Fabry-perot interferometer-based satellite detection of atmospheric trace gases

Abstract

Fabry-Perot interferometer-based optical imaging system for use in detecting atmospheric trace gas emissions from specific target locations that addresses the back reflecting problem of conventional systems. The system comprises a wide-angle Fabry-Perot interferometer that is tilted such that a surface normal of the interferometer gap is at an angle Θ with respect to the angle of incidence that corresponds to zero field angle. In some implementations, the tilt direction is positive; in other implementations, the tilt direction is negative. The tilt angle is preferably larger than half of the angular spread (i.e., angle-of-incidence range) at the position of the Fabry-Perot interferometer in the imaging system. In some embodiments, the system comprises a shutter for blocking the input light aperture. In some embodiments, the system comprises an on-board calibration light source. In one aspect, an output of the system is partitioned into a plurality of data subsets.

Description

FABRY-PEROT INTERFEROMETER-BASED SATELLITE DETECTION OF ATMOSPHERIC TRACE GASESTECHNICAL FIELD

[0001] The present invention relates to optical systems for use in satellites. More specifically, the present invention relates to optical systems for use in detecting atmospheric trace gas emissions from specific target locations.BACKGROUND

[0002] The growing awareness of environmental issues in the last fifty years has led to a greater need for more environmentally friendly systems and devices. This growing awareness has also led to a need for better monitoring of potentially environmentally harmful atmospheric emissions from industrial facilities.

[0003] Current monitoring systems include Fabry-Perot interferometer-based systems and devices. In such systems, satellite-mounted Fabry-Perot interferometers are used in the gathering of image data to determine atmospheric trace gas emissions. The interferometer is a wide-angle interferometer which creates a fringing pattern on the imaging system, enabling measurement of multiple wavelengths in each image. The interferometer also has a large aperture to maximize light throughput and a high finesse to provide high spectral resolution, which enable measurements at precise wavelengths at each pixel in the imaging system. In addition, during a pass of the satellite over the target area, the target area tracks across the field of view of the optical system, thereby allowing the optical system to gather multiple images of the target area. Since the position of the target area within the field of view changes for every image, light at multiple wavelengths is collected from each ground pixel in the target area. In this way, different absorption data for different atmospheric trace gases can be gathered in a single satellite pass over the target area

[0004] However, the geometry of a Fabry-Perot interferometer-based system has certain drawbacks. Namely, the Fabry-Perot is in a collimated beam path, with the beam angle of incidence varying by several degrees for different field angles. As such, a large portion of the rays are at, or close to, normal incidence on the interferometer’s gap, which will reflect back most of the energy. This unwanted ‘backreflection’ can corrupt / obscure the signal. Further, as the standard mathematical instrument model used to model signal levels only includes singlepass rays traveling in the forward direction, using the model to assess real-world data that includes backreflections can be very difficult.

[0005] As such, Fabry-Perot interferometer-based systems that avoid the backreflecting problem are therefore desirable.SUMMARY

[0006] This document discloses a Fabry-Perot interferometer-based optical imaging system for use in detecting atmospheric trace gas emissions from specific target locations by way of an observation platform that addresses the back reflecting problem of conventional systems. The system comprises a wide-angle Fabry- Perot interferometer that is tilted such that a surface normal of the interferometer gap is at an angle 0 with respect to the angle of incidence that corresponds to zero field angle. In some implementations, the tilt direction is positive (i.e., positive 0); while in other implementations, the tilt direction is negative (i.e., negative 0). The tilt angle is preferably larger than half of the angular spread (i.e., angle-of-incidence range) at the position of the Fabry -Perot interferometer in the imaging system. In some embodiments, the tilt angle is less than 10°. In some embodiments, the tilt angle is between 7° and 9°. In some embodiments, the tilt angle is approximately 8°. In some embodiments, the system comprises a shutter for blocking the input light aperture. In some embodiments, the system comprises an on-board calibration light source. In one aspect, an output of the system is partitioned into a plurality of data subsets, which may be retrieved separately. A weighted average of the retrieval data is computed for analysis.

[0007] In a first aspect, this document discloses a system for detecting atmospheric trace gas emissions from a specific target location by way of an observation platform, the system comprising: an image gathering device located at said platform, said platform being for overflying said specific target location, said image gathering device being for gathering multiple images of said specific target location as said platform overflies said specific target location; a wide-angle Fabry -Perot interferometer, said interferometer being located at said platform and being configured such that light gathered from said specific target location passes through said interferometer before being received by said image gathering device; and a filter for filtering incoming light from said specific target location prior to being received by said interferometer, wherein a surface normal of a gap of said interferometer is tilted at an angle 0 with respect to an angle of incidence that corresponds to zero field angle.

[0008] In another embodiment, this document discloses a system wherein said angle 0 is larger than half of the angular spread of said incoming light.

[0009] In another embodiment, this document discloses a system wherein said angle 0 is less than 10°.

[0010] In another embodiment, this document discloses a system wherein said angle 0 is between 7° and 9°.

[0011] In another embodiment, this document discloses a system wherein said angle 0 is approximately 8°.

[0012] In another embodiment, this document discloses a system further comprising: a shutter for blocking light input to the system, wherein said shutter is remotely operable to move between a closed position and an open position.

[0013] In another embodiment, this document discloses a system further comprising: a calibration light source within the system, the calibration light source producing a known illumination.

[0014] In a second aspect, this document discloses a method for detecting atmospheric trace gas emissions from a specific target location by way of an observationplatform, the method comprising: (a) providing an image gathering device at an observation platform, said platform being used to overfly said specific target location; (b) providing a wide-angle Fabry -Perot interferometer at said satellite such that light gathered from a specific target location passes through said interferometer before being received by said image gathering device; (c) providing a filter for filtering incoming light from said specific target location prior to being received by said interferometer; (d) gathering multiple images for said specific target location as said platform passes above said specific target location to thereby simultaneously gather data for multiple atmospheric trace gas emissions; and (e) determining vertical column spectral densities of said atmospheric trace gas emissions from a spectrum resulting from said light in said multiple images, wherein a surface normal of a gap of said interferometer is tilted at an angle 0 with respect to an angle of incidence that corresponds to zero field angle.

[0015] In another embodiment, this document discloses a method wherein step (e) comprises partitioning said data into a plurality of data subsets, each of said data subsets corresponding to one of said multiple images.

[0016] In another embodiment, this document discloses a method wherein step (e) further comprises computing a weighted average of said data subsets, said weighted average being used to determine said vertical column spectral densities.

[0017] In another embodiment, this document discloses a system wherein said angle 0 is larger than half of the angular spread of said incoming light.

[0018] In another embodiment, this document discloses a method wherein said angle 0 is less than 10°.

[0019] In another embodiment, this document discloses a method wherein said angle 0 is between 7° and 9°.

[0020] In another embodiment, this document discloses a method wherein said angle 0 is approximately 8°.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will now be described by reference to the following figures, in which identical reference numerals refer to identical elements and in which:Figures 1A and IB show an optical system using a wide-angle Fabry-Perot interferometer according to the prior art;Figure 2 shows an imaging output from the system of Figures 1A and IB, according to the prior art;Figure 3 is a schematic showing light propagation through a tilted wide-angle Fabry-Perot interferometer;Figure 4A is a schematic of an optical system using a tilted wide-angle Fabry- Perot interferometer according to an aspect of the invention;Figure 4B is a block diagram showing a tilting angle of the wide-angle Fabry- Perot interferometer according to an embodiment of the invention;Figure 5 shows an imaging output from an optical system configured according to an aspect of the invention; andFigure 6 is a flowchart detailing a method according to an aspect of the invention.DETAILED DESCRIPTION

[0022] For a better understanding of the workings of Fabry-Perot interferometer-based optical systems, the reader is directed to USPN 9,228,897 (issued January 5, 2016; titled FABRY-PEROT INTERFEROMETER BASED SATELLITE DETECTION OF ATMOSPHERIC TRACE GASES) and to USPN 10,012,540 (issued July 30, 2018; also titled FABRY-PEROT INTERFEROMETER BASEDSATELLITE DETECTION OF ATMOSPHERIC TRACE GASES), both of which are hereby incorporated by reference in their entirety.

[0023] Conventional Fabry -Perot interferometer-based systems may be configured as shown in Figures 1A and IB. Figure 1A illustrates imaging apparatuses 20, 30 (specifically, a cloud and aerosol telescope 20 and a cloud and aerosol spectrometer 30) side by side within the optical system 10. Of course, different imaging apparatus may be included depending on the embodiment. For example, in some embodiments, a system may include only a single telescope, or a single conventional visible light camera or imager. Nothing in this document should be taken as limiting which imaging apparatus / device(s) may be included in the system.

[0024] Figure IB illustrates additional components of the conventional system 10 of Figure 1 A. A baffle 40 is adjacent to a first telescopic lens 50 at one end of the optical system 10. At another end of the system is a first beam folding mirror 60 adjacent a second beam folding mirror 70. A collimating assembly 80 receives input from the beam folding mirror 70. The collimating assembly 80, in some embodiments, comprises a glued doublet (i.e., comprises two lenses glued together). The output of the collimating assembly 80 is then filtered by an order sorting filter 90. The output of the filter 90 is then received by a further collimating assembly 100. In some embodiments, the collimating assembly 100 comprises two additional lenses. From the collimating assembly 100, light is then passed through a wide-angle Fabry-Perot interferometer 110. Light passing through the interferometer 110 is then received by imaging lenses 120. Finally, what passes through the imaging lenses 120 is then received and recorded by a detector 130, such as a SWIR (short-wave infrared) camera. As should be noted, the interferometer 110 is at normal incidence in the system in this figure. That is, a surface normal vector from the gap of the interferometer is parallel to the angle of incidence of the incoming light.

[0025] Such a Fabry-Perot interferometer-based optical system produces spectral imaging data as shown in Figure 2. That is, absorption lines cause the appearance of a fringing pattern of concentric rings. Fig. 2 illustrates how a conventional system would image the top of atmosphere radiance spectrum overa constant albedo background, with each ring representing an atmospheric absorption line or a solar spectrum line. The circular edges are due to the use of an ideal boxcar model for the order sorting filter.

[0026] The present document discloses an optical system that mitigates back reflection problems common to wide-angle Fabry-Perot interferometer-based optical system. Specifically, the system herein comprises a wide-angle Fabry-Perot interferometer that is tilted with respect to the angle of incidence of the incoming light. The tilt angle means that “ghost ray” (i.e., light reflected out from the detector / imaging lenses and back reflected off the wide-angle Fabry-Perot interferometer) is directed away from the detector and is not counted during analysis.

[0027] The effect of such a tilt is shown in Figure 3. The primary ray 300 enters the Fabry-Perot interferometer 240 from the left and passes through the imaging lens(es) 250 before hitting the detector 260 at right. The interferometer 240 has a gap 241. As can be seen, the primary ray 300 travels at an angle (i.e., nonparallel) to a surface normal 242 of the tilted Fabry-Perot interferometer 240’s gap 241. The surface normal 242 is a vector that is normal to the plane of the gap 241. Also note that the primary ray 300, in the system depicted in Figure 3, travels at an angle that is non-parallel to an axis 320 of the imaging lens(es) 250 and the detector 260.

[0028] A “ghost ray” 310 is also shown in Figure 3. This ghost ray 310 reflects from the detector 260 where the primary ray 300 impinges upon the detector 260. Because of the angle of the incident primary ray 300, the ghost ray 310 is also at an angle to the surface normal 242 of the interferometer 240’s gap 241. The ghost ray then travels back through the imaging lens and into the Fabry-Perot interferometer 240 (again, non-parallel to the interferometer gap 241). The ghost ray 310 is bounced within the Fabry -Perot interferometer 240 and sent back in the direction of the detector 260, again passing through the imaging lens 250. However, because of the angles of the primary ray 300 and of the reflected ghost ray 310, the ghost ray 310 does not re-impinge upon the detector 260, but is directed away from the detector 260. Thus, the detector 260 does not detect the back reflected ghost ray 310, and the ghost ray is not accounted for whenanalyzing the actual signal. This allows for significant benefits in terms of accuracy, by permitting closer correspondence with the data predicted by the standard mathematical instrument model.

[0029] Figure 4A and 4B are schematics that more clearly show the arrangement of the Fabry-Perot interferometer with respect to the other elements of the system. (As should be understood, these figures are not necessarily to scale, nor do they necessarily show all true positions of the components of the invention. These figures should not be considered to limit the scope of the invention in any way.) In Figure 4A, light 210 enters the telescopic lens 200 at one end of the optical system and is passed through mirrors to at least one collimating lens 220. The output of the collimating lens(es) 220 is filtered by the order sorting filter 230. Output of the order sorting filter 230 is passed through the wide-angle Fabry- Perot interferometer 240, which is tilted at an angle 0 with respect to the angle of incidence of incoming rays. Light rays passing through the interferometer 240 are then received by at least one imaging lens 250, and from there are provided to the detector / camera 260.

[0030] Figure 4B shows the tilt angle 0 more clearly. The central angle of incidence (corresponding to zero field angle, and the central position in the image plane) is represented by the line A. As seen in Figure 4B, the Fabry-Perot interferometer 240 of the present invention is positioned such that a surface normal from the interferometer’s central gap 241 (i.e., a normal vector to a plane that longitudinally bisects the interferometer 240) is at an angle 0 to the angle of incidence of incoming light A.

[0031] As should be understood, the tilt angle 0 is limited by mechanical considerations. That is, the tile angle 0 is preferably large enough to reduce or eliminate ghost ray but cannot be so large that the entire aperture of the Fabry -Perot interferometer is blocked. The optimal tilt angle 0 depends on the embodiment and the specific components used. However, the tilt angle is preferably larger than half of the angular spread of the incoming light (z. e. , the range of angles of incidence of the incoming light, also known as “AOI range” or “angle of incidence range”). In some embodiments, the angular spread is approximately ±7°. That is, the angles of incidence of the incoming light can be fromapproximately -7° to approximately +7°, thereby covering a total range of approximately 14° to 15°. In some such embodiments (where the angular spread is approximately ±7°), the tilt angle 0 is larger than half of the angular spread (i.e., the absolute value of the tilt angle 0 is larger than 7°). For example, in such embodiments, the absolute value of the tilt angle 0 can be approximately 8° or between 7° and 9°. The maximum tilt angle 0 depends on mechanical constraints. For example, in some embodiments with an angular spread of ±7°, the absolute value of the tilt angle 0 may be mechanically limited to less than 10°. The maximum tilt angle 0 in any implementation generally depends on the size of the interferometer’s aperture at normal incidence and may vary based on the specific instruments used. Nothing in this disclosure should be construed as limiting the maximum tilt angle 0 or primary ray minimum tilt angle 0 in various embodiments.

[0032] Tilting the wide-angle Fabry-Perot interferometer also changes the shape of the imaging data received. Rather than the series of concentric rings provided by conventional imaging (as shown in Figure 2), an optical system using a tilted wide-angle Fabry -Perot interferometer provides “arcs” of imaging data. Figure 5 shows an exemplary imaging output. Each arc in Figure 5 represents a separate imaging sample of a geographic area — that is, in a single pass of the system over an area, every geographic point is sampled in each arc. The strongest absorption feature is clearly repeated in each arc with significant contrast.

[0033] In particular, the angle of incidence 0 of light on the wide-angle Fabry -Perot interferometer determines the wavelength A of a spectral peak m as A = Amcos 0, where Amis the unshifted wavelength of the peak m. (Note that the angle of incidence (AOI) 6 is related to, but distinct from, the tilt angle 0, and that the angle of incidence is measured relative to the surface normal of the interferometer.) That is, the change in wavelength with the angle of incidence changes as cZA— = — sin 6 dd

[0034] As would be understood, successive Fabry -Perot modes are separated from each other by the free spectral range (FSR), where FSR is measured in wavelength such thatFSR« Am+1— Am. MinimizingFSRthen allows the interferometer to ‘zoom in’ (i.e., focus) on the strongest absorption features. For an optical system that allows a finite range A0 of angles of incidence for incoming light (i.e., A9 is the “AOI range” described above), the number of samples N of the strongest absorption feature(s) whenFSRis minimized will be:

[0035] That is, the number of samples of the strongest feature(s) will be proportional to the angle of incidence. As more samples of the strongest feature(s) are preferable, tilting the wide-angle Fabry -Perot interferometer to greater angles (i.e., increasing 6 and thereby increasing N) is likewise preferable.

[0036] As well, the repeated N samples of the data enable “one-mode” sampling while keeping the arcs narrow, allowing the strongest feature to be easily distinguished from other effects in the images. With only one arc or sample, as in conventional systems, it would be difficult to distinguish between the source of specific effects, as each feature would be spread over more pixels. Specifically, with the strongest absorption feature having a width of A2S. the number of pixels p onto which AASis imaged goes as dd p oc A0 oc — AASoc 1 / sin 6 CIA.

[0037] Thus, when the tilt angle of the wide-angle Fabry-Perot interferometer increases, thereby increasing the angle of incidence 6, the number of pixels p decreases. That is, the strongest feature is imaged on fewer pixels, rather than being spread over more pixels at tower resolution. This also has beneficial effects on signal- to-noise ratios and data accuracy. That is, as is well-known in measurement systems, noise is typically strongest at tower frequencies. (This is referred to as “1 / f” noise.) Lower frequency noise, when translated into a detector such as a wide-angle Fabry-Perot interferometer, has a greater effect on larger pixel spans. Accordingly, reducing the pixel span of interest by tilting the Fabry -Perotinterferometer (thus increasing 9 ) reduces the effect of noise on the gathered data.

[0038] Additionally, the arc-type imaging data may be easily separated into the N component samples. That is, the collected data can be partitioned into multiple subsets of data, each subset corresponding to a specific sample / arc pattern. By analyzing individual data subsets — or, in some embodiments, by merging the data subsets — different information may be obtained. For example, in some embodiments, retrievals are performed on one or more of the partitions and a weighted average of the retrieval data subsets / partitions is taken. This can reduce noise and / or improve accuracy, compared to conventional ring-type output and compared to the output from a single pass / arc pattern. Any suitable weighting of the weighted average may be used, depending on the embodiment. For example, the weighting may be intensity -based (with weights being assigned based on which arc pattern is strongest or weakest).

[0039] Further, partitioned retrievals as described above can also reduce the observation time required to obtain data with good accuracy. For example, consider an observation sequence that has a duration of Tobsv« 20 s. Over those 20 seconds, various factors may change, thereby affecting the data gathered such that the data gathered at the beginning of the sequence is different from the data gathered at the end of the sequence. In some cases, such differences may not pose an issue or may reflect valuable information (e.g , a sudden spike in gas observation during the observation sequence). However, other differences, such as differences resulting purely from changes in observational position as the satellite passes over the area during the sequence, may be undesirable. Accounting for such differences in post-processing, further, may be challenging. Accordingly, reducing the required observational time without sacrificing data quality may be advantageous. By recording multiple samples (i.e., N samples) of the absorption features and assessing the samples using the partitioned retrievals approach described above, the required observational duration may be shortened to Tpart= Tobsv / N, where Tpartis the observational duration for partitioned data. If A is 5 (as shown in Figure 5), the required duration for the standard sequence with Tobsv ~ 20 s would thus only be Tpart= 20 s / 5 = 4 s. A required observationtime of 4 seconds means that far fewer environmental and / or observational effects would need to be accounted for in post-processing and / or analysis.

[0040] Figure 6 is a flowchart detailing a method of data analysis using the arc-type data output of the present system. The imaging data is obtained at step 600 and partitioned into a plurality of data subsets at step 610. At step 620, retrievals are run on the partitions. In some implementations, retrievals of each partition are performed separately. At step 630, the weighted average of the partitions is computed for further analysis. Of course, it should be understood that other forms of analysis / comparison may be used, and that a weighted average is not the only possible approach for extracting meaning from the gathered data.Shutter

[0041] In some embodiments, the system comprises a shutter, i.e., a way of blocking the input light aperture of the optical system. The shutter is controlled such that it can be opened or closed remotely (i.e., based on instructions received from the ground) when the system is in operation on the observation platform. Note that the shutter should be fully closable (i.e., should be configured to block all incident light from outside the system, when in the closed position). When the shutter is closed, the current state / status of the optical system may be assessed, to thereby allow readings to be calibrated based on that current state / status. Since the system will, over time, experience radiation damage (and potentially other forms of damage / wear), the system can be recalibrated such that the recalibration takes into account whatever damage / wear may have been suffered by the system. Closing the shutter allows the “dark images” of the system to be taken, which show the output of the system in a null state, with no incident light generating signal.Calibration Light Source

[0042] In some embodiments, the system comprises a light source used for calibration. The calibration light source is located near the input light aperture of the optical system. When in operation, the calibration light source produces a known, predetermined illumination. Evaluating the system’s response to this known illumination (i.e., assessing the output of the system when the only incident lightis from the calibration light source) allows the system’s response to be determined for other light sources. Calibrating to the calibration light source allows for the calibration of the system’s readings even as the system experiences wear.

[0043] As used herein, the expression “at least one of [x] and [y]” means and should be construed as meaning “[x], [y], or both [x] and [y]”.

[0044] As noted above, the various aspects of the invention may be implemented as a system or as parts of a larger system. Some aspects of the invention may be implemented as being part of a larger monitoring system involving an aerial platform (e.g. , a manned or unmanned aerial vehicle) or a satellite. While the above examples illustrate an embodiment where various aspects of the invention are mounted on a satellite, these aspects may also be mounted on a suitable aerial platform. Such an embodiment may differ from a satellite implementation in the type / capability of the telescope used.

[0045] Additionally, it should be clear that, unless otherwise specified, any references herein to ‘image’ or to ‘images’ refer to a digital image or to digital images, comprising pixels or picture cells.

[0046] It should be clear that various aspects of the present invention may be implemented as software modules in an overall software system. As such, the present invention may thus take the form of computer executable instructions that, when executed, implements various software modules with predefined functions.

[0047] Some embodiments of the invention may be executed by a computer processor or similar device programmed in the manner of method steps or may be executed by an electronic system which is provided with means for executing these steps. Similarly, an electronic memory means such as computer diskettes, CD-ROMs, Random Access Memory (RAM), Read Only Memory (ROM) or similar computer software storage media known in the art, may be programmed to execute such method steps. As well, electronic signals representing these method steps may also be transmitted via a communication network.

[0048] Some embodiments of the invention may be implemented in any conventional computer programming language. For example, preferred embodiments may be implemented in a procedural programming language (e.g, “C” or “Go”) or an object-oriented language (e.g, “C++”, “java”, “PHP”, “PYTHON” or “C#”). Alternative embodiments of the invention may be implemented as preprogrammed hardware elements, other related components, or as a combination of hardware and software components.

[0049] Some embodiments can be implemented as a computer program product for use with a computer system. Such implementations may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g, a diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer system, via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g, optical or electrical communications lines) or a medium implemented with wireless techniques (e.g, microwave, infrared or other transmission techniques). The series of computer instructions embodies all or part of the functionality previously described herein. Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies. It is expected that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g, shrink-wrapped software), preloaded with a computer system (e.g, on system ROM or fixed disk), or distributed from a server over a network (e.g, the Internet or World Wide Web). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g, a computer program product) and hardware.

[0050] A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.

Claims

We claim:

1. A system for detecting atmospheric trace gas emissions from a specific target location by way of an observation platform, the system comprising:- an image gathering device located at said platform, said platform being for overflying said specific target location, said image gathering device being for gathering multiple images of said specific target location as said platform overflies said specific target location;- a wide-angle Fabry-Perot interferometer, said interferometer being located at said platform and being configured such that light gathered from said specific target location passes through said interferometer before being received by said image gathering device; and- a filter for filtering light from said specific target location prior to being received by said interferometer, wherein a surface normal of a gap of said interferometer is tilted at an angle 0 with respect to an angle of incidence that corresponds to zero field angle.

2. The system according to claim 1, wherein said angle 0 is larger than half of the angular spread of said incoming light.

3. The system according to claim 1, wherein said angle 0 is less than 10°.

4. The system according to claim 1, wherein said angle 0 is between 7° and 9°.

5. The system according to claim 1, wherein said angle 0 is approximately 8°.

6. The system according to claim 1, said system further comprising: a shutter for blocking light input to the system, wherein said shutter is remotely operable to move between a closed position and an open position.

7. The system according to claim 1, said system further comprising: a calibration light source within the system, the calibration light source producing a known illumination.

8. A method for detecting atmospheric trace gas emissions from a specific target location by way of an observation platform, the method comprising:(a) providing an image gathering device at an observation platform, said platform being used to overfly said specific target location;(b) providing a wide-angle Fabry-Perot interferometer at said satellite such that light gathered from a specific target location passes through said interferometer before being received by said image gathering device;(c) providing a filter for filtering light from said specific target location prior to being received by said interferometer;(d) gathering multiple samples for said specific target location as said platform passes above said specific target location to thereby simultaneously gather data for multiple atmospheric trace gas emissions; and(e) determining vertical column spectral densities of said atmospheric trace gas emissions from a spectrum resulting from said light in said multiple samples, wherein a surface normal of a gap said interferometer is tilted at an angle 0 with respect to an angle of incidence that corresponds to zero field angle.

9. The method according to claim 8, wherein step (e) comprises partitioning said data into a plurality of data subsets, each of said data subsets corresponding to one of said multiple samples.

10. The method according to claim 9, wherein step (e) further comprises performing retrievals on said data subsets to thereby produce retrieval data.

11. The method according to claim 10, wherein step (e) further comprises computing a weighted average of said retrieval data, said weighted average being used to determine said vertical column spectral densities.

12. The method according to claim 8, wherein said angle 0 is larger than half of the angular spread of said incoming light.

13. The method according to claim 8, wherein said angle 0 is less than 10°.

14. The method according to claim 8, wherein said angle 0 is between 7° and 9°.

15. The method according to claim 8, wherein said angle 0 is approximately 8°.

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