Targets located on the Precision Impact Range Area (PIRA) of Edwards AFB are used to evaluate imaging systems’ sensitivity and spatial resolution to ensure they meet specified requirements. Spectral Sciences, Inc., is developing a field-ready electro-optical sensor calibration/test system for airborne instruments from the visible through longwave infrared. This spectral region is particularly challenging because of the contributions from both solar and thermal fluxes. The system is composed of spectral-spatial ground targets and atmospheric characterization instruments. The design challenges for a new ground target installation applicable over short to long ranges and a broad optical spectrum include: 1) development of an innovative spectral-spatial, high contrast, high uniformity, knife edge target for determination of the spatial characteristics of the imaging system under test, such as the Modulation Transfer Function (MTF) and Relative Edge Response (RER), Noise Equivalent Temperature Difference (NETD), linearity and more; 2) development and implementation of a suite of auxiliary instruments to quantify the atmospheric effects, such as line-of-sight (LOS) turbulence, surface temperatures, humidity, and visibility; 3) development of targets with stable, quantifiable spectral response that can be used for evaluation for the spectral characteristics of multi- or hyperspectral imaging systems; and 4) engineering the target set for simplified long-term maintenance and durability. In this paper we report on the development of a prototype 2m by 2m thermally controlled knife edge target. The target is composed of four 1m by 1m panels each of which has independent temperature control and face surface materials which can be exchanged with other panel faces to produce patterns or spectral features. The full prototype system can be rotated and tipped to maximize the surface area apparent to a sensor system under test. The paper includes initial field measurements of the target array using visible, MWIR and LWIR imaging systems.
Two recent projects at Spectral Sciences Inc. have a goal of benefiting the calibration of overhead optical sensors. In the first, we have developed a vicarious calibration method that utilizes our MODTRAN software, the recognized standard for radiative transport. In the second, we are developing a new array of thermally controlled, square, spectrally characterized panels to support accurate calibration of imagers in the visible through long wavelength infrared (LWIR). Progress and results of both efforts will be described.
The THIA instrument is a visible through extended short-wave infrared (SWIR) imaging spectrometer. Designed using a solid block optical system and a single camera, the sensor is extremely compact with low power requirements. The spectrometer, manufactured by Corning, consists of reflective optical and grating surfaces diamond turned onto a single block of CaF2. The system has been flown repeatedly on a Matrice 600 hexacopter and on small aircraft for data collections. It operates from 0.4-2.45 microns, with high throughput due to the fast f/1.5 optics and has a total weight of 2.4 kg. THIA SNR was designed to exceed 100 over the full spectral range from 400 to 2450 nanometers under normal operating conditions and exceed 250 below 1700 nanometers. The first prototype system exhibits degraded throughput below 500 nanometers, but meets the SNR threshold over the rest of the range. Stray light backgrounds in the initial prototype require software correction. Despite these issues, the system has been used to obtain meaningful data. Here we characterize THIA Signal-to-Noise in flight conditions and compare results to predicted and benchtop performance.
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