Sources of Error

Selective Availability is the degradation of the GPS signal by the DoD. The full system accuracy was only available to authorized users (Selective Availability was deactivated on 1st of May 2000).
Both satellite and receiver clocks are not running synchron. The measured ranges are therefore called pseudoranges.
Multipath occurs when part of the signal from the satellites reaches the antenna after reflecting from the ground, a building or another object.

Figure 5.1: Multipath

Figure 5.1: Multipath

PDOP is a measure of the strenght of the satellite geometry. Satellite geometry is strong when the satellites are well distributed in the sky. Weak geometry is a source of error.

Figure 5.2: Good PDOP

Abbildung 5.2: Good PDOP

Figure 5.3: Bad PDOP

Abbildung 5.3: Bad PDOP

Atmospheric refraction in the ionosphere and troposphere alter the speed and direction of the GPS signal.

$Figure 5.4: Atmosperic Refraction$

Abbildung 5.4: Atmospheric Refraction

Author: Michael Schulz, GEOSOFT