Microwave system
SUMMARY 
     A new invention of a double sideband commutated Doppler system for landing aircraft or spacecraft is described. It is capable of providing angular information in elevation and azimuth, by the measurement of a Doppler frequency in an airborne receiver. The airborne receiver is also able to detect and correct residual multipath errors in the Doppler frequency by means of a beacon signal of known design frequency. The new microwave system is an air-derived navigation system which receives signals from a ground station transmitter and processes them in the airborne receiver. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic arrangement of the azimuth antenna configuration. 
     FIG. 2 is a schematic arrangement of the elevation antenna configuration. 
     FIG. 3 is a schematic arrangement of a ground station transmitter for use with an azimuth or elevation antenna. 
     FIG. 4 is a schematic arrangement of an airborne receiver for receiving signals from either the azimuth or elevation antennas. 
    
    
     DETAILED DESCRIPTION 
     The use of microwave frequencies for landing aircraft has several advantages. Amongst these is the possibility of reducing multipath effects due to signals being reflected from various objects as for example, ground reflection, which causes large errors at lower frequencies. Hence, a considerable amount of work has been undertaken to design a suitable microwave system to replace the present Instrument Landing System (ILS) used at various airports throughout the world. 
     The invention described here, proposes the design of a microwave double sideband commutated Doppler system for landing aircraft or spacecraft. It employs the well-known Doppler effect which is observed when a source of radiation is moved relative to an observer. At a ground station, means are provided for simulating the moving source of radiation along a set of baseline radiators and relative to two stationary reference radiators. This produces a double sideband signal in a distant airborne receiver and each sideband signal is shifted in frequency by the Doppler effect. 
     One feature of this invention relates to the use of the double sideband signal for extracting a Doppler frequency which is closely related to the angular direction of aircraft motion. The angular information in this Doppler signal is used by the aircraft for indicating its position in azimuth or elevation. 
     Another feature of this invention relates to the detection and correction of any multipath errors present in the Doppler signal. This is achieved firstly, by using bi-directional scanning with a single commutating source and a large time-bandwidth product and secondly, by the use of a beacon signal of known design value, for detecting and correcting any residual multipath errors present in the Doppler signal. 
     The new system proposes the use of a number of baseline radiators and two reference radiators as illustrated in FIGS. 1 and 2. FIG. 1 consists of a horizontal baseline of radiating elements 1 some of which are shown at a, b, g and h together with two reference radiators 2 and 3, placed one at either end of the baseline 1, the whole arrangement being suitably positioned above ground level. The polar patterns of all the radiators are designed to cover the required coverage volume which typically is ±60° in azimuth and +30° in elevation. The baseline may vary in length from about 15λ to 60λ, with elements spaced about λ/2 apart, where λ≃6 cm. for C-band operation. The distant point 4 is in the far-field of the radiators and is situated anywhere within the coverage volume. 
     FIG. 2 is a schematic arrangement of the elevation configuration at the ground station. It consists of a vertical baseline of radiating elements 5 some of which are shown at j and k, together with two reference radiators 6 and 7, placed one at either end of the baseline 5, the whole arrangement being suitably positioned above ground level 8. The polar patterns of all the radiators are designed to cover the required coverage volume which typically is ±40° in azimuth and +30° in elevation. The baseline may vary in length from about 15λ to 60λ, with elements spaced about λ/2 apart as for the azimuth configuration. The distant point 9 is in the far-field of the radiators and is situated anywhere within the coverage volume. 
     The operating principles of the system can be described with references to FIGS. 3 and 4. In FIG. 3, the baseline radiators 23 some of which are shown at l, m, r and s are energised one at a time at a microwave carrier frequency f c . It simulates a moving radiator with velocity v and the radiators are energised sequentially in a to-and-fro movement (bi-directional scanning). Means are provided to feed each radiator with energy from a common microwave oscillator 16 and amplifier 17, and the commutation is provided by an electronic switching network 18. 
     The two stationary radiators 21 and 22 which are placed one at either end of the baseline 23, are energised continuously at microwave frequencies (f c  +f o ) and (f c  -f o ) respectively, where f o  is the radio frequency of an offset oscillator 10. The reference frequencies are generated by mixing the carrier frequency f c  and the offset frequency f o  in a balanced modulator 13 and separating the two output frequencies by suitable sideband filters 11 and 12. Means are also provided for independently dithering the phase of the reference frequencies by phase shifters 14 and 15, for interchanging the reference frequencies at scan reversal (not shown) and for amplifying their power levels by amplifiers 19 and 20, so that each power level is about 3 dB greater than that of the commutated carrier level. 
     For an upscan or downscan, an aircraft at a distant point 24 in the upper sector will receive signals at frequencies (f c  -f o ), (f c  +f o ) and (f c  ±f d ) where f d  is the Doppler frequency shift due to the moving radiator with velocity v. The Doppler shift is positive for one direction of scan and negative for the opposite direction of scan and is given by ##EQU1## or 
     
         f.sub.d =K sin θ 
    
     where K=LλT s  is a design constant, L is the length of the baseline, T s  is the single scan time, λ is the wavelength and θ is the angle between the direction of motion of the aircraft and the normal to the baseline radiators. 
     In FIG. 4, the received signals 25 after amplification 26, downconversion 27, joint separation of the carrier and one reference signal 28 and joint separation of the carrier and the other reference signal 29 are then detected by linear detectors 30 and 31. The carrier signal now beats with its appropriate reference signal to yield an upper sideband signal at frequency (f o  +f d ) and a lower sideband signal at frequency (f o  -f d ) where f d  is the Doppler frequency shift which is related to the angular information θ, and it can be extracted by multiplication of the two sideband signals in a product detector 32. The output signal after low-pass filtering 33 is the Doppler signal 35 at the difference frequency 2f d  and this doubling of the Doppler frequency shift f d  leads to some bandwidth economy. 
     Since the Doppler effect is reversed in sign on scan reversal, multipath errors during an upscan tend to cancel those during a downscan. By using a single commutating source, bi-directional scanning is easily employed and by using a suitable measurement period (large time-bandwidth product), residual multipath errors in the Doppler frequency can be reduced to a low average value over several scans (multiscan averaging). 
     In addition to the two sideband signals, the airborne receiver also receives a beacon signal which is independent of its angular position. The beacon signal is due to the beat note between the two reference signals radiated from the reference radiators. It is more conveniently obtained by filtering the output from the product detector 32 by a band-pass filter 34 and the beacon signal 36 has the sum frequency 2f o , which is twice the known offset frequency f o . 
     The great importance of the beacon signal is due to the fact that it can be accurately checked against the known design value 2f o  in the airborne receiver. In the presence of multipath effects due to reflections from various objects, the Doppler frequency 2f d  will be altered by a multipath error. However, this multipath error is due essentially to the ground reflection of the stationary reference signals which interfere with the direct reference signals in the airborne receiver. It can be evaluated at the receiver by a frequency check of the beacon signal frequency 2f o . Its value will differ from the true value 2f o  by the multipath error and a correction can be applied to the Doppler frequency 2f d . 
     The combined use of a Doppler frequency signal of variable value 2f d  and a beacon signal frequency of constant value 2f o  constitutes a frequency-comparison monopulse technique whereby sufficient angular information in elevation or azimuth can be obtained by means of a single unidirectional or bi-directional scan. This unique property is due to the normalizing ability of the beacon signal to detect and correct frequency errors in the Doppler signal. Multiscan averaging is subsequently employed to reduce any residual multipath error to a negligible value.