HITRAN-Trained System for Rapid Measurement of Ground-to-Air Atmospheric Path Transmission

For many scenarios, it is necessary to identify a target by measuring remotely its spectral signature.  This can be done by determining the power emitted by the target as a function of wavelength.  However, the spectrum gets modified due to wavelength dependent absorption through the atmosphere.  In order to infer the actual spectrum of the target emission, it is necessary to account for the propagation loss for each wavelength band.  This can be done by employing a reference target with a known emission spectrum and measuring the spectral distribution after propagation through the atmosphere.  Here we propose to develop a concrete design for such a system.  In our design, the reference black-body source (BBS) will be mounted under a drone.  A spectrum analyzer (SA), covering the wavelengths from 0.2 micron to 14 microns, will be used to measure the emission. A mirror-based telescope mounted under the emitter will be used to enhance the amount of radiation detected.  A camera co-located with the SA will be used to maintain alignment with the BBS.  A spinning wheel with multiple apertures will be used to chop the radiation emerging from the emitter.  A lock-in amplifier, synchronized with the chopping process via remote electronic communication, will be used to detect the radiation with high signal-to-noise ratio.  The degree of uncertainty (DOU) in the Transmission Path Factor (TPFACT) for each band will be determined by comparing the experimental results with the predictions derived from the HITRAN database.  If the DOU for any band is too excessive, investigations will be carried out to determine the cause and steps will be taken to improve the performance.  Such steps may include, for example, changing the detector for that band or cooling it further to reduce thermal noise.  During Phase I, we will carry out extensive studies to determine (a) what types of SA and BBS can be used for the system to gather data from a height of as much as 10000 feet, (b) what type of drones would be suited to carry the proposed BBS along with the telescope, the chopper, and the remote signal transmitter, (c) how the data from the SA would be recorded, and (d) what training, certifications, and FAA approvals would be necessary. Furthermore, we will demonstrate experimentally the efficacy of this approach using a ground-based system.  Specifically, we will measure the spectrum from a BBS placed behind a mirror-based telescope and a chopper wheel, using a lock-in amplifier, for several discrete bands, by making use of narrow bandpass filters and band-optimized detectors, up to a distance of several thousand feet in an open field.  The reliability of the technique will be tested by measuring the TPFACTs for each band over different distances and atmospheric conditions, and comparing the results with predictions made using the HITRAN database.  Northwestern University will serve as a subcontractor for this project.