referring to fig1 , and 3 , the object to be located ( x ) has attached to it or embedded within it a small , low - powered radio transmitter and antenna ( a ) emitting radiation . two or more pairs of antenna elements ( in the figure we have shown three , namely , b , c , and d ) are dispersed around the perimeter of the area in which the object is to be located , and receive signals from the transmitter . within each pair of elements , the two elements , designated as primary ( e ) and secondary ( f ) are spaced approximately ¼ wavelength apart . the total number of antenna elements is at least four ; for improved accuracy six or more elements are used in the preferred embodiment . each of the antenna elements is attached through a cable to a receiver assembly ( g ). each receiver assembly , detailed in fig2 contains a downconvertor ( h ) and intermediate frequency amplifier ( j ) which lower the received signal to a frequency suitable for low - cost phase locking and phase detection . it is possible to omit the downconversion and perform phase locking and detection directly at the transmitter frequency ; however , the preferred embodiment employs downconversion to gain the advantages of a superhetrodyne receiver , familiar to skilled practioners of radio design . the local oscillator signal or signals for each of the downconvertors is derived from a signal source ( i ) in order to maintain phase coherence . in the case of multiple conversion downconvertors , each of the local oscillator frequencies required is derived from a signal source . the output of the intermediate frequency amplifier ( j ) is the input to a phase locked loop ( k ) for each of the receivers , in order to stabilize the amplitude of the signal and reduce the effects of noise . the outputs of the phase locked loops are connected to phase detectors ( l ) which find the phase difference between two signals . they are connected in the following sequence : the two signals ( primary and secondary ) originating with element pair ( b ) are connected to one phase detector , the two signals ( primary and secondary ) originating with element pair ( c ) are connected to a second phase detector , the two signals ( primary and secondary ) originating with element pair ( d ) are connected to a third phase detector . if additional element pairs are present they are connected in a continuation of the above sequence . the outputs of the primary elements in each of the element pairs are simultaneously connected to another series of phase detectors ( m ). they are connected in the following sequence : the signal originating from the primary element of element pair ( b ) is connected along with the signal originating from the primary element of element pair ( c ) to one phase detector . the signal originating from the primary element of element pair ( c ) is connected along with the signal originating from the primary element of element pair ( d ) to a second phase detector . the signal originating from the primary element of element pair ( d ) is connected along with the signal originating from the primary element of element pair ( b ) to a third phase detector . if additional element pairs are present they are connected such that all combinations of two signals originating from primary elements are each fed to a phase detector ( m ). the outputs from the first set of phase detectors ( l ) may be in the form of an analog level proportional to phase or a pulse train with duty cycle proportional to phase , depending on the type of phase detector selected . in either case , the outputs are connected to a data acquisition unit ( n ) and computer ( p ), and are used to compute a direction of arrival of the radio wave from the transmitter , at each of the element pair locations . the resulting direction vectors ( q ) ( fig3 ) can be intersected ( r ) to approximately locate the object of interest ( x ). an accuracy of one degree will correspond to an accuracy of approximately one degree in the direction vector , and the intersection of a number of such direction vectors will yield an accuracy of approximately two meters in an area 100 meters square , yielding the coarse location of the object . the outputs from the second set of phase detectors ( m ) may likewise be in the form of an analog level proportional to phase or a pulse train with duty cycle proportional to phase . the outputs are connected to the same data acquisition unit ( n ) and computer ( p ). the phase differences of each pair define a set of loci of points ( s ) along which the object of interest ( x ) must lie . the loci are separated by a distance of one wavelength ; therefore , if the wavelength is as long as the accuracy of the direction vector intersection ( r ), there will be no ambiguity about the particular locus containing the object of interest . the particular locus computed from the output of each the phase detectors ( m ) may be intersected with the loci of the other phase detectors in set ( m ) to yield a location estimate ( t ) with greatly increased accuracy over the direction vector intersection . an accuracy of one degree will yield an accuracy of one three - hundred - sixtieth ({ fraction ( 1 / 360 )} th ) of one wavelength in physical position . in the preferred embodiment , the object can be located with less than one inch resolution . therefore , the first set of phase detectors ( l ) produce coarse location , and the second set ( m ) are then used to refine the accuracy within the zone defined by the first set ( l ). small variations in phase due to local topography or obstructions such as trees or buildings can be compensated by surveying the site after the system is installed and creating a calibration table . the above method and apparatus can be used alone as one embodiment of the invention . the data can be stored in the computer ( p ) to provide a permanent record of the object &# 39 ; s motion and / or displayed on the computer screen with appropriate software . in another embodiment , for usage at spectator sporting events , extra apparatus is added consisting of a large digital display ( u ) and / or a video overlay interface ( v ), on which the position information is displayed for the convenience and entertainment of officials , players , and spectators . the digital display is for direct viewing , while the video overlay can provide either numerical or graphical indication for television or webcast viewers . in another embodiment , for usage by trainers , officials , or participants , which may be operated independently of or simultaneously with the previous embodiment , the position information can be transmitted using standard wireless techniques to a handheld receiver and display unit . optionally , the handheld receiver and display unit can itself be tracked in the same manner as the ball using the invention to allow relative positions to be determined . the invention and the location information it provides can clearly be applied in a wide range of other applications in which accurate positioning is required , and many alternative designs for each of the component elements of the system will be apparent to skilled practitioners familiar with radio electronics . the object may contain a receiver in addition to the transmitter in the above description of the embodiment . if a receiver is also included , it will be possible to control the object ( eg : switch the system on or off ) and / or communicate with the object . in an alternate embodiment the number of antenna is increased along with the corresponding equipment to increase the resolution and to provide a more accurate location of the game ball and / or players . as the number of antenna increases the accuracy can be reduced to mere millimeters . in a still further embodiment of the present invention a laser tracking system can be utilized , wherein the game ball is coated with a material that can be tracked by a laser , thus further eliminating the potential of radio wave interference of the equipment . in a yet further embodiment of the present invention a magnetic field can be generated by the sensor , such as a magnet ( passive device ), which can be sensed and located , thus increasing the life expectancy of the sensor due to battery life limitations . the present invention can be used in many applications in and outside of the sporting arena where subjects or objects move within a defined territory , such as an airport or prison environment . the advantages of the invention includes low cost , high accuracy and precise resolution . the foregoing detailed description of the invention has been presented for purposes of illustration and description only . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . obviously many modifications and variations are possible in light of the above teaching .
this application describes a method and apparatus for precisely locating moving or stationary objects , with specific application to the real time display of the position of balls and / or players in sporting events and / or monitoring of airplanes in runways , prisoners in prison , children in childcare , and basically subjects in any area with predetermined boundaries .