Higher order ionospheric effects have grown in relevance as the accuracy of geodetic GPS analysis has increased in recent years and have become an active area of research. Considering periods of up to three years at ionospheric maximum, previous studies modelled the effects of the second (12) and third (B) order ionospheric terms arising from the expansion ofthe refractive index of the ionosphere as a series. This study investigates the effects of the 12 and 13 terms and also those of the higher order ionospheric bending terms over a much longer period of 14 years. This allows more extensive consideration of temporal effects and, for the first time, the assessment of velocity biases. Five global reprocessing runs were performed, identical with the exception of changes in the higher order ionospheric terms modelled and the magnetic field model for the 12 term. The velocity bias in the vertical component found when modelling the 12 and 13 terms is in the range 0.0-0.29 mm yr" for 1996.0-2001.0 and -0.34-0.0 mm yr' for 2001.0-2006.0. Mean coordinate differences (2001.0-2004.0) when modelling the magnetic field using the International Geomagnetic Reference Field or using a eo-centric tilted magnetic dipole are sub-millimetre but noticeable around the South Atlantic and in South East Asia. The 13 term is shown to have a negligible effect on translations estimated from the GPS reference frame to ITRF2005 in comparison to the 12 term. Its effect on scale is similar in magnitude to that of the 12 term at less than 0.05 ppb. Finally, a trial implementation of the higher order ionospheric bending terms has been tested on a global GPS network for the first time. Modelling the bending corrections appears to have a minimal effect on site coordinates and tropospheric total zenith delays (TZDs) except for low latitude sites, where mean TZDs are affected by up to ~ 1.7 mm over 5 days at ionospheric maximum (DOY 301-305, 2001). Reference frame effects are mainly limited to the Z component, although the 90-day smoothed Z-translation from the GPS reference frame to ITRF2005 changes by less than 2 mm.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:612620 |
| Date | January 2010 |
| Creators | Petrie, Elizabeth Jane |
| Publisher | University of Newcastle Upon Tyne |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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