Global Navigation Satellite System (GNSS) positioning is based on the pseudorange between satellites and receivers. The ‘time of flight’ of radio signals from several satellites to a receiver is used to calculate pseudorange or pseudo-distances. The term ‘pseudorange’ is used to distinguish it from true range, as it may be affected by various sources of error in time of flight measurement. Even the smallest timing errors can result in large position errors: for example, a millionth of a second timing error can create a distance error 0.16 NM.

Various types of error may degrade precision, including the following:

• Ionospheric and tropospheric errors
• Satellite clock errors
• Ephemeris data errors
• Receiver quality
• Multipath error
• Dilution Of Precision (DOP)

SOURCES OF ERROR

Ionospheric Errors. The ionosphere is the layer of the atmosphere between approximately 50 km to 1000 km above the Earth’s surface. This layer contains ions which are electrically charged. When the GNSS signal passes through this layer, its interaction with theses ions reduces its speed and therefore introduces an error. Ionospheric delay may vary depending on solar activity, the time of year, time of day or location, making it very difficult to predict the induced delay. On the other hand, the troposphere is the layer closest to the earth surface. It is approximately 8 and 14 km deep, depending on the location on the Earth’s surface. Tropospheric errors can also exist, but are far smaller and due to temperature and density factors mostly, and are not mentioned in the Learning Objectives. Ionospheric errors are the greatest errors affecting GNSS accuracy.

Satellite Clock Errors. Although GNSS satellites use the most precise atomic clocks featuring nanosecond accuracy, the clock drift phenomena may cause minute inaccuracies which can produce errors that affect positioning.

Ephemeris Data Error. These are errors induced by the satellite’s location. An ‘ephemeris error’ describes the difference between the expected and actual orbital position of a GNSS satellite. Because GNSS receivers use the satellite's location in pseudorange calculations, orbital error reduces GNSS accuracy. The navigation messages include ephemeris data together with information about the time and status of the entire satellite constellation, called the almanac. Ephemeris are used to calculate the position and the clock deviation of the receiver; the almanac is used to check the visible satellites from a receiver according to the moment of time and its position.

Receiver Quality. The hardware used within the receiver may limit precision by introducing inaccuracies in receiver timing.

Multipath Errors. Multipath errors appear when a GNSS signal arrives at the receiver GNSS antenna after having been reflected from an object such as the surface of a building. The reflected signal clearly has to travel further to reach the antenna and so it arrives with a slight delay. This delay can cause positional error.

Dilution Of Precision (DOP). DOP error may be caused by the relative positions in three-dimensional space of the satellites used to calculate a position. To get a better understanding, the concept of Geometrical DOP (GDOP) is often used. Poor GDOP values mean ‘bad’ positioning of satellites. On the contrary, ‘well’ distributed satellites produce good values. Satellites that are closer together will create a less accurate position measurement, whereas satellites which are well spread around the sky will give a more accurate position of the receiver, as the spheres of pseudorange cut each other at better angles.

The term "User Equivalent Range Error (UERE)" is the combination of all satellite and SiS (Signal-in-Space) errors, and includes all GNSS errors except for Dilution of Precision, which is a different type of error.