Abstract:
A system involving a mechanically or electronically scanned beam of electromagnetic energy and separate antenna means for transmitting a phase reference signal. A remote receiving station, such as on-board an aircraft on landing approach, is able to process received signals coherently to take advantage of the increased accuracy available in such processing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to electronic aids to air navigation, and more particularly, to the so-called scanning-beam microwave landing system. 
     2. Description of the Prior Art 
     The known conventional time-reference scanning-beam microwave landing system includes transmitting apparatus and a mechanically or electrically scanned antenna arranged to scan a single frequency, collimated beam throughout an angular sector of interest. The time of reception of the beam with respect to a reference pulse gives angular information, and the detected signal at the remote receiving station (such as for example, in the aircraft on landing approach) of the system is related directly to the received power radiating from the scanning beam antenna and as such, it is incoherent or non-coherent in nature. The use of coherent signal processing techniques, which may be required or highly desirable to achieve the required accuracies in difficult situations, for example, in respect to low angle coverage in the elevation plane, is not possible. Coherent detection is usually associated with &#34;two-way&#34; radar systems, and in such devices it is possible to use means to &#34;remember&#34; (store) the transmitted phase while waiting for an echo signal which may then be processed against the remembered (stored) transmitted phase. One way of &#34;remembering&#34; the transmitted phase in such 2 way arrangements is to heterodyne-down the microwave transmitted signal and use it to lock-in a coherent oscillator (coho) usually in the IF domain. Received echo signals are then reduced to the IF domain and are coherently (phase) detected. 
     The text Radar Handbook by Merrill I. Skolnik (McGraw Hill 1970), particularly Chapter 17 thereof, provides a background in connection with coherent detection, FIG. 3 of that chapter outlining the concept in a straightforward prior art radar system. 
     Since the so-called scanning-beam landing system is a &#34;one-way&#34; system for providing air-derived angle data using the ground scanning transmission as a beacon, the usual technique of simply &#34;remembering&#34; the transmitted phase, while waiting for a processable echo signal, is not available. 
     The transmitted beam mechanical scanning techniques and also the electronic (inertialess) scanning techniques (such as in connection with phased-array scanning) are prior art, per se, as they may be incorporated in the combination of the present invention. The aforementioned Radar Handbook test is also a useful reference in connection with the various known forms of beam scanning, as may be employed herewith. 
     The manner in which the present invention provides the capability for coherent airborne signal processing will be understood as this description proceeds. 
     SUMMARY OF THE INVENTION 
     According to the invention, a transmitter and scanning antenna arrangement provides for the scanning of a predetermined angular (service) sector of space with a single-frequency collimated radio beam. Separate means including a separate antenna for illuminating at least the aforementioned service sector with a reference signal of frequency offset from the scanning beam frequency, is also provided. From the received signals the remote receiving station, i.e., the aircraft on landing approach, can detect both frequencies and using the reference transmission as a phase reference, can coherently process the scanning beam received signal. 
     According to the invention, the reference signal may be transmitted from a separate radiator, co-sited with the mechanically or electronically scan antenna, or if a phase-array antenna is being used to produce the scanning beam, an element therefrom may be reserved for the transmission of the reference signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts the combination of the present invention in connection with a mechanically scanned antenna arrangement, and also shows a remote receiving station (airborne) in block diagram form. 
     FIG. 2 depicts an alternative scanning antenna in the form of a phased array for the same purpose as FIG. 1, it being understood that the airborne receiving station of FIG. 1 would be employed in connection therewith. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a typical mechanically scanned dish-antenna and feed are shown at 1 and 9, respectively. A sector scanning effect is produced by physically oscillating the dish 1 about the point 8, as depicted by the rotational arrow 2, it being understood that the mechanical drive 7 provides this motion in the well known manner. An RF signal generator 5 generates the signal f c  which is power amplified in 6 and applied through the feed 9 to illuminate the dish 1, thereby transmitting the collimated beam aforementioned. The signal f c  is also applied to an offset reference frequency generator 4 which offsets f c  by f o  to produce the reference frequency f r . The device 4 may be, for example, a beat-type device which mixes f c  with a fixed stable source of f o , thereby obtaining f c  ± f o , one of these sum and difference frequencies being readily discarded by filtering and the other being passed on to illuminate the reference radiator 3, as illustrated. 
     It will be understood that the use of power amplification at 6 is shown by way of example only. Whether or not such an expedient is required will depend on the signal level generated in 5. In any event, the device 4 may be expected to operate satisfactorily at a relatively high or relatively low power level depending on the details of design and it is to be understood that power amplification between 4 and the reference radiator 3 may be applied as an ordinary engineering matter. The remote receiving station (airborne) comprises the receiving antenna 16 which accepts both the reference from radiator 3 and the swept-collimated beam from 9 and 1. The receiver 15 is shown symbolically, it being understood that it is of a type which will beat the two received signals f c  and f r  to produce a video signal of average frequency f o , time-modulated by the scanning beam shape. The instantaneous phase of the beat signal at any point in time depends on the phase relationship between reference and beam, thus providing a coherently detected beam signal. The coherent signal processor 17 may be an MTI canceller of the known prior art, or may be some other form of video signal processing to provide an output at 18. The exact form of coherent signal processing is not a part of the present invention, per se, it being recognized by those skilled in this art that this additional processing may be accomplished in accordance with the particular objective to be accomplished. 
     One important characteristic of the above-described coherence is that it provides a means in the receiver of separating the contributions of difference parts of the transmitting antenna&#39;s aperture, since on a frequency basis, the spectrum of the beat envelope [(f o  - 1/τ) to (f o  + 1/τ, where τ is the beam dwell time] is composed of variously Doppler-shifted contributions corresponding to different regions of the aperture. 
     These separated contributions all contain phase and amplitude information, and allow unrestricted receiver (airborne station) tapering of the antenna aperture. 
     Referring now to FIG. 2, a phased array of elements, typically 10, with associated, controlled, phase shifters in 11, provides an alternative means for transmitting the aforementioned collimated beam at the frequency f c . A separate reference antenna 12 for transmitting the frequency f r  offset in the same way and by the same process typically, as in connection with FIG. 1, by the offset reference generator 13, produces the identical frequency relationships described in connection with FIG. 1. In this case, the power amplification of the output of the RF signal generator feeding the phase shifters of the phased array, has been omitted, however, it is to be understood that the same considerations apply in respect to power amplification and signal levels as were discussed in connection with FIG. 1. 
     From an understanding of the foregoing, those skilled in this art may make modifications or variations falling within the spirit of the invention and accordingly, it is not intended that the drawings or this specification should be regarded as limiting the scope of the invention. The drawings and this description are to be regarded as typical and illustrative only.