Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489294
Title: Ionospheric estimation using tomography and GPS Ll and L2 phase observables
Author: Alshammari, Roghailanm
ISNI:       0000 0001 3418 4635
Awarding Body: Newcastle University
Current Institution: University of Newcastle upon Tyne
Date of Award: 2008
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Abstract:
The ionosphere, extending from 50 km to 1,000 km above the Earth's surface, is a region of free electrons that cause errors in the measurements of Global Positioning System (GPS) signals. For GPS static studies the ionospheric delay can either be modelled or recovered from dual frequency receivers. However, for real-time kinematic GPS the ionosphere is a major obstacle to carrier phase ambiguity resolution over long baselines with possible aliasing between vertical motion and the ionospheric delay. Several researchers have demonstrated that regional tomographic models of the ionosphere can be constructed from a network of static GPS receivers for implementation in kinematic processing. The purpose of this research is to create mathematically a layered ionospheric tomographic model and analyse a three dimensional (3D) one layer ionospheric tomographic model, voxel-based, for the ionosphere based on GPS data observed from dual-frequency GPS tracking stations, using the carrier phase as the principal observable. This model is fully integrated within Kalman filtering that combines all the parameters and updates the corrections at each epoch. The tomographic model observation equations utilise the two carrier phases (L1and L2 ) directly to estimate the ionospheric parameters. The model thus differs from others that typically use a linear combination of L1and L2 • The model will be used for GPS kinematic processing for long baselines to provide an accurate estimation of the electron density with the purpose of obtaining optimal accuracy of the slant delays due to the ionosphere. A double difference method has been implemented in order to perform the ionospheric measurements and to eliminate possible satellite and receiver dependent biases. The Massachusetts Institute of Technology (MIT) kinematic program, TRACK, has been modified to implement the proposed ionospheric tomographic model. The tomographic ionospheric model has been validated using several days of data from North American GPS stations over an area from about 3t'N to 36'N in Latitude and from 242'E to 246'E in Longitude, forming baselines ranging from 230 km to 400 km. The performance of the estimated ionosphere model has been validated against the United States-Total Electron Content (US-TEC) model. RMS comparisons ranged from 1 TECU (Total Electron Content Unit) to 3 TECU. A comparison of the derived ionosphere maps by using the current US-TEC with the corresponding results extracted using the US-TEC of the preceding day, shows a maximum of 1 TECU difference between maps at hourly intervals, a maximum rms differences of less than 0.5 TECU and a maximum formal uncertainty of less than 1.2 TECU over the area of study. The tests also indicated that the ionospheric tomographic model is robust with enhanced capability comparable to the ionospheric free linear combination (Lc) solutions in terms of GPS phase residuals and ambiguity resolution. On utilising the ionospheric tomographic model the phase residuals rms are typically 4 mm compared with 11 mm on using Lc, while the ambiguity resolution increases to about 84% compared with 78% with Lc. In addition, the average difference between the tropospheric results of the ionospheric tomographic model and Lc is about 10 mm.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.489294  DOI: Not available
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