Abstract:
A converter, according to the present invention, operates to convert  pertnt IFF data which is in a digital format to an IFF composite video format. The converter is configured to interface with conventional plan-position indicators (PPIs) and decoders.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is related, in subject matter, to U.S. patent application Ser. No. 392,962 to Callahan, entitled &#34;THREE HUNDRED AND SIXTY DEGREE IFF VIDEO RESYNCHRONIZER UNIT,&#34; filed June 28, 1982, and assigned to the same assignee as the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to Identification Friend or Foe (IFF) systems, but more specifically, it relates to a digital-to-composite IFF video converter for use with IFF systems. 
     2. Description of the Prior Art 
     Identification Friend or Foe (IFF), as a technique for recognizing friendly entities, has long been a vital factor in tactical operations. IFF techniques have progressed from the recognition of coats of arms and flags to the present day electronic systems. Present day IFF systems comprise interrogator subsystems and transponder subsystems. The most important function of an interrogator subsystem is to ask the question of all unknown entities, &#34;Who are you?&#34;. The most important function of a transponder subsystem is to answer the foregoing. Most present day Navy ships use one interrogator subsystem for each radar system. Some present day Navy ships use up to five or more interrogator subsystems. 
     At the present time, no backup interrogator subsystems exist. Thus, if one of the interrogator subsystems should fail, the IFF system for that corresponding radar system would not exist. Consequently, there is a need in the prior art to be able to backup any one of the interrogator subsystems. 
     Moreover, when an interrogator subsystem fails, that particular subsystem must be replaced or repaired. Failure occurs frequently due to the size, weight, complexity and age of the subsystems Therefore, a need exists in the prior art to replace interrogator subsystems with a state-of-the-art unit, such as the present invention, while reducing costs and increasing reliability. In addition, there is a need in the prior art to be able to substitute such a unit in place of an interrogator subsystem for new construction. At least one interrogator subsystem of the prior art type must be present to use the present invention in the foregoing manner. 
     There is one well known prior art IFF system (IFF Central System) which can be used to replace and/or substitute for new construction, but, unlike the present invention, this system can not be used to backup interrogator subsystems. Also, the IFF Central System declares targets in order to accomplish the above mentioned task(s) of replacement and/or substitution. Thus, a need exists in the prior art to accomplish the foregoing task and provide backup capability, but yet be able to process replies or returns rather than targets. 
     The prior art, as indicated hereinabove, includes some important advances in IFF systems. However, insofar as can be determined, no prior art IFF system incorporates all of the features and advantages of the present invention. 
     OBJECTS OF THE INVENTION 
     Accordingly, the single object of the present invention is to configure a converter that will allow any digital computer capable of processing IFF returns to output those returns in an IFF composite video format suitable for display on a PPI/display unit and/or decoder. 
     SUMMARY OF THE INVENTION 
     The digital to IFF composite video converter, according to the present invention, by which the above and other objects, features and advantages are accomplished includes a first-in-first out (FIFO) memory which operates on range, mode, and code information from an associated digital computer. The output of the FIFO memory feeds a range comparator. The range comparator compares the range of IFF returns or replies plus any offset desired with the range of a range counter. When the aforementioned ranges are equal, the range comparator triggers one of a pair of IFF word generators. The IFF word generators get their inputs from the FIFO memory. The IFF word generators perform parallel to serial conversion with reload if the first digital word has not yet been completely outputted. This allows multiple IFF video trains to be interleaved from the IFF word generators, and, accordingly, be in the IFF composite video format for subsequent display on a PPI/display unit and/or decoder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects, other objects, novel features and advantages of the present invention will be more apparent from the following more particular description of the preferred embodiments thereof as illustrated in the accompanying drawings, in which: 
     FIG. 1 block diagram representation of a plurality of IFF interrogator/radar subsystems and a 360° IFF video resynchronizer unit depicted in an interrogator subsystem backup role; 
     FIG. 2 a block diagram representation of a plurality of IFF interrogator/radar systems and a plurality of the 360° IFF video resynchronizer units depicted in an interrogator subsystem replacement role; and 
     FIGS. 3a and 3b show a more particularized block diagram representation of the 360° IFF video resynchronizer unit of FIGS. 1 and 2, according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a plurality of IFF interrogator/radar systems 10 in which the present invention is employed. It comprises a plurality of radar antennas 12 and a plurality of IFF antennas 14. The plurality of radar antennas 12 are operatively connected to a plurality of radars 16. Each one of the plurality of radars 16 provides timing and trigger signals to corresponding ones of a plurality of interrogator subsystems 18. 
     Each one of the plurality of interrogator subsystems 18 comprises a pulse generator/video retimer 20 which is operatively connected to an interrogator unit 22. Interrogation unit 22, in cooperation with an RF switch 24, a mode (4) computer 26, a defruiter unit 28 and a control monitor device 30, generates interrogation signals to a corresponding one of a plurality of transponder systems (not shown) via RF switch 24 and the corresponding one of the plurality of IFF antennas 14. Interrogator unit 22 receives replies or returns from the aforementioned transponder system. Defruiter unit 28 removes non-synchronous replies or returns from the IFF composite video, and mode (4) computer 26 provides the identification function for the associated IFF system. Interrogator unit 22 also provides composite IFF video and mode tags to a radar distribution switch board 32. Control monitor device 30, being operatively connected to interrogator unit 22, mode (4) computer 26 and defruiter unit 28, operates to monitor and control one of the plurality of interrogator subsystems 18. 
     Radar distribution switchboard 32, being operatively connected to plurality of radars 16 and plurality of interrogator subsystems 18, switches the enable, timing, composite IFF and radar video, mode tags and synchro signals to and from any one of a plurality of PPI/display units 34 via a corresponding plurality of IFF decoders 36. The present invention employs a 360° IFF video resynchronizer unit 38, which is operatively connected to radar distribution switchboard 32 so as to backup any one of plurality of interrogator subsystems 18, aforementioned. 
     FIG. 2 shows another embodiment of the present invention in which a plurality of 360° IFF video resynchronizer units 38 (up to N-1 units) can be operatively connected to a corresponding plurality of radars 16. It should be mentioned that at least one of the plurality of interrogator subsystems 18, in combination with one of the plurality of radars 16, is required so as to have available the pertinent IFF data. The plurality of 360° IFF video resynchronizer units 38 are operatively connected to radar distribution switchboard 32 and one of the plurality of interrogator subsystems 18 in such a fashion as to replace the other of the plurality of interrogator subsystems 18. Replacing the other of the plurality of interrogator subsystems 18, as shown, improves the overall IFF system by reducing the number of RF interrogations, increasing system reliability and reducing system costs. 
     FIGS. 3a and 3b show, in block diagram form, a more particularized embodiment of the 360° IFF video resynchronizer unit 38 of FIG. 1 or any one of the plurality of 360° IFF video resynchronizer units 38 of FIG. 2. It comprises a video resynchronizer input device 40, an input computer 42, an output computer 44 and a digital-to-composite IFF video converter 46. 
     Video resynchronizer input device 40 inputs IFF composite video, initiating and terminating triggers, and mode tag and synchro inputs, and formats this pertinent data for all replies or returns in the fashion of range, code, mode and azimuth to input computer 42. Input computer 42 stores this pertinent data. Output computer 44 recalls the pertinent data. Digital-to-composite IFF video converter 46 cooperates with output computer 44 such that the pertinent data is formated into retimed IFF composite video. 
     Video resynchronizer input device 40 comprises a synchro-to-digital converter 48 for converting the synchro input into digital words. It then feeds this information to input computer 42 for further processing. A decoder/encoder 50 converts the mode tag input into an encoded mode tag word which is also fed to input computer 42 for further processing. An internal end of sweep generator 52 inputs the initiating trigger, and should switch SW1 be set, provides an internal trigger to replace the terminating trigger. A logic device 54 is preset by the aforementioned internal trigger and cleared by the terminating trigger. A synchronous clock 56 is controlled by logic device 54. Synchronous clock 56 is connected the clock input to a range counter 58 which is cleared by the initiating trigger. The output of range counter 58 is connected to a range offset generator 60, which allows a range offset to be applied to the range of each reply or return via an optional range offset applied at its input. 
     Still referring to the block diagram representation of FIGS. 3a and 3b video resynchronizer input device 40 further comprises a digital delay line 62 for converting the IFF composite video into parallel digital video. A logic device 64 provides bracket decodes and degarbled bracket decodes for selection by a switch SW2. A switch SW3, in combination with an input AND gate 66, provides end of sweep capability, via logic device 54, to terminate video storage in a first-in-first-out (FIFO) memory 68 upon a trigger from internal end of sweep generator 52 or the terminating trigger as selected by switch SW1. FIFO memory 68 is also operatively connected to input computer 42. FIFO memory 68 stores the range and code information for all replies or returns. A switch SW4 provides the selection for either the initiating trigger or terminating trigger to interrupt input computer 42. The selection depends on the radar system used. 
     Input computer 42 comprises an input/output interface device 70, an interrupt handler device 72, a central processor unit (CPU) 74, a memory aribitrator unit 76, a RAM/ROM memory device 78, and an interface unit 80. 
     Likewise, output computer 44 comprises an input/output interface device 70&#39;, an interrupt handler device 72&#39;, a central processor unit (CPU) 74&#39;, a memory aribitrator unit 76&#39;, a RAM/ROM memory device 78&#39;, and an interface unit 80&#39;. For purposes of the present invention, input computer 42 and output computer 44 each can be an Intel® iSBC 80/30 computer. 
     Continuing, input computer 42 is interrupted by either the initiating trigger or terminating trigger which is selected by switch SW4. Input computer 42 clears FIFO memory 68 and inputs the azimuth from synchro-to-digital converter 48 and the mode tag encoded signal from decoder/encoder 50. The value of the azimuth that is inputted from synchro-to-digital converter 48, points to a location in RAM/ROM memory 78 and/or 78&#39; which contains address pointers to other locations in memory in which all of the returns are stored for 360° of IFF operation. Each time FIFO memory 68 asserts that data is available for input computer 42, it interrupts input computer 42 which stores the next reply in the locations in RAM/ROM memory 78 and/or 78&#39;, aforementioned. Output computer 44 inputs, upon receipt of an interrupt signal from digital-to-composite IFF video converter 46, the azimuth therefor, and then determines where the returns for that azimuth are in RAM/ROM memory 78 and/or 78&#39;. It then outputs the returns to digital-to-composite IFF video converter 46. 
     Still referring to the block diagram of FIGS. 3a and 3b, digital-to-composite IFF video converter 46 comprises a synchro-to-digital converter 48&#39;, an internal end of sweep generator 52&#39;, a logic device 54&#39;, a synchronous clock 56&#39;, a range counter 58&#39;, a range offset generator 60&#39;, and a FIFO memory 68&#39;. Digital-to-composite IFF video converter 46 further comprises an IFF pulse spacing synchronous clock 82, an offset logic device 84, a range comparator 86, a mode tag generator 88, an IFF word generator 90, an additional IFF word generator 90&#39;, a logic device 92, an output OR gate 94, and an output AND gate 96. 
     For purpose of the present invention, logic devices 54, 54&#39; and 92 can be &#34;D&#34; flip-flop type integrated circuits or their discrete component counterparts. In a &#34;D&#34; flip-flop, data at the &#34;D&#34; input (internally connected and not shown) are transferred to the Q and Q outputs on the positive going edges of clock pulses at the clock (CLK) input, unless there is activity at the preset (PR) input or clear (CLR) input. If the PR input is active, then the Q output is set and the Q output is reset. If the CLR input is active, then the reverse of the foregoing occurs. Also, the functions of logic devices 64 and 84 and the circuitry to carry out same are well known in the prior art. Circuitry to generate the bracket decode and degarbled bracket decode signals of logic device 64 in cooperation with a digital delay line 62 or similar device is well known in the prior art. A device for carrying-out the foregoing function is well documented in the publication edited by Merril Skolnik entitled &#34;Radar Handbook&#34;, McGraw-Hill Book Company, pages 38-18 through 38-24, 1970. The foregoing or similar circuitry is also available from other sources in the broad range of IFF publications. Finally, an example of circuitry to perform the function of logic device 84 is a monostable multivibrator, which is a circuit having one stable state and one unstable state. A trigger signal is required to drive the circuit into the unstable state, where it remains for a predetermined time before returning to the stable state. 
     To continue, synchro-to-digital converter 48&#39; inputs synchro information from the radar antenna (see FIGS. 1 and 2) and outputs this information to input/output interface device 70&#39; of output computer 44. An internal end of sweep generator 52&#39; inputs an initiating trigger and outputs an internal end of sweep trigger which can be selected by switch SW5 for interfacing with certain radars. Logic device 54&#39; starts upon the initiating trigger being present and stops or is cleared upon the terminating trigger being selected by switch SW5. Range counter 58&#39; is cleared directly by the initiating trigger or by a trigger from output computer 44 as selected by switch SW6. Range offset generator 60&#39; adds an additional offset to the range of all replies or returns via an optional range offset applied at its input. 
     FIFO memory 68&#39; receives range and code information from input/output interface device 70&#39; of output computer 44. IFF pulse spacing synchronous clock 82 delivers the appropriate pulse spacing clocks for proper operation of IFF word generator 90 and IFF word generator 90&#39; via offset logic device 84. The foregoing clock connections allow both IFF word generators 90 and 90&#39; to operate to output interleaved IFF video trains. Range comparator 86 compares the range information outputted from FIFO memory 68&#39;  with the range information from range offset generator 60&#39;. Mode tag generator 88 receives information from input/output interface device 70&#39; of output computer 44. Mode tag generator 88 has as an additional input, information from range counter 58&#39;. The output of mode tag generator 88 comprises mode tags that have been retimed by input and output computers 42 and 44. 
     Continuing with the description of digital-to-composite IFF video converter 46, IFF word generators 90 and 90&#39; have as inputs, code from FIFO memory 68&#39; and load inputs from a logic device 92. Logic device 92 is driven by range comparator 86. Consequently, IFF word generator 90 outputs a digital signal to an output OR gate 94. Similarly, IFF word generator 90&#39; outputs a digital signal to output OR gate 94, but offset by a sufficient amount to allow recognition of both interleaved selective identification feature (SIF) trains. (This is the standard IFF train for non-mode (4) replies.) Thus, output OR gate 94 &#34;OR&#34;s the previously mentioned digital signals. Switch SW7 allows termination of 360° retimed IFF composite video upon an end of sweep trigger in conjunction with an output AND gate 96. 
     To make clear how the various inputs and outputs of FIGS. 3a and 3b are related to those of FIGS. 1 and 2, reference should first be made to FIGS. 1, 2 and 3a as viewed concurrently. Thus, the inputs to a particular 360° IFF video resynchronizer unit 38 are synchro, mode tag in, terminating trigger, inititing trigger, optional range offset and IFF composite video in. Still referring to FIGS. 1 and 2, but now to FIG. 3b as viewed concurrently, the outputs of a particular 360° IFF video resynchronizer unit 38 are 360° retimed mode tages and 360° retimed IFF composite video. Also, additional inputs to a particular 360° IFF video resynchronizer unit 38, as shown in FIG. 3b, are another synchro, another optional range offset, another initiating trigger and another terminating trigger. The origin of the aforementioned input signals are clearly shown for the first and second embodiments of the present invention in FIG. 1 and FIG. 2, respectively. 
     STATEMENT OF THE OPERATION 
     Details of the operation, according to the present invention, are now described primarily in conjunction with the block diagram of FIG. 3b. 
     Referring then to the digital-to-composite IFF video converter 46 portion of 360° IFF video resynchronizing unit 38, digital computer 44 sends range, mode, and code information thereto subsequent to the receipt of an initiating trigger from one of associated radars 16, for example. Digital-to-composite IFF video converter 46 stores the range and code in FIFO memory 68&#39;. It stores the encoded mode in mode tag generator 88. Upon receipt of the initiating trigger, synchronous clock 56&#39; and IFF pulse spacing synchronous clock 82 begin operating. Accordingly, range counter 58&#39; is cleared and begins to count. When range comparator 86 detects that the range of range offset generator 60&#39; is equal to the range of the digital range word in FIFO memory 68&#39;, it generates a load signal to one or the other of IFF word generators 90 and 90&#39; via logic device 92. The particular IFF word generator 90 or 90&#39; chosen, then loads the code of FIFO memory 68&#39;. FIFO memory 68&#39; is instructed to dump one entry (range or code) from among its entries. IFF word generator 90 or 90&#39; may be thought of as a parallel to serial shift register with an optional logical &#34;OR&#34;ing of present data upon reload. IFF pulse spacing synchronous clock 82, which is started by the initiating trigger and halted by the terminating trigger, and offset logic device 84 function to allow two replies or returns to be interleaved one with the other. The outputs of IFF word generator 90 and 90&#39; are logically &#34;OR&#34;ed by output OR gate 94, and &#34;AND&#34;ed with the output of logic device 54&#39;, when switch SW7 is set. Switches SW5, SW6, and SW7 allow digital-to-composite IFF video converter 46, according to the present invention, to be interfaced with different radar systems. 
     To those skilled in the art, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the present invention can be practiced otherwise than as specifically described herein and still be within the spirit and scope of the appended claims.