Introducing My Ionospheric Research Work Using Various Modelling Techniques
Geodetic Research Laboratory, Department of Geodesy and Geomatics Engineering
University of New Brunswick, P.O. Box 4400, Fredericton, N.B. E3B 5A3 Canada
Phone: 1-506-453-4698, Fax: 1-506-453-4943
The electromagnetic waves propagating from the satellites of the Navstar Global Positioning System (GPS) to a GPS receiver on or near the earth's surface must travel through the earth's ionosphere. GPS receiver users must correct for the carrier phase advance and pseudorange group delay imposed on the signals by the ionosphere to achieve the highest possible positioning accuracies. Since a new solar cycle has just begun, this effect will become increasingly important.
In accounting for the effect of the ionosphere using a single-frequency GPS receiver, it is possible to use global ionospheric models [Langley, 1996]. Numerous studies have been undertaken using different empirical and physics-based ionospheric models for such a purpose. At UNB, we are conducting an on-going study to assess the accuracy and efficacy of such models. We decided to include the IRI-90 model [Bilitza, 1990] in our ionospheric research after Newby [1992] investigated the International Reference Ionosphere 1986 (IRI-86) model's performance. Earlier we compared the Broadcast model of the GPS navigation message [Klobuchar, 1986] and the IRI-90 model with vertical ionospheric range error corrections inferred by using Faraday rotation data. We concluded that the IRI-90 model appeared to be more accurate than the Broadcast model, both for day-time and night-time periods, during a low solar activity period, for mid-latitude conditions [Komjathy et al., 1995 and 1996]. The Faraday rotation measurements for use as "ground-truth" provided by GOES geostationary satellites are no longer readily available. We have, therefore, decided to use dual-frequency pseudorange and carrier phase GPS measurements to infer ionospheric total electron content (TEC).
Recently, UNB participated along with several other research groups in an experiment to assess the capabilities of GPS dual-frequency observations to provide TEC values. The measurement campaign was organized by the Orbit Attitude Division of the European Space Agency's European Space Operations Centre (ESA/ESOC) Darmstadt, Germany, under the auspices of the IGS. The initial results of the comparison of ionospheric products between different processing centers were reported at the IGS Workshop in Silver Spring, MD, 19-21 March 1996. The experiment involved the processing and analysis of a 5 week long data set of dual-frequency GPS data from stations of the IGS network (GPS weeks 823 through 827). UNB analysed GPS data sets from 6 of the European IGS stations. The European region was chosen so that UNB and the IGS processing and analysis centers producing regional ionospheric maps would have a common region for comparison. Our regional model uses the following stations: Madrid, Grasse, Matera, Brussels, Wettzell, and Onsala. In the context of geomagnetic latitudes, three distinct latitude regions can be identified in our test network (1. Madrid, Grasse, Matera; 2. Brussels, Wettzell; 3. Onsala). All 6 stations use Allen Osborne Associates Inc. TurboRogue receivers.
Some of the results of the processing have been reported previously by Komjathy and Langley [1996a], where we concluded that after processing data from the 6 European stations collected over a 7 day period (the first 7 days of the ionospheric experiment organized by ESA/ESOC), we were able to detect highly varying ionospheric conditions associated with a geomagnetic disturbance. After investigating the effect of using different elevation cutoff angles and ionospheric shell heights on the TEC estimates and satellite-receiver differential delays, we discovered that using different elevation cutoff angles had an impact on TEC estimates at the 2 TEC unit (total electron content unit - TECU) level. We also concluded that using different ionospheric shell heights has an effect on the ionospheric TEC estimates also at about the 2 TECU level depending on geographic location and time of the day. We discovered that there are no significant changes in the satellite-receiver differential delay estimates computed using different elevation cutoff angles. We also compared our TEC estimates with TEC predictions obtained by using the latest IRI model enhancement also known as IRI-95. The results of this comparison are similar to those of other studies (e.g., Newby [1992]), which also investigated data sets at low solar activity times and for mid-latitude conditions.
As a continuation of this initial study, we used 21 days' worth of data with a more rigorous approach for ionospheric shell height determination as derived from IRI-95. The results of this study have been reported in Komjathy and Langley [1996b]. In the case of TEC estimation using dual-frequency GPS data, the ionospheric shell height determination is one of the potential error sources that could bias our estimates. We introduced the notion of using varying ionospheric shell heights derived from the IRI-95 model as opposed to using an ionospheric shell height fixed at a commonly adopted altitude (400 km). We found differences in the differential delays between the two approaches of up to the 0.3 ns (1 TECU) level and differences in the TEC estimates up to the 1 TECU ( 0.16 m delay on L1) level. We also found that with an inappropriate setting of the ionospheric shell height, it is possible to introduce a 0.5 TECU level error for every 50 km error in the shell height. In the case of differential delays, the equivalent error is about 0.14 ns. After comparing our differential delay estimates with those obtained by other research groups participating in the experiment, we found agreement in the differential delays between the three participating analysis centers which are involved in analysing regional ionospheric maps, at the 1 ns level. The relatively large bias differences were also confirmed by Feltens et al. [1996] and Wilson et al. [1996]. These differences may be caused by the use of different ionospheric mapping functions by the different analysis and processing centers. The comparison of the TEC maps performed by the Deutsche Forschungsanstalt für Luft und Raumfahrt (DLR) Fernerkundungsstation, Neustrelitz, Germany concluded that there was a good agreement between DLR's and UNB's results for 12 of the 21 days under comparison [Jakowski and Sardon, 1996]. However, for the rest of the data, 2 to 4 TECU level differences were reported. An analysis performed by ESA/ESOC showed a 1 TECU mean bias between ESA/ESOC and UNB results with a standard deviation of about 2 TECU [Feltens et al., 1996].
The ionospheric estimation technique currently used at UNB is described in detail in publications such as Komjathy and Langley [1996a and 1996b]. A brief description of the model is as follows: we estimate three stochastic parameters for each IGS station in a network mode tied to a solar-geomagnetic coordinate system assuming a Gauss-Markov stochastic process. The three parameters use a spatial linear approximation of TEC above each IGS station. The L1-L2 phase-levelled geometry-free observable is used to estimate the stochastic parameters along with other biases such as the satellite-receiver differential delays using a Kalman filter approach.
We have finished processing all 5 weeks' worth of GPS data from the experiment and have produced hourly TEC maps at a 1 degree by 1 degree grid spacing for the European region spanning from -10 to 30 degrees in east longitude and 30 to 60 degrees in north latitude. In our current study we also investigated the practicability and efficacy of using the IRI-95 model to provide ionospheric range error corrections for single-frequency GPS users. The above described GPS-derived TEC maps have been used as "ground-truth" to provide updates to the IRI-95 model on an hourly basis. Once the IRI-95 update is completed, the new (updated) coefficient set for the IRI-95 is used to compute TEC predictions between two updates. For validation purposes, the updated IRI-95 model was used to compare the model performance with the GPS-derived TEC. We also compared predictions by the original IRI-95 with the GPS-derived TEC values. The results of this investigation is described by Komjathy and Langley [1996c]. and Langley and Komjathy [1996d].
We have recently made significant enhancements to our software to be able to independently produce global total electron content (TEC) maps on an hourly basis. The UNB global TEC maps can be input directly into a modified version of the International Reference Ionosphere (IRI-95) model to update its CCIR/URSI coefficient sets on an hourly basis. These updated IRI-95 coefficient sets serve as a basis for improved IRI-95 predictions by using the modified IRI-95 model as a sophisticated interpolator between two GPS-derived TEC updates.
For the ionospheric workshop at the University of Colorado held 24-25 September 1996, we processed 3 days' worth of global GPS data (33 IGS stations for each day) at a medium solar activity time (year 1993) and 3 days' worth of global GPS data (74 IGS stations for each day) at a low solar activity time (year 1995). An additional day's worth of global GPS data was also processed to help validate the UNB global TEC maps using Faraday rotation data. We produced hourly snapshots of the global ionosphere using GPS data only (mpeg movies of these maps can be accessed via the Web at <http://gauss.gge.unb.ca/grads/attila/movie/>). We have also compared the updated IRI-95 predictions using UNB's global TEC maps, the original IRI-95 predictions, and JPL-derived TEC maps (GIM) against 6 days' worth of TOPEX-derived TEC data which has been provided by the workshop organizers for comparison purposes. The UNB results show that based on 3 days' worth of global GPS data during a medium solar activity time in 1993, there was better than a 9 TECU level (1 sigma) agreement in the total electron content on a global scale with the TOPEX-derived TEC data using UNB's technique. For the low solar activity 1995 data, the UNB's results agreed with the TOPEX data at better than the 5 TECU level (1 sigma). The UNB technique has been demonstrated to be a viable alternative to provide independently-derived ground-based ionospheric delay corrections for future single-frequency radar altimeter missions. The results of this investigation is descrbed by Komjathy et al. [1996e] and Komjathy and Langley [1997].
REFERENCES
Bilitza, D. (ed.) (1990). International Reference Ionosphere 1990. National Space Science Center/World Data Center A for Rockets and Satellites, Lanham, MD. Report Number NSSDC/WDC-A-R&S 90-22.
Feltens, J., J.M. Dow, T.J. Martin-Mur, C. Garcia Martinez and Bayona-Perez, M.A. (1996). "Verification of ESOC Ionospheric Modeling and Status of IGS Intercomparison Activity." Proceedings of the 1996 IGS Workshop, Silver Spring, MD, 19-21 March (in press).
Jakowski, N. and E. Sardon (1996). "Comparison of GPS/IGS-derived TEC Data with Parameters Measured by Independent Ionospheric Probing Techniques." Proceedings of the 1996 IGS Workshop, Silver Spring, MD, 19-21 March (in press).
Klobuchar, J.A. (1986). "Design and Characteristics of the GPS Ionospheric Time Delay Algorithm for Single-Frequency Users." Proceedings of the PLANS-86 Conference, Las Vegas, NV, 4-7 November, pp. 280-286.
Komjathy, A., R.B. Langley, and F. Vejrazka (1995). "A Comparison of Predicted and Measured Ionospheric Range Error Corrections." EOS Transactions of the American Geophysical Union, Vol. 76, No. 17, Spring Meeting Supplement, S87.
Komjathy, A., R.B. Langley, and F. Vejrazka (1996). "Assessment of Two Methods to Provide Ionospheric Range Error Corrections for Single-frequency GPS Users." In GPS Trends in Precise Terrestrial, Airborne, and Spaceborne Applications, the Proceedings of International Association of Geodesy Symposium, No. 115, Boulder, CO, 3-4 July 1995, Springer Verlag, New York, pp. 253-257.
Komjathy, A. and R.B. Langley (1996a). "An Assessment of Predicted and Measured Ionospheric Total Electron Content Using a Regional GPS Network." The Proceedings of the National Technical Meeting of the Institute of Navigation, Santa Monica, CA, 22-24 January 1996. pp. 615-624.
Komjathy, A. and R.B. Langley (1996b). "The Effect of Shell Height on High Precision Ionospheric Modelling Using GPS." Proceedings of the 1996 IGS Workshop, Silver Spring, MD, 19-21 March (in press).
Komjathy, A. and R.B. Langley (1996c)."Improvement of a Global Ionospheric Model to Provide Ionospheric Range Error Corrections for Single-frequency GPS Users." Proceedings of ION 52nd Annual Meeting, Cambridge,MA, 19-21 June 1996 (in press).
Langley, R.B. and A. Komjathy (1996d). "High Precision Ionospheric Total Electron Count Mapping Using the Navstar Global Positioning System." Presented at the American Geophysical Union Western Pacific Geophysical Meeting, Brisbane, Australia, 23-27 July 1996.
Komjathy A., R. B. Langley and D. Bilitza (1996e). "Updating the International Reference Ionosphere Using a Global Network of GPS Stations: Preliminary Results." Presented at the workshop for Ionospheric Delay Correction for Single-frequency Radar Altimetry at the Colorado Center for Astrodynamics Research of the University of Colorado, Boulder, Colorado, U.S.A., 24-25 September 1996.
Komjathy A., R.B. Langley (1997). "Ionospheric Delay Correction for Single-Frequency Altimetry." Presented at the Jet Propulsion Laboratory, Pasadena, California, 12 March 1997.
Langley, R.B. (1996). "Propagation of the GPS Signals" in GPS for Geodesy, International School, Delft, The Netherlands, 26 March - 1 April, 1995. Springer-Verlag, New York.
Newby, S.P. (1992). An Assessment of Empirical Models for the Prediction of the Transionospheric Propagation Delay of Radio Signals. M.Sc.E. thesis, Department of Surveying Engineering Technical Report No. 160, University of New Brunswick, Fredericton, N.B., Canada.
Wilson, B., A. Mannucci, D. Yuan, H. Christian, X. Pi, T. Runge and U. Lindqwister (1996). "Global Ionospheric Mapping Using GPS: Validation and Future Prospects." Proceedings of the 1996 IGS Workshop, Silver Spring, MD, 19-21 March (in press).
Other Major Research Project I Was Involved In
Transport Canada Aviation, in conjunction with Cougar Helicopters Inc., initiated an in-service trial of helicopter precision approaches using local differential GPS (LDGPS). Cougar Helicopters, a Halifax-based commercial helicopter operator, equipped a Sikorsky S-76A helicopter with a Trimble TNL 3100 DZUS-mounted GPS/Loran-C avionics receiver. A high power TNL-2800G landing system base station at Halifax International Airport provided DGPS signals. Simultaneously, both at the base station and on board the helicopter, Ashtech LM-XII single frequency geodetic receivers collected data using the same antennas supplying the Trimble equipment. During the period of the 2nd of February 1994 to the 3rd of March 1994, ten different Cougar pilots flew a total of 65 approaches at the Halifax International Airport to test the performance of the system and collect data to analyze the accuracy of LDGPS guidance. It was necessary to carry out a detailed analysis of the Ashtech LM-XII ground truth system before using it as benchmark for evaluating the Trimble LDGPS. It is important to know how reliable the ground truth system is. We described the software used for processing the Ashtech LM-XII data, methods developed to evaluate the Ashtech solution, the results of the evaluation, and the conclusions about the reliability of the ground truth system. Once the data from the Ashtech ground truth system was processed and its solution and accuracy validated, we were able to use the C/A-code/carrier phase solution to compare with the Trimble local differential GPS (LDGPS) solutions. We also described the software developed to perform the analysis of the comparisons. We provided cross-track and vertical error assessment for a variety of approaches and develop statistics to quantify the agreement.
REFERENCES
Attila Komjathy and Richard B. Langley (1995). "The Halifax GPS Precision Approach Trials: A Report on the In-Depth Data Analysis". Annex C of In-Service Trial Helicopter Precision Approach Using Local Differential GPS (LDGPS) Final Report. Transport Canada Aviation. The executive summary of the final research report is available for on-line viewing.
Attila Komjathy and Richard B. Langley (1996). "A Summary of the Data Analysis for Halifax GPS Precision Approach Trials." The seminar has been presented at the Department of Geodesy of the Faculty of Civil Engineering at the Technical University of Budapest, Hungary, 4 March 1996.
This page was compiled by Attila Komjathy and last updated
15 September 1997. If you have any questions, comments
please feel free to get in touch with me.
You
can reach me at: komjathy@ocean.colorado.edu