Solar heating effects on balloon-borne microthermal probes for the airborne laser program

Abstract

Atmospheric optical turbulence induces phase fluctuations in a propagating electromagnetic wave. The resulting degradation in coherence limits the capability of any laser, target acquisition, or surveillance system. Past data collection methods for the parameterization of atmospheric turbulence profiles, in support of critical Theater Ballistic Missile Defense (TBM) systems, from ground level to 30 km, have depended on meteorological balloon- thermosonde systems, probes carried on the U.S. Air Force Argus aircraft, as well as radar and optical measurements. The balloon and aircraft systems measure temperature fluctuations and compute the temperature structure function, CT2 and the related index of refraction structure parameter, Cn2. It has recently become critical to explain why turbulence profiles from daytime thermosonde data consistently show a two order of magnitude increase over that taken during the night, primarily between 12-20 km. This thesis analyzed the TSI 3.8 micron platinum coated tungsten thermosonde probe used by the USAF Research Laboratory (AFRL) to quantity the magnitude of the solar heating and to investigate other heat transfer mechanisms in the probe. A model of the thin wire probe was developed to identity each of the contributions to the temperature error and its significance. Experimental measurements where collected to verify most aspects of the final model. We found that the sun induces a temperature rise in the TSI 3.8 micron fine wire probe, during the day, that can vary from near zero to 0. 175 K. It is strongly dependent on probe orientation with respect to the sun and on variations in the air flow over the probe. This then causes an apparent increase by two orders of magnitude in the daytime measurements of the optical turbulence parameters CT2 and Cn2