Measuring and testing – Gas analysis
Reexamination Certificate
1999-09-07
2001-10-02
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
C073S023280, C073S030010, C250S339070, C250S339130
Reexamination Certificate
active
06295859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to digital filters, and more particularly to a method for detecting and monitoring atmospheric gases, such as gases emitted from smoke stacks or tail pipes, through use of a gas sensor and a digital filter constructed to correlate only with the spectrum of the gas of interest to yield the density of the gas.
2. Discussion
The detection and measurement of trace gases is often essential in order to adhere to environmental standards and regulations, or process control standards. As a result, there is a need by civilian and government agencies for a technique of passively detecting and measuring trace gases in a plume using target plume sensors. The sensors may be as close to an emissions source as the base of a smoke stack, or as far away as on an orbiting satellite.
One problem associated with detecting and measuring trace gases is that the spectral signal of interest associated with the trace gas is typically a small part of the overall signal measured by the sensor. It is often difficult for a basic correlation filter to detect this small target signal unless the background component of the measured spectrum is removed.
Several conventional spectral measurement techniques exist. One technique, known as two spectrum differencing, involves measuring a scene and subtracting the background spectrum from the target spectrum. However, the two pixel differencing does not eliminate spectrally correlated background features in a target spectrum, therefore leading to large errors in gas amount quantification, and leaving a large residual which masks the desired signal.
A second technique, known as model matching, takes a spectral measurement and generates a complex numerical model of the spectrum. The technique then adjusts model parameters until a spectral match is found. However, model matching techniques typically require long, complex computer runs that require a large amount of operator involvement to adjust the model parameters.
A third known technique is known as an orthogonal background suppression (OBS) technique. OBS techniques are used to measure the column density/thermal radiance contrast product of a gas plume using a passive thermal/infrared emission spectrometer. Column density refers to the number of molecules per unit area seen by the sensor, while thermal radiance contrast is the difference between the radiance of the scene behind a plume, and a Planck function generated with the temperature of the plume. Such a technique facilitates target signal detection in the presence of low signal to spectral clutter ratio. OBS techniques are based upon the assumption that background spectral clutter can be assumed to be a linear combination of background spectra taken with no target gas in the field of view. Such a technique finds the proper combination of background scene components and removes the components completely from the target spectrum, thereby leaving only the spectrum of the gas of interest, along with associated random noise.
One advantage of the OBS technique is that an optimal linear filter associated with the technique is able to suppress a large background component of a measurement, while yielding the column density thermal radiance contrast product (DCP) of a gas plume in the atmosphere. However, while present OBS techniques exhibit certain desirable results, the techniques are limited in effectiveness in that the techniques cannot separate the column density measurement from the thermal radiance contrast measurement.
Therefore, there is a need for an OBS based technique of passively detecting and measuring trace gases and a gas plume that allows the value of plume column density to be separated from the thermal radiance contrast.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an improved OBS based technique that monitors gas emissions from smoke stacks, tail pipes, and other sources, and that separates gas column density from thermal radiance contrast. The technique of the present invention utilizes software generated digital filter constructed to correlate with only the spectrum of gas of interest, and have zero correlation with background components. Through using the improved OBS technique of the present invention, high performance and more positive cost effective gas monitoring systems and sensors may be built and implemented.
More particularly, the present invention provides a method of measuring trace gases in a gas plume. The method comprises the steps of: detecting a target scene including gas plume information and background information; filtering the background information from the gas plume information to yield a density contrast product including both gas plume column density and gas plume thermal radiance contrast values; filtering the gas plume thermal radiance contrast value from the density contrast product to yield a gas plume column density estimate; and outputting the gas plume column density estimate for gas plume analysis purposes.
In addition, the present invention provides a system for measuring column density of a plume of gas. The system provides a sensor that measures target pixel information including background and gas plume information. A processor is connected to the sensor to store and process the target pixel information, and filter the background information from the target pixel information to yield a gas plume column density/thermal radiance contrast product. The processor also filters the gas plume column density/thermal radiance contrast product to separate the thermal radiance contrast from the gas plume column density to yield gas plume column density information. An output associated with the processor outputs the gas plume column density information from the processor for data analysis purposes.
REFERENCES:
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patent: 4320975 (1982-03-01), Lilienfeld
patent: 5371542 (1994-12-01), Pauli et al.
patent: 5528037 (1996-06-01), Whitsitt
patent: 0 604 124 (1994-06-01), None
Hayden, A., E. Niple, B. Boyce, “Determination of Trace-Gas Amounts in Plumes by the Use of Orthogonal Digital filtering of Thermal-Emission Spectra”, Applied Optics, Jun. 1996.
Hayden Andreas F.
Noll Robert J.
Calfee Halter & Griswold LLP
Larkin Daniel S.
Rashid James M.
The B. F. Goodrich Co.
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