Development of GPS orbit forecasting system for differential positioning in forest area

与农田或海洋相比，在林区进行GPS定位比较困难。造成这一现象的主要原因被认为是地形和树冠切断了GPS信号。因此，需要制定GPS测量计划，通过了解GPS卫星何时通过特定定位点的树冠空隙，从而确定在该点进行GPS定位的可能性。本研究旨在结合点位照片和GPS轨道信息，开发用于林区差分定位的GPS轨道预报系统。研究领域分别设在北海道的东京大学森林和千叶的东京大学森林。在森林中的每个定位点，都拍摄了一张高分辨率的鱼眼数字天空照片和一个多小时的GPS信号。利用数字图像分析技术对数字鱼眼照片进行分析，对照片中的天空、树叶、树枝、地面等元素进行分类。然后将这些元素转换为表示…信号穿透程度的更加开放的元素，生成开放的图像。观测卫星的GPS轨道是从年历数据中计算出来的，并将它们绘制在“开放”图像上，以提取每10秒一次的“开放”数据。分析每颗GPS卫星的信号，计算每10秒的信号强度。然后，以“时间”为关键，将“开放”数据和信号强度数据结合起来，分析了两者之间的关系。结果表明，“开放性”与信号强度总体上存在相关性，但不能解释信号强度的全部波动。这是由于镜头水平度、北向位置误差等拍摄条件较差造成的鱼眼照片与GPS轨道的不对应。但这些错误通常发生在森林拍照时。因此，开发了在用户定义的“开放”值和时间间隔下预测GPS卫星数量的程序。通过使用该程序，用户可以预测他(或她)何时可以在特定点处进行GPS定位，如果他(或她)拍摄该点的鱼眼照片。这将极大地促进GPS在林区的使用。较少

GPS positioning is difficult in forest area compared to agricultural field or ocean. The main reason for this supposed to be cut-off of GPS signal by terrain and tree canopies. So, it is needed to make GPS survey plan, by knowing when GPS satellites will pass through tree canopy gap of particular positioning point, to find out when GPS positioning is possible at that point. This research aims to develop GPS orbit forecasting system for differential positioning in forest area, by combining photograph of the point and GPS orbital information. Research fields were set at Tokyo University Forest in Hokkaido and Tokyo University Forest in Chiba. In each positioning point in the forest, a high-resolution fish-eye digital photograph of the sky and GPS signal of more than one hour were taken. Digital fish-eye photograph was analyzed by digital image analysis technic, to classify the elements of the photograph, such as sky, leaves, branches, and ground. Then those elements were converted into " … More openness", which denotes degree of penetration of GPS signal, to generate "openness" image. GPS orbits of the observed satellites were calculated from almanac data, and they were plotted on the "openness" image to extract every 10 seconds' "openness" data. Each GPS satellites' signal were analyzed to calculate signal strength of every 10 seconds. Then "openness" data and signal strength data were combined by using "time" as the key, and relationship of these two data were analyzed. The result showed correlation of "openness" and signal strength on the whole, but does not explain all of fluctuation of signal strength. This owes to uncorrespondance of fish-eye photo and GPS orbit, which caused by bad photographing condition such as unhorizontalness of lens and north position error. But these error may happen usually when taking photographs in forest. So, the program was developed for forecasting number of GPS satellites under user-defined "openness" value and time interval. By using this program, the user can forecast when he (or she) can do GPS positioning at particular point, if he (or she) takes fish-eye photo of that point. This will help greatly to promote use of GPS in the forest area. Less