GPS World, February 2014

Algorithms Methods INNOVATION Given the nature of its basis functions the SH model is an excellent tool for global modeling but it has some shortcomings for localized variability modeling As to larger regional networks with longer baselines such as those used for WAAS which covers North America the difference of the sensitivities between the batch and the station by station solutions should be larger than the results we have obtained However we cannot conclude that the sensitivity of 3D UNB IMT is better than that of the 3D SH model with the batch processing strategy for such large regional networks before more tests are conducted Still it is clearly seen in our tests that the 3D SH model is not always ideal for regional networks in terms of sensitivity We reached similar conclusions for October 25 2011 where the residuals spread more widely compared with quiet condition residuals In the storm conditions the residuals of the quadratic 3D UNB IMT spread relatively less than those of other modeling strategies This is especially the case for the several hours at the beginning of the day which corresponds to the peak of the Dst and Kp indices shown in Figure 3 The quadratic 3D UNB IMT seems to have the capacity to handle the ionospheric spatial and temporal variation even during severe storm conditions Repeatability of Estimated DCBs The DCBs not only have influence on the quality accuracy of the vTEC estimation but their repeatability can also provide information to evaluate ionospheric models This implies that the ionospheric models that have the capability to estimate eliminate more accurate DCBs independent of ionospheric variability are preferable We carried out a number of tests to evaluate the repeatability of estimated DCB values using the 2D and 3D UNB IMT approaches as well as the 3D SH technique under both quiet and disturbed ionospheric conditions For quiet ionospheric conditions the performance of all the tested models looks comparable although the quadratic 3D UNB IMT performs slightly better than the others As to the disturbed conditions the quadratic 2D 3D UNB IMT seems be able to provide more stable DCBs than the other models However the improvement of the extension from 2D to 3D is slight for the quadratic models although it is VLJQL FDQW IRU WKH OLQHDU PRGHOV 7KH performance of the 3D SH model looks fairly poor compared to 3D UNB IMT for regional modeling Consult the conference paper for further details Conclusions and Future Research In the work described in this article we extended the UNB IMT from 2D to 3D and compared the performance between them in station by station and batch processing scenarios for both quiet and storm ionospheric conditions We used the data from a small regional network of dual frequency GPS receivers The DCBs and ionospheric delays were estimated at the same time by a DOPDQ OWHU 7KH QHZO GHYHORSHG approach was evaluated by analyzing WKH SRVW W UHVLGXDOV 7 RI WKH VWDWH estimation process and the repeatability of estimates of DCBs In the single station processing the improvement of 3D UNB IMT has been demonstrated in both quiet and disturbed ionospheric conditions LQ WHUPV RI SRVW W UHVLGXDOV 7KH UNB IMT with more parameters allows the depiction of more complex vertical variability of the ionosphere The 3D UNB IMT is able to better deal with the measurements from lowelevation angle satellites owing to EOFs replacing the mapping function The artificial jumps with 2D UNBIMT when satellites come into or go out of view of the receiver have been properly handled by the 3D UNB IMT In addition the time series of estimated DCBs with 3D UNB IMT exhibit less perturbation than the results with 2D UNB IMT As to the multi station network processing it is confirmed that the station by station solution is more sensitive to localized information than the batch solution Based on the results from our research station by station processing with 3D UNB IMT is suggested to increase chances to catch localized ionospheric structures The repeatability of estimated DCBs was investigated as another indicator to evaluate the viability of ionospheric models Before the 3D UNB IMT is tested in the positioning domain for singlefrequency positioning it is worth validating the model with other data sources In addition the potential benefits of 3D UNB IMT during extremely disturbed ionospheric conditions is worth investigating further Acknowledgments We would like to thank the IGS and the Crustal Dynamics Data Information System for providing the GPS data and we acknowledge the financial contribution of the Natural Sciences and Engineering Research Council RI DQDGD IRU VXSSRUWLQJ WKH UVW DQG last authors This article is based on the paper Eliminating Potential Errors Caused by the Thin Shell Assumption An Extended 3D UNB Ionospheric Modelling Technique presented at the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation Nashville Tennessee September 16 20 2013 WEI ZHANG received his M Sc degree in space science in 2009 from the School of Earth and Space Science of Peking University China He is currently an M Sc E student in the Department of Geodesy and Geomatics Engineering at University of New Brunswick UNB under the supervision of Dr Richard B Langley ATTILA KOMJATHY is a principal investigator at the California Institute of Technology Jet Propulsion Laboratory and an adjunct professor at UNB specializing in remote sensing techniques using GPS He received his Ph D from the Department of Geodesy and Geomatics Engineering of UNB in 1997 SIMON BANVILLE works for the Geodetic Survey Division of Natural Resources Canada on realtime precise point positioning PPP using global navigation satellite systems He is also in the process of completing his Ph D degree at UNB under the supervision of Dr Langley MORE ONLINE Further Reading For references related to this article go to gpsworld com and click on Innovation in the navigation bar www gpsworld com February 2014 GPS World 65