Abstract:
A passive radio frequency signal-enabled aircraft altimeter employing signals of one or more global position system satellites as a source of terrain illumination. The altimeter determines altitude of the host aircraft with respect to specific terrain features beneath the aircraft rather than an altitude above a mean or nominal level of the earth&#39;s surface—as is already provided in a global position system signal. The altimeter employs two signal paths between the global position system satellite and the host aircraft, one direct signal path and one earth-reflected signal path, together with elementary geometric/trigonometric relationships, involving length difference in these paths and signal angle of arrival, in determining aircraft altitude. Equal angles of satellite signal incidence and reflection at the point of satellite signal reflection from the earth is an enabling principle in the altimeter.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of aircraft altitude determination as accomplished electronically from within the considered aircraft by a passive, non signal emitting and global positioning system signal-based technique. 
     The conventional radar altimeter transmits a radar signal from its host aircraft to the ground and processes a reflected or returned radar signal to determine the altitude of the host aircraft. A transmitted signal of this nature can, however, be detected by hostile persons to reveal the presence of the aircraft and to provide guidance for ordnance intended to intercept the aircraft. Similar non-atmospheric pressure altitude measuring arrangements based on electromagnetic energy of differing wavelengths including, for example, laser energy emissions are possible under some weather conditions but have the related disadvantages of being detectable and providing ordnance target identification. The present invention provides an uncomplicated measurement of aircraft altitude free of these signal emission and target identification disadvantages. The invention employs signals readily available in most parts of the world and makes additional use of some apparatus possibly already available on the host aircraft. 
     SUMMARY OF THE INVENTION 
     The present invention provides a passive, global positioning system signal-operated altitude determination for an aircraft. The invention uses the global positioning system signals received at a pair of locations on the host aircraft and signal processing based on ground reflection of one received signal together with simple geometric relationships between reflected and direct signals to determine the distance between the host aircraft and the earth. 
     It is an object of the present invention, therefore, to provide a passive electronic altimeter for an aircraft. 
     It is another object of the invention to provide a global positioning system signal operated aircraft altimeter. 
     It is another object of the invention to provide a non signal-emitting electronic altimeter for a military aircraft. 
     It is another object of the invention to provide an aircraft altimeter based on the relationship between directly incident and reflected global positioning system signals received from a satellite in earth orbit. 
     It is another object of the invention to provide a confirming non atmospheric pressure-related altitude determination for an aircraft. 
     It is another object of the invention to provide an accurate low-cost radio altimeter usable in a variety of large and small aircraft types. 
     It is another object of the invention to provide a global positioning system signal-enabled aircraft altimeter operable with either a single or a plurality of global positioning system signals. 
     Additional objects and features of the invention will be understood from the following description and claims and the accompanying drawings. 
     These and other objects of the invention are achieved by the combination of: 
     a host aircraft having first and second global positioning system signal reception antennas disposed on opposite extremities thereof, said first and second antennas having direct signal and earth-reflected signal reception capabilities respectively with respect to radio frequency signals originating in a global positioning system satellite; 
     radio frequency signal receiver apparatus connected with said first and second global positioning system signal reception antennas and generating output signals representing first and second samples of global positioning system data received from said global positioning system satellite via said first and second antennas respectively; 
     signal processing apparatus connected with said radio frequency signal receiver apparatus and generating, from said first and second samples of global positioning system data, first distance signals representative of direct physical distance between said aircraft first global positioning system signal reception antenna and said satellite and second distance signals representative of greater ground reflection-inclusive indirect physical distance between said aircraft second global position system signal reception antenna and said satellite; 
     said second distance signals representative of greater ground reflection-inclusive indirect physical distance between said aircraft second global positioning system signal reception antenna and said satellite including equal-angled incident and reflected signal paths to and from an earth point of satellite signal illumination attending said aircraft: and 
     geometric algorithm signal processing apparatus generating from said first and second distance signals and angular relationships attending said equal-angled incident and reflected signal paths an aircraft altitude signal representative of orthogonal distance between said aircraft and a ground point below said aircraft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a representation of signal source and signal receiving apparatus, i.e., a satellite and an aircraft, relevant to the present invention. 
     FIG. 2 shows a smaller scale representation of the FIG. 1 elements together with the signal paths relevant to the invention. 
     FIG. 3 shows an enlarged view of terminal portions of the FIG. 2 signal paths and several geometric/trigonometric constructions helpful in describing the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 in the drawings shows the major components of the present invention altimeter system. These components include a host aircraft  101  whose altitude is to be determined, an aircraft represented by the tactical military aircraft appearing in the FIG. 1 drawing. Signals used in accomplishing the altitude determination of the aircraft  101  are provided by one or more satellites of the global positioning system type, satellites such as are represented at  100  in FIG.  1 . Signals provided by the satellite  100  are represented at  108  and  110  in FIG.  1  and are ultimately received at the two aircraft extremity-mounted antennas  104  and  106 . Preferably these antennas  104  and  106  are disposed on top and bottom fuselage extremities of the aircraft  101  as indicated, however, other aircraft locations capable of receiving radio frequency signals from above and below the aircraft are possible. As indicated by the break lines at  112 , the satellite  100  and the aircraft  101  are in reality separated by a much greater distance than would otherwise be understood from the FIG. 1 drawing, since satellite orbit altitudes in the range of 20,192 to 25,785 kilometers and aircraft altitudes under 30 kilometers, or 100,000 feet are most common. Such altitude or distance differences inherently result in the satellite signal paths, such as the paths represented in FIG. 1 at  108  and  110  for example, being essentially parallel when encountered at the aircraft  101 . Additional details regarding the satellite  100  and its emitted signal characteristics are provided in the patents and other materials identified and incorporated by reference in later paragraphs of this document. 
     It is now well known in the art that signals, such as the signals  108  and  110 , from a global positioning system satellite, e.g. the satellite  100 , are capable of being processed to determine a satellite-to-receiver signal path length measurement. Such path length measurement capability is additionally discussed in, U.S. Pat. No. 5,187,485 titled “Passive Ranging Through Global Positioning System” (issued in the name of myself and a co-inventor colleague on Feb. 16, 1993). The U.S. Pat. No. 5,187,485 discloses two different propagation time-based arrangements for determining the distance between an aircraft and a target object. In addition to the aircraft and target position relationships present in the invention of the U.S. Pat. No. 5,187,485, the observer in this patent must know the direction of the target from the aircraft and must use a large high gain antenna to aim at the target during a range determination. 
     When compared with the invention of the U.S. Pat. No. 5,187,485 the present invention&#39;s use of two small, specifically disposed, fixed, broad beam antennas rather than a large directed antenna, its use of two parallel path signals from a satellite and its determination of a vertical distance between aircraft and ground appear to preclude a significant relationship with the invention of the U.S. Pat. No. 5,187,485. These differences are considered significant even though both inventions make use of global positioning system signal illuminations in their operation. 
     FIG. 2 in the drawings shows a differently scale representation of the FIG. 1 aircraft  101  and satellite  100  elements together with the direct and reflected satellite signal paths  200  and  202  relevant to the invention. The direct signal path  200  in FIG. 2 is represented to terminate in the upward facing antenna  104  of the aircraft  101  while the reflected signal path,  202 , is shown to illuminate a point  204  located on the ground  208  somewhere below the aircraft  101 . Signal reflections from this point  204  include a signal traveling along the path  206  to the bottom-mounted antenna  106  of aircraft  101 . In view of the drawing-imposed close spacing between the satellite  100  and the aircraft  101  in the FIG. 2 view, the essentially parallel dispositions of the direct and reflected paths  200  and  202  are not well represented and the absence of representative spacing relationship in FIG. 2 is again indicated by the break lines at  212 . 
     The signals emitted by satellite  100  in FIG.  1  and FIG. 2, the signals along each of the direct and reflected paths  200  and  202 , are known in the art to be provided with right hand circular polarization. As may be somewhat less well known in the art, the signals reflected from the point  204  along the path  206  are altered in this polarization to being of a left hand circular polarization nature. See, for example, the textbook “Radar Cross Section” (Second Edition), by E. F. Knott, J. F. Shaeffer and M. T Tubey, Artech House Inc., 1993 at chapter 3, section 2, page 74 regarding this point. Although these polarizations are believed to be of secondary importance with respect to the present invention, they do influence one characteristic of the aircraft  101  antennas  104  and  106  used in receiving these signals. With the correct circular polarization characteristics present in these antennas, greater signal strength i.e., decreased transmission losses, is achievable for the satellite-to-aircraft signals and therefore more reliable and less interruptible operation of the altimeter and possibly greater polar region range of operation from the satellite are to be expected. In the “patch” antennas (of perhaps less than three inches in overall dimension) preferred for use in the antenna locations  104  and  106  of aircraft  101 , such differences in signal circular polarization are often accommodated by altered locations of the antenna element to transmission line conductor node in the antenna configuration and tentatively through other physical feature distinctions in the antenna conductive element. 
     FIG. 3 in the drawings shows an enlarged view of terminal portions of the FIG. 2 direct and reflected signal paths  200  and  202  and several geometric/trigonometric constructions helpful in describing the theoretical basis for use of signals from these paths in the present invention. This use is of course to determine the length of an aircraft altitude representation i.e., length of the path h at  310  in FIG.  3 . 
     In the FIG. 3 drawing the direct signal path  200  is shown to arrive at the topmost antenna  104  of the aircraft  101  with an angle of arrival θ measured with respect to the vertical axis of a set of coordinate axes located at the aircraft  101 . In the FIG. 3 drawing the indirect signal path  202  is shown to arrive at the bottom fuselage of the aircraft  101  after being reflected from the ground  208  at the point  204  and then continuing along path  206 . The relative length of the two satellite-to-aircraft signal paths  200  and  202  can be measured through use of global positioning system signals because of the nature of these signals, as is known in the electronic/navigation art, and as may also be understood from the below incorporated by reference patents and other documents. 
     Relative distance can in fact be measured from the coarse/acquisition (C/A) code or the carrier frequency portions of the global positioning system signal. A C/A code distance measurement can provide a distance error measurable in a few meters of length and a carrier frequency based measurement can provide error measurable in centimeters. Accuracy of this latter degree may not be required by an altimeter however. A yet more accurate measurement arrangement is also vested in a third component of the global positioning system signals, i.e., in the P(y) code of the global positioning system. Since matters relating to this code are at the time of present application filing the subject of military security classification and this code is not believed necessary to obtain useful accuracy from the present system, discussion of this code and its use is omitted from the present document. For interested readers, however, certain basic discussion of this code and its use is contained in the below incorporated by reference patents and other documents and in additional materials available in the open literature and doubtless on the world wide web. 
     The position of a global positioning system satellite with respect to the earth is contained in its transmitted signal, thus, the position of the global positioning system satellite can be obtained from a global positioning system receiver. However, in the present invention altimeter, only the direction of the global positioning system satellite is needed, as may be appreciated from the equations below. In FIG. 3 the satellite is presumed to be disposed at an angle θ from the aircraft  101 . The difference, d, between the direct signal path to the top antenna  104  and the reflected signal path to the bottom antenna  106  can be measured from the global positioning system signal and is considered in the equations 1-3 disclosed below. Before describing these equations, however, some discussion of a concept involved in the FIG. 3 signal paths appears warranted and a detailed definition of terms appearing in the FIG. 3 drawing is believed helpful. 
     It is well known in the technical arts that an optical beam received along, for example, the reflected path  202  in FIG. 3 will be reflected from a hypothetical mirror located at the point  204  in such manner as to make the angle of beam incidence (with respect to a normal to the mirror surface) equal to the angle of beam reflection. Such angles of incidence and reflection with respect to a normal line  304  are indicated at  306  and  308  respectively in the FIG. 3 drawing. (Such reflection characteristics are often demonstrated in the course of the classic Newtonian physics optical galvanometer demonstration, for example, wherein tilting of the galvanometer reflection mirror surface through an angle σ accomplishes a 2σ angular rotation of the galvanometer output beam.) It is also known in the electronic art that the equal angle of incidence and reflection concept in the optics field is duplicated in the case of a reflecting radar or other microwave spectrum signal - eg. in a reflection from the earth at point  204 . See for example the textbook “Electromagnetic Waves and Radiating Systems” by E. C. Jordan, Prentice-Hall Inc., sixth printing, 1960, at chapter 5, section 5.09, pages 138 and 139 regarding this point. This equal incidence and reflection angle concept provides a basis for analysis of the FIG. 3 drawing and a measure of theoretical support for the present invention altitude determination. 
     Turning now to geometric/trigonometric relationships appearing in the FIG. 3 drawing, it may be determined by geometric reason that the angle  306  between the mirror-earth normal  304  and the reflected path  202  is equal to the angle θ, the angle of arrival of the direct path  200  signal. This follows from the parallel relationship between normal lines  304  and  314  and from the parallel relationship described above between indirect and reflected paths  200  and  202 . Since the angles of incidence and reflection in the reflected path  202 , the angles  306  and  308  in FIG. 3, are equal, as described in the previous paragraph, it follows that the angle  302  between incident and reflected paths  202  and  206  in FIG. 3 is of θ+θ or 2θ size. 
     Additionally in FIG. 3 it follows that the angle  316  between the path connecting the reflection point  204  and the aircraft  101  (the path  206 ) and an extension of the direct path  200  at  320  is also of 2θ size (since the path  206  forms a geometric “transversal” intercepting the two parallel lines of the paths  200  and  202 ). Moreover the angle  318  in FIG. 3 is equal to the angle  300  in size since the path  200  and the normal  314  are in fact intersecting straight lines. Therefore the angle  322  in FIG. 3 is also of 0 size and the angle  324  must be of (90−θ) size since the angle  326  is a right angle. 
     With this knowledge of the angles  324  and  322  in mind, it is possible to consider the following equations which provide a usable mathematical expression for the aircraft altitude dimension h, at  310  in FIG.  3 . In these equations the reflection path  206  is identified with the letter “b”, the perpendicular between paths  200  and  202  with the letter “c” and the additional length of path  202 , beyond the length of path  200 , the length at  326 , with the letter “a” all as are shown in the FIG. 3 drawing. 
     The total length difference, d, between the direct signal path to the top antenna  104  and he reflected signal path to the bottom antenna  106  can be measured in FIG. 3 as 
     
       
           d=a+b   (1)  
       
     
     In the right triangle abc the distances a and b are related as 
     
       
           a=b  cos 2θ  (2)  
       
     
     
       
         Thus  
       
     
     
       
           b (1+cos 2θ)= d  or        b   =     d     1   +     cos                 2                 θ                               
      
     
     Therefore once the distance b is found from the global positioning system signals, the desired altitude h can be obtained as              h   =       b                   sin        (     90   -   θ     )         =       b                 cos                 θ     =       d                 cos                 θ       1   +     cos                 2      θ                     (   3   )                                
     In equation (3) the latter fractional quantity provides a convenient expression for determining the desired altitude of the aircraft  101 . This expression requires only knowledge of the angle of arrival of the satellite signal and the total difference in path  200  and  202  lengths as input signals to a software or hardware processor. 
     Theoretically therefore, the altitude of the aircraft  101  can be measured using signals from one global positioning system satellite. However, in general, there are several global positioning system satellites in view at any instant at any point on the earth. One can therefore use the altitude measurement results from multiple satellites to confirm or average the accomplished altitude determination and thereby improve the accuracy of the determination. The distance between the two antennas  104  and  106  on the aircraft  101  is known and can be calibrated out in the altitude measurement. 
     The present invention has been thusly described herein using references to the global positioning system or GPS system and with reference to the signals employed in this system. Although the existence of and the capability of this system and its provision of both high precision location information intended for military use and lower precision information usable at lesser expense for ordinary purposes is now well known in the art, additional information regarding this system is readily available. The U.S. Pat. No. 6,012,013 of one Paul McBumey, for example, provides an interesting and informative discussion of the (American Department of Defense-originated) global positioning system and of its Russian counterpart the Global Orbiting Navigation Satellite System, i.e., the GLONASS, in the text of columns 7 and 8 in the patent. In addition to this discussion the McBurney patent also identifies several publications, including specification-like and textbook publications, descriptive of these systems. The disclosure of the McBurney U.S. Pat. No. 6,012,013 and also the disclosure of the references identified in this patent are hereby incorporated by reference herein. 
     Another patent, U.S. Pat. No. 6,011,510 of D. M. Yee et al. is also informative with respect to technical details of the global positioning system, see the text in columns 3 and 4 of the patent, and is also hereby incorporated by reference herein. In a similar manner the disclosure of the prior patent of the present inventor and his colleague, i.e., U.S. Pat. No. 5,187,485 issued to James B. Y. Tsui and Rudy L. Shaw, is of interest as a global positioning system-based measurement system and is also incorporated by reference herein. In addition to these supplementations of the present disclosure, applicant intends that references to the global positioning system or GPS in this document be understood to be generic in nature and inclusive of the GLONASS system or other mapping and locating systems, for example, rather than be strictly interpreted. 
     While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.