RF Refraction on Atmospheric Paths from Raman Lidar
Document ID: 16
Master's Thesis
The Pennsylvania State University
The Graduate School
Department of Electrical Engineering
Abstract
By combining three measurements of atmospheric properties, profiles of RF refractivity can be calculated, and the refraction of an electromagnetic wave caused by molecular scattering is determined. Raman lidar has been demonstrated to provide measurements of temperature and water vapor content in the lower troposphere. Using a ground-based pressure sensor, and applying the hydrostatic equation allows an accurate profile of pressure to be obtained through the lower atmospheric region. The PSU LAPS (Lidar Atmospheric Profile Sensor) system is able to provide this important parameter with continuous real-time profiles. A method is suggested which improves the detail seen in visual inspection by forming a ratio of the measured refractivity to values calculated from the U.S. Standard Atmosphere. The variations in refraction cause significant bending of the path of electromagnetic waves. The possible propagation paths result in the phenomena referred to as subrefraction and super-refraction. Ducting is an extreme case in which refraction of an electromagnetic wave causes the wave to bend to such a degree that it behaves as if in a waveguide along the surface of the Earth. This process causes radiation to travel much further along the Earth’s surface than expected and greatly distorts the information in the backscatter signals. Modeling this refractive behavior is accomplished using the RPO (Radio Physical Optics) program. Two separate measurement campaigns are analyzed, one from the southeast coastal United States, and one from the Persian Gulf region. In the first campaign, examples are shown in which the LAPS system was able to measure minor ducting events in the lower troposphere. In the data sets from the Persian Gulf, major ducting events are described based upon weather model data from four separate locations. Because the results demonstrate that refractive conditions happen on a regional scale, an argument is made that a relatively small number of lidar systems, deployed on ships in a coastal region, would provide a reliable picture of refractivity in a region.
Keywords: Raman lidar, water vapor, RF refraction, radar tracking
| Citation: | P. J. Collier, "RF Refraction on Atmospheric Paths from Raman Lidar", The Pennsylvania State University, Master's Thesis, August 2004, 79 pages |