document: BACKGROUND OF THE INVENTION 
       1. Field of the Invention 
       The invention relates to arrangements of antenna elements to measure the direction of propagation of electromagnetic radiation. 
       2. Related Art 
       The directional properties of antennae and the processing of phase and amplitude information from separate antenna elements has long been used for determining the angle of arrival of signals, and thus determining the direction of the source transmitter. However, low elevation angles are difficult to measure because of interference from strong unquantifiable reflections from the ground. Moreover, radio direction finders and navigation systems are constrained by weather conditions, to use relatively low operating frequencies, where ground reflections cannot be excluded by the natural directivity of practical antennae. 
       A unique equi-spaced triplet arrangement of similar antenna elements was identified in U.K. Patent No. 1449196 (incorporated herein by reference), which when placed orthogonal to a reflecting ground plane, could be used to determine a function of elevation angle which was independent of the unknown ground reflection coefficient. Patent No. 1449196 describes the use of a triplet to define fixed angles in the elevation plane, within which angle measurements were made by other means. This invention extends these ideas to optimize angle measurements, using the triplet itself, notably over selected sectors at low elevation angles. 
       The prior art of UK 1449196 is illustrated in FIG. 1, which shows a single equi-spaced triplet for which the two R.F. outputs A+C and B are shown in the above patent to be; 
       
           A+C=2f(a,p,r,h,P,θ)cos(2·II·d·sinθ/.lambda.) . . .                                             (1) 
      
       
           B=f(a,p,r,h,P,θ) . . .                               (2) 
      
       Where: 
       a represents the antenna element pattern. 
       P represents the transmitted power. 
       r represents the range to the transmitter. 
       h represents the mid-height of the triplet. 
       p represents the ground reflection coefficient. 
       θ represents the elevation angle. 
       d represents the spacing between triplet elements,; and 
       λ represents the wavelength of the transmitted signal. The function f(..) represents the essential strength of the radio signal and is designated S. 
       Whence, the quotient of (1) and (2) above is independent of a,p,r,h and P. The quotient of other functions of (1) and (2), for example the square or the modulus, is also independent of a,p,r,h, and P. The square of the signal amplitude is the natural output of a radio receiver, and takes only positive values, which is advantageous for post detection signal processing. 
       At low elevation angles, p approximates to -1, and in this case S (representing the function (f)) simplifies to; 
       
           S=f&#39;(a,P,r)sin(2·II·h·sinθ/λ) . . . (3) 
      
       so that h may be chosen to maximize the amplitude of S. 
       Measurement of elevation angle can only be made, when S is non-zero, and is best made when S is changing slowly, with angle, near its maximum value. 
       It will also be noted that when d =λ/2, (A+C)/2B takes values from 1 to -1 as the elevation angle changes over 90 degrees, typically, from the horizontal to the vertical. Larger values of d and other functions of (A+C)/2B may increase the sensitivity with which the elevation angle can be measured. However, for a single triplet, the angle measurement may become ambiguous over the angular range of interest. 
       SUMMARY OF THE INVENTION 
       It is one object or this invention to overcome such ambiguity. 
       According to this invention, an antenna array for operation by radio interferometric techniques comprises; at least four antennae spaced so as to provide at least two equi-spaced linear triplets perpendicular to a ground plane, each triplet will be characterized by the spacing of its elements and the height of the centre element above the ground plane; radio receiver means to obtain from each antenna of each triplet an information signal of which both amplitude and phase relative to the amplitude and phase of information signals obtained from other antennae of each triplet are functions of the elevation angles θ of incidence upon the array of a radio wave arriving from a remote source and of the ground reflection coefficient p; first logic means (56 in FIG. 5) associated with each triplet to combine vectorially the information signals obtained from the outermost antennae of that triplet to provide a first derived signal of which the amplitude represents the modulus of such combination; second logic means (58 in FIG. 5) associated with each triplet arranged to derive from an information signal obtained from the centre antenna of each triplet a second derived signal of which the amplitude represents the modulus of that information signal from the said centre antenna; dividing means associated with each triplet arranged to divide one derived signal by the other to provide a quotient signal for each triplet which is a function of θ but not of p; and selection means (60 in FIG. 5) to provide at least one quotient signal which provides a measure of the elevation angle. 
       The array may either operate in a receive mode, in which case unambiguous angle measurement is provided by the signal processing circuits embodied in the antennae receivers, as described, or in a transmit mode in which case the transmitted signal from each antenna in the array must be coded so that it can be identified to enable a quotient representing angle to be derived for each triplet in a remote receiver. It will be noted that a common coherent carrier is required for the transmissions from each element of any triplet to achieve the interferometric performance, but that coherence between triplets is not required. 
      
      
       BREIF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example only with reference to the accompanying drawings, in which; 
       FIG. 1 illustrates the prior art arrangement using a single triplet. 
       FIG. 2 illustrates the principle of operation of an antenna array arranged as triplets. 
       FIG. 3 illustrated a practical embodiment of an antenna arranged as three triplets. 
       FIG. 4 is a plot of signal against elevation for the three triplets illustrated in FIG. 3. 
       FIG. 5 illustrates a possible circuit for measuring the quotient signal. FIG. 6 illustrates the possible use of the array shown in FIG. 3 for determining the range, identity and elevation angle of an aircraft carrying a suitable transponder. 
      
      
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       In FIG. 2, three triplets of a general set are shown, each having three antennae A, B and C spaced in line at right angles to, and above a ground plane, shown shaded. The three triplets are respectively referenced a, b and q, and have different spacings d, and different heights h above the ground plane of the central antenna B in each triplet. 
       Only three triplets are shown, but in principle two or more can be provided. 
       In each triplet, the two output signals (B) and (A+C) are indicated. 
       The triplet arrays may be used in receiving or transmitting mode, and one application of each will be described by way of example. 
       FIG. 3 shows seven antenna elements deployed as three triplets arranged to operate on responses from secondary surveillance radar interrogations, at a wavelength of about 30 centimeters. It is assumed, for this example, that processing the signals to determine the quotient is best carried out on the signals A+C and B after detection by &#34;square law&#34; detectors. 
       The design aims in this example, are; 
       to provide unambiguous angle measurements up to about 7 degrees. 
       to provide measurement accuracy and resolution of the order +/-0.05 degrees from 3 degrees, at which aircraft normally approach to land, down to the lowest angle possible (about +0.5 degrees in this case). 
       use the smallest number of antenna elements possible, with the lowest element not so close to the ground, that it might be obscured, and the highest antenna element at an acceptable height. 
       d is chosen for the elevation angle, at which most measurement sensitivity is required, and h is chosen for the elevation angle at which S is close to its maximum value. For these criteria values for h and d are calculated from the relationships; 
       
           2·II·d·sinθ/λ=(n+1)II/4. . . (4) 
      
       
           2·II·h·sinθ/λ=(n+1)II /2. . . (5) 
      
       where n is the integer 0,1,2,3,. . . 
       In this example values of d and h are optimized for approximately the same elevation angles, so that amplitude S is at a maximum and altering slowly at angles where sensitive measurements are needed. It will also be noted that in this example, three triplets are sufficient to give the unambiguous sector coverage required (approximately 0.5 degrees to 7.0 degrees), and that by examining and averaging the three measurements obtained a judgement on optimum measurement integrity can be made. 
       Referring to FIG. 3, the triplet (n) is arranged to give its best measurements at around one degree and three degrees, the triplet (m), to give its best measurements at around two degrees and the triplet (l), its best at around five degrees. All readings are examined at each elevation angle to decide which triplet measurement is to be preferred, and the extent of the agreement between the three readings is a measure of the integrity of the system. In general, a suitably weighted average of the three readings will give the optimum result. 
       Typical values that might be chosen for d and h, in wavelengths, are then as follows; 
       
           ______________________________________(d) (l) = 1.5        h (l) = 2.5(d) (m) = 3.75       h (m) = 7.75d (n) = 7.5          h (n) = 15.25______________________________________ 
      
       The highest element is at 22.75 wavelengths, which is about 7 metetrs from the ground. 
       It will be noted that, subject to a small variation to allow antenna elements to be shared between triplets, h=2d, and that common values of sin θ satisfy both equations (4) and (5) for all values of n. Thus, a very long array with a suitable ambiguity resolving system, is capable of giving very high angle measurement accuracy over each of its high amplitude regions. 
       The thick lines on FIG. 4 show the preferred measurement ranges for each triplet l, m and n. 
       This example is appropriate to the important application of monitoring the height of aircraft approaching to land, which, hitherto, has not been easily achieved with the standard secondary surveillance radar system. However, many other arrangements are possible for civil and military applications, where the measurement of the elevation angle of an emitting or reflecting object is required. 
       FIG. 5 shows one of many possible arrangements for measuring the quotient {(A+C)/B}, all squared. Superheterodyne receivers (e.g. superheterodyne frequency converters 50 and 52 fed by a common local oscillator. will normally be necessary to achieve the sensitivity required and the dynamic range of function (S), for example, for an aircraft flying from a range of 20,000 metres to one of 200 metres will be large. However, 2B&gt;(A+C) and the squared quotient is always positive and, normally, will be in the range 0.33 to 0.67. In FIG. 5, a divider 54 is used, where B sets the gain of two balanced amplifiers (A1 and A2), and a timer (T) and comparator (C) measure the decay time of the CR circuit from a charged voltage corresponding to 4{B squared} from second circuit 58, to the voltage corresponding to {(A+C) squared} from first circuit 56. It is well known that the exponential nature of the decay, ensures that the time delay measured at the output of divider 54, is a function of the quotient required, and is independent of the absolute amplitude of the signals. In a further refinement the received signals are sampled as quickly as possible after their arrival, so that multi-path interference effects from lateral, and therefore delayed, reflections are minimized. The particular desired one of the triplets 1, m, n may be selected by selector 60 for further processing. 
       However, other suitable circuits may be used. 
       FIG. 6 shows a system in which a directional secondary surveillance radar interrogator (I), measures the range of identified aircraft (T), and triggers a measurement, by the elevation measuring system (E) described, on part or all of the reply message from the selected aircraft. Thus angles are firmly associated with particular identified aircraft, at a known range. In one possible embodiment, the high directivity of the interrogator antenna (A) is also used to set the seven degree upper coverage limit, by blanking the system, when the signals it receives, fall below those received by any antenna in the elevation array. 
       An alternative application of the antenna array shown in FIG. 3, is to provide coding, by radio transmissions, of angles in space. Triplet l may be energized by a coherent carrier in which elements A and C are amplitude modulated at a frequency l(a) and B at a frequency l(b). The modulation sidebands will carry the effective amplitude of the carrier, and the quotient of the amplitude of l(a) divided by the amplitude of l(b) may be derived, after demodulation, in a remote receiver. Likewise quotients m(a) divided by m(b) and n(a) divided by n(b) may be derived, and the optimum value of the elevation angle of the receiver with respect to the array ground plane, obtained.
summary: An antenna array for radio interferometry uses three equi-spaced triplets set vertically above the ground with different respective spacings, the center antenna of each triplet being at a different height. Signal processing circuits provide for each triplet a signal which is a function of the elevation angle θ but is independent of the ground reflection coefficient, P. the signals are weighted to give the optimum value of θ, e.g., by selecting the signal varying most rapidly with θ. Some antennae can be shared and for example three triplets may be provided by seven antenna elements.