Site-Dependent Electromagnetic Effects in High-Accuracy Applications of GNSS

1. Site-Dependent Electromagnetic Effects in High-Accuracy Applications of GNSS Jan Johansson and Tong Ning Chalmers University of Technology, Onsala Space Observatory Camilla Granström and Per Jarlemark SP Technical Research Institute of Sweden Martin Lidberg and Gunnar Hedling National Land Survey of Sweden Presented by Gunnar Hedling IGS Analysis Center Workshop June 2-6, 2008, Miami Beach

2. Presentation Structure The SWEPOS network Site-dependent effects Observables and Processing strategies Investigation of site-dependent effects in SWEPOS Calibration of GNSS stations - statistical calibration method - in-situ calibration method Summary

3. The Swedish Ground-Based GNSS NetworkSWEPOS • Operated by the National Land Survey of Sweden. • Collecting data since 1993 • 21 original stations and 140 additional roof-top stations for RTK-applications. • Original station separation ~200 km 3 m Roof-top Station Original Station

4. The Design of the Original SWEPOS Station

5. Satellite Extended signal path Antenna Site-Dependent EM Effects • Antenna type • Pillar design • Radome type • Station environment Snow on the antenna Multipath

6. Motives for this study? • Attaching Eccosorb on SWEPOS antenna-pillar system changed the estimated vertical component up to 10 mm depending on elevation cut off angle • What happens when new GNSS, new signals, and new ground and satellite antennas are introduced?

7. Site-Dependent EM Effects in SWEPOS Purpose To detect and investigate common and individual site dependent electromagnetic effects • All original SWEPOS stations and some • additional roof-top stations • Precise Point Positioning (PPP) solution strategy using • GIPSY-OASIS II • Residual analysis • Elevation cut-off angle set to 0 degrees • 6 years of reprocessed data (1999-2004)

8. Analysis Method Each observation in the PPP analysis gives a residual The direction in azimuth and elevation angles for the observations are known. Elevation dependent Azimuth and elevation dependent Daily residuals

9. Investigation of Original SWEPOS Station Arjeplog Onsala Hässleholm

10. Investigation of Original SWEPOS Station Arjeplog Onsala Hässleholm

11. The typical SWEPOS pattern Överkalix Vänersborg Simulation of multipath contribution following Elosegui et al , 1995 and Jaldehag 1995. α=0.06 H=0.15 m Vilhelmina Östersund LC multipath contribution [mm] α=0.06 H=0.5 m Our theory is that the similar pattern is due to the pillar-top design (concrete, metal plate etc.) Elevation angle [o]

12. Investigation of Roof-Top Stations Nynäshamn Stavsnäs

13. Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Fences Fences Fences Fences Fences Fences Fences Fences Fences Fences Fences Objects close Objects close Objects close Objects close Objects close Objects close Objects close Objects close Objects close Objects close Objects close Flat ground Flat ground Flat ground Flat ground Flat ground Flat ground Flat ground Flat ground Flat ground Flat ground Flat ground Special Effects and Possible Causes Fences Close objects Flat ground Effects seen in the SWEPOS network and possible cause Effects seen in the SWEPOS network and possible cause Fences Objects close Fences Objects close Flat ground Flat ground

14. Different characteristics among IGS Stations Tromsö Wettzell Metsähovi

15. GNSS Station Calibration Two methods of calibration where developed and evaluated for the SWEPOS sites 1. In-situ 2. Statistical Reference antenna [cm] Elevation ~ 5 m Residual plots

16. Statistical Station Calibration Results Averaged PPP residuals for the Onsala station with and without calibration implemented Uncalibrated based on 6 years data Calibrated based on 1 year data Mean residual [cm] Mean residual [cm]

17. In-Situ Station Calibration in Norrköping Between ”tripods” Three reference antenna of type Dorn Margolin type T

18. The New Calibration Station at Onsala • Construction during fall 2005 at Onsala • Antenna movable in X, Y and Z inside radome • Radome movable in Z direction • Eccosorb may be added Radome Antenna Z Z Y Investigation of effects from misalignment of antenna, radome and pillar X Results show clear “SWEPOS-like” residual pattern Pillar

19. ONTE without ECCOSORB ONTE with ECCOSORB ONSA MR – Microwave Radiometer ONTE

21. Stations with similar design and equipment suffers from similar elevation dependent effects Surfaces and objects in the near-field of the station could cause disturbances in the post-fit phase residuals correlated to its distance to the antenna. The additional unique features in the residual patterns for each station is related to its local environment. Guidelines are to avoid reflecting surfaces close to the antenna and keep vegetation below the horizon mask. Summary Site-Dependent Effects in SWEPOS GNSS station calibration • Mitigation of site dependent effects is important in order to achieve higher accuracy in positioning. (but also for the estimation of the water vapour content in the atmos-phere) • Two calibration methods were tested and both were found to suppress site effects. However the methods are not directly comparable since they correct for different error sources. • The reference antenna in the in-situ calibration needs to be well isolated from site-dependent error sources.