The invention relates to the field of radar antenna systems, in general, and more particularly to a two beam array radar antenna system having illumination characteristics for providing amplitude and phase differences between the two beams which when taken together uniquely and accurately identify the height of a detected target. 
Radar antenna systems have traditionally been classified into one of either a two dimensional or three dimensional type.  The two dimensional radar antenna systems are typically simple and obtain range-azimuth information by generating a fan beam and rotating the fan beam about an azimuth angle.  On the other hand, three dimensional radar antenna systems are complex and obtain range-azimuth-elevation information by generating scanning or multiple pencil beams.  Presently, there is a growing demand for an intermediate type of antenna system that is basically two dimensional in nature, but has the capabilities of providing approximate elevation information.  Some examples of where these intermediate systems may be used are: for classifying high versus low flying threats, for air borne versus surface targets; for directing interceptors that have their own radar; for correcting fan beams' azimuth in the presence of roll and pitch; and for correcting azimuth on antennas with skewed elevation beams.  In most of these cases, the expense of using a conventional three-dimensional radar antenna system is not justified. 
One known proposed radar antenna system for obtaining height information utilizes illumination characteristics of two beams in a shaped reflector to provide height information comparable to a four beam system.  This proposed system comprises a dual feed horn driven shaped reflector type antenna for providing the desired illumination characteristics.  In this system, the two formed beams have characteristics which not only provide an amplitude difference therebetween which may be used to determine the height of the target, but they also have a phase difference which when taken together with the amplitude difference yields relatively accurate height information of a detected target in the lower elevation angles and longest range generally where accuracy is most needed.  The phase variation of the formed beams comes primarily from the predetermined shape of the reflector, more or less naturally. 
However, as with simple feed horn driven reflector type radar antennas, the problems connected with illuminating the ground to produce ground clutter and ground reflection interference still remain.  That is, since the beam can't be made with square corners, the simple feed horn radars have limited rate at which the shape of the beam may be cut off.  Consequently, some of the generated power illuminates the ground and generates returns from ground obstacles, such as buildings and trees, for example; and some of these ground returns bounce up in the air and interfere with direct returns causing false or inaccurate information, at times. 
To improve upon these conventional simple feed horn radar antenna systems, parabolic cylinder reflector antennas were designed and fed by a linear array of feed horns, say 30-40 feed horns, for example.  The linear array is normally driven by a power divider wherein the power is distributed across the aperture to provide a sharp cutoff in the fan beam to reduce the ground undercut of the beam.  It is understood that the same sharp cut-off in the fan beam may be achieved using a planar or cylindrical array of antenna elements thus eliminating the need for the parabolic cylinder reflector.  However, in most cases economic factors dictate, and it is less expensive to build the parabolic reflector than it is to expand the linear array into an area array of radiating elements.  A conventional two dimensional surveillance radar antenna of this type generally provides a sharp underside cutoff region of long maximum range coverage which falls off commonly as cosecant squared (CSC.sup.2) with elevation above that constant amplitude versus range coverage.  In addition, the elevation coverage may span from 20.degree.  to 80.degree.,  in general, wherein 40.degree.  may be considered a typical value.  Usually, the linear array/parabolic cylinder reflector or area array radars generate power across the elements of the array in such a fashion that the power is fairly highly concentrated in the middle of the array and has a phase center approximately at the point of maximum power concentration. 
To expand the capabilities of these array type radar antenna systems to provide height information as well as azimuth and range information, they must be adapted to generate illumination characteristics of at least two beams.  One proposed way of accomplishing this is to provide a beam forming network of microwave couplers, for example, wherein all of the elements in the array are fed with different phases.  More specifically, for a two phased beam forming network, a microwave network may be comprised of two inputs wherein one input may feed all of the array elements through the microwave network to effect a certain phase slope across the elements for directing a first beam upward and the second input may feed all the array elements to effect a different phase slope for directing a second beam downward.  Thus, the capabilities of a high and low beam are provided.  These two beam radar antenna systems are usually unattractive costwise, and, in addition, accuracy is usually limited to a certain fraction of the beamwidth.  Probably the best accuracy one might except is one tenth of a beamwidth.  That is, out of a 20.degree.  elevation beamwidth, for example, accuracy may only be good to within 2.degree.,  optimistically.  This accuracy limitation may be attributed to the fact that most two beam systems depend primarily on amplitude differences between the two beams to provide the height information of the detected target.  They normally don't have phase difference variations because the two phase slopes of the beams are normally generated with respect to the same phase center of the array. 
Another way of generating two beams in an array type of radar antenna system is to separate the array elements into two groups, one being used to form a beam upward and the other being driven to form a beam downward.  However, in this case, each of the beams will incur a loss of sharpness at the beam cutoff regions.  Moreover, each beam may lose approximately half of its power because it is being generated by only half of the array; therefore, both beams are generated at less than full strength with respect to the power capabilities of the overall array antenna system. 
From the above, it appears that in each of the proposed radar antenna systems, the adaptation for increasing its capability to provide accurate height information of a detected target has generally resulted in undesirable side effects.  Therefore, what appears to be needed is a low cost two dimensional type radar antenna system which may provide, without significant loss of illuminating power, relatively accurate target height information, while maintaining the sharp fan beam cutoff characteristics normally associated with the array type radar antenna systems.