Quantifying Global Position System signal attenuation as a function of three-dimensional forest canopy structure

The NAVSTAR Global Positioning System (GPS) has in recent years become a critical tool in fields ranging from military applications, to scientific earth measurement. GPS satellites transmit signals at 1575.42 MHz on the L1 band and 1227.60 MHz on the L2 band, with a wavelength of approximately 19.0 cm and 24.4 cm respectively. At these frequencies, the signals are attenuated by vegetation, making it problematical to anticipate if GPS will work at all, and if so, how the positioning may be degraded by various types and densities of forest canopies. At the same time, precisely measuring the degree to which the GPS signal is affected by a forest canopy may provide useful information about signal degradation. The Civil and Coastal Engineering Department and the Electrical Engineering Department at the University of Florida are interested in developing a method to predict GPS signal attenuation caused by forest canopy by relating field measurements of GPS signals in forested terrain to three dimensional forest structure as determined from Airborne Laser Swath Mapping (ALSM) data collected by the Geo-Sensing program. This study investigates the impact of GPS signal to noise ratio levels under different vegetation types. Results of the GPS data such as 13 14 signal reception, position accuracy are compared with ALSM data in order to determine any relationships among these variables. To compare GPS data to ALSM data for the modeling of signal attenuation, GPS data were collected in areas around the University of Florida where recent ALSM data is currently on hand. These locations include the Intensive Management Practice Assessment Center (IMPAC), a managed forest north of the Airport Gainesville Regional, a natural forest in Hogtown, and a base station on the Gainesville campus. Data collection devices include two identical Ashtech Z Surveyor GPS receivers, two antennas, cables, and computers for data capture. A total of eleven forest locations, six points located in IMPAC and five located in the Hogtown natural forest, were measured with GPS data capture covering in excess of twenty minutes at each location. Upon analysis of the data I found that 3D positional accuracy is inversely proportional to point cloud density of the ALSM data and it is directly proportional to the signal to noise measurements taken at the site. I was also able to verify that as a satellite approaches the horizon with respect to the GPS receiver the signal to noise measurements decrease exponentially. With the above findings further work on developing a method to predict positional accuracy was conducted. The prediction of position accuracy under complex forested terrain is of significant interest to the Army research center. Given ALSM data the model developed attempts to predict the level of position accuracy a user can obtain over a period of tim