Sources of error in GNSS

Unfortunately, GNSS measurements are not perfect and absolute. The measurement system is subject to several sources of error, as indicated in the diagrams below.

GNSS signals are very weak: typically less than 100 watts transmitted from a distance of 20,000 km to 25,000 km. When received at the surface of the earth, the signal strength may be as low as -160 dBW (1 x 10-16) watts.

Satellite orbit

Orbital biases occur within the ephemeris transmitted, mostly as a result of un-modelled gravitational forces.

Satellite clock

The satellite clocks experience drift and noise which are modelled and included as part of the broadcast message, although residual error remains.

Ionosphere and Plasmasphere

The Ionosphere is the region of the atmosphere between around 80km – 600km above the earth. The signals are delayed in the region above an altitude of 80km by an amount proportional to the number of free electrons given off by the Sun. The effect is lower when the satellite is at the zenith than when it is near the horizon and it is frequency dependent. Uncorrected this is the largest error source.

Troposphere

Delay in the signal caused by varying temperature and humidity levels at up to 12km in height. Basic models can correct up to 90%.

Receiver noise

Inherent noise within the receiver which causes jitter in the signal.

Multipath

In addition to the direct satellite-to-receiver path, the signals are also reflected from the ground and other objects. These cause multiple copies of the signal or a broadening of the signal arrival time both of which reduce precision.

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