SBIR-STTR Award

The measurement of atmospheric refractive index, its variations, and humidity fluctuations is very important in understanding and predicting the propagation of electromagnetic waves through the atmosphere. This is important to a variety of Air Force technologies such as the Airborne Laser and ground-based lasers. It is also important in explaining the behavior of clear-air radars. QEI proposes to develop a lightght microwave refractometer for airborne use which will measure the above quantities. The instrument is designed to directly measure the speed of propagation of the microwaves, as opposed to previous indirect methods. It will be constructed using commercial off-the-shelf electronics and advanced materials which nearly eliminate thermal considerations which affect other designs. It will make high-speed measurements of the radio refractivity of the atmosphere, the radio refractive index structure constant, and humidity. In Phase I, a prototype refractometer will be fabricated and tested. In Phase II, a final ruggedized design will be completed for Air Force use. Extensive field tests will be carried out with the system to ensure it meets Air Force needs.

This document presents a proposal to fabricate a revolutionary, compact microwave refractometer system. The refractometer will be capable of accurately (better than 01.0 ppm) sensing atmospheric refractive index gradients and turbulent fluctuations at sufficiently small scales so that it can be used to validate signal propagation models used/proposed to predict performance of a variety of VHF, UHF microwave and optical communications and sensing systems. Solid state microwave and signal processing electronics will be used to activate a sensing cavity, to process its output signal and to display atmospheric refractive index fluctuations in real time. The sensing cavity will be modified using a low cost, high stability material, such as electroplated ceramic or glaze. To minimize adverse effects of turbulence at high air speeds, the sensing cavity will be enclosed in a flow controlling aerodynamic housing. Comparative evaluation of the instrument will be done using critical components from a Thompson-Vetter laboratory standard refractometer. Calibration and field testing will be performed at the Rock Springs micrometeorological/turbulence field site at Penn State University. As research progresses, we will work with the Air Force to arrange a high altitude aircraft test of the prototype unit. This will require a source of additional funding.

Benefits:
The measurement of atmospheric refractive index and its variations is very important in understanding the propagation of electromagnetic waves through the atmosphere. These measurements are crucial to Air Force technologies, particularly the Airborne Laser, as well as other DoD directed-energy programs. These measurements may also be used to improve the operational performance of civil and military radars; for example, the Navy is currently interested in optimizing AEGIS radar performance using such measurements. In Phase I, QEI built a proof-of-concept prototype microwave refractometer using off-the-shelf components that successfully demonstrated a new, direct-measurement technique for measuring atmospheric refractive index. The prototype also incorporated the ability to compute Cn2 (the refractive index structure function parameter) in real time. Outdoor measurements gathered by the prototype were in close agreement with those predicted by atmospheric turbulence theory, thus proving the theory and design of the refractometer. In Phase II, the refractometer design will be refined and optimized for use in a variety of environments. The Phase I prototype design provides a proven starting point, with demonstrated components and design features that will be carried into Phase II. Extensive field tests will be carried out with several prototypes to ensure Air Force needs are met.

Keywords:
Microwave Refractometer, Clear-Air Turbulence, Index Of Refraction Propagation