Abstract
On carrying out the static surveying using GPS for local control at forest, it is very useful to know how much of the signals will reach the antenna after penetrating tree canopy. The objective of this study is to determine the observation periods in consideration of signals weaken by tree canopy using the sky photograph as full-frame fisheye image with picture angle of 180 degrees and the ephemeris of satellites at observation point. The collected data at the point were the observation time, the sky coordinates and the signal strength of each satellite. We related the possibility of signal penetrating tree canopy to the luminance ratio of each pixel to neighboring sky pixel using the digitized sky photograph. Regression analysis of the collected data shows that there is a very straight relationship between the signal strength and the elevation of satellite under no satellite obstruction and a linear formulation is justified. Also it shows that there is a relationship between the estimated signal strength with elevation of satellite to observed signal strength ratio under some satellite obstructions with tree canopy and the luminance ratio at the sky coordinates of satellite. Then, in method using these relationships we could presuppose how much of the signals would reach the antenna after penetrating tree canopy and determine the observation periods. This procedure is built in a computer program called EYE.
