Abstract:
A passive target positioning system utilizes three stations positioned on a linear baseline with a central master station positioned predetermined distances from two slave stations. Angle to the target is determined from the time differences of arrival at the master station and each of the slave stations. Range to the target, in one embodiment, is determined from the difference of the two time difference of arrival measurements and the angle-to-target, while in a second embodiment from the time differences of arrival. A scannable antenna with frequency selection capability provides a spatial sector and frequency band selectivity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to passive target location and more specifically to the location of targets by determining the time difference of arrival of electromagnetic emissions between a master station and two stations linearly positioned on either side thereof. 
     2. Description of the Prior Art 
     Early in the history of the radar art passive systems, e.g. systems that utilize target generated transmissions as signal sources, were employed to determine the location of radiating sources. Angles to the radiating sources at the two locations, a known distance apart, were determined with receiving antennas having cardioid patterns. The angle to the source being determined when the apex of the cusp of the cardioid was in the direction of the radiating source. The two measured angles and the known distance between the antenna locations were then triangulated to establish the position of the target. Since the angular range of the cardioid pattern cusp were relatively broad, inaccurate angular measurements resulted giving rise to relatively imprecise target location determinations. 
     Additionally, these systems were operative only in a single source environment. If two or more sources at different locations were emitting signals within the frequency reception band of the system, the system performance was seriously impaired. 
     Improved accuracy was realized with the advent of monopulse receiving systems. These systems provided significant improvements in the measurement of the angle to the radiated source, thus permitting a more accurate determination of the target location. In a multiple simultaneous signal environment operating within a monopulse beamwidth and within the bandwidth of the system, however, monopulse systems will provide an indication of a single target at a location determined by the angular centroid of the radiating sources. Thus these systems provide useful information of the location of emitters radiating time overlapping signals and operating within the receiver bandwidth only when one emitter is within the monopulse beamwidth. Though monopulse systems may provide target location information when the received signals are time separated, target location ambiguities exist when only two receivers are employed on a baseline. 
     In addition to the multiple target limitations, prior art systems do not have broadband capabilities and operate only within relatively narrow frequencies band. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the present invention a radiating source location is determined from the reception of emissions from the source at three colinearly positioned receiving stations. The time difference of arrival between the reception of an emission at a centrally positioned receiver, the master station, and two outer receiving locations is determined and utilized with the known receiver separation distances to calculate the angles of arrivals at each of the outer receivers. These angles and the length of the base leg between the two outer receivers determine a triangle that unambiguously positions the target to an accuracy that is primarily a function of the base leg length between stations, improving as this length increases. 
     Target resolution in a high density target environment may be achieved by restricting the measurement of the time difference of arrival to narrow frequency and space angle bands. This may be achieved with a scannable antenna operable over a multiplicity of selectable radiation bands. Each selected radiation band is further divided into a multiplicity of narrow frequency bands, thus restricting observations to narrow frequency and space angle ranges prior to the determination of time differences of arrival between the master and slave stations. This frequency and angled selection establishes a high probability that the signals processed at the master and slave stations were emitted from a common source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an illustration of the geometry for a three station passive detection system. 
     FIG. 1B is a list of equations used to explain the operation of the invention. 
     FIG. 2 is a block diagram of a preferred antenna system having angular sector and frequency selection capability. 
     FIGS. 3A and 3B are schematic diagrams of switches useable with the antenna of FIG.  2 . 
     FIG. 4 is an illustration of an external configuration of the antenna of FIG.  2 . 
     FIG. 5A is a block diagram of a preferred embodiment of the invention. 
     FIG. 5B illustrates a second method for determining range to target that may be utilized with the embodiment of FIG.  5 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer to FIG. 1A wherein the geometrical representation of a preferred three station passive receiving system, in accordance with the present invention, is shown. Three receiving stations are positioned along a base line  11  with a master station  12  located an equal distance L from slave station  13  and  14 . It will be recognized by those skilled in the art that equal distances between stations is not a requirement and that satisfactory operation may be realized with unequal master-slave station operation. Signals emitted from a target at a distance R from the master station  12  propagate along paths  15 ,  16 , and  17  which respectively form angles ,  1 ,  2  with the baseline  11 . These signals are correspondingly received at the master station  12  and the slave stations  13 , and  14 . The distance R from the master station to the target is very much greater than the overall length 2L of the baseline. To those well versed in the art it will be apparent that the paths  15 ,  16 , and  17  are near parallel and that angles ,  1 ,and  2  are near equal. For purposes of clarity the deviation from parallelism and the angle differences have been exaggerated in the Figure. 
     The direction to the target from the slave stations  13  and  14  may be established from the time difference of arrival (TDOA) of the signals at the slave and master stations. TDOA designated T 1  for slave station  13  and T 2  for slave station  14  are deemed positive when the arrival of the signals at the slave stations  13  and  14  respectively lead and lag the arrival of the signal at the master station  12 . The time differences T 1  and T 2  determine the angles to the target from the slave stations  13  and  14  respectively in accordance with equations  1   a  and  1   b  of FIG.  1 B. In these equations C is the free spaced velocity of propagation of the signal. Since TDOA is utilized, the determination of angle and range to the target is independent of frequency. This is apparent from the equations in FIG.  1 B. 
     The range R to the target may then be determined from the length of the baseline and the angles  1  and  2  determined from the TDOA measurements T 1  and T 2 . This range is given by equation  2   a  in FIG.  1 B. Since  1  is approximately equal to  2  as previously discussed, equation  2   a  may be written to a very good approximation as equation  2   b . Further, the difference between the angles  l  and  2  may be closely approximated by equation  3 . Since the angle  l  and  2  differ by only small angular measurements equation  3  may be combined with equation  2   b  to provide an alternate expression for the range from the master station to the target that is given by equation  4 . In these equations the angle  is the angle to the target from the master station as shown in FIG.  1 A. Target position in the plane determined by the target and the baseline  11  may be specified by the range of equations  2   b  or equation  4  and the angle  as evaluated from the measured angles  1  and  2  by equation  5   a . Since the difference between the angles  l  and  2  is small equation  5   a  may be approximated to a high level of accuracy by equation  5   b . 
     An antenna system that may be employed at each of the three stations along the baseline for passively searching specified angular sectors for targets radiating at an unknown frequency within one of three frequency bands, is shown in FIG. 2. A receiving antenna is provided for each frequency band as for example, antenna  21  for the lowest of the three frequency bands. Antenna  21  may comprise a plurality of receiving elements  21   a  circumferentially positioned about a Luneberg lens  21   b . Each receiving element  21   a  is coupled to a port of a three port circulator  21   c  having a second port  21   d  coupled to the Luneberg lens  21   b  and a third port coupled via amplifiers  21   e  to a beam selector  22   a . Electromagnetic energy received from a given direction is coupled from the receiving elements  21   a , therefrom coupled via the circulator  21   c  to the Luneberg lens which, by a manner well known in the art, couples substantially all the received energy to one of the ports  21   d  corresponding to the angular sector of reception, and therefrom via circulator  21   c  and the amplifier  21   e  to the beam selector  22   a . The beam selector, which may comprise a plurality of switches as shown in FIGS. 3A and 3B to be discussed subsequently, selectively couples each beam port of the Luneberg lens  21   b  to a frequency band selector  23 . 
     Receiving antennas  24 ,  25  for the other frequency bands of interest are similarly constructed and operative, with the beam ports respectively coupled to beam selectors  22   b  and  22   c . The output ports of the beam selectors  22   b ,  22   c  are respectively coupled to mixers  26   a ,  26   b , whereto local oscillators  27   a ,  27   b , having selectable frequencies, are correspondingly coupled. Frequency selection of the local oscillators  27   a ,  27   b  cause mixers  26   a ,  26   b  to couple signals that have frequencies within the first frequency band to the band selector  23 . 
     Band selector  23  switchably couples the output ports of beam selector  22   a , mixer  26   a  and mixer  26   b  to a third mixer  31 . A sub-band of frequencies within the first frequency band is selected by tuning a third selectable frequency local oscillator  32  coupled to the mixer  31 . The output of the mixer  31  may be coupled via an amplifier  33  to a bank of filters  34   a  through  34   n , which may be of the surface acoustic wave (SAW) type, for further frequency selection. Each of the SAW filters  34  output ports may be coupled via amplifiers  35   a  through  35   n  to detectors  36   a ,  36   b  wherein the signals may be converted to inphase I and quadrature Q components relative to the phase of a coherent oscillator  37 , analog-to-digitally converted in convertors  38   a  and  38   b  respectively and digitally stored in memory  39 . Each I, Q digital pair stored in memory  39  corresponds to a beam selected by the beam selector  22  and a frequency selected by the frequency selection process described above. The stored digital signals corresponding to a selected frequency within a selected beam are coupled to a digital beam former wherein signals at a selected frequency within a plurality of selected beams are weighted and appropriately combined for beam shaping to establish a signal detection within a narrow angular spatial sector. For each signal so detected the frequency, spatial sector, and time of arrival is noted and stored. 
     Antenna operation for the track (measurement) mode at each of the three stations  12 ,  13 , and  14  of FIG. 1A are as described above. Initial location of the target, however, is performed at the master station  12 . At this station the beam selector  22  additionally functions to switch the beam ports of the Luneberg lens  21  to search mode circuitry  40 . This circuitry may be substantially equivalent to the track mode circuitry through the filter banks  34 ,  35  and may operate in a similar manner. When target emissions are detected at the master station, the spatial sector and frequency thereof are coupled to the system control  41  to establish greater dwell time in the frequency band and spatial sector of the detection. Additionally, control  41  relays beam and band selector signals to the slave station to establish synchronism for all three stations. When all three stations are switched to the spatial sector and frequency band of the initial detection, the times of arrival of subsequent transmissions from the target that emitted the initially detected signal are thereafter noted at all three stations and processed accordingly. 
     A schematic diagram of a switch that may be utilized with the antenna at the master station is shown in FIG.  3 A. Terminals  45   a  through  45   n  are correspondingly coupled to the circulators at each beam port of the Luneberg lens. For the switch configuration shown in FIG. 3A, each beam port couples to three switches, though for other applications additional switches may be stacked. The first  46   a  through  46   n  couples a matched termination  47   a  through  47   n  across the beam port when the system has been commanded to listen to another spatial sector. The second switch  48   a  through  48   n  couples the selected beam port to the search circuitry  40 , while the third switch  49   a  and  49   n  couples the selected beam port to the track circuitry. Only one beam port may be activated at a time. Thus, all terminating switches  46  save the one coupled to the selected beam port are closed and all search  48  and track  49  switches are open save that coupled to the selected beam port. While only one beam port may be activated at a time the search and track switches at that port may be simulatenously closed, permitting search while track operation. 
     Since the slave stations do not perform the search function a simplified version of the switch as shown in FIG. 3B may be employed. In this configuration the terminals  52   a  through  52   n  are correspondingly coupled to the beam port circulators of the Luneberg lens and respectively to termination switches  53   a  through  53   n  and receiver coupling switches  54   a  through  54   n . She switch network of FIG. 3B operates in the same manner as that of the network in FIG. 3A, only one of the receiver switches  54  being actuated for coupling a selected beam port to the receiver while only the terminating switch  53  of the selected beam port is open, all other beam ports being terminated through switches  53 . 
     It is desirable that the antennas covering each frequency band be at a common location at each station. This may be accomplished by stacking the Luneberg lenses as shown in FIG.  4 . Since it is desired that the beam width at each beam port be substantially equal, the diameter of the Luneberg lens  55  for the lowest frequency band must be greater than the diameter of the Luneberg lens  56  of the intermediate frequency band which in turn must have a larger diameter than that of the Luneberg lens  57  for the highest frequency band. 
     Referring now to FIG. 5A, the output signal from the beam and frequency selector at the master station  61  is coupled to a detector  62  which is timed by a master clock  63  wherein the receive signal is detected and its time of arrival noted. Sync pulses from the master clock  63  are transmitted to clock  64  at slave station  1  and clock  65  at slave station  2 . Clock  64  times a detector  66  wherein signals received from the beam and frequency selector  67  of slave station  1  are detected and the time of arrival noted. Similarly, at slave station  2  signals from the beam and frequency selector  68  are detected and the time of arrival noted in detector  71  which is timed by the clock  65 . The time difference of arrival T 1  between the signals coupled to detector  62  and  66  is determined by a differencing circuit  72 , while the time difference of arrival T 2  between the signals coupled to detector  62  and  71  is determined in differencing circuit  73 . Signals representing the time differences of arrival are coupled to angle of arrival measurement units  74  and  75  wherein the angle to the target at slave station  1  and slave station  2  are respectively determined. Signals representative of the angles  1  and  2  are coupled to an angle-to-target determining unit  76  and a range-to-target determining unit  77  wherein the angle and range to the target from the master station are determined in accordance with the equations of FIG.  1 B. 
     To increase the probability that the signals received at the three stations were emitted from a common target the angle of arrival determining units  74 ,  75  may be respectively gated by angle gates  81 ,  82 . Angle gate  81  may be activated by the first arriving signal from detector  62  and  66 , while angle gate  82  may be activated by the first arriving signal from detector  62  and  71 . Gates remain open, after activation, for a period of time corresponding to selected angular interval. The gated time interval is a function of the selected spatial sector, since the time intervals for constant angular intervals vary from sector to sector. To maintain a constant angular interval signals from control  41  may be coupled via lines  83 ,  84  respectively to the angular gates  81 ,  82  to vary the time interval in accordance with the selected spatial sector. 
     The configuration of FIG. 5A provides a determination of the range-to-the target with the utilization of a calculated angles  1  and  2  in equations  2   a  and  2   b . Range to the target may also be calculated from the angle to the target at the master station and the difference between T 2  and T 1  in accordance with equation  4 . Referring to FIG. 5B, signals representative of T 2 , T 1  may be coupled to a differencing network  85  wherefrom a signal representative of the difference between T 2  and T 1  is coupled to a range-to-target determining unit  86 . Also coupled to the range-to-target determining unit is a signal from the angle-to-target determining unit  86 . The range-to-target is then s determined in accordance with equation  4  of FIG.  1 B. 
     While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made within the purview of the appended claims without departing from the true scope and spirit of the invention in its broader aspects.