Airborne Spectroscopic Static Temperature Sensor

The goal of this proposal is to develop a high speed, high accuracy method to measure the static air temperature outside an airplane while in flight. This is in response to SBIR sub-topic S11.05 Suborbital Instruments and Sensor Systems for Earth Science Measurements which seeks a method to measure static air temperature “from aircraft to better than 0.1 °C accuracy”. We will use open path, high resolution infrared absorption spectroscopy. We will project a frequency-swept laser beam outside of the aircraft and reflect it back to its source, monitoring the resulting infrared spectrum. The laser will primarily probe undisturbed air which has not yet been affected by the approach of the aircraft. The static air temperature will be extracted from the infrared spectrum by monitoring the relative intensity of two spectral lines which have dramatically different temperature dependence. We expect to achieve measurement precision of at least 25 mK and measurement accuracy of at least 100 mK. The measurement rate can be as fast as 100 Hz which would provide time resolution of 10 ms and spatial resolution of a few meters while flying under cruise conditions. During the Phase I project we will choose the optimal spectral lines and test the spectroscopic precision and accuracy in laboratory tests designed to simulate flight conditions. We will also develop a preliminary optical design for deployment on research aircraft. Finally, we will investigate the structure of the boundary layer around the aircraft in consultation with Boeing engineers to help us choose the optimal optical beam path for a Boeing 777 airframe. The intended application for this open path thermometer will be NASA research flights investigating the formation of persistent contrails in the upper troposphere.
Benefits: Persistent contrails have been identified as a key environmental impact of commercial aviation, representing roughly 2/3 of the radiative impact of the current commercial fleet. While there are significant uncertainties in this estimate, it emphasizes that contrails are a major contribution of commercial aviation to climate change. Our proposed technology will directly support NASA’s research into the mechanism and possible avoidance of persistent aircraft contrails. Persistent contrails are formed in the upper troposphere under conditions of water vapor super saturation. Very small temperature changes can alter the formation conditions from favorable to unfavorable. Therefore, a very accurate measurement of static temperature is required to understand and avoid persistent contrail formation. Existing temperature measurements are not adequate due to the presence of large ram temperature effects. Having better and more complete temperature data for the atmosphere is a NASA mission priority for this and other reasons. This is demonstrated by the existence of two calls in the current solicitation seeking a high accuracy measurement of static air temperature: S11.05 and A1.03. NASA missions that were at least partially concerned with persistent contrail formation include SUCCESS, ACCESS, ECLIF2/ND-MAX and EcoDemonstrator [2021, 2022, 2023]). In addition, NASA actively supports research into contrail modeling. The most significant non-NASA application will likely be in commercial aviation. This will be driven by interest in reducing persistent contrail formation from commercial aircraft. Real-time, accurate, and precise measurements of water vapor and temperature will be required to guide commercial aircraft to avoid locations and altitudes where they would produce persistent contrails. In some cases the required altitude shifts can be very small (300 meters), but taking action requires highly accurate, real-time information. This is akin to the methods that are currently used to avoid turbulence based on reports from other aircraft. Developing this live database requires mounting measurement hardware aboard commercial air platforms, which we would envision occurring in collaboration with aircraft manufacturers and government agencies aiming to reduce persistent contrails. This commercial market could become significant in years to come, resulting in 100’s or 1000’s of aircraft being outfitted with Aerodyne’s static air temperature sensor. We will interface with Boeing engineers during Phase I and Phase II to better understand what will be required for this system to be mounted on commercial aircraft.