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EGNOS User Segment
| EGNOS | |
|---|---|
| Title | EGNOS User Segment |
| Edited by | 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 with integrity.
To receive EGNOS signals, an EGNOS compatible receiver is required; they 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.[1]
EGNOS Receivers and User Applications
The European GNSS Agency (GSA)[2] 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 within 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.[3]
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.[4]
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:[4]
- 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 the ionised layer of the Earth atmosphere located from around 60 kilometres to several thousand kilometres. When propagation through the ionosphere, navigation signals are disturbed, resulting in range measurement errors or reduced availability. Please refer to the article Ionospheric Delay for further information.
- 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. Tropospheric effects are further described in the article Tropospheric Delay.
- Local effects: When propagating in the local environment of a user receiver, navigation signals are prone to reflections or obstructions 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).
Credits
This article contains some verbatim extracts taken from ESA navigation web pages[1] and EGNOS SoL Service Definition Document[4] according to the references cited.