A Norwegian Ionosphere Model Based on GPS Data

Anna B.O. Jensen

Nordic Institute of NavigationOslo, June 2008

Outline

Navigation in the arctic Ionospheric activity at high latitudes SATREFTM Ionosphere Model Verification towards IGS GIM Comparison with EGNOS ionosphere

model Summary

Navigation in the arctic (1) Norway is located at high

latitudes, from 60° to 80° N,

mainly in the arctic region

GPS is used extensively as a navigational mean in the arctic

Unfortunately, GNSS performance is limited in the arctic compared to mid latitudes

Navigation in the arctic (2) Much offshore activity

in the Norwegian Sea Need for reliable

navigation

In the future: Increasing activity and

more traffic due to global warming and more oil and gas exploration

Increasing need

for navigation

Ionospheric activity at high latitudes

At high latitudes characteristics of the ionospheric activity are different than at mid latitudes: Higher ionospheric variability Increased amount of scintillation

This does affect navigation users e.g. by: Reduced accuracy Poor signal tracking (loss of lock)

60°N network

Trimble gpsnet

software

Ionospheric activity at high latitudes

Feb. 28, 2008

70°N network

SATREFTM Ionosphere Model (1)

Several ionosphere models exist, but they are generally poor for high latitudes

In 2007 the NMA therefore started development of a regional Norwegian ionosphere model based on the SATREFTM network of GNSS stations

SATREFTM Ionosphere Model (2) The model is based on:

GPS data from selected SATREFTM stations

Estimation of ionospheric delays in the stations

Spatial interpolation to obtain nationwide grid model

Test area

Test data

Day of year Date K-index Iono. activity

DOY 015 Jan. 15, 2008 2 – 4 Low

DOY 033 Feb. 02, 2008 2 – 6 Medium

DOY 059 Feb. 28, 2008 2 – 6 Medium

DOY 324 Nov. 20, 2007 1 - 5 Medium

Verification towards GIM (1)

Verification with respect to the Global Ionosphere Model (GIM) of the IGS

IONEX files retrieved from the IGS web site, and L1 ionosphere delays extracted for comparison with SATREFTM Ionosphere Model

20 grid points used for verification

Verification towards GIM (2)

Date Mean

[ meter ]

Std. dev.

[ meter ]Jan. 15, 2008 -0.02 0.12

Feb. 02, 2008 -0.03 0.12

Feb. 28, 2008 -0.04 0.18

Nov. 20, 2007 -0.05 0.19

Differences, SATREFTM minus IGS GIM

30 second sampling, 20 grid points

Verification towards GIM (3)

Summing up: Mean of differences of 2 - 5 cm is

basically negligible• Indicates no offset between the two models

Standard deviation of 12 – 19 cm• Occur mainly because no filtering is applied

to the SATREFTM model• Lower standard deviation on the day with low

ionospheric activity

Comparison with the EGNOS iono. model

Verification of the SATREFTM model towards the IGS GIM showed acceptable results

Therefore, the SATREFTM model is now used for a preliminary evaluation of the performance of the EGNOS ionosphere model in the arctic

Comparison with EGNOS iono. model

Date Mean

[ meter ]

Std. dev.

[ meter ]

Samples

Jan. 15, 2008 -0.12 0.15 6868

Feb. 02, 2008 -0.10 0.20 6230

Feb. 28, 2008 -0.15 0.20 6435

Nov. 20, 2007 -0.12 0.23 6189

Differences, SATREFTM minus EGNOS

16 grid points

Selected grid point – Feb. 28, 2008

EGNOS: blue, GIM: green, SATREFTM: red

EGNOS model is biased

Test area

Selected grid points – Feb. 28, 2008

EGNOS bias for upper grid point

Selected grid points – Feb. 2, 2008

Another day - again EGNOS bias for same point

16 grid points, Jan. 15, 2008

Future work Modify model to run in real time

Lots of programming

Further investigations to decide on: Coverage area Grid spacing Number of SATREFTM stations to include Temporal update interval Information to users – web application

Summary Development of the SATREFTM Ionosphere

Model has been initiated

Verification of the SATREFTM Ionosphere Model towards the IGS GIM show very good results

Comparison with EGNOS model show deviations for some grid points Improvement expected with new EGNOS version this

summer

Acknowledgments

Thanks to the Norwegian Space Centre for providing support for the work

Thanks to Ola Øvstedal, Norwegian University of Life Sciences in Ås, for valuable discussions during the development phase