Earth's surface, their strength 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
GPS signals must be received from a minimum of four satellites in order to fix a position. However, the more the number of satellites available for positioning, the more accurate is the determined position. Although early GPS receivers were limited to just a few satellites they could track at any one time, modern GPS receivers have enough "tracking channels" to follow all satellites in view. More channels are also helpful in reducing the time it takes to get an initial fix (cold start) and to reduce power consumption.
To calculate the distance between the GPS receiver and 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 specified by the manufacturer, it is important that the GPS receiver includes good error correction algorithms.
2. POSITION OF GPS SATELLITES
As stated above, the greater the number of satellites used for calculating the position, the greater the level of accuracy. As they orbit around Earth, the number of GPS satellites in view (under optimal conditions) naturally varies. Although the user has no control over this, it is still worth understanding how this influences 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 through the standard menus. Unfortunately with hundreds of GPS receivers available, it is impossible to provide documentation for all devices. Refer to the manual that came with your device or try searching online. Smartphone apps come with this "satellite view" feature.
3. SURROUNDING LANDSCAPE
GPS requires a direct line of sight between the receiver and the satellite. 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 inaccuracies in calculated positions. Even when the obstructions are less obvious (tree cover, car roof, your body), reflection and weakening of signals may still occur.
When carrying a GPS device, generally, the higher the antenna is held, 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.
In closed spaces, such as a steep sided valley or a high rise urban environment, the area of sky visible to the GPS receiver is greatly reduced. This gives rise to two problems. Firstly, it reduces the number of satellites that are in direct line of sight of the receiver, and secondly, it prevents the GPS device from receiving GPS signals from a dispersed 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 (GDOP), 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. If a track is to be recorded from a vehicle, it is a good idea to get a very good fix before entering the vehicle.
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