Chalmers University of Technology

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 JarlemarkSP Technical Research Institute of Sweden

Martin Lidberg and Gunnar HedlingNational Land Survey of Sweden

Presented byGunnar Hedling

IGS Analysis Center WorkshopJune 2-6, 2008, Miami Beach

Chalmers University of Technology

Presentation Structure

The SWEPOS networkSite-dependent effectsObservables and Processing strategiesInvestigation of site-dependent effects in SWEPOSCalibration of GNSS stations

- statistical calibration method- in-situ calibration method

Summary

Chalmers University of Technology

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

Original Station Roof-top Station

Chalmers University of Technology

The Design of the Original SWEPOS Station

Chalmers University of Technology

Site-Dependent EM Effects

Antenna

Satellite

Extended signal path

• Antenna type• Pillar design• Radome type• Station environment

Snow on the antenna

Multipath

Chalmers University of Technology

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?

Chalmers University of Technology

Site-Dependent EM Effects in SWEPOS

• 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)

PurposeTo detect and investigate common and individual

site dependent electromagnetic effects

Chalmers University of Technology

Analysis Method

Each observation in the PPP analysis gives a residual

The direction in azimuth and elevation angles for the observations are known.

Daily residuals

Elevation dependent

Azimuth andelevation dependent

Chalmers University of Technology

Investigation of Original SWEPOS Station

Arjeplog Hässleholm Onsala

Chalmers University of Technology

Investigation of Original SWEPOS Station

Arjeplog Hässleholm Onsala

Chalmers University of Technology

The typical SWEPOS pattern

Östersund

Överkalix Vänersborg

Vilhelmina

Our theory is that the similar pattern is due to the pillar-top design (concrete, metal plate etc.)

Simulation of multipath contributionfollowing Elosegui et al , 1995 and Jaldehag 1995.

α=0.06H=0.15 m

α=0.06H=0.5 m

Elevation angle [o]L

C m

ulti

path

con

trib

utio

n [m

m]

Chalmers University of Technology

Investigation of Roof-Top Stations

Nynäshamn

Stavsnäs

Chalmers University of Technology

Special Effects and Possible Causes

Fences

Flat ground

Close objects

Chalmers University of Technology

Different characteristics among IGS Stations

Wettzell Tromsö Metsähovi

Chalmers University of Technology

GNSS Station Calibration

~ 5 m

Elevation

[cm]

Residual plots

1. In-situ 2. Statistical

Reference antenna

Two methods of calibration where developed and evaluated for the SWEPOS sites

Chalmers University of Technology

Statistical Station Calibration Results

Uncalibrated based on 6 years data

Mean residual [cm]

Calibrated based on 1 year data

Mean residual [cm]

Averaged PPP residuals for the Onsala station with and without calibration implemented

Chalmers University of Technology

In-Situ Station Calibration in Norrköping

Three reference antenna of type Dorn Margolin type T

Between ”tripods”

Chalmers University of Technology

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

Investigation of effects from misalignment of antenna, radome and pillar

Radome

Antenna

Pillar

Z

XY

Z

Results show clear“SWEPOS-like” residual pattern

Chalmers University of Technology

ONSA

MR – Microwave Radiometer

ONTE

ONTE without ECCOSORB ONTE with ECCOSORB

Chalmers University of Technology

Chalmers University of Technology

• 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.