Beam steering unit real time angular monitor.

0001The present invention relates to a method of and a system for monitoring the operation of a beam steering unit for a phased array antenna, during a scanning operation of the beam steering unit. In particular, according to the invention, the pattern of wave energy which would be radiated from the antenna to an observation point in space during the scanning operation is simulated by processing phase angle data provided by the beam steering unit and combining it with observation angle data corresponding to the observation point.

0002In order to verify proper operation of a beam steering unit associated with a scanning phased array antenna, it has ordinarily been required to monitor the wave energy actually radiated by the antenna to near and/or far observation point, and then compare the monitored energy levels with a reference standard. For example, in United States patent 4,520,361 issued May 28, 1985, to R.F. Frazita and assigned to the assignee of the present invention, phase angle data provided from a beam steering unit to each of a number of radiating elements of a phased array antenna, is verified separately for each of the elements by coupling some of the element radiation to a manifold at the antenna, mixing with manifold output with a sample of the RF power source to obtain a beat frequency signal, and measuring the phase shift between the beat frequency signal and a reference pattern signal.

0003United States patent 4,536,766 issued August 20, 1985, to R.F. Frazita and assigned to the assignee of the present invention, discloses a beam pointing correction arrangement which also entails the use of a manifold proximate the radiating elements of a scanning phased array antenna, wherein the manifold output is detected and decoded to provide an indication of the actual beam pointing angle. The start and stop time of the beam steering unit scanning operation is then adjusted to eliminate or minimize any detected beam pointing error. A system is also known from United States patent 4,532,517 issued July 30, 1985, in which output data from a beam steering unit is subjected to a cyclic redundancy check employing algebraic methods commonly used to verify accuracy of information transmitted in digital form.

0004As far as is known, no method or system has been disclosed by which the pattern of wave energy radiated from a phased array antenna to an observation point during operation of an associated beam steering unit, can be simulated to allow for a comparison with a standard reference pattern. The desirability for such a method or system is especially great in microwave landing systems (MLS) in which precise timing of the beam steering operation must be maintained continuously to assure that an aircraft at a certain point in space relative to the system antenna will receive the antenna beams at the proper timings as the antenna beams are scanned "to and fro" and "up and down".

0005Basically, a MLS employs at least two phased array antennas each having a number of equally spaced radiating elements which are excited with microwave energy at a generally uniform amplitude but at a phase determined by the setting of the individual phase shifters associated with the elements. The function of setting the phase shifts for the individual phase shifters is accomplished by the beam steering unit (BSU). As is well-understood by those skilled in the art, a main energy beam which is radiated from the excited antenna elements can be steered or scanned in a direction relative to the antenna, in accordance with predetermined incremental changes of the phase shifters by the BSU over successive time intervals.

0006In MLS applications, an azimuth (AZ) phased array antenna scans its radiated beam to and fro periodically in the horizontal direction, the beam-width being relatively broad in the vertical direction but narrow in the horizontal direction, so that an aircraft within the scanning Field of the AZ antenna will be able to detect a passage of the scanning beam from the AZ antenna from ground level to a relatively high altitude. An elevation (EL) phased array antenna scans its beam up and down periodically in the vertical direction, the beam width being relatively broad in the horizontal direction but narrow in the vertical direction, so that an aircraft within the scanning field of the EL antenna will be able to detect the passage of the scanning beam from the EL antenna from an approach which is head-on to the antenna to one which is about ±40° relative to the antenna axis.

0007Prior to a scanning operation of the AZ antenna, a "preamble" signal is radiated broadly from a third antenna for reception by an aircraft within the operating range of the MLS. The preamble signifies, <u style="single">inter alia</u>, that a horizontal scan of the beam from the AZ antenna is to begin at a certain time from one side (e.g., -40°) of the AZ antenna, to the opposite side (+40°), and back again to the starting side (-40°). Equipment on board the aircraft detects and decodes the preamble, and counts the time period between reception of the beam from the AZ antenna on its "to" scan and reception of the beam on the "fro" scan. The counted time difference corresponds to a unique azimuth heading of the aircraft relative to the AZ antenna. The MLS then broadly radiates a preamble signifying that a scanning operation of the EL antenna is about to begin and, by a corresponding time difference counting operation, the equipment on board the aircraft determines a unique elevation angle for the craft relative to the EL antenna. Since both the AZ and EL antennas are located in the vicinity of a runway employing the MLS, the aircraft pilot thus receives information which is critical to assure a proper glide path for a safe landing on the runway.

0008From the foregoing, it will be appreciated that precise timing of the scanning operations of both the AZ and EL antennas is essential to ensure accurate glide path information will be provided to the aircraft pilot. Any malfunction which results in a deviation of the time difference between to and fro or up and down scanning beams at a given point in space, from a predetermined difference which defines the location of the point in space when the MLS is functioning properly, will cause the on-board equipment to produce erroneous heading information.

0009A major source of such potential system malfunction is the BSU which controls the direction and rate of scan of the beams from the AZ and EL antennas in the MLS. Thus, it is imperative that the BSU be monitored continuously with respect to the phase angle data which it provides to the phase shifters associated with the antenna elements, causing the beams to be swept at the desired predetermined rates.

0010An object of the present invention is to overcome the above and other shortcomings in the known techniques by which operation of a BSU can be monitored in real time.

0011Another object of the invention is to provide a technique by which the accuracy of the BSU can be ascertained without providing field monitors in the vicinity of or at points located remote from the antenna with which the BSU is associated.

0012A further object of the invention is to simulate, in real time, the pattern of wave energy which would be radiated to an aircraft from a MLS antenna during operation of the associated BSU.

0013A further object of the invention is to simulate, in real time, the scanning of a beam of a MLS antenna as received by an aircraft at a certain point in space during a scanning operation of the BSU, and to compare the time difference between successive beams with a preset time difference to confirm proper operation of the BSU.

0014According to one aspect of the present invention, a method of simulating the pattern of wave energy which would be radiated to an observation point in space from a scanning phased array antenna during operation of an associated beam steering unit, the beam steering unit providing phase angle data at certain time intervals to set a number of phase shifters associated with elements of the phased array antenna, is characterized by the steps of:    storing initial phase angle data in memory areas each of which corresponds to a phase shifter to be driven by the beam steering unit;    sequentially reading out phase angle data from said memory areas and updating the phase angle data from each memory area in accordance with the phase angle data from the beam steering unit, and storing the updated phase angle data in the corresponding memory areas over each successive time interval;    selecting an observation angle relative to the antenna at which the pattern of wave energy radiated from the antenna to a point in space at said selected observation angle is to be simulated during a scanning operation of the beam steering unit;    generating observation angle data which is a function of said selected observation angle, the distance between adjacent antenna elements and the wavelength of the wave energy;    obtaining composite angle data which is a function of the addition of said updated phase angle data and said generated observation angle data;    subtracting from the composite angle data for each time interval the composite angle data for the immediately preceding time interval and accumulating resulting differences with initial value composite angle data to provide accumulated composite angle data; and    determining the relative amplitude of wave energy which would be radiated to the point in space at the selected observation angle during operation of the beam steering unit as a function of the accumulated composite angle data.

0015According to another aspect of the invention, there is provided a system for testing the operation of a beam steering unit by simulating the pattern of wave energy which would be radiated to an observation point in space from a scanning phased array antenna including phase shifters associated with substantially equally spaced elements of the antenna, the beam steering unit providing phase angle data at certain time intervals to set the phase shifters over a scanning operation, comprising:    memory means for storing phase angle data in memory areas each corresponding to a phase shifter to be driven by the beam steering unit;    logic means coupled to said memory means and adapted to be responsive to the phase angle data provided by said beam steering unit, for addressing and controlling data flow into and out of said memory areas, said logic means including means for setting initial phase angle data in the areas of said memory means to correspond with initial phase settings for the phase shifters prior to a scanning operation of the beam steering unit;    data increment means coupled to said memory means for updating the value of phase angle data when read out of each of said memory areas in accordance with the phase angle data from the beam steering unit, the updated phase angle data being stored in the corresponding memory area by said logic means for each successive time interval;    means for generating observation angle data in accordance with a selected observation angle at which said observation point is located relative to the antenna, said observation angle data being a function of said selected observation angle, the spacing between adjacent antenna elements and the wavelength of the wave energy;    means coupled to said data increment means and said observation angle data generating means for obtaining composite angle data which is a function of the addition of said updated phase angle data and said generated observation angle data;    means for subtracting from the composite angle data for each time interval the composite angle data for the immediately preceding time interval;    means coupled to said subtracting means for accumulating resulting differences with initial value composite angle data to produce accumulated composite data; and    means coupled to said accumulating means for determining the relative amplitude of wave energy which would be radiated to said observation point during a scanning operation of the beam steering unit in accordance with said accumulated composite angle data, and for producing a corresponding output.

0016An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a conceptual block diagram of a system for testing operation of a BSU, according to the invention;</li><li>Figure 2A is a block diagram of a BSU interface portion 12a of an antenna pattern simulator 12 according to the invention; and</li><li>Figure 2B is a block diagram of a phase angle and observation angle processing portion 12b of the present antenna pattern simulator 12.</li></ul>

0017Figure 1 represents a technique for monitoring in real time a pattern of wave energy which would be radiated to a given point in space by a phased array antenna which is scanned by a given beam steering unit (BSU) 10. The beam steering unit may be, for example, one which is intended for MLS applications such as, e.g., the type MLS 2600 manufactured by Hazeltine Corporation of Commack, New York. The BSU may have separate phase angle data outputs φA and φB corresponding to differential phase angle information to be conveyed to phase shifters associated with an "A" and a "B" side of a MLS phased array antenna. The differential phase data supplied by the BSU 10 during a scanning operation is coupled to an array antenna pattern simulator 12, rather than or in addition to the phase shifters of the MLS antenna. As explained below in regard to Figures 2A and 2B, the simulator 12 will appear to the BSU 10 as the phase shifters themselves insofar as the addressing and phase angle data outputting functions of the BSU are concerned.

0018By processing the phase angle data provided by BSU 10 and observation angle data generated upon setting of an observation angle select switch 14, the simulator 12 provides a digital-to-analog converted output signal which, if connected to the V input of an oscilloscope 16, causes a real time display of a MLS antenna beam were the antenna to be steered by the BSU. A "start scan" signal provided from the BSU 10 to the trigger (T) terminal of the scope 16 thus would cause the display to represent the time at which the main scanning beam of the antenna would be received at an observation point at the selected angle, after the start of a single scan.

0019Assuming that the phased array antenna to be associated with the BSU 10 comprises a number (e.g., 112) of equally spaced, uniformly illuminated radiating elements, the far-field pattern of the antenna at a point in space at an angle ϑ relative to the antenna axis can be represented by <maths id="math0001"><math display="block"><mrow><mtext>Σ exp j (</mtext><mfrac><mrow><mtext>2π</mtext></mrow><mrow><mtext>λ </mtext></mrow></mfrac><msub><mrow><mtext> nd sin ϑ + φ</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP0247780B1_D0001.tif" /></maths> wherein: <dl id="dl0001"><dt>n</dt><dd>is the element number</dd><dt>d</dt><dd>is the spacing between elements</dd><dt>φ_n</dt><dd>is the relative phase shift introduced to the nth element by its associated phase shifter, and</dd><dt>λ</dt><dd>is the wavelength of energy to be radiated by the antenna.</dd></dl>

0020Expansion of the foregoing yields: <maths id="math0002"><math display="inline"><mrow><msub><mrow><mtext>Σ (cos x</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><msub><mrow><mtext> + j sin x</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP0247780B1_D0002.tif" /></maths> , where:<maths id="math0003"><math display="block"><mrow><msub><mrow><mtext>x</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><mtext> = 2 </mtext><mfrac><mrow><mtext>π</mtext></mrow><mrow><mtext>λ</mtext></mrow></mfrac><msub><mrow><mtext> nd sin ϑ + φ</mtext></mrow><mrow><mtext>n</mtext></mrow></msub></mrow></math><img file="EP0247780B1_D0003.tif" /></maths> .

0021The relative power at the observation point ϑ thus may be expressed as: <maths id="math0004"><math display="inline"><mrow><msub><mrow><mtext>|Σ cos x</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><msub><mrow><mtext>|² + |Σ sin x</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><mtext>|²</mtext></mrow></math><img file="EP0247780B1_D0004.tif" /></maths> .

0022By obtaining a continuous real time summation of the values for the cos(x_n) and the sin (x_n) for all the antenna elements or phase shifters n, squaring the sums and then summing the squares, the relative power radiated by the antenna to the far-field observation point at the set angle ϑ is obtained.

0023Each of the φ_n may be changed or updated at a rate of, e.g., 5 MHz or every 200 nanoseconds as in the MLS 2600 BSU. The summations must therefore be performed, then squared and added to one another as the values are updated to enable a faithful reproduction of the scanning pattern which would be obtained at the observation point.

0024The antenna pattern simulator 12 of Figures 2A and 2B performs the necessary operations on the phase angle data from the BSU 10 as updated, without the requirement for a large summing network having inputs (e.g., 112) corresponding to the settings of phase shifters coupled to the BSU output.

0025The BSU interface portion 12a of Figure 2A includes control logic 20 for buffering the output from the BSU 10 and supplying it to a random access memory 27 having memory areas the addresses of which correspond to phase shifters which would be driven by the BSU 10 when operating with a phased array antenna. As mentioned, the BSU 10 provides only differential phase angle data, i.e., data indicative of the change, if any, to be made to a particular phase shifter setting from the setting of the immediately preceding update interval. In actual practice, the BSU 10 provides initial absolute value phase shift settings for each of the n phase shifters, followed by differential data in, e.g., 22 1/2° increments to alter the phase shifter settings up or down in certain time intervals. In Figure 2A, the initial setting phase angle data is transferred through control logic 20 directly to the memory areas of RAM 27 corresponding to the phase shifters to be set. The contents of the memory areas are then successively added in adder 24 to any differential phase angle data produced by BSU 10 as passed by control logic 20 to a second input of adder 24. Since no differential data is provided at the start of a scan, the initial phase shifter setting data is unaffected and passed to an input of a second adder 26. The remaining input of adder 26 is coupled to a universal preset/count circuit 28 which provides a function corresponding to one which is available on MLS antennas and well-known in the art. The adder 26 and circuit 28 may, however, be eliminated in some cases.

0026When the first differential data for a phase shifter n is provided from BSU 10, it is routed to adder 24 wherein the previous (or initial) phase angle data for the phase shifter n is incremented according to the differential data. The result is stored at the memory area corresponding to the phase shifter n in the RAM 27, and provided to the second adder 26 or directly as output data corresponding to the absolute phase shift value set in each phase shifter n during a time interval t.

0027Each time new differential data for a phase shifter n is produced by the BSU 10, it is combined in the adder 24 with the immediately previous absolute phase shift value as stored in the corresponding memory area in RAM 27, and the thus incremented (or decremented) absolute value data is rewritten in the same memory area while being provided as output data from the interface portion of Figure 2.

0028Figure 2B is a phase shifter angle and observation angle processing portion 12b of an antenna pattern simulator 12 according to the invention.

0029An observation angle select circuit 30 which may be in the form of DIP switches is connected to a programmable observation angle memory (PROM) 32. PROM 32 provides an output corresponding to the sine of the selected observation angle ϑ multiplied by the antenna element spacing d, the factor <maths id="math0005"><math display="inline"><mrow><mfrac><mrow><mtext>2π</mtext></mrow><mrow><mtext>λ </mtext></mrow></mfrac></mrow></math><img file="EP0247780B1_D0005.tif" /></maths> , and the phase shifter number n. The result is combined in adder 34 with the absolute phase setting for each phase shifter n to produce composite phase angle data for the phase shifter n at a given update interval t. In order to carry out the required summations of the cosine and the sine of the composite angle data, differences between the cosine of said data for a phase shifter n at a time interval t and the data for the same phase shifter n at the immediately preceding time interval (t-1) are determined by cosine circuit 36 and supplied for each of the phase shifters to a cosine accumulator circuit 38. A sine subtraction circuit 40 and sine accumulator circuit 42 carry out similar operations for the required sine summation. An output I of cosine accumulator 38 corresponds to the sum of the in-phase field contributions of each phase shifter (antenna element) n at a far-field point at the selected observation angle. An output Q of the sine accumulator 42 corresponds to the quadrature far field effects of the antenna elements as combined. By squaring each of the I and Q outputs, summing the squares and taking the Log of the result, a signal P corresponding to the relative power at the observation point during a scanning operation of the BSU 10 is produced. Since the signal P is in digital form, it may be necessary to provide a D/A converter 46 to provide a corresponding analog signal for observation and/or further processing.

0030It will be appreciated that in accordance with the invention, the absolute phase angle settings for each of a great number of phase shifters is stored in corresponding memory areas of the RAM 27. The in-phase and quadrature far field effect of each phase shifter at a certain observation angle is determined and accumulated in the accumulators 38, 42 at the start of a scanning operation of the BSU 10. As differential phase angle data is produced by the BSU 10, the previous field contribution of each phase shifter is subtracted by the circuits 36, 40 from the new contribution and the result accumulated.

0031A highly desirable instrument for monitoring the operation of phased array antennas with a particular beam steering unit is disclosed herein, with a relatively small amount of circuit devices required for its implementation.