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GNSS measurements are affected by various error sources. Some of them are described here.
If the influence from the error sources is large, it also means that the uncertainty of the GNSS receiver's position increases. In the worst case, no position may be available at all.
Satellite and receiver clocks
Clock error means that the signal is not transmitted or received exactly at the time specified. However, satellite clock errors are much smaller in absolute numbers because the satellites have atomic clocks with very high precision. Clock errors can be reduced or eliminated using relative measurements.
Satellite orbits
Orbital errors occur when the GNSS satellite is not exactly in the position that was predicted and transmitted through orbit data. This error source can be reduced by means of relative measurement, or by post-processing when observed and post-processed orbit data of good quality ar available.
Ionosphere
The ionosphere is the upper part of the atmosphere which contains electronically charged particles due to solar radiation. When the GNSS signal travels through the ionosphere it is affected in a way that is directly proportional to the frequency of the signal. This is generally the largest error source in GNSS measurement, but it can be reduced by using relative or multi-frequency measurement.
Troposphere
The troposphere is the lower part of the atmosphere, where weather occurs, and extends up to 7-17 kilometers above the Earth's surface depending on latitude. Here, the GNSS signal is mainly affected by water vapour, which varies is a lot in time and space. The troposphere error is slightly smaller than the ionosphere error under normal Swedish conditions but is more difficult to reduce because it is not frequency dependent.
Multipath
Sometimes the GNSS signals do not travel the shortest distance between satellite and receiver because the signal is reflected on other objects along the way. The phenomenon is called multipath and is highly dependent on local conditions. In an environment with tall houses or trees, multipath errors are more common. This error source can be partially reduced by observing over a longer period and with good quality GNSS equipment.
Other error sources
There are also other error sources that must be considered when doing GNSS measurements, such as signal interference, hardware delay and relativistic effects. In most cases, these are handled automatically in the receiver.
In addition, the user's handling of equipment and the execution of the measurement affect the standard uncertainty that can be expected in the GNSS measurement. Advice on measurement methodology and instrument handling can be found in the HMK documents on GNSS-based measurements.
Questions and answers
With the low-cost receivers available in smartphones and car navigation systems, the uncertainty will be in the order of 5-20 meters, depending on how much the satellite signals are disturbed.
GNSS measurements for professional use, such as construction work, require lower uncertainty. For such applications more expensive GNSS receivers are used in combination with correction data from other GNSS receivers or services.
With GNSS measurements it is not really the height above sea level that is determined, but the height above a reference surface called the ellipsoid. These different heights differ from each other by 20-40 meters in Sweden. To convert from height above the ellipsoid to height above sea level, a so-called geoid model is needed.
In addition, the uncertainty of the GNSS measurement is greater in the height component than in the horizontal, depending on how the satellites are located in relation to the receiver. The ideal situation would be to have satellites also "under you". The fact that the GNSS signals are disturbed by the atmosphere and reflections in the local environment also means that determination of vertical positions become more uncertain than horizontal coordinate determination.