HR: 1330h
AN: G43B-04    [Abstracts]
TI: A semi-analytic evaluation of the effect of second-order ionosphere term on GPS positioning
AU: * Munekane, H
EM: munekane@gsi.go.jp
AF: Geographical Survey Institute, 1 Kitasato, Tsukuba, 3050811 Japan
AB: We developed a method to evaluate the effect of the second-order ionosphere term on GPS positioning. The method is based on the semi-analytic positioning error simulation method developed by Geiger (1988), which assumes the continuous distribution of the GPS satellites and maps the ranging error to the positioning error using the normal equation. We expanded the method to incorporate the satellite positioning error due to the second-order ionospheric term, which is estimated in a similar manner as the site positioning error, assuming the continuous distribution of the ground tracking stations instead of the continuous satellite distribution in the case of the site positioning error estimation. The method is first applied to simulate the positioning errors on three IGS sites (BAHR, COCO, GALA) which were investigated in Kedar et al. (2002) by analyzing the observed GPS data using the GIPSY software with the correction for second-order ionospheric term. We considered three cases, namely, 1) without satellite positioning error, 2) with satellite positioning error, and 3) with satellite positioning error whose spatial average of each component is corrected for. The third case corresponds to the situation where there are other observations available such as SLR and the center of the mass of the GPS satellite network is corrected properly. For the first case, we found that our method reproduced the positioning errors observed at these stations as well as Kedar et al. (2002). For the second case, however, we found that the positioning error is almost canceled. For the third case, we found that the error is reproduced as well as in the first case, though the spatial distribution of the error is different. These results indicate that 1) the semi-analytic method developed in this paper is accurate enough to simulate the position error due to the second-order ionospheric term, and 2) the satellite positioning error due to the second-order ionospheric term may have significant effect on the site position error and should be evaluated properly. Next, we applied the method to investigate the seasonal coordinate timeseries variability of the GEONET sites in Japan. Heki et al. (2001, 2003) fully analyzed the GEONET seasonal coordinate variability and found that most of the variability is explained by the snow loading, except the large annual scale change (5.6 ppb) whose source is unknown. We re-analyzed the GEONET data using the GIPSY software and found that the large annual scale change is actually semi-annual rather than annual. We simulated the position error time series of the GEONET sites using our method, and determined the scale parameter. We found that our method successfully reproduced the characteristics of the observed scale change of the GEONET, indicating that the large seasonal scale change of the GEONET is artifact caused by neglecting the second-order ionospheric term.
DE: 1243 Space geodetic surveys
DE: 2400 IONOSPHERE
SC: Geodesy [G]
MN: 2005 Joint Assembly