| dc.description.abstract | At the Center for Remote Sensing of Ice Sheets (CReSIS), two Ultra-Wideband (UWB) Frequency Modulated Continuous Wave (FMCW) radars are used for remote sensing of snow. The 12-18 GHz Ku-Band Radar altimeters provides high resolution surface elevation measurements, while the 2-8 GHz Snow Radar measures snow thickness over sea ice. In order for the Intergovernmental Panel on Climate Change (IPCC) to continuously develop more accurate models, additional snow characterization over sea ice is needed. Employing a constrained optimization approach, the snow water equivalent (SWE) can be estimated directly from the measured radar backscatter. Using the current Snow Radar depth measurement ability along with modified SWE approximation, remotely sensed snow density data can be gathered over large areas using airborne microwave sensors. These additional snow parameters will allow scientists to more accurately model a given area of snow and its effect on polar climate change. To meet this demand, a new “Airborne” Multi-Channel, Quad-Polarized 2-18GHz Snow Radar has been proposed. With tight size and weight constraints from the airborne platforms deploying with the Navy Research Laboratory (NRL), the need for integrated and miniaturized receivers for cost and size reduction is crucial for future deployments. A set of heterodyne microwave receivers were developed as part of the new 2-18 GHz Snow radar to satisfy the March 2015 NRL deployment. The receivers were designed to enable snow thickness measurements from a survey altitude of 500 feet to 5000 feet while nadir looking, and estimation of SWE from polarimetric backscattered signals at low elevation 30 degree off nadir. The individual receiver has undergone a five times size reduction with respect to initial prototype design, while achieving a sensitivity of -125 dBm on average across the 2-18 GHz bandwidth, enabling measurements with a vertical range resolution of 1.64 cm in snow. The design of a compact enclosure was defined to accommodate up to 18 individual receiver modules allowing for multi-channel quad-polarized measurements of snow backscatter over the entire 16 GHz bandwidth. With the new receiver and enclosure design, a one-fourth size reduction of the overall receiver chassis has been accomplished. The receiver bank was tested individually and with the entire system in a full multi-channel loop-back measurement, using a 2.95 μs optical delay line, resulting in a beat frequency of 200 MHz with 20dB range side lobes. Due to the multi-angle, multi-polarization, and multi-frequency content from the data, the number of free parameters in the SWE estimation can thus be significantly reduced resulting in more accurate estimation of SWE. In addition to the receiver design, several UWB passive components were designed, fabricated, and tested for future implementation to reduce cost and allow for quick lead time due to in-house assembly. Design equations have been derived and a new method for modeling Suspended Substrate Stripline (SSS) filters in ADS for rapid-prototyping has been accomplished. Two SSS filters were designed which include an Optimized Chebyshev SSS Low Pass Filter (LPF) with an 18 GHz cutoff frequency and a Broadside Coupled SSS High Pass Filter (HPF) with a 2 GHz cutoff frequency. These filters were designed and modeled in house and sent out for professional fabrication. Mechanical design, fabrication, and assembly were all completed at CReSIS. Measurements were taken with a Vector Network Analyzer (VNA) and compared with HFSS simulations. Also, a 2-18 GHz three- port Transverse Electromagnetic (TEM) Mode Hybrid 8:1 power combiner was designed and modeled at CReSIS. This design will be integrated into the Vivaldi Dual Polarized antenna array with 8 active dual-polarized elements to implement a lightweight and compact array structure, eliminating cable and connector cost and losses. | |
