EGNOS User Segment

EGNOS
Title	EGNOS User Segment
Author(s)	GMV.
Level	Basic
Year of Publication	2011

The EGNOS User segment is made of EGNOS receivers that enable their users to accurately compute their positions.

To receive EGNOS signals an EGNOS compatible receiver is required which are already available on the market from a variety of manufacturers. An EGNOS receiver is like a GPS receiver but with special software inside that allows the receiver to lock onto the code used by the EGNOS satellites and compute the EGNOS corrections to the GPS signals. An EGNOS receiver is the same size as a GPS receiver and uses the same type of antenna.

EGNOS Receivers and User Applications

The European GNSS Agency (GSA) and other institutions involved in the GNSS Industry have extensively worked on the definition of communication protocols, specification and user performances for EGNOS Compatible Receivers.

Among the endless applications withing the EGNOS User Segment, SISNeT is a remarkable technology that combines the powerful capabilities of satellite navigation and the Internet. The highly accurate navigation information that comes from the EGNOS (European Geostationary Navigation Overlay Service) Signal-In-Space (SIS) is now available over the Internet and in real time via SISNeT.

Errors Affecting User Positioning

A GNSS receiver processes the individual satellite range measurements and combines them to compute an estimate of the user position (latitude, longitude, altitude, and user clock bias) in a given geographical coordinate reference frame.

The estimation of the satellite-to-user range is based on the measurement of the propagation time of the signal and a number of error sources affect the accuracy of these measurements:

• Satellite clocks: any error in the synchronisation of the different satellite clocks will have a direct effect on the range measurement accuracy. These errors are similar for all users able to view a given satellite.
• Signal distortions: any failure affecting the shape of the broadcast signal may have an impact on the propagation time determination in the user receiver.
• Satellite position errors: if the spacecraft orbits are not properly determined by the system’s ground segment, the user will not be able to precisely establish the spacecraft location at any given point in time. This will introduce an error when computing the user position. The size of the error affecting the range measurements depends on the user’s location.
• Ionospheric effects: The Ionosphere is an ionized layer of the atmosphere located a few hundred kilometres above the surface of the Earth. When transiting through the ionosphere, the satellite navigation signals are perturbed, resulting in range measurement errors.
• Tropospheric effects: The troposphere is the lower part of the atmosphere where most weather phenomena take place. The signal propagation in this region will be affected by specific atmospheric conditions (e.g. temperature, humidity…) and will result in range measurement errors.
• Reflections: When propagating towards the user receiver, navigation signals are prone to refl ections from the ground or nearby objects (buildings, vehicles...).
• Thermal noise, Interference and User receiver design: the navigation signals have an extremely low power level when they reach the user receiver.

When computing its position the user receiver combines the range measurements from the different satellites in view. Through this process, the individual errors affecting each range measurement are combined which results in an aggregate error in the position domain. The statistical relationship between the average range domain error and the position error is given by a factor that depends on the satellite geometry; this factor is named DOP (Dilution Of Precision).