"Design, Development, Calibration and Validation of 205 MHz Wind Profiler Radar"

Under the guidance of
Prof. P. Mohanan

Abstract:

Frequent occurrence of natural disasters like cyclone, tornado, severe storms, etc., highlight the need for research in atmospheric science. In that perspective, a clear and vivid understanding of 3 dimensional wind velocities at different layers of atmosphere is essential for the analysis and prediction of such extreme events. Therefore continuous monitoring of 3 dimensional wind velocities in the entire troposphere and lower stratosphere are significant in the present scenario. Currently, radiosondes and wind profiler radars are widely employed for the measurement of high resolution wind informations. Radiosonde cannot provide continuous observation and also drift the observational region with the background wind. On the other hand, wind profiler radars provides continuous high temporal resolution for horizontal and vertical velocities of wind. The well established wind profilers used for
atmospheric research in the world are centered around 50 MHz. Such radars cannot provide informations about the wind for lower atmosphere (<1 km above ground). Acknowledging this fact as well as other disadvantages like high input power, large Power Aperture Product and high background noise, a new frequency band centered at 205 MHz was identified for the wind profiler recently commissioned at Cochin University of Science and Technology.

This thesis explores the design, testing and validation of wind profiler radar operating at 205 MHz at Cochin University of Science and Technology. This is an active phased array radar system controlled by a radar processing computer, measuring wind velocities from 315 m to 20 km. The required power aperture product for faithful processing of backscattered signal at an altitude is calculated by considering the low reflectivity of atmosphere. The system is designed to provide a Half Power Beam Width (HPBW) of 3 by employing 619 numbers of 3 element Yagi-Uda antenna array in a circular aperture with an approximate diameter of 27 m. The array is divided into 13 clusters with 7 clusters of 49 elements each and 6 in periphery with 46 elements. Each cluster is again grouped into 7 clusters of 7 elements each.

Each antenna element is excited using individual TR module with a peak power output of 57 dBm (500 W). Using the inbuilt 6 bit phase shifter in the Transmit Receive Module (TRM), the beam can be pointed in the entire azimuth and 30 off-zenith with a step size of 1. To ensure the beam pointing accuracy the necessary and essential phase calibration is carried out manually using a current probe. The half power beam width of beam is validated using Virgo star while pointing accuracy is validated using moon tracking. Finally, the wind velocities measured from the ST radar at Cochin is validated with radiosonde observations. By suitably configuring the radar, it can be used for various other applications like i. Imaging different layers in the atmosphere, ii. Precipitation observations, iii. Thunderstorm studies, iv. Upper atmospheric lightning studies, v. Field aligned irregularity observations, vi. Aircraft tracking, vii. Meteor observation, viii. Satellite tracking, etc. The data acquired from the wind profiler can be used by IMD, Cochin Airport, climate change studies, weather prediction, etc.