The ability to provide location is a need in our societies. We need to locate items, places, business, customers, resources etc. Maps are the means to place and represent an object into a geographic context, and for producing maps objects need to be positioned. In Europe, European Geostationary Navigation Overlay Service (EGNOS) can help to locate great amount of points with sub-meter accuracy in an easy, affordable and flexible way and in real time. It can boost the data capture, usually the most time-consuming process in the development of a mapping or GIS application. How does EGNOS work?
EGNOS is the European SBAS (Satellite Based Augmentation System) and it represents EU’s first concrete venture into satellite navigation. It uses geostationary satellites and a network of ground stations to receive, analyze and augment GPS, and in the next version Galileo signals. It augments the L1 (1575.42 MHz) Coarse/Acquisition (C/A) GPS civilian signal by providing corrections and integrity information for safety of life applications.
The EGNOS signal is free of charge and its accuracy compliance area covers most of Europe. There are different means for users to access EGNOS corrections:
- EGNOS Signal in Space, directly via the geostationary EGNOS satellites (same L1 frequency as GPS satellites) using a GPS receiver that is EGNOS-enabled, without any additional cost.
- Via EDAS (EGNOS Data Access Service), through the Internet in real time or in form of archive data by the EDAS FTP Service. This option is beneficial for the users when the satellites are not in view, e.g., at high latitudes, urban canyons, etc.
Satellite based corrections: EGNOS Open Service
EGNOS has been designed and developed with the general goal to improve GPS performance in Europe. EGNOS Open Service is intended to offer these benefits for the users of general-purpose applications. It is freely accessible through a GPS/SBAS compatible receiver within the area of coverage and no specific authorisation is required.
EGNOS Open Service can be widely used for numerous applications. However, it is not suitable for safety critical applications: despite the performant, state-of-the-art design, implementation and operations the system, as well as service provision, it is not meant to offer a service guarantee or liability from the EGNOS service provider, the European Union, GSA or ESA.
In the particular case of mapping, EGNOS is employed for sub-metre level data collection. The positioning accuracy is defined mainly for horizontal coordinates (although EGNOS provides also corrected vertical position). The following parameters are key for users:
- The Error Ellipse defines a confidence region for the horizontal coordinates of a point. It is an approximate graphical representation of the standard deviation in two directions (lowest and highest precision directions). The most common confidence level used for error ellipses calculation is 95%.
- The 2DRMS represents the twice the Distance Root Mean Square. The DRMS is the root mean square of the radial distances from the true position to the observed positions obtained from a number of trials.
The following figures show typical accuracy one can reach with EGNOS.
Figure 1: – EGNOS 95% error ellipse 26.05.2017
Figure 2: – EGNOS & GPS standalone horizontal deviation 26.05.2017 – Santiago de Compostela (left) and Berlin (right)
EGNOS Open Service accuracy for mapping shows very good values over all EGNOS reference stations and Figure 1 demonstrate the EGNOS added value with respect to GPS standalone positioning: the observed horizontal error 95th-percentile is almost half than the for GPS standalone case. (Figure 2).
EDAS: EGNOS Data Access Service
EDAS (EGNOS Data Access Service) is the EGNOS Internet-based service, which provides free of charge access to all the data generated and collected by the EGNOS infrastructure. EDAS gathers all the raw data coming from the GPS, GLONASS and EGNOS GEO satellites collected by all the receivers located at the EGNOS stations, which are mainly distributed over Europe and North Africa.
Once the data are received, EDAS disseminates them over the Internet in real time and through an FTP archive, resulting in the different services depending on the protocol and format used, along with the type of information available to users.
EDAS users, equipped with compatible software applications and/or GNSS receivers and having access to the Internet, can obtain improved accuracy with respect to GPS standalone by implementing advanced positioning techniques.
The real-time positioning techniques supported by EDAS are the following ones:
- EGNOS augmented position: EGNOS corrections can be retrieved from the EDAS SISNET Service. This option is of special interest in areas where the orography obstructs the visibility of the EGNOS Geostationary satellite. The EGNOS messages provided by EDAS are the same that are broadcast by the EGNOS satellites and hence the achieved positioning performance would be equivalent to EGNOS OS (previous sections).
- RTK positioning: centimetre level accuracy can be achieved using EDAS Ntrip service when located in the surroundings of an EGNOS station (up to 40-50 km typically).
- Differential GNSS solution: DGNSS technique can be used through the EDAS Ntrip Service, applying DGNSS corrections from a close EGNOS station to improve the accuracy of the standalone solution.
For multiple land based applications, EDAS based DGNSS could be an interesting solution since the density of the EGNOS stations network is enough to provide significant coverage over Europe. In turn, considering sub-metre accuracies are sufficient to support a wide range of applications in the transportation, mapping and emergency services domains, EDAS DGNSS corrections could be an alternative for users located within a reasonable distance from the designated EGNOS reference station (for baseline distances below 260 km, EDAS DGPS is expected to support provide horizontal accuracies below 1 meter -95th percentile-).