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The Navigation Support Office (NSO) is operating a permanent Global Navigation Satellite System (GNSS) station on-site at ESOC as support for the European Geostationary Navigation Overlay System (EGNOS). The station is part of a European-wide ground network of 12 continuously-operating monitoring stations aiming to provide near-real time information on EGNOS key performance values for PERFECT, the “PERformance-website For Egnos Continuous Tracking”, hosted by ESA’s EGNOS Project Office (EPO) in Toulouse, France.

The site’s core component is a high-quality 48-channel dual-frequency Septentrio  PolaRx2 receiver, configured to gather Global Positioning System (GPS) and EGNOS data at a sampling rate of 1 Hz and an elevation cut-off angle of 5º. The receiver is connected to a geodetic dual-frequency antenna from Ashtech (“ASH701945C_M”) comprising a Dorne/Margolin element arranged on a choke ring ground  plane in order to weaken undesired multipath signals coming from below the horizon. The antenna is housed in a sealed, conically shaped radome to protect it against snow and ice. It is mounted on a steel mast located on the rooftop of the ESOC H building allowing a clear view of the sky with no obstructions above the selected elevation mask of 5º (Fig. 1). A lightning rod is installed close to the antenna in order to minimize the risk of lightning damage.

Fig. 1: GPS/EGNOS receiving antenna located on the rooftop of ESOC’s H building.

The antenna’s mean phase centre position with respect to the International Terrestrial Reference Frame 2005 (ITRF 2005) is precisely known within a few millimetres. The coordinates were obtained from a long-term static Precise Point Positioning (PPP) analysis using high-accurate satellite orbits and clocks from the International GNSS Service (IGS). The derived position serves as “ground-truth” for the EGNOS performance monitoring activities.

Although the PolaRx2 receiver offers dual-frequency capability, positioning is based on carrier-smoothed GPS L1 C/A code ranges only. Ionospheric propagation delays and GPS system biases (orbit, clock) are removed through the use of differential wide-area corrections provided by EGNOS. The corrections are broadcasted by navigation transponders onboard three geostationary satellites: Inmarsat AOR-E (PRN 120) located east of the Atlantic, Inmarsat IOR-W (PRN 126) above East Africa and ESA’s Artemis satellite (PRN 124) above Central Africa. The receiver at ESOC, however, is configured to only accept corrections from Inmarsat AOR-E (PRN 120).

Comparing the “kinematic” coordinate estimates with the “ground-truth” values from the static PPP analysis allows an evaluation of the positioning accuracy provided by GPS/EGNOS under real-life conditions. A long-term analysis including around 26 Million samples collected during the year 2010 indicates a horizontal and vertical accuracy (RMS) for EGNOS-aided single-frequency positioning of ±0.32 m and ±0.50 cm, respectively (Fig. 2). The number of GPS satellites being tracked by the station during 2010 ranges from 4 to 14.

 Fig. 2: Horizontal and vertical position errors from EGNOS-aided single-frequency positioning during the year 2010.

To have a more comprehensive view of the EGNOS system performance, a special kind of 2D histogram is used. The diagram, better known as the “Stanford-ESA Integrity Diagram”, illustrates the relationship between positioning error (XPE) against protection level (XPL). The protection level indicates the expected user error with an underlying integrity of 10-7. The XPE/XPL pairs are plotted as pixels with a colour coding representing the frequency of their occurrence. The plots are routinely generated on a daily basis with updates every hour.

A simplified form of the diagram showing the overall performance for the year 2010 is given below (Fig. 3). It clearly demonstrates that actually all EGNOS-aided coordinate solutions generated in 2010 were well within the EGNOS integrity margins meaning that there was not a single case when the position error exceeded the protection level (XPE > XPL), whether in horizontal or vertical direction.

Fig. 3: Relationship between positioning error against protection level during the year 2010.