KEYWORDS: Sensors, Refractive index, Phase measurement, Interferometers, Interference (communication), Atmospheric turbulence, Signal detection, Interferometry, Temperature metrology, Monte Carlo methods
Measurement of atmospheric turbulence has long been carried out using differential temperature sensors, DTS. DTS measurements of small temperature fluctuation differences between two spatially separated thermocouples can be ensemble averaged to calculate the temperature structure constant which can be used to approximate the refractive index structure constant. This technique has been widely applied and used with reasonable success in spite of several important limitations. One of the primary limitations is that the temperature structure constant alone does not fully define the refractive index structure constant; because the water vapor structure and water vapor to temperature cross correlation terms, which are much more difficult to measure, are ignored. This paper presents an initial feasibility analysis and experimental validation for a new type of refractive index structure constant sensor that is based on a compact, low cost interferometric sensor. This method is being explored as a potential alternative to DTS, because it measures index of refraction directly and thus offers the possibility of significant improvement in measurement accuracy. The paper outlines the sensor concept, its key components, and an analytical and experimental validation of the refractive index structure measurement accuracy.
Herein are discussed five straightforward field tests that are appropriate for evaluation of the performance of focal plane array (FPA) based ladar systems capable of generating high-resolution 3D imagery. The tests assess system level performance using traditional imaging targets and ladar specific targets. In addition, the tests allow comparisons to be made between the predicted performance of a ladar system and the actual performance. Analysis of actual field test ladar data is included based on appropriateness and availability of data. In the first test, range resolution is examined when the target is obscured by camouflage; the intent is to provide two pulse returns within the same instantaneous field of view (IFOV) and determine the source of the range report from different pixels within the range image with the emphasis on determining performance based on the pulse detection approach that is implemented. The second series of tests evaluates the lateral and range resolution of the FPA using standard modulation transfer function (MTF) and statistical approaches. The third test (Sect. 3.4) involves a moving target to introduce a dynamic version of the previous spatial frequency dependent tests. The fourth test (Sect. 3.5) assesses the system range performance as a function of received signal, essentially determining the performance of the system as signal-to-noise ratio (SNR) is varied. The fifth test (Sect. 3.6) assesses the uniformity of the range resolution and range accuracy of the FPA.
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