Abstract:
A system for measuring a parameter of a medium with a vehicle moving in a traveling direction through the medium includes four detecting portions and a calculating portion. The four detecting portions respectively detect first through fourth values of the parameter from first through fourth lines-of-sight in first through fourth directions at first through fourth positions of the vehicle at first through fourth times. The first line-of-sight and the third line-of-sight are in a first plane and intersect at a first intersection, while the second line-of-sight and the fourth line-of-sight are in a second plane and intersect at a second intersection. The calculating portion calculates the parameter based on the first through fourth values and the first and second intersections.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 61/353,707, filed Jun. 11, 2010, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods for determining ionic or neutral concentrations within a medium using a passive remote sensing technique. 
     2. Description of the Related Art 
       FIG. 1  illustrates different atmospheric layers of the Earth&#39;s atmosphere. 
     As shown in the figure, up to about 14 kilometers (km) directly above the Earth  100  is the troposphere  102 . After the troposphere  102  is the tropopause  104 , which is only about 4 km thick. The stratosphere  106  is directly above the tropopause  104 . Within the stratosphere  106  is the ozone layer  108 . Next is the mesosphere  110 , which is about 40 km thick. Finally, the ionosphere  112 , directly above the mesosphere  110 , is hundreds of kilometers thick. The neutral component of the atmosphere above the mesosphere is referred to as the thermosphere. Together, the ionosphere, thermosphere and mesosphere are commonly referred to as the upper atmosphere. 
     With respect to telecommunications, the ionosphere is particularly important. 
     The ionosphere is the ionized part of the atmosphere produced primarily by the absorption of solar radiation. The principle component of the upper part of the ionosphere is singly ionized atomic oxygen (O+). The ionosphere has practical importance because, among other functions, it influences radio wave propagation to distant places on the Earth. The influence extends across a wide range of radio frequency bands, well above the high frequency band, considered to be 3-30 megahertz (MHz.) The effects include impacts on radio transmissions in all bands, e.g., amplitude modulation (AM), frequency modulation (FM), shortwave, etc., and radars (including over the horizon radars). 
     Satellite-borne remote sensing of the ionosphere observe emissions by atomic ions (singly ionized atomic oxygen (O+)) and the neutral components of the upper atmosphere such as atomic oxygen (O), molecular oxygen (O 2 ), molecular nitrogen (N 2 ), nitric oxide (NO), ozone (O 3 ), helium (He), hydrogen (H).  FIGS. 2A-2C  illustrate such a system. 
       FIGS. 2A-C  illustrate a conventional satellite-based method of measuring ionic concentrations within the Earth&#39;s ionosphere.  FIG. 2A  illustrates measurements taken at a first time t 1 .  FIG. 2B  illustrate measurements taken at a second time t 2 .  FIG. 2C  illustrate locations of calculated ionic concentrations using the measurements at times t 1  and t 2 . 
     As shown in  FIG. 2A , a satellite  202  and a satellite  204  are located in space  206  above the Earth&#39;s ionosphere  208 , which is illustrated as having a lower boundary  210  and an upper boundary  212 . 
     At time t 1 , satellite  202  measures the total emissions of a particular ion along a line-of-sight (LOS)  214 , whereas satellite  204  measures the total emissions of the ion along a LOS  216 , a LOS  218  and a LOS  220 . 
     In the conventional method of  FIG. 2A , satellite  202  is able to detect a total of emissions by a particular ion, for example, atomic oxygen ions (O + ), within ionosphere  208  along LOS  214 . 
     What is more valuable for radio wave communications is an altitude profile of the amount of the particular ion, in this example atomic oxygen ions (O + ) at each altitude z, or [O + ](z). In other words, in addition to the total amount of emission along LOS  214 , an altitude function [O + ](z) of the O +  number density along LOS  214  would be valuable. A mapping of such altitude functions along the path of a vehicle traveling above the earth would greatly enable high frequency (HF) communication systems to compensate for negative impacts of our otherwise imperfect knowledge of the altitude profile of atomic oxygen ions on HF and radio frequency signals. 
     The altitude function of the particular ion is formulated by tomographic retrieval. The mathematical basis for tomographic retrieval is applied to obtain cross-sectional images and is based on the notion that a projection of an object at a given angle θ is made up of a set of line integrals. In ionospheric observations, the line integral represents the total emissions along a line-of-sight (LOS) through the ionosphere. It is known that if there are an infinite number of one-dimensional projections of an object taken at an infinite number of angles, the original object can be reconstructed. To accomplish this, a filtered back projection algorithm is used. Accordingly, to find the altitude function of the particular ion, the individual ion concentrations along LOS  214  via satellite  204  are first determined. For example, satellite  204  is able to detect a total of emissions by the same ion as satellite  202 , in this example O + , within ionosphere  208  along LOSs  216 ,  218  and  220 . 
     Here, LOSs  214 ,  216 ,  218  and  220  are in the same plane, i.e. the plane of the figure, such that: LOS  214  intersects with LOS  216  at location  222 ; LOS  214  intersects with LOS  218  at location  224 ; and LOS  214  intersects with LOS  220  at location  226 . Clearly, satellite  204  may detect total emissions within ionosphere  208  along more LOSs, however, for purposes of discussion, a sampling of LOSs  216 ,  218  and  220  are provided. 
     In order to tomographically retrieve the ion altitude function of the entire plane of ionosphere  208  (a ribbon in the plane of the figure), satellites  202  and  204  must scan additional areas. This will be described with reference to  FIG. 2B . 
     As shown in  FIG. 2B , satellite  202  and satellite  204  are located at new locations in space  206  above ionosphere  208 . 
     At time t 2 , satellite  202  measures the total emissions of the particular ion along a LOS  228 , whereas satellite  204  measures the total emissions of the ion along a LOS  230 , a LOS  232  and a LOS  234 . 
     Here, LOSs  228 ,  230 ,  232  and  234  are in the same plane, i.e. the plane of the figure, such that: LOS  228  intersects with LOS  230  at location  236 ; LOS  228  intersects with LOS  232  at location  238 ; and LOS  228  intersects with LOS  234  at location  240 . Clearly, satellite  204  may detect total emissions within ionosphere  208  along more LOSs, however, for purposes of discussion, a sampling of LOSs  230 ,  232  and  234  are provided. 
     The detected total emissions along a LOS includes emission contributions from ions within the LOS in addition to emission contributions from neighboring ions, taking into account secondary emission issues related to resonance, fluorescence, etc. This will be described with reference to  FIG. 2C . 
     As shown in  FIG. 2C , locations  222 ,  224  and  226  are determined from the intersecting LOSs of  FIG. 2A , whereas locations  236 ,  238  and  240  are determined from the intersecting LOSs of  FIG. 2B . Here the emission detected by satellite  202  (and  204  for that matter) at location  222  includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at locations  224 ,  226 ,  236 ,  238  and  240 . Similarly, emission detected by satellite  202  (and  204  for that matter) at location  236  includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at locations  222 ,  224 ,  226 ,  238  and  240 . 
     As satellites  202  and  204  scan the remainder of the plane within ionosphere  208 , an array of emission values will be determined. If more LOSs are used, then more emission values will be determined, i.e., the larger the array. Once the emission values are determined, any known method may be used to determine the ion altitude function for the entire plane of ionosphere  208 . 
     Once the ion altitude function for the entire plane of ionosphere  208  is known, it may be taken into account when transmitting/receiving signals therethrough. 
     All conventional systems for measuring ionic concentrations within the Earth&#39;s ionosphere are not satellite-based. 
       FIGS. 3A-C  illustrate a conventional system of ground-based detectors used to deduce the properties of the ionosphere. The geometry illustrated in  FIG. 3A-C  has been applied to radio-based remote sensing of ionospheric properties.  FIG. 3A  illustrates measurements taken at a first time t 1 .  FIG. 3B  illustrate measurements taken at a second time t 2 .  FIG. 3C  illustrates locations of calculated ionic concentrations using the measurements at times t 1  and t 2 . 
     As shown in  FIG. 3A , a ground-based detector  302  and a ground-based detector  304  are located below ionosphere  208 . The system of  FIG. 3A  operates in a similar manner to that of the system of  FIG. 2A . However, in the system of  FIG. 3A , the LOSs are directed from the Earth to ionosphere  208 . 
     At time t 1 , ground-based detector  302  measures the total emissions of a particular ion along a LOS  314 , whereas ground-based detector  304  measures the total emissions of the ion along a LOS  316 , a LOS  318  and a LOS  320 . 
     The altitude function of the particular ion is formulated by initially finding individual ion concentrations along LOS  314  via ground-based detector  302 . Ground-based detector  304  is able to detect a total of emissions by the same ion ground-based detector  302 , in this example O + , within ionosphere  208  along LOSs  316 ,  318  and  320 . 
     Here, LOSs  314 ,  316 ,  318  and  320  are in the same plane, i.e. the plane of the figure, such that: LOS  314  intersects with LOS  316  at location  322 ; LOS  314  intersects with LOS  318  at location  324 ; and LOS  314  intersects with LOS  320  at location  326 . Clearly, ground-based detector  304  may detect total emissions within ionosphere  208  along more LOSs, however, for purposes of discussion, a sampling of LOSs  316 ,  318  and  320  are provided. 
     As shown in  FIG. 3B , ground-based detector  302  and ground-based detector  304  are located in the same positions as described above with reference to  FIG. 3A . However, in this situation, ground-based detector  302  is detecting along a new LOS and ground-based detector  304  is detecting along new LOSs. 
     At time t 2 , ground-based detector  302  measures the total emissions of the particular ion along a LOS  328 , whereas ground-based detector  304  measures the total emissions of the ion along LOS  330 , a LOS  332  and a LOS  334 . 
     Here, LOSs  328 ,  330 ,  332  and  334  are in the same plane, i.e. the plane of the figure, such that: LOS  328  intersects with LOS  330  at location  336 ; LOS  328  intersects with LOS  332  at location  338 ; and LOS  328  intersects with LOS  334  at location  340 . Clearly, ground-based detector  304  may detect total emissions within ionosphere  208  along more LOSs, however, for purposes of discussion, a sampling of LOSs  330 ,  332  and  334  are provided. 
     As mentioned previously, the detected total emission along a LOS includes emission contributions from ions within the LOS in addition to emission contributions from neighboring ions, taking into account secondary emission issues related to resonance, fluorescence, etc. This will be further described with reference to  FIG. 3C . 
     As shown in  FIG. 3C , locations  322 ,  324  and  326  are determined from the intersecting LOSs of  FIG. 3A , whereas locations  336 ,  338  and  340  are determined from the intersecting LOSs of  FIG. 3B . Here the emission detected by ground-based detector  302  (and  304  for that matter) at location  322  includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at location  324 ,  326 ,  336 ,  338  and  340 . Similarly, emission detected by ground-based detector  302  (and  304  for that matter) at location  336  includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at location  322 ,  324 ,  326 ,  338  and  340 . 
     As ground-based detectors  302  and  304  scan the remainder of the plane within ionosphere  208 , an array of emission values will be determined. If more LOSs are used, then more emission values will be determined, i.e., the larger the array. Once the emission values are determined, any known method may be used to determine the ion altitude function for the entire plane of ionosphere  208 . 
     Once the ion altitude function for the entire plane of ionosphere  208  is known, it may be taken into account when transmitting/receiving signals therethrough. 
     Of the conventional systems discussed above, they are limited to determining the ion altitude function from above a medium or from below a medium. 
     What is needed is system and method for determining the ion altitude function of a medium from within the medium. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for determining the ion altitude function of a medium from within the medium. 
     In accordance with aspects of the present invention, a system measures a parameter of a medium with a vehicle moving in a traveling direction through the medium. The system includes four detecting portions and a calculating portion. The first detecting portion detects a first value of the parameter from a first line-of-sight in a first direction at a first position of the vehicle at a first time. The second detecting portion detects a second value of the parameter from a second line-of-sight in a second direction at a second position of the vehicle at a second time. The third detecting portion detects a third value of the parameter from a third line-of-sight in a third direction at a third position of the vehicle at a third time. The fourth detecting portion detects a fourth value of the parameter from a fourth line-of-sight in a fourth direction at a fourth position of the vehicle at a fourth time. The calculating portion calculates the parameter based on the first value, the second value, the third value, and the fourth value. The first line-of-sight and the third line-of-sight are in a first plane and intersect at a first intersection. The second line-of-sight and the fourth line-of-sight are in a second plane and intersect at a second intersection. The calculating portion calculates the parameter based, additionally, on the first intersection and the second intersection. 
     Additional advantages and novel features of the present invention are set forth in the various embodiments described in more detail in the description which follows, and will become more readily apparent to those of ordinary skill in the art upon examination of the following, or may be learned by practice of the invention. The numerous advantages of the invention are realized and attained by the instrumentalities and combinations particularly pointed out in the appended claims. It will be understood that the embodiments described herein are exemplary, and thus do not restrict the scope of the invention. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate exemplary embodiments of the present invention and, together with the description, explain, but do not restrict, the present invention. In the drawings: 
         FIG. 1  illustrates the different atmospheric layers of the Earth&#39;s atmosphere; 
         FIGS. 2A-C  illustrate a conventional satellite-based method of measuring ionic concentrations within the Earth&#39;s ionosphere;  FIG. 2A  illustrates measurements taken at a first time t 1 ;  FIG. 2B  illustrate measurements taken at a second time t 2 ; and  FIG. 2C  illustrate locations of calculated ionic concentrations using the measurements at times t 1  and t 2 ; 
         FIGS. 3A-C  illustrate a conventional system of ground-based detectors used to deduce the properties of the ionosphere;  FIG. 3A  illustrates measurements taken at a first time t 1 ;  FIG. 3B  illustrate measurements taken at a second time t 2 ; and  FIG. 3C  illustrate locations of calculated ionic concentrations using the measurements at times t 1  and t 2 ; 
         FIGS. 4A-C  illustrate a system and method of measuring ionic concentrations within the Earth&#39;s ionosphere in accordance with the present invention;  FIG. 4A  illustrates measurements taken at a first time t 1 ;  FIG. 4B  illustrates measurements taken at a second time t 2 ; and  FIG. 4C  illustrates locations of calculated ionic concentrations using the measurements at times t 1  and t 2 ; 
         FIG. 5  illustrates an example resulting grid of calculated ionic concentrations within the Earth&#39;s ionosphere in accordance with the present invention; 
         FIG. 6  illustrates an example system for calculating ionic concentrations within the Earth&#39;s ionosphere, in accordance with the present invention; and 
         FIG. 7  illustrates an example method for calculating ionic concentrations within the Earth&#39;s ionosphere, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with exemplary embodiments of the present invention, a sensor design permits imaging of structures in a layer of the atmosphere from a vehicle traveling within the layer of the atmosphere. An example embodiment uses a hyperspectral imager that scans from near zenith to below the local horizon to image the atmosphere in predetermined wavelengths. The spectral signatures imaged contain information about the line-of-sight (LOS) density of the neutral and ionized constituents. These spectral signatures, when combined with the viewing geometry, enable retrieval of the ion altitude function of the layer of the atmosphere. These data can be used to evaluate the electron density profiles (EDP) and scintillation profiles that impact space operations. 
     In an example embodiment, a spectrograph is used as a sensor that operates in the ultraviolet range from about 40 nanometers (nm) to 300 nm. This spectral region contains the signatures of the major species in the upper atmosphere (also known as the thermosphere) and the ionosphere. In particular, atomic oxygen (O), molecular oxygen (O 2 ), molecular nitrogen (N 2 ), nitric oxide (NO), ozone (O 3 ), helium (He), hydrogen (H) and singly ionized atomic oxygen (O+) can be observed. From these species, electron density profiles can be retrieved. The sensor includes an imaging spectrograph (e.g., a spectrograph with the capability of producing spatial information along the slit direction) coupled to a mirror that scans the field of regard of the instrument. The field of regard is scanned in the vertical plane. 
     The system design and operation produces a two-dimensional set of intersecting lines-of-site (LOSs). This set of intersecting LOSs specifies the two dimensional structure of the emitting layer. This technique works when the sensor is immersed in the radiating medium, for example. If the system is above the radiating layer the field of regard of the sensor may be changed in order to achieve a sampling density sufficient to uniquely specify the emitting region. Multispectral imagery may be used in order to be able to accurately account for other emission mechanisms. 
     An example system and method for calculating ionic concentrations within the Earth&#39;s ionosphere, in accordance with aspects of the present invention will now be described in further detail with reference to  FIGS. 4A-7 . 
       FIGS. 4A-C  illustrate a system and method of measuring ionic concentrations within the Earth&#39;s ionosphere in accordance with one or more exemplary embodiments of the present invention.  FIG. 4A  illustrates measurements taken at a first time t 1 .  FIG. 4B  illustrates measurements taken at a second time t 2 .  FIG. 4C  illustrates locations of calculated ionic concentrations using the measurements at times t 1  and t 2 . 
     As shown in  FIG. 4A , a detecting vehicle  402  is traveling through ionosphere  208 , along a path indicated by dotted line  404 . A zenith direction indicated by the dotted line  406  is normal to the traveling path, i.e., dotted line  406  is perpendicular to dotted line  404 . 
     At time t 1 , detecting vehicle  402  is operable to measure the total emissions of a particular ion along a LOS  408 , a LOS  410 , a LOS  412 , a LOS  414  and a LOS  216 . LOS  408  and LOS  410  are below the traveling direction. LOS  408 , in particular is below the traveling direction by an angle φ, i.e., LOS  408  is below dotted line  404  by angle φ. LOS  416  is beyond the zenith direction by an angle θ, i.e., LOS  416  is beyond dotted line  406  by angle θ. 
     The scan range of detecting vehicle  402  is below detecting vehicle  402  and above detecting vehicle  402 . 
     The altitude function of the particular ion is formulated by initially finding individual ion concentrations along a “fan” of LOSs  408 ,  410 ,  412 ,  414  and  416  via detecting vehicle  402 . Another fan of LOSs will then be used, as will be described with reference to  FIG. 4B . 
     As shown in  FIG. 4B , detecting vehicle  402  is located at a new position along the path indicated by dotted line  404 . 
     At time t 2 , detecting vehicle  402  is operable to measure the total emissions of the particular ion along a LOS  418 , a LOS  420 , a LOS  422 , a LOS  424  and a LOS  426 . LOS  418  and LOS  420  are below the traveling direction. In particular, LOS  418  is below the traveling direction by angle φ, i.e., LOS  418  is below dotted line  404  by angle φ. LOS  426  is beyond the zenith direction by angle θ, i.e., LOS  426  is beyond dotted line  406  by angle θ. 
     Here, LOSs  408 ,  410 ,  412 ,  414  and  416  of  FIG. 4A  are in the same plane, i.e. the plane of the figure, and LOSs  418 ,  420 ,  422 ,  424  and  426  of  FIG. 4B  are in the same plane such that: LOS  410  intersects with LOS  418  at location  428 ; LOS  412  intersects with LOS  424  at location  430 ; and LOS  414  intersects with LOS  426  at location  432 . Clearly, detecting vehicle  402  may detect total emissions within ionosphere  208  along more LOSs, however, for purposes of discussion, a sampling of LOSs is provided. 
     The detecting instrument on detecting vehicle  402  scans as detecting vehicle  402  moves along the path indicated by dotted line  404 . Successive scans overlap. The multiple-overlapping LOSs provide the input to a tomographic retrieval of the ion altitude function above and below detecting vehicle  402 , i.e., for the entire plane of ionosphere  208 . 
     In order to map the ion altitude function for the entire plane of ionosphere  208 , i.e., a ribbon in the plane of the figure, detecting vehicle  402  must scan additional areas. 
     As described previously, the detected total emission along a LOS includes emission contributions from ions within the LOS in addition to emission contributions from neighboring ions, taking into account secondary emission issues related to resonance, fluorescence, etc. This will be further described with reference to  FIG. 4C . 
     As shown in  FIG. 4C , locations  432 ,  430  and  428  are determined from the intersecting LOSs of  FIG. 4B . Here the emission detected by detecting vehicle  402  includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at other locations as discussed above, for example with reference to  FIGS. 2C and 3C . 
     As detecting vehicle  402  scans the remainder of the plane within ionosphere  208 , an array of emission values will be determined. If more LOSs are used, then more emission values will be determined, i.e., the larger the array. Once the emission values are determined, any known method may be used to determine the ion altitude function for the entire plane of ionosphere  208 . This will now be described with reference to  FIG. 5 . 
       FIG. 5  illustrates an example resulting grid of calculated ionic concentrations within the Earth&#39;s ionosphere in accordance with aspects of the present invention. 
     As shown in the figure, a detector is operable to detect along a plurality of “fans” of LOSs, a sampling of which is indicated as fan  502  and fan  504 . Fan  502  spreads from a first LOS  506  through an oblique angle to an LOS  508 . A grid  510  represents intersections of LOSs from the plurality of fans of LOSs. Grid  510 , in this illustrative case, spans a longitude of 10° along an x-axis  512  and spans an altitude from 200 Km to 600 Km along a y-axis  514 . An example sampling of ionic concentrations is shown by dotted line  516 . 
     Once the ion altitude function for the entire plane of ionosphere  208  is known, it may be taken into account when transmitting/receiving signals therethrough. 
       FIG. 6  illustrates an example system  600  for calculating ionic concentrations within the Earth&#39;s ionosphere, in accordance with aspects of the present invention. 
     As shown in the figure, system  600  includes a controlling portion  602 , a detector  604 , a calculating portion  606  and an output portion  608 . Controlling portion  602 , detector  604 , calculating portion  606  and output portion  608  are illustrated as individual devices. However, in some embodiments, at least two of controlling portion  602 , detector  604 , calculating portion  606  and output portion  608  may be combined as a unitary device. Further, in some embodiments, at least one of controlling portion  602 , detector  604 , calculating portion  606  and output portion  608  may be implemented as a tangible computer-readable media for carrying or having computer-executable instructions or data structures stored thereon. Such tangible computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer. Non-limiting examples of tangible computer-readable media include physical storage and/or memory media such as RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. For information transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer may properly view the connection as a computer-readable medium. Thus, any such connection may be properly termed a computer-readable medium. Combinations of the above should also be included within the scope of tangible computer-readable media. 
     Detector  604  includes a detecting portion  610 , a detecting portion  612 , a detecting portion  614  and a detecting portion  616 . Detecting portion  610 , detecting portion  612 , detecting portion  614  and detecting portion  616  are illustrated as individual devices. However, in some embodiments, at least two of detecting portion  610 , detecting portion  612 , detecting portion  614  and detecting portion  616  may be combined as a unitary device. Further, in some embodiments, at least one of detecting portion  610 , detecting portion  612 , detecting portion  614  and detecting portion  616  may be implemented as a tangible computer-readable media for carrying or having computer-executable instructions or data structures stored thereon. 
     Controlling portion is arranged to provide control signals  618 ,  620  and  622 . Detector  604  is arranged to receive control signal  618  and output detected signal  624 . Calculating portion  606  is arranged to receive control signal  620  and signal  624  and to output a calculated signal  626 . Output portion  608  is arranged to receive control signal  622  and signal  626 . 
     Operation of  600  will now be described with additional reference to  FIG. 7 . 
       FIG. 7  illustrates an example method  700  for calculating ionic concentrations within the Earth&#39;s ionosphere, in accordance with aspects of the present invention. 
     Method  700  starts (S 702 ) and the signatures for detection are determined (S 704 ). 
     In an example embodiment, the emission signature for atomic oxygen (O) is selected. For example, detecting portion  610  may be operable to detect the signature of atomic oxygen. However, in other embodiments, the emission signature for molecular oxygen (O 2 ), molecular nitrogen (N 2 ), nitric oxide (NO), ozone (O 3 ), helium (He), hydrogen (H) and singly ionized atomic oxygen (O+) may be selected. For example, detecting portion  610  may be a hyperspectral imaging device operable to detect the signature of at least one of the group of molecular oxygen (O 2 ), molecular nitrogen (N 2 ), nitric oxide (NO), ozone (O 3 ), helium (He), hydrogen (H) and singly ionized atomic oxygen (O+). In such a case, controlling portion  602  may instruct detecting portion  610  as to which signatures it should detect. Still further, in the event that another medium is to be scanned, other emission signatures may be selected. In particular, scanning of the ionosphere is described herein as a non-limiting example—merely for purposes of explanation. Any medium, non-limiting examples of which include other layers of the atmosphere, or fluids such as oceans, may be scanned for predetermined emission signatures which for the purposes of explanation have been described as “light” or “optical emissions” herein but may consist of acoustic or other forms of energy. 
     The examples discussed above additionally include a single signature as a non-limiting example—merely for purposes of explanation. In other embodiments, a plurality of signatures may be detected, e.g., a hyperspectral scanning. This is described in greater detail below. 
     Once the signature is determined, then the scanning fan is determined (S 706 ). For example, returning to  FIG. 4A , the scanning fan includes five LOSs—LOS  408 , LOS  410 , LOS  412 , LOS  414  and LOS  416 . However, as seen in  FIG. 5 , the scanning fan includes many more LOSs, for example as seen in fan  502 . 
     As the number of LOSs in a fan increases, required data processing resources increase. However, as the number of LOSs in a fan increases, the spacing between LOS intersections decreases, which ultimately provides a more precise ion altitude function. 
     With respect to the maximum scanning angle of the scanning fan, it may be of any angle. Returning to  FIG. 4A , in example embodiments, the scanning fan should include an angle above the path indicated by dotted line  404 , for example any one of LOS  412 , LOS  414  and LOS  416 . In the example of  FIG. 4A , the maximum scanning angle is from LOS  408  to LOS  416  (including angles φ and θ). The scanning fan should additionally include an angle below the traveling direction indicated by dotted line  404 , for example any one of LOS  410  and LOS  408 . Geometrically speaking, including an angle above the direction of travel and including an angle below the direction of travel will ensure scanning of the entire medium in which detecting vehicle  402  is traveling. 
     In an example embodiment, if the upper bound of the scanning fan is beyond the zenith direction (dotted line  406 ), this ensures adequate sampling of the medium above the detector.  FIG. 3  illustrates this principle in action for the case of stationary ground sensors—lines of sight  330 ,  332 , and  334  can, for the purposes of illustration, be thought of as the LOS past the vertical. The measurements from ground-based position  302  defining LOS  328  (which is not past vertical), when combined with those measurements from position  304 , define points  336 ,  338  and  340 . The exact range beyond the vertical is determined by the vertical resolution requirements of the measurements. 
     Returning to  FIG. 6 , controlling portion  602  may set maximum scan angle of the scanning fan. In some embodiments, the maximum scan angle may be predetermined and programmed within controlling portion  602 . In other embodiments, the maximum scan angle may be remotely entered into controlling portion  602 . 
     Controlling portion  602  may additionally set the number of LOSs. For example, as shown in  FIG. 4A , controlling portion  602  would have set the number of LOSs to five, whereas as shown in  FIG. 5 , controlling portion  602  would have set the number of LOSs to a much larger number. In some embodiments, the number of LOSs may be predetermined and programmed within controlling portion  602 . In other embodiments, the number of LOSs may be remotely entered into controlling portion  602 . 
     Controlling portion  602  instructs detector  604  to scan via control signal  618 . Detector  604  may be any known detecting system for detecting a desired parameter. In example embodiments, detector  604  is operable to detect emissions by atomic ions such as atomic oxygen (O), molecular oxygen (O 2 ), molecular nitrogen (N 2 ), nitric oxide (NO), ozone (O 3 ), helium (He), hydrogen (H) and singly ionized atomic oxygen (O+). Further, in some embodiments, detector  604  may be a hyperspectral detector operable to detect emission by atomic ions of any combination of the group of atomic oxygen (O), molecular oxygen (O 2 ), molecular nitrogen (N 2 ), nitric oxide (NO), ozone (O 3 ), helium (He), hydrogen (H) and singly ionized atomic oxygen (O+). 
     Detector  604  may scan by any known beam steering system and method. Non-limiting examples of beam steering systems and methods include electrical and mechanical beam steering systems and methods. 
     Returning to  FIG. 7 , once the scanning fan is determined, the medium is scanned (S 708 ). For example, as shown in  FIG. 4A , detecting vehicle detects an intensity value from LOS  408 . This may be accomplished, as shown in  FIG. 6 , by detecting portion  610 . Detector  604  knows where to start its scanning fan as instructed by controlling portion  602  via control signal  618 . Accordingly, detecting portion  610  is directed to detect an intensity value along LOS  408 . Detecting portion  610  may be any known type of intensity detector, a non-limiting example of which includes a photodiode. As mentioned previously, in some embodiments, detecting portion  610  may be a one or two dimensional array detector able to detect wavelength dependent intensity measurements over a wavelength range simultaneously. A system operable to detect an intensity value corresponding to a plurality of distinct wavelengths is commonly called a “hyperspectral” sensor and would be in operation along LOS  408 . 
     Returning to  FIG. 4A , the detected intensity value I, for example of LOS  408 , corresponds to a summation of the emissions from all the ions along LOS  408  from detecting vehicle  402  to lower boundary  210  of ionosphere  208 . The detected intensity value I and the geometry (direction of the vector) of LOS  408  are passed to calculating portion  606  via detected signal  624 . 
     Returning to  FIG. 7 , once the scan is complete, it is determined whether the most recent scan is the last scan to be performed (S 710 ). Continuing with the example discussed above, and returning to  FIG. 4A , presume that LOS  410  is to be scanned next. With reference to  FIG. 6 , control signal  618  from controlling portion  602  had instructed detector  604  of the scanning fan, which includes the number and placement of LOSs. Accordingly, at this point, detector  604  would know that LOS  410  is to be scanned after LOS  408 . 
     In this example, since LOS  408  is not the last scan to be performed, it then scans LOS  410  (S 708 ). For example, as shown in  FIG. 4A , detecting vehicle detects an intensity value from LOS  410 . This may be accomplished, as shown in  FIG. 6 , by detecting portion  612 . Detector  604  knows where to start its scanning fan as instructed by controlling portion  602  via control signal  618 . Accordingly, detecting portion  612  is directed to detect an intensity value along LOS  410 . Detecting portion  612  may be any known type of intensity detector, a non-limiting example of which includes a photodiode. As mentioned previously, in some embodiments, detecting portion  612  may be a hyperspectral detector, operable to detect an intensity values corresponding to a plurality of distinct wavelengths, along LOS  410 . 
     Returning to  FIG. 4A , the detected intensity value I, for example of LOS  410 , corresponds to a summation of the emissions from all the ions along LOS  410  from detecting vehicle  402  to lower boundary  210  of ionosphere  208 . The detected intensity value I and the geometry (direction of the vector) of LOS  410  are passed to calculating portion  606  via detected signal  624 . 
     It should be noted that the scan of LOS  408  occurs at a first time t 1  whereas the scan of LOS  410  occurs at a second later time t 2 . Accordingly, when scanning LOS  408 , detecting vehicle  402  is at a first position (presuming it is moving at a velocity), whereas when scanning LOS  410 , detecting vehicle  402  is at a second position. For purposes of discussion simplification, presume that the rate of scanning is much larger than the velocity of detecting vehicle  402 . In such a case, when scanning the fan that includes LOS  408 , LOS  410 , LOS  412 , LOS  414  and LOS  416 , presume that detecting vehicle  402  (and therefore detector  604 ) is at the same location. 
     The process of scanning (S 708 ) and determining whether the most recent scan is the last scan (S 710 ) continues throughout a scanning fan. For example, after the scanning of LOS  408 , LOS  410 , LOS  412 , LOS  414  and LOS  416  of  FIG. 4A , detector  604  will scan LOS  418 , LOS  420 , LOS  422 , LOS  424  and LOS  426  of  FIG. 4B . In particular, detector  604  will know the number of scans it is to perform based on instruction from controlling portion  602 . In the example illustrated in  FIG. 5 , detector  604  scans a plurality of fans, with a sample shown as fan  502  and fan  504 . In the example of  FIG. 5 , the scanning is complete, when a sufficient number of fans are scanned to obtain data points for grid  510 . 
     In the present example embodiment, detector  604  includes four detecting portions, each scanning a LOS in turn. Of course in other embodiments, detector  604  may include additional detecting portions, one for each predetermined scanned LOS. In still other embodiments, a single detecting portion is used to scan all LOSs. 
     Returning to  FIG. 7 , once it is determined that the scanning is complete, then the ion altitude function for the entire scan plane is calculated (S 712 ). 
     In simple terms, the observed intensity [y], is related to the geometric factor [a] and the ion altitude function [v] as follows:
 
 [y]=[a][x];  
 
where [y] is a vector of the observed intensity values [y 0 , y 1 , . . . , y n-1 , y n ], e.g., the intensity values measured from LOS  408 - 426  of  FIGS. 4A-B , where [a] is the corresponding tensor (a two dimensional matrix) of the geometries of the LOSs as they pass through each cell in the retrieval grid [a 00 , a 10 , . . . , a n-1,n , an n ], e.g., the corresponding directions of LOSs  408 - 426  of  FIGS. 4A-4B , and [x] is the corresponding vector of emission rates [x 0 , x 1 , . . . , x n-1 , x n ].
 
     Since [y] is measured and since [a] is known, for example, as instructed from controlling portion  602 , then [v] may be determined conceptually as follows by determining the “inverse” of the geometries of the LOSs:
 
[ a]   −1   [y]=[x] 
 
     The above description for determining [x] is purely a conceptual one: prior art defines many techniques for solving a general class of problems known as inverse problems by a technique known as tomographic inversion. Inverse problems are referred to as such because one seeks the distribution of a parameter that creates, by emission and/or absorption the feature or features detected. Tomographic reconstruction of the ionosphere from UV brightness measurements requires inversion of a discrete forward model that relates the observed brightness values to ionospheric electron density. The brightness value recorded by the instrument within detecting vehicle  402  is proportional to the square of the electron density integrated along the instrument&#39;s LOS. The line integral can be discretized by dividing the two-dimensional ionosphere into a series of basis functions that are non-zero over a cell, e.g., a 10 km by 10 km cell, of the ionosphere. The electron density value is considered constant within individual cells. A LOS measurement y i  is then related to the squared electron density values x j  by the following equation: 
                 y   i     =       ∑   j             ⁢       a   ij     ⁢     x   j           ,         
where a ij  is proportional to the length of the LOS i in cell j. A series of LOS measurements can then be related to ionospheric electron density by the matrix equation:
 
 y=Ax,  
 
where y is a vector of LOS measurements, x is a vector of squared electron densities, and A is a projection matrix determined from a geometrical forward model of LOSs from detecting vehicle  402 .
 
     The inverse problem can be solved by any known technique. For the purposes of illustration, assume that the individual contribution from each of the idealized cells is determined by determining the solution set that minimizes the cost function:
 
 J ( x )=∥ y−Ax∥   2 +λφ( Dx )
 
     For illustrative purposes, the minimization problem may be solved using a conjugate gradient approach. The two terms in the cost function are a least-squares term which enforces data fidelity and a regularization function that ensures a smooth ionosphere and reduces the impact of noise on the solution. The regularization function includes a weighted gradient term φ(Dx) that preserves edges in the image. The regularization parameter) balances data fidelity and smoothness in the reconstruction. 
     Returning to  FIG. 7 , once the ion altitude function is calculated, the result is output (S 714 ). For example, the ion altitude function may be provided as an image on a screen or provided to a transmitter for further processing. Returning to  FIG. 5 , the output may be grid  510 , wherein a graphical user interface enables a user to select any one column. In this example, let the column selected be that corresponding to dotted line  516 . Accordingly, the user may be provided with the ion altitude function of the column of ionosphere  208  corresponding to the location of dotted line  516 . 
     A system, for example one including a passive ultraviolet sensor, and its associated concept of operations in accordance with aspects of the present invention can recover the spatial structure of an inhomogeneous radiating layer when immersed within the medium. This design enables the recovery of the two dimensional structure of the upper atmosphere without requiring that the instrument rotate or be above the medium. 
     Aspects of the present invention are novel in that they address the problem of making these measurements from a vehicle that is immersed with the atmosphere. For ionospheric observations, that means an altitude from about 300 km through about 500 km. It will be noted, however, that additional aspects of the present invention may be applied to any medium and that the atmosphere, and the ionosphere in particular, are merely non-limiting examples used for purposes of discussion. 
     The foregoing description of various embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching without departing from the spirit or scope of the present invention. The example embodiments, as described herein, were chosen and described to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. The scope of the invention is defined by the claims appended hereto.