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.
Extremely thick haze caused by air pollution is observed in many satellite images of the earth, and in particular over eastern China. Standard image display software typically provides satisfactory visualization of the ground through automated or user-driven scaling to enhance contrast; however, it does not perform well with these highly polluted scenes, where the haze is spatially non-uniform. Furthermore, estimation of surface reflectance using standard atmospheric correction software is highly problematic under these conditions due to very low visible transmission of the haze coupled with lack of knowledge of its optical properties, which may not conform to the haze or aerosol models in the software. In this paper we show that a version of the empirical Quick Atmospheric Correction (QUAC) algorithm, adapted for spatially dependent scattering, produces visually satisfying imagery of the entire ground in multispectral satellite scenes containing thick haze, and that the output reflectance spectra appear to be realistic enough for performing basic surface classification. The QUAC algorithm is applicable to multispectral and hyperspectral imagery with any number of wavelength bands, including true color (RGB) imagery, and does not require radiometrically calibrated data.
A miniaturized, lightweight turn-key hyperspectral sensor package incorporating a single, monolithic spectrograph, telescope and navigation system is being built for airborne applications on small, Unmanned Aircraft Systems (UAS). The sensor is based on Corning’s existing MicroHSI 410 Vis/NIR Selectable Hyperspectral Airborne Remote sensing Kit (SHARK) currently used for airborne agricultural monitoring. Under DOE sponsorship, we are extending the approach to cover the full spectral range from 0.4-2.5 microns with a single spectrograph. This will enable rapid aerial surveys of vegetative mass, quality, and carbon sequestration. Other applications include mineralogy, agriculture, and intelligence/surveillance/reconnaissance (ISR).
The sensor features an Offner-type spectrograph machined from a single transmissive block. The monolithic construction provides an unprecedented combination of high performance, low cost and low size, weight, and power. It has an f/1.4 aperture, 5 nm resolution, and measures only 46mm x 60mm x 76mm. The spectrograph block is coupled to a sterling-cooled, back-thinned, HgCdTe FPA covering 0.4-2.5 micron spectral range. The flight package, including spectrograph, camera, telescope, and navigation system weighs less than 2.4kg and can fit on group 1 UASs.
In this paper, we present the design and optical performance of the sensor, and a detailed physical model of detection performance in standard, airborne hyperspectral sensing applications. At 100 Hz data rate, the sensor will achieve shotnoise limited performance with SNR > 250 from 0.4-1.7 microns and SNR<100 between 2-2.3 microns. Operating procedures for airborne monitoring of vegetative properties are also discussed. Initial test flights on a UAS are scheduled for next summer.
KEYWORDS: Cameras, Fourier transforms, Signal detection, Image filtering, Video, Sensors, Remote sensing, Signal to noise ratio, Modulation, Principal component analysis
We present progress being made in the passive optical remote detection of ground surface vibration. With proper design, minute seismic surface waves may be captured using remote visible imagery. The utility of subband steerable filters to the detection of surface vibrations in the absence of inherent image contrast is demonstrated. Detections with the filters are shown with laboratory data and compared to Fourier transform results over a range of surface vibrational amplitudes. We present an analysis of the optical measurements of ground surfaces performed during the passing of nearby trains with discussion of the hardware, software, and detection clutter sources. Results from optical remote sensing are interpreted using additional accelerometer measurements and image processing.
Vibration waveforms in materials appear in video as a minuscule fluctuation in the light scattered into the camera. By inferring from processed video how vibrational energy propagates through an article to be inspected, we may detect local material anomalies. We report progress in developing measurement protocols and technologies to perform standoff nondestructive inspection of materials for defects using video image processing. In particular we show promising results from a protocol that conforms to relatively inexpensive hardware.
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