Abstract:
A passive ranging system measured spectra of solar radiation off an adjacent spot and a distal target. Reflected solar radiation and differential attenuation are used to estimate target range. A comparison of absorption spectra from the solar illuminated distal target compared to the adjacent location is performed. Since the sun&#39;s position is always known, the increased absorption due to a distant target is derived from the differential between the adjacent spot and the distal target.

Description:
RELATED APPLICATION(S) 
     This application is a Continuation-In-Part (CIP) of U.S. patent application Ser. No. 08/949,503 entitled “Passive Ranging to Source of known Spectral Emission”, filed Oct. 14, 1997 now U.S. Pat. No. 6,222,018 which is a CIP of Ser. No. 08,506,847 filed Jul. 25, 1995 now U.S. Pat. No. 5,677,761, the entire teachings of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a measurement of the range of a source of electromagnetic radiation and, more particularly, to the use of passive ranging by examination of relative attenuation among a plurality of spectral lines wherein differences in attenuation among various portions of the radiation spectrum arise from selective atmospheric absorption of radiation at various frequencies as a function of propagation distance of the radiation through the atmosphere. The foregoing attenuation is in addition to the attenuation arising from the spreading of the waves of radiation through increasing regions of space, the latter attenuation following the well-known relationship of intensity varying as the inverse square of the range from a point source of the radiation. 
     Various objects, such as the plume of a rocket or other fire, or a hot filament or gas discharge of a lamp, are known to act as sources of radiation having characteristic spectra. There are situations in which it is desirable to determine the location of such a source from a viewing site distant from the source, the location data including range, elevation and azimuth of the target source from the viewing site. However, a problem arises in that the usual apparatuses for determination of target location, such as active radar, are not operative with the foregoing type of radiant energy signal for a passive determination of the range of the source. A further problem arises when a target conceals its source of radiation or when the source of radiation is inactive. 
     SUMMARY OF THE INVENTION 
     The aforementioned problem is overcome and other advantages are provided by a system and method of passive ranging, in accordance with the invention, wherein a suitable target, or distant source of radiation, is identified by its electromagnetic spectrum during a target acquisition procedure and, thereafter, the spectrum of the radiation is analyzed to determine the effects of atmospheric attenuation on various parts of the spectrum. In the practice of the invention, prior knowledge of the spectrum, as emitted by the target, is employed in both the acquisition and the analysis stages. The invention is particularly useful in the situation wherein a source of radiation, on or near the ground, illuminates a cloud above the source, and a distant observer obtains range of the source by observation of radiation scattered from the cloud. 
     A typical spectrum includes both a continue distribution of spectral energies in an emission band or in each of a plurality of emission bands, as well as a line spectrum wherein individual ones of the lines are characteristic of certain constituent substances in a source of the radiation, such as the various gasses in a rocket plume. In accordance with the theory of the invention, a source of radiation, such as a rocket plume, emits radiation characterized by a known set of spectral emission lines and/or emission bands. The lines of the line spectrum, as well as an amplitude profile of the continuous spectrum, are useful in identifying the source of the radiation. Generally, the spectrum of a received radiation signal will be shifted in frequency by a Doppler shift due to motion of the source, and there will be a broadening of one or more of the spectral lines due to movement of the gasses and particles thereof within the rocket plume. To identify the spectrum of a received radiation signal automatically, as by use of a computer or other signal processor, the received spectrum may be correlated against known spectra from a set of previously stored spectra. The previously stored spectra correspond to respective ones of known rocket plumes and other sources of radiation which may be of interest. A match is obtained between the received spectrum and one of the known spectra, the match serving to identify the source of the radiation. The correlation also indicates a frequency offset between the two matching spectra and, hence, is useful in providing the additional information of Doppler shift. 
     In accordance with a feature of the invention, a continuous portion of the received spectrum can be employed to determine range of a target, such as the plume of a rocket, independently of whether or not there be any Doppler frequency shift. Operation of the invention to obtain the range may be explained as follows. As the radiation propagates through the atmosphere from the source to optical receiving apparatus employed by the invention, there may be interaction between the radiation and various substances dependent on the frequency of the radiation. The interaction results in a relative attenuation of various spectral components by the atmosphere as a function of frequency and a function of distance of propagation of the radiation through the atmosphere. Thus, the attenuation is indicative of target range. 
     Measurement of the ratios of intensities of radiations at the various spectral bands at a distance from the source will differ from the same measurements performed at the location of the source because of the selective absorption of the radiation at its various spectral bands. In the practice of the invention, a correlation is made between variation of an intensity ratio of any two spectral lines as a function of distance between source and the receiving apparatus. The range to the source is thereby obtainable from spectrometric measurements of the radiation, computation of the intensity ratio, and association of the specific range with a specific intensity ratio, or an average value of ranges obtained from sets of intensity ratios. A succession of range measurements may be differentiated to obtain range rate. 
     Another aspect of the present invention uses reflected solar radiation, or other radiation source (e.g., tunable laser, search light) located at a known position in the atmosphere, to determine the range and/or rate of a target. In the case of using the sun as the source, a reference measurement of solar radiation is made at a spot adjacent (i.e., proximal target) to an optical apparatus employing the present invention. The reflected solar radiation is also measured as a reflection off the target (i.e., distal target). As the reflected solar radiation propagates through the atmosphere from the target to optical receiving apparatus employed by the invention, there may be interaction between the solar radiation and various substances dependent on the frequency of the reflected solar radiation. The interaction results in a relative attenuation of various spectral components by the atmosphere as a function of optical frequency and a function of distance of propagation and look angle or orientation of the reflected solar radiation through the atmosphere. Thus, the attenuation is indicative of target range. In an alternative embodiment, because the position of the radiation source, adjacent spot, and atmosphere between the radiation source and radiation source adjacent spot are known, the reflected radiation from the adjacent spot can be calculated rather than measured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein: 
     FIG. 1 is a block diagram of an electrooptic system useful in the practice of the invention; 
     FIG. 2 is a block diagram of a signal processor forming a part of the system of FIG. 1; 
     FIG. 3 is a stylized representation of the frequency spectrum of electromagnetic radiation, having both continuous and line spectral portions, emitted by a target at zero range with three significant frequency components of the continuous spectral portion being identified by the letters A, B, and C; 
     FIG. 4 is a stylized graph representing the relative amplitudes of the frequency components A, B, and C of FIG. 3 after the radiation has propagated through a distance in clear air, the amplitudes of the spectral components having been attenuated by the environment; 
     FIG. 5 shows the ratio of amplitudes of the A component versus the B component, and the A component versus the C component of the graph of FIG. 4 as a function of distance from the target; 
     FIG. 6 is a diagram of method steps in obtaining target range from spectral data; and 
     FIG. 7 shows, diagrammatically, a viewing of target radiation reflected from a cloud by the electrooptic system of the invention, wherein the system may be carried by a vehicle on the ground or an airborne vehicle; 
     FIG. 8 is a diagram indicating the use of solar radiation by the present invention; and 
     FIG. 9 is a stylized graph representing the relative amplitudes of the frequency components A and B after the solar radiation of FIG. 8 has reflected off the target. 
    
    
     Identically labeled elements appearing in different ones of the figures refer to the same element in the different figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
     FIG. 1 shows an electrooptic system  10  for obtaining passively spectral data of electromagnetic radiation emitted by a distant target  12 . In accordance with the invention, the electromagnetic radiation emitted by the target  12  has a known spectrum, or target signature, which is stored in a signature memory  14 . The system  10  includes a telescope  16  which views electromagnetic radiation, indicated as a plurality of light rays  18 , which propagates through the atmosphere  20  to be incident upon the telescope  16 . The telescope  16  is steered mechanically in azimuth and in elevation by a scanner  22  which enables the telescope  16  to scan through space to determine whether the target  12  as well as other targets may be present. By way of example, the telescope  16  is shown in a Cassegrain form having a main mirror  24  and a secondary mirror  26 , the latter reflecting light through an aperture  28  to an optical assembly  30 . 
     The optical assembly  30  provides an optical path from the telescope  16  to a signal processor  32  of the system  10 . The signal processor  32 , operates in a manner to be described with reference to FIG. 2, for extracting spectral data from the target radiation, and for determining the range of the target  12  to the telescope  16  from the spectral data. The optical assembly  30  comprises a collimating lens  34  for establishing a beam  36  of parallel rays suitable for operation of the signal processor  32 . In addition, the optical assembly  30  comprises four partially reflecting mirrors  38 ,  40 ,  42 , and  44  for tapping off portions of the optical energy of the beam  36  to be used for purposes of acquiring and tracking the target  12 . 
     The system  10  further comprises three spectral line filters  46 ,  48 , and  50 , three detectors  52 ,  54 , and  56  of target radiation received by the telescope  16 , and a correlation unit  58 . In operation, a portion of the optical energy of the beam  36  is reflected by the mirror  38  via the filter  46  to the detector  52 , the detector  52  converting the optical energy to an electrical signal which is applied to the correlation unit  58 . In similar fashion, optical energy reflected by the mirror  40  propagates via the filter  48  to the detector  54  to be converted to an electrical signal which is applied to the correlation unit  58 . Also, optical energy reflected by the mirror  42  propagates through the filter  50  to be converted by the detector  56  to an electrical signal which is applied to the correlation unit  58 . 
     The filters  46 ,  48 , and  50  provide different specific passbands for the propagation of the optical energy of the beam  36 . This enables each of the filters  46 ,  48 , and  50 , in conjunction with the respective detectors  52 ,  54  and  56 , to view only a specific portion of the spectrum of the target radiation while discarding the balance of the radiation. Thereby, the detectors  52 ,  54 , and  56  signal the presence of specific spectral lines. The absence of a signal outputted by any one of the detectors  52 ,  54 , and  56  is an indication of the absence of the corresponding spectral line from the spectrum of the target radiation. It is to be noted that the use of three signal channels provided by the three mirrors  38 ,  40 , and  42  in combination with the three filters  46 ,  48 , and  50 , and the three detectors  52 ,  54 , and  56  is presented by way of example and that, in practice, more of these signal channels may be employed for observation of additional spectral lines of the target spectrum. The correlation unit  58  obtains best fit between incoming spectral data, which may be Doppler shifted in the event of target motion, and the known spectrum of the target radiation stored in the signature memory  14 . Thresholds, stored in a memory  60 , are employed by the correlation unit  58  in a decision process of the correlation unit  58  for deciding if a specific spectral line is considered to be present. 
     The system  10  includes a memory  62  for storing the locations of possible targets in terms of azimuth and elevation address, a switch  64  operated by the correlation unit  54 , a Faraday filter  66 , a detector assembly  68  comprising an array of charge-coupled devices (CCD) providing a two-dimensional viewing of target image data on the beam  36 , a track-mode electronics unit  70 , and an acquisition-mode electronics unit  72 . In operation, optical energy extracted from the beam  36  by the mirror  44  is directed by the mirror  44  via the Faraday filter  66  to the detector assembly  68 . The use of the Faraday filter  66  is well known, such use being described in an article entitled HELICOPTER PLUME DETECTION BY USING AN ULTRANARROW-BAND NONCOHERENT LASER DOPPLER VELOCIMETER by S. H. Bloom et al, appearing in OPTICS LETTERS, Vol. 18, No. 3, Feb. 1, 1993 at pages 244-246. 
     The optical passband of the Faraday filter  66  is dependent on the strength of the magnetic field of the filter, and a specific spectral region of the incoming radiation may be selected for viewing via the filter  66  by adjustment of the magnetic field strength. The magnetic field strength is set by a passband signal outputted by the correlation unit  58  corresponding to the detection of a desired spectral line by one or more of the detectors  52 ,  54 , and  56 . The rays of light passing through the filter  66  retain their relative directions of orientation so that the detector assembly  68  is able to determine whether the source of the target radiation appears to be above or below the boresight axis of the telescope  16 , or to the right or the left of the boresight axis. Thereby, the detector assembly  68  provides an error signal to the track-mode electronics unit  70  which indicates whether the telescope  16  is to be repositioned or oriented by the scanner  22  during a tracking of the target 
     The acquisition-mode electronics unit  72  is operative to provide electric signals to the scanner  22  for directing the telescope  16  to view a designated portion of space during a scanning of space in the acquisition mode. The decision as to whether to enter the acquisition mode or the tracking mode is made by the correlation unit  58 . Initially, the switch  64  is in the acquisition position for coupling signals from the acquisition-mode electronics unit  72  to the scanner  22 . During the acquisition process, any possible targets noted by the correlation unit  58  are entered into the memory  62 . This is accomplished by an output signal of the correlation unit  58  which strobes the memory  62  to store the azimuth and elevation command signals outputted by the acquisition-mode electronics unit  72  to the scanner  22 . The storage of the possible target locations in the memory  62  is useful for entering a reacquisition mode wherein the electronics unit  72  scans a region of space around a possible target to ascertain the target coordinates in azimuth and in elevation. 
     Additionally, the acquisition-mode electronics unit  72  outputs the target coordinates to the track-mode electronics unit  70  during a hand-off procedure wherein the switch  64  is operated to disconnect the acquisition-mode electronics unit  72  from the scanner  22  and to connect the track-mode electronics unit  70  to the scanner  22 . This operation of the switch  64  occurs upon the determination by the correlation unit  58  that a target is present. The azimuth and elevation (AZ/EL) coordinates of the target being tracked are applied by the track-mode electronics unit  70  to the signal processor  32 , via line  74 , for use in identifying a specific target by its angular coordinates. 
     As shown in FIG. 2, the signal processor  32  comprises an address generator  76  driven by a clock  78 , and spectrum analyzer  80  which receives the beam  36  (FIG. 1) and is driven by a scanning drive  82 . The signal processor  32  further comprises two memories  84  and  86  which are addressed by the address generator  76 . The memory  84  stores known target spectral data for the target  12  (FIG. 1) as well as for other targets which may be viewed by the telescope  16  (FIG.  1 ). The memory  86  stores spectral data of the target  12  obtained by operation of the spectrum analyzer  80 . The address provided by the generator  76  is in terms of the frequency coordinate in a graph of amplitude versus frequency for the target spectral data. The generator  76  is operative also to address the scanning drive  82  for directing the drive  82  to drive the spectrum analyzer  80  to a specific frequency during a scanning of the spectrum. Operation of the drive  82  may be either mechanical or electrical depending on the construction of the spectrum analyzer  80 . 
     By way of explanation of the operation of the invention for the measurement of range to a source of radiation, it is noted that in a hypothetical case, in the absence of selective atmospheric attenuation of various portions of the target spectrum, such as for the propagation of radiation in vacuum, it is apparent that the relative amplitudes of various frequency components in the reference spectrum of the memory  84  would be the same as those being measured by the spectrum analyzer  80 . However, due to the presence of the atmosphere  20  (FIG.  1 ), the selective attenuation results in a distortion of the measured spectrum such that the relative intensities of the spectral lines differ between the measured and the reference spectra. The nature of the distortion depends on the propagation distance of the radiation through the atmosphere. The invention employs a relatively small continuous portion of the electromagnetic spectrum wherein the influence of clouds, rainfall, aerosols, or dust can be discounted because they present a substantially uniform attenuation, as a function of frequency, across the small portion of the spectrum employed for the range measurement. 
     With a knowledge of the atmospheric attenuation rates as a function of distance at various frequencies of the spectrum, the signal processor  32  can derive the target range by analysis of distortion in the received spectrum as compared to the reference spectrum. Assuming that the continuous portion of the target spectrum, utilized in the measurement, is essentially constant in amplitude at zero range, before attenuation by the atmosphere, a measurement of spectral distortion from attenuation can be accomplished without regard to Doppler frequency shift. The use of the continuous spectrum avoids any effect of a broadening of spectral lines by collisions among particles in the constituent substances of a rocket plume. 
     In the event that the nature of the target  12  is unknown, or in the event that any one of a plurality of targets (not shown) may be present, it is useful to provide means for identification of the target  12 . In order to identify the target  12 , the signal processor  32  further comprises a correlator  88  which correlates measured spectral data stored in the memory  86  with the known spectral data stored in the memory  84  to determine if a match can be made. Identification of the type of target is made by use of the target spectrum as a signature. A match between the spectra identifies the nature of the source of radiation, such as the plume of a rocket, and thereby serves to identify the target  12 . For example, the spectrum may indicate a combustion of a certain type of fuel which serves to identify the target. 
     Also included in the signal processor  32  is computational equipment for calculation of target range, as well as for utilization of the target range to calculate range rate and trajectory. The computational equipment, for convenience in explaining operation of the processor  32 , is portrayed as three separate computers  94 ,  96 , and  98 . The computer  94  receives input signals from the memories  86  and  84 , and also receives atmospheric data stored at  100  in order to compare various ratios of intensities of selected spectral components of the measured target spectrum with corresponding ratios of intensities of the target reference spectrum. This will be described hereinafter in greater detail. 
     The range of the target produced by the computer  94 , and the identity of the target produced by the correlator  88  are applied to the computer  96 . Range rate can then be computed by the computer  96  by observing changes in range over an interval of time. Target range, range rate, and identity are then outputted by the computer  96  to the computer  98 . The computer  98  receives the target azimuth and elevation coordinates via line  74  and, in conjunction with the range and the range rate, computes target trajectory. The azimuth and elevation coordinates also serve to identify the target by location. The target identity and trajectory data are outputted by the computer  98  to a display  102  for outputting data relative to each of the targets selected by the correlation unit  58  (FIG. 1) for analysis. The display  102  may include recording apparatus (not shown) for recording the data. 
     By way of example, in the use of the spectrum analyzer,  80 , emission lines of sodium and potassium are discerned readily in the hot plumes of rockets by atomic line filters, such as the filters  46 ,  48 , and  50  (FIG. 1) and by the spectrum analyzer  80 . Such spectral lines facilitate identification of the target. The use of a spectrum analyzer, such as the analyzer  80 , is disclosed in the aforementioned article of S. H. Bloom et al. The sodium and the potassium spectral lines are presented by way of example, and numerous other lines may be observed, depending on chemistries of the sources of radiation. In atomic spectroscopy, there are well-known doublet lines appearing in the spectrum which also serve to identify a source of the radiation. It is also recognized that the effect of atmospheric attenuation may vary with elevation angle and, accordingly, in FIG. 2, the target coordinates on line  74  are applied also for addressing the atmospheric data store  100  to select an atmospheric attenuation profile consonant with a specific value of target elevation. 
     FIGS. 3-5 demonstrate attenuation of various frequency components of target radiation as a function of distance from the target. By way of example, FIG. 3 shows a stylized representation of the radiation wherein the spectrum of the radiation has both a continuous portion and a set of spectral lines. This is the spectrum which would be measured at the location of the target, namely, at zero range. In the continuous portion of the spectrum, three components are identified by the legends A, B, and C by way of example. The frequency of component B is greater than the frequency of component A, and the frequency of component C is greater than the frequency of component B. In FIG. 4, the spectrum has been simplified to show only the continuous portion with the components A, B, and C. FIG. 4 depicts relative attenuation of the components A, B, and C at a distance from the target of five kilometers, by way of example. 
     Upon comparing the graphs of FIGS. 3 and 4, it is noted that in FIG. 3, the amplitudes of the components A, B, and C are equal. In FIG. 4, component B is substantially smaller than component A, and component C is substantially smaller than component B due to atmospheric absorption. Thus, upon comparing the amplitudes of the various components, such as the ratio of A to B, and the ratio of A to C, by way of example, it is observed that these ratios, which are unity in FIG. 3, differ markedly in FIG.  4 . 
     Due to the selective attenuation of the radiation by the atmosphere as a function of frequency, the intensity ratio changes with increasing distance from the target. This is portrayed in FIG. 5 wherein the lines represent the amplitude ratios A/C and A/B. The attenuation of the radiation by the atmosphere is described by the slopes of the graph of FIG. 5, the slopes being determined by the atmospheric attenuation factors. Information from the memory  84  (FIG. 2) is employed by the computer  94  (FIG. 2) to compute the attenuation factors. The attenuation factors are based on experimental evidence as is stored in the memory  84  wherein spectra are stored for measurements conducted at various distances from each of a plurality of radiation sources. The computer  94  then computes the amplitude ratios of the selected frequency components of the measured continuous spectrum from the memory  86 , and employs the amplitude ratios of the frequency components to compute the target range. 
     Calculation of the target range can be accomplished by (1) establishing an initial value of the amplitude ratios of the essential spectral components at zero range as is depicted in the left side of FIG. 5, (2) establishing the ratio of the spectral components at a nonzero distance such as at five kilometers presented in FIG. 5, (3) establishing the slope of a specific graph of FIG. 5 from prior knowledge of atmospheric attenuation, and (4) solvine mathematically the graphically portrayed relationship of FIG. 5 for the propagation distance along the horizontal axis of FIG.  5 . 
     FIG. 6 outlines the essential steps of the foregoing procedure wherein, in block  104  the weather or other atmospheric condition is noted, and at block  106  the spectrum of a suitable target is identified. The weather conditions of block  104  is stored in the atmospheric data storage  100  (FIG.  2 ), and the identification of a suitable target spectrum is obtained during the acquisition and track modes of FIG. 1, by operation of the correlation unit  58 . At block  108 , a selection is made of spectral lines to be used in forming the ratios, the selection being made by the computer  94  (FIG. 2) which uses, by way of example, the strongest spectral lines, such as the three strongest spectral lines in the continuous spectrum of FIG.  3 . Then, at block  110 , the amplitude ratios of the spectral lines are formed for the measured spectrum stored in the memory  86  (FIG.  2 ). The ratios are compared at block  112 , the comparing being done in the computer  94 . This is followed by a computation of the range at block  114 . The computation of the range is accomplished by the computer  94  following the procedure outlined above with reference to the graphs of FIG.  5 . Thereby, the invention has accomplished the attainment of target range by a passive observation of radiation emitted by the target. It is noted that the foregoing measurement of rang can be accomplished by use of a value of intensity ratio obtained from the spectral components A and B or from the spectral components A and C. Alternatively, in the practice of the invention, plural intensity ratios can be employed for improved accuracy of measurement. For example, in the operation of the computer  94 , a first measurement of the range can be accomplished by use of the spectral components A and B, and a second measurement of the range can be accomplished by the use of the spectral components A and C. The two measurements are then averaged by the computer  94  to provide an average value of the range measurement for improved accuracy in the determination of the range. As has been noted above, the graphs of FIGS. 3-4 depict the three spectral lines, A, B, and C, by way of example, and that additional lines such as lines D and E (not shown) may be employed for determination of still further intensity ratios for yet additional measurement of range. Any pairs of frequencies, such as the ratio of B and C, or the ratio of C and D, and the ratio of A and D, may be employed, assuming that the ratios are statistically independent. Thus, three spectral lines provide two statistically independent ratios, and four spectral lines provide three statistically independent ratios, by way of example. In this manner, the use of plural ratios from both the received and the reference spectra may be employed for improved accuracy in the determination of the range. 
     In the foregoing analysis, it has been assumed that the spectral components of the continuous spectrum have equal amplitude. In the event that these spectral components differ in amplitude, such amplitude difference appears in the reference spectrum provided by the memory  84  to the computer  94 . In such case, the computer  94  introduces an additional multiplicative factor to each of the amplitude ratios of the selected frequency components to compensate for the differences of amplitude in the spectral components at zero range. 
     With reference to FIG. 7, the system  10  may view radiation from the target  12  in a situation, wherein the target  12  is located beyond the earth&#39;s horizon, by observation of radiant energy emitted by the target  12  and reflected from a cloud  116  via rays  18  of radiation. Typically, the system  10  is located on the earth&#39;s surface, as indicated in solid lines, or is provided as an airborne system  10 ′ carried by an aircraft  120 , as indicated in phantom view. FIG. 7 shows the situation wherein the target  12  is a rocket  122  emitting a plume  124  which is a source of radiation  126  reflected via the rays  118  from the cloud  116  to be viewed by the system  10 , or the system  10 ′. 
     FIG. 8 is a diagram of an application in which an embodiment of the present invention passively determines a range to a target reflecting solar radiation or other suitable radiation source, such as a search light or tunable laser. In this application, the target need not be self-luminous since the solar radiation is providing the electromagnetic spectrum used during a target acquisition. 
     In the case of using the sun as the source of the radiation, the position of the sun  150  is static within the time frame of the measurement. The sun  150  illuminates a target  154  and a spot  156  adjacent to a sensor (i.e., optical assembly employing the present invention) at a known position, for example, on a tank  158 . Solar radiation  152  reflects off the target  154  and is represented as reflected radiation  160   a.  Solar radiation  152  further reflects off the adjacent spot  156  and is represented as reflected radiation  160   b.  The sensor in the tank  158  measures the reflected radiation  160   a  and  160   b  and calculates a differential attenuation between the two spectra. 
     The tank  158 , employing the principles of the present invention, compares the absorption spectrum off the adjacent spot  156  (e.g., ground) with the reflected radiation  160   a  off the target  154 , thus, using Beer&#39;s Attenuation Law and known attenuation of the atmosphere. If operating at higher elevations, consideration for the lower absorption of the higher atmosphere must be taken into account. It should be understood that since the sun  150  is illuminating the adjacent spot  156 , and the sun  150  and adjacent spot  156  both are at known positions, it is possible for the present invention to calculate the solar radiation spectra rather than measuring it. If, however, measurement of the adjacent spot  156  is to be made, a small flip mirror may be employed in the optical assembly apparatus to switch the look angle between the target  154  and the adjacent spot  156 . 
     It is desirable for the differential attenuation between the reflected radiation from the adjacent spot  160   b  and the reflected radiation  160   a  from the target  154  to be large. A large differential attenuation results in more sensitivity than if the differential attenuation were small. 
     FIG. 9 provides a graph indicating relative amplitudes between spectra of the reflected radiation  160   b  from the adjacent spot  156  and the reflected radiation  160   a  from the target  154 . The wavelengths of interest fall between λ 1  and λ 2 . At λ A , the range from the adjacent spot  156  (unabsorbed radiation) to the target  154  (absorbed radiation) is determined as a function of distance and angle, F(R, ζ, φ). Relative attenuation of the components A and B can be determined in a manner set forth above. Further, rate and angle can also be determined in a manner as set forth above. 
     It is to be understood that the above described embodiment of the invention is illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiment disclosed herein, but is to be limited only as defined by the appended claims.