Abstract:
Disclosed is a means for synthesizing many different antenna patterns with different beamwidths and elliptical symmetries, being derived from a common data set corresponding to a planar cut or slice of an arbitrary antenna pattern which is stored in an addressable memory such as a programmable read only memory (PROM). A pattern surface or envelope is modeled by an algorithm which takes generated azimuth and elevation values simulating a desired scanning motion from which a memory address is produced according to a predetermined mathematical relationship including factors which determine the degree of ellipticity as well as the beamwidth relative to the stored planar cut of the arbitrary antenna pattern.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to signal generators and more particularly to radar antenna pattern signal generators utilized in electronic test equipment to simulate time varying emissions from a number of different transmitters and where the relative positions of the transmitters or emitters from the receiver may also be varying with time. This type of environment with time varying transmitter parameters and/or time varying transmitter/receiver distances and relative spatial relationships is often referred to a dynamic electromagnetic environment. 
     One known type of apparatus for simulating in real time the electromagnetic signal received from an RF transmitter in a dynamic electromagnetic environment is disclosed in U.S. Pat. No. 3,719,812 entitled &#34;Dynamic Electromagnetic Environment Simulator&#34; issued to G. Bishop et al. on Mar. 6, 1970 and assigned to the assignee of the present invention. Disclosure therein is a general purpose computer wherein there is stored data relating to the varying relative positions of the environmental emitters and the equipment under test, the parameters of the emission from environmental emitters, the transfer functions of the simulator test equipment hardware, and the characteristics of the equipment under test upstream of the point of insertion of the simulated signals from the test set. The computer is programmed to use this information to provide on a medium, preferably a magnetic tape, binary coded digital information to control the generation by the test equipment signals in real time that would be received by the equipment under test from the emitters in the environment. In performing a simulation, the tape is placed in the test system hardware and run, whereupon the test equipment hardware translates the signals on the tape into a signal simulating the result, at the equipment under test, of the environmental emitters, which resulting signal is fed into the equipment under test as it is being operated in a normal manner. While the simulated radiation signal is being fed in the equipment under test, an operator of the equipment can observe the effect of environmental radiation upon its operation. 
     More recently fusible link read only memories have been utilized for producing an electronical signal of the desired antenna pattern. Such apparatus is disclosed in U.S. Pat. No. 4,800,476, entitled &#34;Digital Antenna Pattern Generator For Radar Simulation&#34; issued to A. B. Evans on Feb. 15, 1977. In the simulation apparatus disclosed there is a requirement that every pattern to be simulated must first be fed into memory and thereafter read out on demand. It can be seen therefore that such a system requires a relatively large storge capacity since each antenna design, inter alia, must have its own individual or respective &#34;signature&#34; stored in the memory. 
     It is an object of the present invention therefore to provide a digital antenna pattern generator which obviates the necessity of storing each and every antenna pattern which is to be simulated. 
     It is another object of the present invention to provide a digital antenna pattern generator which significantly reduces the data storage requirement by synthesizing many different patterns from one generalized pattern stored in memory. 
     SUMMARY OF THE INVENTION 
     Briefly these and other objects of the present invention are achieved by a system which synthesizes in real time a three dimensional power pattern surface or envelope which is cirlularly or elliptically symmetrical about boresight from an arbitrarilly generated two dimensional antenna characteristic which is stored in a programmable read only memory (PROM) as a data set. A microprocessor coupled to the read only memory is operable upon receiving input parameters from a programmer such as a support computer to implement a desired boresight motion algorithm and a pattern surface algorithm to generate a lookup value which is utilized to address the memory which accordingly reads out a digital attenuation word from the data set. The microprocessor generates these values in equal sequential operational time intervals. An interpolation algorithm is also implemented in each operational time interval to smooth the transition between successive attenuation values. The pattern surface algorithm utilizes a square root of the sum of the squares calculation based on the instantaneous spatial azimuthal and elevational relationship between the emitter and receiver as well as predetermined beamwidth factors to produce a complex three dimensional pattern surface which is moving in space from a static two dimensional planar pattern cut or slice which is contained in the PROM as an addressable data set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description of the preferred embodiment will become understood when taken in conjunction with the drawings which form a part of this specification and in which: 
     FIG. 1 is a diagram illustrative of a three dimensional radar antenna pattern emitted from a transmitter which is in spatial relationship with a receiver under test; 
     FIG. 2 is a diagram illustrative of a two dimensional planar cut of the antenna pattern shown in FIG. 1 and which is helpful in understanding the subject invention; 
     FIG. 3 is an electrical block diagram illustrative of the preferred embodiment of the subject invention; 
     FIG. 4 is a diagram illustrative of the interpolation algorithm provided by the subject invention; 
     FIG. 5 is a top level flow diagram illustration of the modal sequence of the algorithms implemented by the subject invention; 
     FIG. 6 is a flow diagram illustrative of the interpolation algorithm referred to in FIG. 5; 
     FIGS. 7A and 7B are flow diagrams illustrative of two typical types of antenna scans implemented by the boresight motion algorithms referred to in FIG. 5; 
     FIG. 8 is a flow diagram illustrative of the pattern surface algorithm referred to in FIG. 5; 
     FIG. 9 is a diagram illustrating the geometrical relationships in elevation between an emitter and receiver under test and the pertinent angles utilized in effecting boresight motion algorithm and the pattern surface algorithm shown in FIGS. 7A and B and FIG. 8; 
     FIG. 10 is a diagram illustrative of the geometrical relationships in azimuth between the emitter and receiver and the pertinent angles utilized in effecting the boresight motion algorithm and the pattern surface algorithm shown in FIGS. 7A and 7B and FIG. 8; 
     FIG. 11 is a diagram illustrative of a first arbitrary two dimensional planar cut of an antenna beam surface pattern stored in the PROM shown in FIG. 3 and utilized for synthesizing a first plurality of antenna patterns; 
     FIG. 12 is illustrative of a second arbitrary two dimensional planar cut of an antenna beam surface pattern stored in the PROM for synthesizing a second plurality of antenna patterns. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention described herein is designed primarily for testing an RF receiver which is responsive to a radiation pattern emitted for example from a radar set. Moreover, in such apparatus it has become desirable to subject the receiver to various types of antenna patterns and scan types. It is to this task that the subject invention is directed. 
     Referring now to the drawings and more particularly to FIG. 1 there is shown a three dimensional coordinate axis system in which the X-Y axis defines an elevational plane while the X-Z axis defines an azimuth plane. The reference character E at the origin designates the position of an emitter (transmitter) antenna, not shown, which radiates a three dimensional RF envelope or pattern consisting of a main lobe 20 which has a relatively large amplitude accompanyed by a plurality of lesser powered side lobes 22 1 , 22 2  etc. This power distribution is typical of all radar emitters; however, the specific shape of the pattern itself is peculiar to each individual type of system and exhibits a &#34;signature&#34; which can be sensed and plotted in a well known fashion. In prior art simulator apparatus each such &#34;signature&#34; is stored and read out upon demand to provide the required test signal. What is significant about the subject invention, however, is that this requirement is obviated by means of apparatus which will operate to synthesize a required output pattern from a generalized typical antenna characteristic created in the X-Y plane. From this two dimensional planar characteristic or &#34;cut&#34; of the antenna pattern which is shown in FIG. 2 a three dimensional pattern will be generated by a synthesis process making an assumption that the resulting pattern will be circularly or elliptically symmetrical about the boresight axis, which constitutes the center line of the main lobe 20, and which in FIG. 1 comprises the Y axis. 
     FIG. 1 additionally includes a reference character R which designates a position in space of a receiver, not shown, whose line of sight axis to the emitter E is designated by LOS with the off boresight angle being designated by OB. The receiver in effect senses the RF amplitude (attenuation) at the point where the LOS intersects the three dimensional envelope or surface pattern of the radiated beam. As is well known if the relative positions of the emitter E and the receiver R are fixed any type of scan motion effected by the antenna at E will cause the angle OB to change in accordance with the scan pattern generated. 
     The present invention has for its purpose synthesizing an antenna surface pattern for seventeen (17) types of scans which are listed in the following Table 1. 
     
                       TABLE 1______________________________________List of Scan TypesType Number Description______________________________________0           Circular1           Bidirectional Azimuth Sector2           Bidirectional Elevation Sector3           Unidirectional Azimuth Sector4           Unidirectional Elevation Sector5           Unidirectional Azimuth Sector with       Dead Time6           Unidirectional Elevation Sector       with Dead Time7           Steady8           Conic9           Bidirectional Raster10          Unidirectional Raster with Dead Time11          Palmer Sector12          Spiral13          Bidirectional Helical14          Unidirectional Helical15          Sequential Lobing16          Palmer Circular______________________________________ 
    
     By storing a single data set of digital values in an addressable memory of a planar two dimensional cut, for example, as shown in FIG. 2 whose address comprises a lookup angle, θ, a character which results from a square root of the sum of the squares calculation (FIG. 2) of instantaneous off boresight angles for azimuth and elevation i.e. OBAZ i  and OBEL i  and beamwidth factors BWAZ and BWEL, the latter being adapted to determine ellipticity, a plurality of different pattern surfaces can be synthesized in increments of real time as will become evident as this discussion continues. If for example the beamwidth factors range from integer values 1 to 10, a total of 100 unique pattern surfaces can be generated from each data set of the planar cut. 
     Referring now to FIG. 3 there is disclosed in block diagramatic form apparatus for modeling a desired antenna boresight scanning motion and surface pattern of an emitter dictated from an external programming device such as a support computer 24. The computer 24, a typical example of which is shown in the above referenced U.S. Pat. No. 3,719,812 is adapted to provide a system &#34;scenario&#34; in that it has within its capability the outputting a set of parameters indicative of a desired shape of surface pattern, scan type, scan rate, beamwidth factor, sector sizes, etc. which are provided as a set of initial parameters on data bus 26 to a random access memory (RAM) 28 each time a new operational mode is selected at the support computer. Additionally, a set of update parameters which consists of, for example, a set of offset angles OB for azimuth and elevation are continuously supplied to the RAM 28 via data line 30. 
     The RAM 28 is adapted to operate so as to feed the parameters inputted thereto from the support computer 24 to a microprocessor unit (MPU) 30, typically comprising a type 6802 MPU via a bidirectional input/output data bus 32 and a system data bus 34. The MPU 30 has an output address bus 36 which couples to a system address bus 37 and a clock bus 38. A real time interrupt processing clock pulse is periodically applied to the clock bus 38 from a ΔT counter 40 located in an interval timer 42. The timer 42 is adapted to receive an external master clock signal on line 44. This clock signal when desirable may be generated in the support computer 24. The MPU 30 operates in conjunction with a computational speed enhancing support arithmetic logic unit (ALU) 46 to perform two major functions during each operational time interval ΔT established by the interrupt clock pulses generated in counter 40. These two functions comprise implementing a boresight motion algorithm and a beam pattern surface algorithm and are controlled by program instruction sets contained in at least one programmable read only memory (PROM) 48, e.g. a type 2716 erasable EPROM. The PROM 48 in fact contains instructions for modeling the seventeen types of scans noted in Table 1. In addition program instruction sets are included in the PROM 48 which establish an operational logic for the system. The PROM 48 also contains at least one data set defining an arbitrarilly generated two dimensional planar beam surface pattern having certain predetermined characteristics which is typically illustrative of a generalized type of pattern, meaning that it is not specific to any one individual known pattern. One such pattern cut is shown in FIG. 11. In the instant embodiment of the subject invention a second data set for another generalized pattern cut is shown in FIG. 12 is also contained as a data set in the PROM 48. Where the storage capability of one PROM is insufficient to contain the stored program sets referred to above two or more additional PROMS may be utilized in a well known fashion. 
     As mentioned above, the MPU 30, the ALU 40 and the PROM 48 are adapted to first model a predetermined boresight motion for a particular scan type and secondly model a predetermined antenna pattern surface in three dimensional form from one of the two dimensional pattern sets stored in the PROM 48. What is ultimately produced is a digital output word P which is an attenuation word simulating in real time the RF value of the emitted radiation that the receiver (FIG. 1) would sense. Additionally an interpolation algorithm is also implemented by the subject invention during each ΔT period for smoothing the transition between successive P values. This is provided by an interpolation counter 50 shown in FIG. 3 and comprises an up/down counter whose count direction is controlled by an address decode and control circuit 52. The interpolation counter 50 moreover is driven by means of a ΔI counter 52 which generates a variable clock output governed in accordance with a calculation performed in the MPU 30 every interrupt operational period ΔT. 
     Referring now to FIG. 4, shown therein is a diagram illustrating the interpolation algorithm. P 1 , P 2 , P 3  and P 4  represent four successive attenuation values produced in respective operational time intervals ΔT. It is to be noted that the amplitude change ΔA between values P 1  and P 2  is relatively greater than between the values P 2  and P 3  also P 3  and P 4 . The interpolation process operates to effect a linear ramp of discrete equal amplitude steps at a rate ΔI determined by the difference between successive P values. Such a determination results from a calculation performed in the MPU 30 according to the equation ΔI=ΔT/ΔA. The result of this calculation causes the MPU 30 to address the ΔI counter 54 and instruct a ΔI clock output from the counter 52 to increase or decrease in repetition rate. It should also be noted that in any ΔT i  period the calculation for ΔI is made for the two preceeding values of calculated P in the two preceeding ΔT time periods. This will become evident when FIG. 6 is considered. Thus in any ΔT time period the interpolation counter 50 will be fed a new value of P from the MPU 30 which will count up or down or remain unchanged. It merely permits the output attenuation word to exhibit a relatively smooth transition between successive calculated digital values of the synthesized pattern surface attenuation. The interpolated output appears on data bus 50 and can, when desirable, be fed to a digital to analog converter 58 which in turn can be utilized to drive an RF generator, not shown. 
     In operation, the motion and pattern generator shown in FIG. 3 operates in a sequence as shown by the top level flow chart of FIG. 5. There reference numeral 60 designates the beginning of an i th  interrupt processing interval ΔT. First the P value interpolation algorithm 62 is performed wherein a ΔI calculation is made from the two previously calculated P values during the i-1 and i-2 ΔT intervals in accordance with the procedure outlined in FIG. 4. Following this a boresight motion position algorithm 64 is effected to produce instantaneous azimuth and elevation angle values AZ i  and EL i  which are used in a antenna pattern surface algorithm 66 to generate a new value of P. At the end of the algorithm period the system waits for a new interrupt clock signal as indicated by block 68. Meanwhile the ΔI counter 54 (FIG. 3) continues to output the ΔI clock signal to the interpolation counter 50 until the end of the specific ΔT interval. 
     Considering now the interpolation algorithm as outlined in FIG. 6 the first step 70 comprises the determination of the difference between the new value of P and the previous value of P following which a query is made as indicated by block 72 as to whether the value ΔA is equal to 0, -1 or +1. If this condition exists the output of the interpolation counter 50 as shown in FIG. 3 is caused to remain unchanged. This is effected by turning off the ΔI counter 54 as indicated in step 74 and putting in the previous or old value of P i.e. P old  from the MPU 30 to the interpolation counter 50 as shown in step 76. 
     If the output of query 72 is false a query 78 is next made to determine whether ΔA is greater than 1. If so the interpolation counter 54 is set into the &#34;up&#34; direction per step 80. Otherwise, it is set into the &#34;down&#34; direction in step 82. Following this a calculation step 84 is entered into to determine the value of ΔI whereupon the MPU 30 turns off the ΔI counter 54 and loads it with a new ΔI value as shown in step 86. Next the previous value of P i.e. P old  is loaded into the interpolation counter 50 in step 88 after which the ΔI counter 54 is turned on in step 90 which will increment up or down to the new value of P i.e. P new  as shown in FIG. 4. Following this a step 92 is entered into whereby P new  will become P old  in the next ΔT interrupt interval. 
     The foregoing interpolation algorithm is ancillary to the more important function of simulating an emitter which is scanning a volume of space in real time relative to the position of the receiver and involves the boresight motion algorithm and the pattern surface attenuation algorithm referred to previously. Of the seventeen types of scans (Table I) desired to be implemented by the subject invention, FIGS. 7A and 7B disclose two typical types of scans, namely a circular scan and a bidirection azimuth sector scan which are disclosed in the respective flow diagrams. Referring to FIG. 7A, the circular scan is relatively simple in that following a start step 94 the MPU program outputs a fixed value of azimuth angle ΔAZ which is added to the previous instantaneous azimuth angle AZ i  in each ΔT period for a three hundred and sixty degree (modulo 360°) scan as shown in block 96. For a circular scan the values of the instantaneous elevation angle EL i  =0. The new AZ i  values are utilized in the pattern surface calculations to be considered in the algorithms shown in FIG. 8. This is shown as step 98 in FIG. 7A. 
     The bidirectional azimuth sector scan as shown in FIG. 7b also involves a situation where the instantaneous elevation angles EL i  are 0; however, it is desired to operate the boresight sweep within the confines of a sector shown in FIG. 10. Following a start step 100 during the ΔT interval, a query step 102 determines the present scan direction. In steps 104 or 106 the azimuth delta (ΔAz) is either added or subtracted with the Az i  value. The resultant Az i  after these steps 104 or 106 may be outside or equal to bound±SS/2 and queries 110 and 108 determine this. If the Az i  value is outside the SS/2 bound steps 114 or 112 will calculate a new Az i  value that is within the SS/2 boundary so that no Az i  values used in step 116 will ever be outside the sector boundaries. As an example, consider a sector of 20 degrees (so that SS/2=10 degrees) and a ΔAz of 3 degrees. If Az i   were equal to 9 degrees upon entry to step 104, the result of the addition in step 104 would be a new Az i  of 12 degrees. In query 108 this would be tested against SS/2 and the TRUE path would be taken to step 112 where a new Az i  value of 8 degrees would be calculated. The amount that the Az i  value exceeds the sector bound (in this case 2 degrees) is subtracted from SS/2 in step 112 to determine the Az i  used in step 116. If step 114 were encountered, the amount that Az i  exceeds the sector bound would be added to -SS/2 to determine the new Az i  used in step 116. 
     It should be pointed out that the instantaneous values of EL i  and AZ i  as shown by the geometrical relationships in FIGS. 9 and 10 comprise boresight directions relative to the center of the sector irrespective of the line of sight (LOS). 
     Considering now the pattern surface attenuation algorithm shown in FIG. 8, the line of sight offset from the center of the sector is considered first by having the offset angles AZOF and ELOF shown in FIGS. 9 and 10 respectively applied as update parameters to the MPU 30 from the support computer 24 (FIG. 3) via the RAM 28 from which instantaneous off boresight angles OBAZ i  and OBEL i  are calculated in steps 118 and 120 wherein the respective instantaneous azimuth and elevation angles AZ i  and EL i  are subtracted from respective offset angles AZOF and ELOF inputted thereto. Next a very important calculation is made in the MPU 30 along with the ALU 46 involving the instantaneous off boresight angles in both azimuth OBAZ i  and elevation OBEL i  as well as beamwidth factors BWAZ and BWEL. These values are utilized in a square root of the sum of the squares calculation shown in step 122 which results in an angle look up value θ being generated. This value is used in step 124 to address either data set for the pattern 0 or pattern  1 shown in FIGS. 11 and 12 which are stored in the PROM 48 (FIG. 3) and which results in a new attenuation value P new  being read out as evidenced in step 126. Finally the new P value is saved as shown in step 128 for the interpolation algorithm (FIG. 6) which will begin after the next ΔT interrupt. Thus during each ΔT interrupt interval a value of P is calculated in accordance with the offset angles and the desired beamwidth factors which affect ellipticity to synthesize a desired surface pattern for the relative position in space between the emitter and receiver for a particular scan type. What is significant however, is that many three dimensional surface patterns can be developed from a single two dimensional pattern which is stored in a programmable read only memory. 
     Thus what has been shown and described is a relatively simple, yet efficient means of simulating in real time the RF amplitude variations of a symmetrical radiated electromagnetic signal beam pattern as seen by a receiver located in space with respect to a transmitting antenna having a predetermined type of scanning motion. 
     Appendix I and II attached hereto comprise a specific program listings for effecting the algorithms shown in FIG. 5, while Appendix III comprises program listings for the two antenna patterns 0 and 1 shown in FIGS. 11 and 12 and the PROM 48 for synthesizing any desired antenna pattern. These Appendices are intended to form a part of this specification. 
     While the present invention has thus far been described with what is considered to be a preferred embodiment of the subject invention, it should be understood that desirable attenuation and modifications may be resorted to without departing from the spirit and scope of the invention which is defined in the following claims. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5##