Abstract:
An electronic warfare apparatus for determining the location of for example a ground based source of electromagnetic radiation from a platform such as an aircraft. Location is determined using angle of arrival based vector determinations provided by signal differences detected in the ground based signals arriving at platform antennas. Elimination of angle of arrival errors arising from imprecise knowledge of platform electronic warfare antenna characteristics is a focal point of the invention and is accomplished through precision use of global position system information received via the same electronic warfare antennas. Accurate determination of electronic warfare antenna characteristics prior to consideration of the large distance multiplication factors imposed by aircraft to distant signal source geometry enables accurate distant signal source location using the invention.

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. 

   CROSS REFERENCE TO RELATED PATENT DOCUMENT 
   The present document is somewhat related to the co pending and commonly assigned patent application document “GPS RECEIVER WITH ELECTRONIC WARFARE RECEIVER FRONT END”, Ser. No. 10/815,600 filed of even date herewith. The contents of this related even filing date application are hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   In many electronic warfare applications, it is desirable determine the location of a certain distantly located signal emitter. Such a determination may be made in relative terms with respect to a known reference position and may also be transferred from this relative position to a fixed or global position with the aid of additional processing. In making such position determinations, the angle of arrival (AOA) of the distant signal with respect to a present reference frame is often used as an intermediary step in determining a range vector connecting the present reference location and the distant signal source. Even where it is possible to interpret data relating to the angle of arrival with significant accuracy (for example where a desirable electronically implemented algorithm is available), the relative locations of the receiving antennas used in collecting this data significantly determines the accuracy of the overall distant signal locating process. If the vectors connecting the receiving antennas are determined with errors, these errors can cause enormous emitter location determination errors. 
   As shown in  FIG. 1 , the distance between two electronic warfare receiving antennas  102  and  104  disposed on the same platform or aircraft  100  is very short in comparison with the range distance  108  to the signal emitter  106 . For example, the distance between the two aircraft-wing mounted antennas  102  and  104  in  FIG. 1  can be in the range of tens of meters, even for the largest of possible host aircraft, while the range  108  to the electronic warfare emitter can be hundreds of kilometers in a typical scene. Let us assume for example that the  FIG. 1  distance between the antennas  102  and  104  is ten meters and the range  108  is one hundred kilometers. Under these conditions a one-millimeter error in the believed location of an antenna  102  or  104  can cause approximately a 10 meter error in the determination of range  108  (the distance ratios being 10,000 in each instance). Therefore in order to minimize such error, the vector between the electronic warfare antennas  102  and  104  must be measured very accurately. 
   From this discussion it may be appreciated that differing arrangements of the  FIG. 1  measuring event may require the use of either a single electronic warfare antenna on the aircraft  100  or multiple of such antennas.  FIG. 1  and the discussion in the preceding paragraph thus may be interpreted to imply that a plurality of electronic warfare antennas, especially two such antennas, are so included and that it is the distance between the effective electrical center point of these antennas that is of a major accuracy concern in the invention. While these implications and interpretations are justified and indeed provide a most easily appreciated understanding of the invention, it should be realized that the invention is not limited to such a two antenna situation and that it may also extend to a single antenna and to the more than two antennas situations where even the instantaneous global positions of a single or more than two movable platform antennas are of concern because of their possible effect on distant signal source location determinations. 
   According to the present state of the electronic warfare art the locations of antennas such as the  FIG. 1  antennas  102  and  104  used for electronic warfare purposes are determined using global position system receivers and the overall capabilities residing in the global position system. In this present art arrangement an electronic warfare antenna location vector is determined from measurements accomplished using an additional global position system antenna that is dedicated to this specific purpose; i.e., a global position system antenna is placed physically close to each electronic warfare antenna under consideration and the relative positions of the global position system antennas are then measured accurately by global position system techniques. Once the location of each thus added global position system antenna is measured, the vector between the electronic warfare antennas can be determined—by physical measurements for example. 
   In addition to the added complexity of supplying, mounting and measuring associated with the added global position system antennas used in this present state of the electronic warfare art, this procedure can also encounter measurement error as a result of the phase center or the electrical center of the electronic warfare antenna being located in a position differing significantly from the physical center of the antenna. The present invention is believed to provide an arrangement overcoming each of these difficulties with the present state of the electronic warfare art and the determination of a vector between electronic warfare antennas. 
   SUMMARY OF THE INVENTION 
   The present invention provides a procedure and an apparatus enabling accurate measuring of distances and angles between a movable platform such as an aircraft and a distant signal source such as a ground-located electromagnetic source. 
   It is therefore an object of the present invention to provide for the accurate measurement of bearing angles and large distances using an electronic measurement arrangement that is accuracy controlled by a small reference measurement. 
   It is another object of the invention to provide for the convenient measurement of small critical dimensions in an electronic measurement apparatus using global position system procedures. 
   It is another object of the invention to provide accurate knowledge of small antenna dimension effects in an electronic warfare system in order to preclude multiplication of errors attending such dimensions in antenna-accomplished measurements. 
   It is another object of the invention to provide a beneficial usage of a combined global position system apparatus and electronic warfare apparatus that is disclosed in a companion patent document hereto. 
   It is another object of the invention to provide for accuracy improvement in measurements made with an electronic warfare signal reception apparatus. 
   It is another object of the invention to measure accurately a vector connecting the phase center of two EW antennas using GPS signals received in an electronic warfare receiver. 
   It is another object of the invention to provide accommodation for a plurality of real world accuracy difficulties affecting measurements made with an electronic warfare radio receiver apparatus. 
   It is another object of the invention to provide an additional use for global position system signals. 
   These and other objects of the invention will become apparent as the description of the representative embodiments proceeds. 
   These and other objects of the invention are achieved by signal source locating apparatus comprising the combination of:
         a ground based source of microwave frequency electrical signals;   an aircraft carried radio frequency receiving and signal processing apparatus compatible with said ground based source of microwave frequency electrical signals;   said radio frequency receiving and signal processing apparatus including first and second signal receiving antenna members carried by said aircraft and generating angle of arrival related electrical signals in response to signals received from said ground based source of microwave frequency electrical signals;   a source of standardized physical measurement calibration electrical signals of compatible frequency with said ground based source of microwave frequency electrical signals;   means for calibrating selected portions of said electronic locating apparatus including location error generating dimensional portions of said first and second signal receiving antenna members carried by said aircraft in response to said standardized physical measurement calibration electrical signals;   signal location processing apparatus operative on said angle of arrival related electrical signals and generating electrical signals representing relative location of said ground based source of microwave frequency electrical signals with respect to said aircraft.       

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  shows an aircraft locating a ground sourced electromagnetic signal. 
       FIG. 2  shows the processing of a global position system signal by a conventional global position system radio receiver in block diagram form. 
       FIG. 3  shows the processing of a global position system signal by a portion of a combination electronic warfare radio receiver in block diagram form. 
       FIG. 4  shows a modified  FIG. 1  aircraft locating a ground sourced electromagnetic signal through use of electronic warfare antennas mounted in an alternate aircraft location. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the above identified patent document “GPS RECEIVER WITH ELECTRONIC WARFARE RECEIVER FONT END” two of the inventors named in the present patent document and a third colleague have disclosed an invention in which certain functions of an electronic warfare radio receiver and a global position system radio receiver are advantageously combined. The present invention enlarges upon the concept of this receiver combination and provides specific use details for the combination receiver in solving a frequent electronic warfare signal-locating problem. The “GPS RECEIVER WITH ELECTRONIC WARFARE RECEIVER FONT END” document has been incorporated by reference herein in the material above. 
   Generally the underlying concept for the present invention may be stated in simple terms. In lieu of the presently practiced procedure wherein a global position system antenna is disposed adjacent an electronic warfare antenna for antenna dimensional and location calibration purposes we find it is possible to use the electronic warfare antenna itself for receiving calibrating global position system signals. In this arrangement the electronic warfare antenna location determined by the received global position system signals is certain to involve the correct phase center of the electronic warfare antenna. Details attending this procedure including signal frequency considerations and others are included in the following discussion. 
   The band pass of an electronic warfare antenna is typically from 0.5 to 18 GHz. The global position system signals of possible present interest are the L1 and L2 signals located at 1.57542 and 1.2276 gigahertz respectively, frequencies that are thus covered by a typical electronic warfare antenna. A typical electronic warfare receiver moreover has an instantaneous bandwidth of 1 GHz or more. The bandwidth of the global position system signal is around 2 megahertz at the L1 frequency for the C/A code civilian signal and 20 megahertz at the L1 and L2 frequencies for the P(Y) code military signals. Suppose that only the L1 frequency is of interest for present purposes; at this frequency a bandwidth of 20 MHz is adequate and also accommodates both codes. In addition it has been demonstrated that the civilian global position system signal with a bandwidth of 2 MHz can be processed through a single electronic warfare receiver output with a bandwidth of 9.77 megahertz. 
   Many different approaches may be used to process the global position system signal received in an electronic warfare antenna such as antenna  102  or  104  in  FIG. 1  in order to accomplish the present vector between antennas determination function. Two of these possible approaches are discussed herein; the first by way of the  FIG. 2  drawing. In  FIG. 2  the GPS signal is received by the electronic warfare antenna  200  and filtered through a 20 megahertz narrow band filter  202  for application to a conventional global position system receiver  204 . The 1-gigahertz filter bank at  206 , the analog-to-digital converter  208 , and the digital baseband receiver  210  in the  FIG. 2  apparatus are parts of the electronic warfare receiver and its signal search and identification function. Signals processed by the global position system receiver  204  are additionally processed as discussed below to accomplish the antenna location determining function of the present invention. 
   A second arrangement for processing the global position system signals received in an electronic warfare antenna, such as antenna  102  or  104  in  FIG. 1 , to accomplish the present antenna location function, is represented in  FIG. 3  of the drawings herein. In the  FIG. 3  drawing the 20 megahertz bandwidth L1 global position system signal is filtered through the band pass filter  300  and frequency up-converted to a 1980 to 2000 megahertz signal (i.e., a 1.98 to 2.0 gigahertz signal also of 20 megahertz bandwidth) in the mixer circuit  302 —in order that it may be combined at  304  with the 2 to 3 gigahertz baseband signal from the filter bank  306 . This combination involves only a 20 megahertz overall bandwidth increase in the receiver baseband, a small two percent increase with respect to a 1000 megahertz bandwidth that is normal for an electronic warfare receiver. Because of this adding signal to an existing path this second approach may be thought-of as tagging the global position system signal to one of the channels of an EW receiver. The digital baseband receiver at  320  in  FIG. 3  includes an embodied Fourier transformation algorithm. The combined electronic warfare and global position system receiver is indicated at  312  in the  FIG. 3  drawing. 
   In the  FIG. 3  receiver arrangement the electronic warfare receiver function may be switched from band to band by the switch  308  in order to for example search for incoming radar signals of differing frequency (i.e., to perform the receiver&#39;s electronic warfare function) while the 20 megahertz global position system signal from the mixer  302  is always available for antenna location determination usage. Such combining of electronic warfare and global position system signal reception into a single apparatus is in fact the subject matter of the above identified and incorporated by reference herein patent document “GPS RECEIVER WITH ELECTRONIC WARFARE RECEIVER FONT END”. 
   After the global position system signals from the antennas  102  and  104  in  FIG. 1  are initially processed according to one of the approaches described herein or some other approach (including possible use of a monobit receiver arrangement described in the GPS RECEIVER WITH ELECTRONIC WARFARE RECEIVER FONT END” patent document) the sought after vector between two antennas can be determined. The accurate measurement of vectors between such antennas is preferably accomplished through use of global position system carrier phase measurement techniques wherein measurement accuracies into the range of millimeters are available. Such global position system carrier phase measurement techniques have been developed as a part of the differential GPS technical area and are believed well known in the art. Numerous publications describing both the differential global position system and the carrier phase measurement techniques subdivision of this system exist in the technical literature. One such publication of this nature is the 1996 two volume text set titled “Global Positioning System: Theory and Applications” Edited by Bradford W. Parkinson and James J. Spilker Jr. and published by the American Institute of Aeronautics and Astronautics Inc. of Washington D.C. Chapter 1 in the second volume of this text set discusses the differential global position system and section IV commencing at page 27 in this Chapter 1 discusses the carrier phase techniques of interest here. The contents of the “Global Positioning System: Theory and Applications” texts are hereby incorporated by reference herein. 
   In performing such carrier phase processing of signals from the antennas  102  and  104  in  FIG. 1  the processing of signals from two electronic warfare receivers connected to these antennas can be treated as slave processing. The main or master receiver of the global position system receivers can provide ephemeris and navigation data, and initial code phase. This master receiver can also calculate the Doppler frequencies for each slave receiver in order to allow the slave receiver to keep the phase tracking loop tight. A tight tracking loop can assist the slave receivers in performing long coherent integrations and overcome a weak global position system signal condition. A coarse vector between the two electronic warfare antennas also can be calculated in order to ease vector measurements accomplished through carrier phase processing by reducing the number of ambiguity integer sets to be considered. 
   After reflecting on the present invention over a period of time a person of skill in the electronic warfare and global position system arts may come upon a concern that an electronic warfare antenna is not normally disposed in the posture of facing upward toward the location of a global position system satellite or of even having reasonable upward exposure—as has been depicted in the  FIG. 1  drawing herein for drawing and discussion convenience. Although such an actual practice arrangement may indeed reduce the signal strength of signals received from satellites, such signals can nevertheless be received by way of side lobes existing in an electronic warfare antenna pattern. It has for example been demonstrated that even a left circular polarized global position system antenna mounted under the fuselage of an aircraft and facing the ground can successfully receive signals directly from satellites. (This notwithstanding the fact that such a left circular polarized global position system antenna is deliberately configured not to receive signal directly from a satellite.) In addition, an electronic warfare antenna as espoused herein is usually mounted on an aircraft facing somewhat laterally or side-ways from the fuselage as is represented in the modification of the  FIG. 1  drawing appearing in  FIG. 4  herein. With this antenna mounting arrangement, global position system signals thusly received from satellites are sufficiently strong for determining an electronic warfare antenna location under most conditions encountered in a real world environment. 
   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.