Abstract:
A system for remote measurement of wind velocity in the atmosphere, operating from ground level. An array of acoustic transducer elements which provide for transmitting a beam of acoustic energy along a path and receiving such transmitted energy as scattered by wind in the path. The system includes signal transmitting means for driving the arrays, and signal receiving means, with a switching system for connecting the elements of the array to the transmitting means and to the receiving means, for operation as a monostatic system. Phase control for the transmitted signals comprising phase shifters and switches for selectively connecting driving signals to the elements of the array for driving selected elements at different phases to produce first, second and third beams in sequence at three different predetermined angles from a single antenna.

Description:
BACKGROUND OF THE INVENTION 
     The subject invention relates to an atmospheric wind measuring apparatus utilizing acoustic beams, and more specifically, to a monostatic system with a fixed array of acoustic transducer elements. 
     Doppler acoustic systems are used to measure atmospheric winds remotely from a site on the ground. Valid measurements are obtained typically to heights of many hundreds of meters, occasionally to well over a kilometer. Such instruments are therefore of considerable value in obtaining continuous wind profiles for use in connection with pollution monitoring, wind power, meteorology, atmospheric modeling and other applications. 
     A full description of the wind vector at a given location and time requires the determination of three components. These are frequently taken to be the east, north and vertical components. Different component resolutions, e.g., speed, direction and vertical, are equivalent, and each set can be readily converted to the other. 
     On configuration of a doppler acoustic system for measurement of wind velocity and direction is shown in U.S. Pat. No. 3,889,533. This is a bistatic system with a transmitter and two or three receivers spaced from the transmitter. Typically the transmitter directs a beam vertically upward and the receivers provide a measure of scattering at various elevations along the beam. The receiver outputs are processed to provide a velocity vector indicating wind velocity and direction. 
     Another configuration of a doppler acoustic system that measures three independent components of wind is the monostatic system, i.e., one in which the same antenna serves as transmitter and receiver of the acoustic energy. FIG. 1 shows schematically how this is normally accomplished. 
     Three different antennas 11, 12, 13 are employed, each generating one beam to measure one of the three wind components. The beam of antenna 13 points vertically, the beam of antenna 11 points towards the east, and the beam from antenna 12 toward the north, the latter two at some angle γ from the vertical. The details of pointing angles are not critical; any set of three independent measurements can be converted to the desired wind components. Most commonly, each antenna consists of a reflecting dish, on the order of 1 meter in diameter, irradiated by an acoustic driver, or possibly a small cluster of drivers. 
     Such systems suffer from two limiting disadvantages. First, the three antennas with their acoustic shields are very large and bulky, and require considerable effort to move and align. Second, the radiated power available from a single acoustic driver limits the range (altitude) from which usable returns are obtained. 
     A monostatic system using four antennas, each comprising an array of drivers or acoustic transducer elements in the form of commercial loud speakers is shown in U.S. Pat. No. 3,675,191. The beams from the four antennas are fixed in direction, with the return signals from the four antennas being mixed in various combinations to provide the desired output. 
     It is an object of the present invention to provide a new and improved acoustic wind velocity measuring system utilizing only a single antenna having an array of acoustic transducer elements or drivers. This provides a significant advantage over prior art systems in that the size, cost and weight of the antenna arrays is a significant factor in manufacture, installation and operation of wind measuring systems. 
     It is a further object of the invention to provide such a system which can provide beams of acoustic energy at three different predetermined angles from a single antenna for transmitting acoustic energy along three different paths and receiving the transmitted energy as scattered by wind in the paths, with the received energy being converted to electrical signals for processing to provide the wind velocity vector. An additional object of the invention is to provide such a system wherein the three beams are produced in sequence by controlling the phase of the driving input to the individual elements of the antenna array. 
     It is a particular object of the invention to provide such a system wherein the desired beam paths and wind velocity information can be achieved while requiring only a single 90° phase shift in the driving signals. 
     Other objects, advantages, features and results will appear in the course of the following description. 
     SUMMARY OF THE INVENTION 
     A single antenna monostatic system for remote measurement of wind velocity utilizing acoustic doppler. An antenna in the form of an array of acoustic transducer elements providing for transmitting a beam of acoustic energy along a path and receiving such transmitted energy as scattered by wind in the path. Signal transmitting means including an electrical signal generator providing an electrical signal for driving the elements, and phase shifting means having the electrical signal as input and providing as output one or more shifted electrical signals, with switching for selectively connecting the signals to the elements for driving selected elements at different phases to produce three beams in sequence at three different predetermined angles. One or more receivers and a signal processor, with the transducer elements being selectively connected to the transmitting means and to the receivers, with the signal processor providing appropriate manipulation of the received signal or signals to compute the desire vector wind velocity. An antenna with a specific orientation or boresight angle permitting simple phase shift requirements, particularly a 90° phase shift which is readily achieved with a minimum of electrical circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating three beams used in measuring three wind components; 
     FIG. 2 is a plan view of an antenna with N columns and N rows of transducer elements; 
     FIG. 3 is a side view of the antenna of FIG. 2; 
     FIG. 4 and 5 are views similar to FIG. 2 showing some alternative antenna configurations; 
     FIG. 6 is an electrical schematic in block diagram form illustrating signal transmitting means and signal receiving means for use in conjunction with the antenna array; and 
     FIG. 7 is a view similar to that of FIG. 6 showing the presently preferred embodiment of the invention utilizing a single 90° phase shift. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention uses a phased array of acoustic transducer elements or drivers to function as a single antenna. In its simplest form, an array comprises a number of identical elements, each of which radiates an amplitude pattern f 1  (θ,φ). Then, each element is assigned a location in the array, generally linear or planar, a relative phase, and if desired, a weight. Taken as a set of point sources, the array then yields an amplitude pattern f 2  (θ,φ). The overall radiated (power) antenna pattern is then 
     
         F(θ,φ)=|f.sub.1 (θ,φ)f.sub.2 (θ,φ)|.sup.2 
    
     By changing the relative phases assigned the elements of the array, different beams can be formed. The desired phases are produced electrically, and thus electrical switching obviates the need for separate antennas or mechanical motion of an antenna. Further, each element of the array can be subject to a relatively low power level, while the full antenna can radiate much more power than could be supported by a feed in a conventional reflecting antenna. 
     The present invention utilizes a single antenna, such as the antenna 11 of FIG. 1. This antenna comprises an array 16 of acoustic transducer elements 17. In the embodiment shown in FIGS. 2 and 3, the array comprises N columns and N rows of elements, where N is a small whole number. In the embodiment illustrated, N is 7. Each acoustic transducer element typically comprises a driver 18 with coil and diaphram and a horn 19 and operates in the same general manner as a radio speaker. 
     In contrast with the conventional antenna configuration of a single source or feed illuminating a reflecting dish that creates the wave front, in the antenna of the present invention the mouth of the horn of the transducer element is directly a section of the radiating wave front. Typically, its dimensions are some fraction of a wavelength, thus on the order of 12-20 cm; elements separated in an array by a large fraction of a wavelength or more tend to create undesirable grating lobes. 
     In the array of FIG. 2, square elements of side d are arranged in the N×N array, providing an antenna aperture of D=Nd. The array of FIG. 2 is both convenient and economical, but is not a limitation of the generality of array configurations. 
     The most straightforward use of an acoustic array is to mount it in a horizontal plane. When all of the elements are fed in phase, a vertical (first) beam is created, corresponding to the illumination of a D×D aperture. 
     This antenna with no further capability is already well suited to be the transmitter for an acoustic sounder or a bistatic doppler acoustic system, both of which employ only a vertical transmitted beam. The greater transmitted power available from the array would benefit all such systems by increasing the altitude range of their useful operation. 
     Next consider the phased array of the present invention. Suppose that the transmitted signal, typically a pulse at the radiated frequency f, is fed to all elements in column 1, the same signal with a time delay τ is fed to all elements of column 2, and so forth to a delay of (N-1) τ to all elements of column N. Phase shifts of σ=2πfτ in column 2 to one of (N-1)σ in column N would be eqivalent, for systems of interest that employ a narrow band. The (second) beam thus generated would tilt toward the right, say eastward, at an angle from the vertical determined by ##EQU1## 
     By proper choice of τ, the desired beam direction is achieved. if on a succeeding pulse, the delay τ (phase shift σ) is applied to row 2, etc., the (third) beam is directed at the angle θ from the vertical, but in a northward direction. Thus are the three desired beams obtained. 
     As an example, if we desire θ=30°, then one beam, the vertical, would be generated at boresight, which is the direction perpendicular to the face of the array. The other two beams are generated at angle 30° from boresight, in orthogonal planes that intersect along the boresight. 
     In a preferrred beam configuration, the three beams are more symmetrical with respect to boresight and possess certain practical advantages as well over the preceding configuration where the array was horizontal with the antenna boresight vertical. 
     Suppose that the boresight is pointed at an angle α from the vertical, such as in the direction of northeast. Suppose further that the columns of the array are aligned parallel to the plane determined by the vertical and the boresight. If the observation coordinates are x, y, z, which have been identified with east, north and vertical respectively, then the coordinate system of the antenna is given by ##EQU2## Then a beam transmitted in the x&#39;, z&#39; plane at an angle α from the z&#39; axis is pointed vertically, i.e., along the z axis. Also if a pair of beams is transmitted at angles ±β from the z&#39; axis in the y&#39;z&#39; plane, for some angle β these beams will be in the x, z and y, z planes at some angle γ with respect to the z axis. This set of three beams is again the desired set. 
     The three angles α, β and γ are related by 
     
         tan β=sin α 
    
     
         tan γ=2 tan α 
    
     and for small angles ##EQU3## For example, if we choose γ=30°, then α=22° and β=21°. To a very good approximation (considering that typical beam widths are 10° or more), the desired transmissions are obtained by tilting the antenna at an angle of 21.5° and generating the three beams at an angle of 21° from boresight as described above. 
     This arrangement has two advantages over that with the boresight vertical. First, all three beams are generated using the same phasing of the array, but switched to different sets of elements. Second, the angle off boresight of the oblique beams is considerably reduced, thereby decreasing both the loss due to the element pattern f 1  (θ,φ) and the potential problem with grating lobes. 
     The number of elements in an array of the type shown in FIG. 2 does present some practical difficulty. The 7×7 array, for example, has 49 elements. 
     Alternative designs for an array are shown in FIGS. 4 and 5, which are subsets of the full square array of FIG. 2. In addition to the obvious advantage of requiring fewer elements, the smaller arrays provide a natural weighting of the antenna illumination. If each element is given the same power, then the aperture D effectively is illuminated more strongly at the center, less so at the edges. This weight substantially reduces the sidelobes of the antenna at the cost of a slight increase in the beam width of the main lobe. 
     The general technique for utilizing a phased array is to feed the array elements with the specific phase pattern that creates the desired beam, and then change that pattern to create the next desired beam. The block diagram of FIG. 6 illustrates schematically the technique of producing the three desired beams in an N×N array of tranducers as accomplished in this application. 
     The system as illustrated in FIG. 6 includes a signal generator 21, one or more phase shifters 22, 23, 24, a switch control unit 25, a receiver 26, a signal processor 27, and an output unit 28. Electrical connnections to the elements of the row and columns of the array are indicated at 32. A set of switches 34 operated by the switch control 25 provides for connecting signals from the signal generator 21 and phase shifters 22 etc. to the various rows and colums of the array 32. In operation the switches 34 are selectively actuated to energize predetermined elements of the array to produce a beam in a predetermined direction. A cycle through the switch positions produces the three desired beams in sequence. 
     The signal from the transducer elements operated as a receiver returns through the switches 34, which remain set in the same position from the initiation of the transmitted pulse through the type typically 5-10 sec. later when the last useful return is received. Typically an amplifier or set of amplifiers is used to generate the power necessary to operate the transducer elements. Amplification may take place either ahead of or following the phase shifters, and one such amplifer is shown at 33. A transmit/receive switching device 36 is used to bypass the amplifer on the return path and also to protect the sensitive receiver 26 from the high power levels generated by the amplifier. The received signals pass through the same phase shifters, which thereby creates the same beam for reception as was used for transmission. The received signal is fed into the receiver 26, followed by the signal processor 27 and the output unit 28. The signal processor may be conventional, such as is disclosed in the aforementioned U.S. Pat. No. 3,889,533. 
     Further simplification of the equipment can be achieved by special choice of the phase shifts. This can be done for cases of interest with little constraint on system design, and allows considerable economy in producing the system. The phase shift σ 1  is give by ##EQU4## where d is the distance between rows or columns, λ is the wavelength of the radiated sound, and θ is the angle off boresight of the beam. 
     The combination given in the equation can be chosen to yield σ 1  =π/2 (or 90°) or another desired value. Specifically, if d/λ is taken to be 2/3 (which gives d=14 cm for a 1600 Hz frequency) then sin θ=3/8 (or θ=22°) corresponds to σ 1  =π/2. Other combinations of parameters also lead to convenient phase shifts. 
     With the choice of σ 1  =90°, the set of phase shifts depicted in FIG. 6 reduces in effect to a single phase shift of 90°, no matter how large the array size N. The second phase shift, 180°, is achieved by a phase reversal (change of sign) of the original signal, the third phase shift, 270°, is a phase reversal of the 90° signal, and the cycle is then repeated as often as necessary. 
     A further simplification and presently preferred embodiment is shown in FIG. 7. The symmetry of the two beams involving columns of elements results in some phases being in common for those two beams, and thus reduces switching requirements. Elements in FIG. 7 corresponding to those of FIG. 6 are identified by the same reference numerals. The configuration of FIG. 7 additionally includes phase reversals units 38, 39, each of which provides a 180° phase shift, and another phase shifter 40 which provides a 90° phase shift. The output of the 90° phase shift unit 40 is connected to one moving arm of a double pole, double throw switch 41, and the output of the phase reversal unit 39 is connected to the other moving arm of this switch. 
     In operation, for the first beam, the switches 34 are connected to the rows of the array and the switch 41 is connected as shown in FIG. 7. For the second beam, the switches 34 are actuated to connect signals of given phase to given columns. For the third beam, the switch 41 is actuated to reverse the phase trend across the columns. This configuration requires only 90° and 180° phase shift units, which are relatively simple to produce, the 180° unit calling for merely a reversal in polarity. In a still further simplification, certain elements of the array always share the same phase and can therefore be permanently connected and fed by the same input.