Project Description: 

Major: BS Engineering, Physics, BA Mathematics Company: SSAI Mentor: Mark Stephen Code: 544 - Electro- Mechanical Systems

The system that is being developed will eventually fly in orbit and is completely laser based. An amplified and frequency doubled infrared laser will be shot at the surface, and then a detector will read the laser light reflected back. As a laser beam passes through the atmosphere, gas particles absorb some of the radiation. The result is an absorption spectrum. For various reasons, the Oxygen A absorption band has been chosen as our characteristic to measure. Assuming zero molecular motion, the oxygen A-band should be at a discrete frequency, or a delta function distribution. However, as particles in a gas move relative to the laser emitter, there is a resulting Doppler shift in the absorption spectrum. Molecules moving toward the laser emitter will encounter what they perceive as a higher frequency, and so will absorb radiation at a lower frequency according to the emitter’s frame of reference. Likewise, molecules moving away will absorb radiation at what the emitter would perceive as a higher frequency. The net result is a widening of the absorption spectrum lines, specifically the Oxygen A-band line. The higher the temperature, the greater the average molecular velocity, thus greater Doppler shifts, ultimately resulting in a wider spreading of the oxygen A-band. Under these circumstances, the oxygen A-band itself takes the form of a Gaussian distribution, with mean at the typical “textbook” A-band frequency. In essence, then, the standard deviation of the A-band Gaussian distribution becomes our measure of the molecular velocity distribution from which the atmospheric temperature and pressure can be calculated.

Contribution:

During his internship, Joe mounted a traditional weather station on the roof of building 33 and ran the information cable back to a computer in the division 554 lab. This weather station is to serve as a reference to compare the accuracy of the LIDAR system once it has been developed to the point where a target can be fired on from across the parking lot. In the lab, Joe set up a pump system that could both pressurize and evacuate a small test cell that could be used for the initial experiments. Most of his time, though, was spent on developing the software necessary to acquire absorption data from a detector, fit it to a Voigt profile with known parameters, and calculate the actual temperature and pressure , all in real time (or as close as could be obtained on Mac OS 10 systems). Later actual readings will be taken in LabView, while the fitting routine will be performed in Matlab for efficiency. The system Joe was developing was intended to have Labview and Matlab working seamlessly in real time, acquiring data and fitting it while keeping a data archive throughout successive automated trials.