Design and Development of Low-Cost Differential Temperature Sensors for Estimation of Refractive INDEX Coefficient

As engineers and scientists work to refine free-space optical systems that operate over long terrestrial ranges, there is a need to better understand, characterize, and quantify the atmospheric properties of the environment. One such atmospheric property of interest is the refractive-index structure coefficient, or , which describes small changes in the base atmospheric refractive index. Over very short distances, small index of refraction changes usually cause negligible problems to all but the most demanding optical systems, such as interferometric systems, but can have a large effect on Optical Path Length (OPL) as short as 1 km. This paper presents a low cost, non-optical, instrument that provides accurate measures of the refractive-index structure coefficient.

At the lowest level of understanding, index of refraction fluctuations in the atmosphere originate with turbulent air motion (Hunt & Roggemann, 1996). The source of atmospheric turbulence originates from temperature gradients on the surface of the earth as solar radiation and daily weather patterns cause a heating and cooling cycle. The large scale temperature gradient from the surface of the earth to upper atmosphere that is both easily measured and causes atmospheric turbulence also applies to very small temperature gradients that are not separated by such vast distances. These small temperature gradients are considered randomly distributed throughout a larger temperature gradient. The index of refraction of air is sensitive to fluctuations in temperature yielding a randomly distributed index of refraction for air through a slant or horizontal path of small temperature gradients, setting the groundwork for understanding the differential temperature impact on the refractive-index structure coefficient.

To facilitate low cost, portable, low sensor mass differential temperature measurement systems EngeniusMicro is on the fourth iteration of custom hardware designed to make high resolution temperature sensors measurements with a variety of sample rates extending to 1 kHz sample speeds. Challenges associated with development of low cost high speed temperature sensor systems require consideration of the sensor mass, the sensor substrate mass, methods of transiting analog signals to digital data, data speeds associated with digital data protocols, and the digital speed of each individual component. Once sensor systems are fabricated the challenge of calibrating sensors is not trivial and requires careful oversight and process.

The low cost and small size of individual differential temperature sensor systems has facilitated atmospheric tests and measurements that were not previously practical or cost effective. Recent testing at ISARRA Flight Week demonstrated the utility of integrating low cost differential temperature sensors on a small unmanned vehicle system to make vertical profile atmospheric measurements, similar to a balloon thermosond. The low cost and portability of the differential temperature sensor systems allowed a ground based differential temperature sensor system to operate at the unmanned vehicle system launch site around the clock and the unmanned vehicle system payload to operate 30 minutes out of every hour at a hardware cost of less than $10,000.00.