| Summary: | 碩士 === 國立政治大學 === 地政學系 === 106 === Ionospheric delays are one of the main error sources of the Global Positioning System (GPS). The ionospheric delays include first-order, second-order and third-order items. Those second-order and third-order items are denoted as higher-order ionospheric delays. Supposed that L1 and L2 bands GPS data are available, the first-order item can be eliminated through ionosphere-free linear combination. However, the higher-order ionospheric delays are left. For higher precision GPS applications, such as national coordinate frame maintenance and deformation monitoring, the higher-order ionospheric delays should be taken into account.
In order to study the higher-order ionospheric delays on the GPS relative positioning, the main goals of this work include: (1) studying the relationship among the double differences of higher-order ionospheric delays of carrier phases of each baseline, and the baseline length, baseline orientation, geomagnetic latitude, season and solar activity level, etc., (2) studying the effects of the higher-order ionospheric delays on baseline vectors, both with and without considering the higher-oder ionospheric delay correction. (3) On the basis of the coordinates of the GPS relative positioning solution, studying the effects of the higher-order ionospheric delays on the accuracy of the point position.
The GPS data from five satellite tracking stations in the region of Taiwan and six satellite tracking stations in the region of Asia covering the years 2009 to 2015 will be used as test data. The software RINEX_HO is used to compute and correct the higher-order ionospheric delays on the GPS L1/L2 data, and the software Bernese 5.2 is used to process the GPS relative positioning.
According to the experiment results: (1) There are several characteristics among the double differences of higher-order ionospheric delays of each baseline, and the baseline length, baseline orientation, geomagnetic latitude, season and solar activity level. ∆∇I_H can reach 16.3mm. (2) The higher-order ionospheric delays still be influenced by season and solar activity. The higher-order ionospheric delays have positive and negative characteristics, as observation time increase, the higher-order ionospheric delays will be offset. In contrast, as observation time decrease, the ability of offset will be unstable. The effect of I_H can reach 6.94mm. (3) After correcting the higher-order ionospheric delays, the improvement ratio of each direction is about 20%~80%, and the best improvement ratio is east-west direction, which is better than that of north-south direction and up-down direction.
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