外文翻譯---gps的精密單點(diǎn)定位技術(shù)在空中三角測(cè)量的應(yīng)用-文庫(kù)吧
2025-01-02 23:07 本頁(yè)面
【正文】 nvestigation did not achieve an implementing result because of themany technological limitations at that time (Li and Shan, 1989). In the 1970s, with the application of Global Positioning System (GPS), the situation changed a lot. GPS can provide 3dimensional coordinates of surveying points with centimeter accuracy in differential mode, it was therefore applied in AT to measure the spatial position coordinates of the projection centers (referred to as GPS camera stations or airborne GPS control points). In this way, the number of GCPs could be significantly reduced. Block adjustment of bined photogrammetric observations and GPSdetermined positions of perspective centers is regarded as GPSsupported AT. Since the beginning of the 1980s, many papers have presented the significant research and experimental results of GPSsupported AT (Ackermann, 1984。 Friess, 1986。 Lucas, 1987). After about 20 years of these efforts, GPSsupported AT was extensively applied in aerial triangulation at many scales and in all types of terrain. It is particularly beneficial in areas where they are difficult to establish ground control (Ackermann, 1994). In the late 1990s, with the development of sensor technology, an integrated systemof GPS / Inertial Navigation System(POS)was first used in AT to obtain the position and attitude information of aerial images directly. This technology, in theory, can eliminate the need for GCPs. However, research indicates that the digital orthophoto map can be made directly by image orientation parameters obtained via a POS (Cannon and Sun, 1996。 Cramer et al., 2000。 Heipke et al., 2001), but there will be larger vertical parallax when stereo models are reconstructed using these image orientation parameters and the height accuracy cannot satisfy the requirement of large scale topographic mapping. Therefore, a bundle block adjustment should be made, bined image orientation parameters obtained via the POS and photogrammetric observations (Greening et al., 2000). Whether exploiting GPS data or POS data in AT, DGPS positioning is necessary to provide the GPS camera stations at present. In the DGPS mode, one or more GPS reference stations should be emplaced on the ground and observed synchronously and continuously together with the airborne GPS receiver during the entire flight mission. Additionally, signals from GPS satellites should be received as few transmission interruptions as possible. Initialization surveying is also required before aircraft takes off and static surveying should be performed after landing. In the processing of GPS observations, carrier phase differential technique is used to eliminate or reduce GPS positioning errors, including satellite clock error, satellite orbit error, atmospheric delay error, and so on. Generally speaking, it is difficult to emplace proper GPS reference stations when the aerial photographic region is with large scope or difficult to access and municate. In order to guarantee the quality of aerial images, a survey area must be photographed for a long period, which is result from the shortage of weather suitable for photography. GPS reference stations must therefore remain in place for a long time. Moreover, the accuracy of DGPS positioning is relevant to the length of baseline. The longer the baseline, the weaker the correlation between ionospheric refraction error and tropospheric delay error. Due to the need for spatial correlation of atmospheric delay errors, the lengths of GPS differential baselines are
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