The ionosphere is the layer of atmosphere between 50 and 1,000 km above the Earth. This layer contains electrically charged particles called ions due to solar radiation. These ions alter the transmission time of the satellite signals and can cause a significant amount of satellite position error (typically ±5 meters). Modelling the ionosphere is highly variable and more difficult to model. This is generally the largest error source in GNSS measurement, but it can be reduced by using relative or multi-frequency measurement.

The troposphere is the layer of atmosphere closest to the surface of the Earth. It is approximately 8 and 14 km deep, depending on the location on the Earth’s surface. Tropospheric errors are caused by temperature, density, pressure or humidity changes.. Here, the GNSS signal is mainly affected by water vapour, which varies is a lot in time and space.

Multipath error occurs when a signal from the same satellite reaches a GNSS antenna via two or more paths, such as reflected off the wall of a building. The reflected signal clearly has to travel further to reach the antenna and so it arrives with a slight delay. 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. Also, to reduce multipath errors place the GNSS antenna in a location that is away from the reflective surface.

Receiver noise refers to the position error caused by the GNSS receiver hardware and software. All GNSS receivers are subject to other radio waves that occur naturally or through other processes. These other radio waves, along with other types of electrical interference are what is known as background noise, and impact the ability of the receiver to ‘hear’ the GNSS signals. High-end GNSS receivers tend to have less receiver noise than lower-cost GNSS receivers. Calibration and signal processing techniques are employed to minimize these errors, but they can still impact accuracy.