Stephen Keith Holland

Measurements of atmospheric brightness temperature fluctuations and their implications on passive remote sensing.

Passive remote sensing of airborne chemicals at infrared wavelengths may be limited by temporal fluctuations in atmospheric brightness temperatures deltaT(t). Brightness temperatures in two infrared spectral bands were simultaneously measured on clear and cloudy days along three lines of sights. For time windows t < 3-5 s, deltaT(t) remained constant at the sensor noise level and rapidly increased as t increased. The fluctuation time scale for the cloudy day was longer than for the clear day. The long correlation time for T(t) limits the utility of signal averaging in improving detection signal-to-noise ratio (SNR). The simultaneous outputs of the two spectral channels during the clear day exhibited no spectral coherence at t < 3 s and limited coherence at t > 30 s. Measurements during the cloudy day were largely coherent. Consequently, band-by-band subtraction may have limited benefits.

Effect of temperature on passive remote sensing of chemicals by differential absorption radiometry

Differential absorption radiometry DAR, using uncooled de- tectors, is a simple, low-cost method for passive remote sensing of haz- ardous chemicals for domestic security applications. However, radiomet- ric temperature differences Teffective between a target gas species and its background affect detection sensitivity. Two DARs with sensitivities to methanol, diisopropyl methylphosphonate DIMP, and dimethyl meth- ylphosphonate DMMP, all spectral or physical simulants of hazardous chemicals, were developed and used to experimentally determine the effect of Teffective on detection sensitivity. An analytical model was also developed and compared with the experimental results. With a signal-to- noise ratio SNR5, a Teffective2 K is sufficient for rapid 1s detection of methanol at 0.03 atm cm and DMMP and DIMP at 0.001 atm cm. These measured sensitivities suggest that rapid detec- tion of hazardous chemical vapor clouds below lethal dose concentra- tions can be achieved using room-temperature pyroelectric detectors. Measurements were within 3% of the analytical predictions.

Demonstration of an uncooled LiTaO3-detector-based differential absorption radiometer for remote sensing of chemical effluents

Differential absorption radiometers (DARs) using uncooled detectors are introduced as a simple method for low-cost remote sensing of chemical vapors for domestic security. A DAR consisting of a pair of uncooled LiTaO3 pyroelectric detectors integrated with bandpass filters selected to detect methanol, a simulant of many hazardous vapors, was demonstrated. At a signal-to-noise ratio (SNR) 5, the measured detection limit for methanol was 0.014 atm cm. This corresponds to a detection limit of 5.70×10–4 atm cm (31.5 mg m–2) for dimethyl methylphosphonate (DMMP). For comparison, a DAR consisting of a pair of cryogenically cooled HgCdTe (MCT) detectors was also tested. The detector-limited noise equivalent temperature differential (NETD) of the MCT detectors was measured to be 0.38 mK, whereas for the pyroelectric detector it was 115 mK. Despite the much lower detector noise, the MCT-based DAR provided a detection limit of only 0.005 atm cm for methanol, corresponding to 2.04×10–4 atm cm (11.25 mg m–2) for DMMP. The relatively poor sensitivity of the MCT-based DAR was shown to be limited by small temperature gradients of 15 mK across the noncoincident fields of view of the detectors in the DAR and by environmental fluctuations, which contributed a total NETD = 37 mK.

Wide-field-of-view solar occultation gas filter correlation radiometer for stratospheric methane measurements from a sounding rocket

A wide field of view Gas Filter Correlation Radiometer (GFCR) has been developed to make solar occultation measurements of the vertical methane distribution in the stratosphere from a sounding rocket platform. The GFCR has demonstrated a 50° solar acceptance angle that allows for a GFCR measurement during every rotation of the payload without active orientation control. The flat surface of a plano-convex ZnSe lens was etched to diffuse the projected image of the sun. By diffusing the incident solar radiation through a wide angle, sufficient radiation could be directed to the collimating GFCR optics even when the optical axis points as far as ± 25° away from the Sun. The system can be configured to measure other gaseous species with spectral bands in the 2 - 6 μm region by simply changing the bandpass filter and the correlation gas. In a laboratory calibration, the optical density of methane in a test cell was varied from 10^-4 to 10-2 atm·m as the GFCR correlation cell optical density was held at 2.5×10-3 atm-m. The process showed that measurements with a signal to noise ratio > 30:1 can be expected when the system operates in altitudes from 25 to 40 km. The GFCR performed with a correlation of 99.7% to the prediction of a theoretical model created with the HITRAN database. Sensitivity to gas distributions at other altitudes can be optimized by changing the gas pressure in the correlation cell. The payload featuring the GFCR is scheduled to be launched on an Enhanced Orion sub-orbital sounding rocket from NASA Wallops Flight Facility in April 2003. Future applications include validation and truthing for space-born remote sensing systems.

A low-cost chemical sensor for fixed security applications

A low-cost infrared sensor that uses room temperature pyroelectric detectors integrated with bandpass filters to provide low-resolution spectral scans of the absorption characteristics of hazardous chemicals was developed for fixed security applications. The sensor provides fast (1 s) and continuous monitoring, detection, and identification capabilities. A unique detection and identification algorithm that uses non-linear computation techniques to account for the exponential nature of optical absorption was developed. Chemical detection and identification is achieved by matching the recorded sensor response vector to an updatable signature library that currently includes the signatures of 14 chemicals. The sensor and algorithm were tested by introducing methanol vapor at optical depths between 225 - 270 ppm-m. Using 1 s signal samples obtained during approximately 20 min. test, resulted in no false positive alarms and 3.4% of false negatives. All false negatives were shown to be due to misidentification of methanol as isopropanol, which is spectrally similar to methanol. By grouping isopropanol with methanol the rate of false negatives was reduced to 0%. Results of the same test using a 30 s signal integration time resulted in no false positive and no false negative alarms.

Algorithms for chemical detection with a low-cost multi-spectral sensor

A 16-channel, cross-reactive remote infrared chemical sensor for detection of toxic industrial chemicals in fixed-location applications is being developed. The outputs of the 16 channels, uncooled pyroelectric detectors fitted with infrared bandpass filters, can be viewed as a coarse spectrum of the chemical(s) in the field of view. This spectrum must be unmixed, wherein the identity and optical depth of the chemical(s) are estimated by processing the spectrum with a library of known signatures for the chemical(s) of interest. Several unmixing methods are presented, including enhancements to linear projection methods, parameterization (curve fitting) of the system response, and non-linear, iterative techniques. It is found that linear methods and simple curve parameterizations produce excessive unmixing errors. Higher-order parameterization and iterative methods provide much better estimates, with the latter being more computationally intensive. The suitability of the methods for the application at hand is discussed.

Low Cost Passive Remote Sensor of Chemicals for Expendable UAVs

An inexpensive passive remote sensor of weapon and toxic industrial chemicals is developed for expendable UAVs. Detection and identification are achieved by a “cross-reactive” sensor sensitive to large groups of chemicals and updatable databases.

Demonstration of multispectral remote chemical sensing and identification using uncooled detectors

Response and mitigation following a confirmed release of hazardous chemicals requires mapping of affected areas to determine evacuation and response procedures. In-situ sensors sample only locally while mapping requires remote sensors which can rapidly monitor large volumes from a distance. For use by first responders at all levels, sensors must be low-cost, simple, robust, battery operated, and relatively fast (< 1 s). A low-cost simple passive remote sensor based on multi-spectral infrared radiometry was demonstrated under laboratory conditions. The sensor consists of 8 uncooled pyroelectric infrared detectors with integrated bandpass filters selected to transmit radiation at bands that coincide with prominent spectral features of selected chemicals. Large radiative throughput achieved by detecting radiation through relatively broadband filters (20-30 cm-1) permitted the use of low-cost, uncooled detectors without a significant loss of system sensitivity relative to high-specificity remote sensors, which require cryogenic cooling. A new modulation and radiation distribution technique was developed to provide well registered imaging by the detectors and the amplitude modulation that is necessary for detection with pyroelectric detectors. Results show that uncooled sensors can provide sufficient sensitivity to simulants of toxic chemicals (methanol, DMMP, and DIMP) with spectral features in the 8-12 micron region. In addition, the detector array provides signatures of the tested chemical simulants sufficient for identification.

Comparison of HgCdTe and LiTaO3 detector based differential absorption radiometers (DARS) for remote chemical effluent detection

The need for the development of a low-cost, low-energy, portable remote sensor of hazardous chemicals for first responders and facility protection has been recognized. Differential absorption radiometry (DAR) based on uncooled detectors has been identified as a possible solution. However, uncooled detectors have lower detectivity than cooled detectors and thus require efficient light management. Two prototype DARs, one consisting of two cryogenically cooled HgCdTe detectors and the other consisting of two LiTaO3 pyroelectric detectors, designed to detect methanol vapor, were built and tested in the laboratory to compare their relative performance by measuring detectivity limits under controlled conditions. With ΔT = 8.3 K between the methanol vapor and a radiation source having an emissivity of εs = 0.92, methanol detection limits of 3.14x10-4 atm-cm and 3.5x10-3 atm-cm were projected for the HgCdTe and pyroelectric based DARs with similar optics, respectively, assuming that a minimum SNR less than or equal to 5 is required for positive detection and identification. Evaluation of the individual detectors in each DAR demonstrated that the detector limited noise equivalent temperature difference (NETD) for the HgCdTe detector was 381 μK whereas the detector limited NETD for the pyroelectric detector was 110 mK. With a 1 s exposure to the source, temperature fluctuations in the environment increased the NETD of the HgCdTe detector to 31.0 mK whereas the NETD of the pyroelectric detector was 115 mK. These results indicated that the advantage of the HgCdTe based DAR relative to the pyroelectric based DAR is much smaller than the advantage projected by their detector limited characteristics such as D*. Thus for remote sensing applications where cost is critical, the use of pyroelectric detectors can provide acceptable performance characteristics when the signal incident on the detector is increased only by x10 relative to the signal required for similar sensitivity using HgCdTe detectors.

University of Virginia suborbital infrared sensing experiment

An Orion sounding rocket launched from Wallops Flight Facility carried a University of Virginia payload to an altitude of 47 km and returned infrared measurements of the Earths upper atmosphere and video images of the ocean. The payload launch was the result of a three-year undergraduate design project by a multi-disciplinary student group from the University of Virginia and James Madison University. As part of a new multi-year design course, undergraduate students designed, built, tested, and participated in the launch of a suborbital platform from which atmospheric remote sensors and other scientific experiments could operate. The first launch included a simplified atmospheric measurement system intended to demonstrate full system operation and remote sensing capabilities during suborbital flight. A thermoelectrically cooled HgCdTe infrared detector, with peak sensitivity at 10 micrometers , measured upwelling radiation and a small camera and VCR system, aligned with the infrared sensor, provided a ground reference. Additionally, a simple orientation sensor, consisting of three photodiodes, equipped with red, green, and blue light with dichroic filters, was tested. Temperature measurements of the upper atmosphere were successfully obtained during the flight. Video images were successfully recorded on-board the payload and proved a valuable tool in the data analysis process. The photodiode system, intended as a replacement for the camera and VCR system, functioned well, despite low signal amplification. This fully integrated and flight tested payload will serve as a platform for future atmospheric sensing experiments. It is currently being modified for a second suborbital flight that will incorporate a gas filter correlation radiometry (GFCR) instrument to measure the distribution of stratospheric methane and imaging capabilities to record the chlorophyll distribution in the Metompkin Bay as an indicator of pollution runoff.