[1]lir Ir 41%0 390772594 UlLiLlted States Patent Office Patented Feb. 12, 1963 2 more scatterin ,@ occurs. The scattering is of such a nature that the reflection or echo strength of a nlicrowave beam, as observed at the aircraft, decreases rapidly as the an,@le of incidence is increased. At angles of incidence of interest, between 25' and 30', the high rate of decrease of echo strength skews the frequency spectrum materially. As a result, convention al frequency tracker operation to find the spectrum center gives a result about 1% low. The principal purpose of this invention is to 10 provide means for automatica lly correcting for errors introduced by overwater operation. Instrum entation for this purpose includes a resonant frequenc y tracker of the type using a frequenc ymodulat ed heterody ning oscillator . To carry out the inven15 tion the oscillator is additiona lly freqlenc y modulat ed at a second, different modulati on rate. The frequenc y tracker filter output is detected and applied to a gated phase detector from which a signal is secured having an ariiplitud e which differs in overwat er oderatio n from overland 20 operatio n. This signal, herein termed a landwater signal, is then employe d to correct the frequenc y output of the frequenc y tracker loop. A furthe r under standi ng of this invent on may be secur ed from the detaile d descri ption and drawi n.-s, in 25 which: FIGU RE I is a block diagra m of the circuit of the inventi on. FIGUR ES 2, 3 and 7 are charaele ristic curves of Doppler spectral returns from land and waler surfaces. 30 FIGUR ES 4 and 5 are curves illustratin g resonant fr,- quency tracker operatio n. FIGU RES 6, 8 and 9 are curve s illustra ting the operat ion of the inve@ ition in disting uishin g betwe en land and water reflect ion. 35 Refer ring now to FIGU RE 1, a modul ator 11 receiv es a Doppl er signal at input 12. This sig-,ial is to be tracke d and the center freque ncy is to be measu red. The modul ator output is applie d to a filter 13 having a narro w trans missio n band, for exam ple, 200 cycles wide, center ed at 40 3 6 k c. p. s. T h e o ut p ut of th e fil te r is a p pl ie d to a n a m pl it u d e d et e ct o r 1 1 4 a n d a 2 5 c. p. s. p h a s e d et e ct o r 1 7 c o n n e ct e d in th at o r d e r. T h e p h a s e r ef e r e n c e fo r th e p h a s e d et e ct o r 1 7 is ta k e n fr o m a 2 5 c. p. s. s o u r c e te r m in al 1 8. T h e o ut p ut of p h a s e d et e ct o r 1 7 is a p pl ie d th r o u gi i a r e45 si st o r 1 9 to a n in te g r at o r 2 1, a n d th e in te , @ r at e d o ut p ut is in tu r n a p pl ie d th r o u. ,h a c o n d u ct o r 2 3 to a p r o p o rt io r.i n g ci r c ui t 2 2. T h e in st r u m e nt o ut p ut m a y b e ta k e n fr o m c o n d u ct o r 2 3, th e di r e ct p ot e r @ ti al 'L h e r e of b ei n g s u b st a nt in il y p r o p o rt io n al to th e m e di a n fr e q u e n c y of th -. D o p50 pl e r si g n al in pt it s p e ct r u m . A lt e r n at iv el y, th e m e a n o s ci ll at o r 2 7 fr e q u e n c y m a y b e m e a s u r e d. T h e i n p u t a p p l i e d t o t h e p r o p o r t i o n i p g e i r c u i t 2 2 f r o m t h e i n t e g r a t o r 2 1 c o n s i s t s o f d i r e c t c u r r e n t o f a s e l e c t e d p o l a r i t y h a v i n g a v a r i a b l e l e v e l . T h e p r o p o r t i o n i n g c i r - 55 cuit 22 is also provided witli -an alternati ng current input tpplied over conduct or 24. The funrtion of the proporti on@ing circuit is to emit at its conduct or 26 a direct currelit modulat ed by the - alterrati ng current whose amplitude is a selected constant fraction of the direct current 60 amplit@i de. This proporti oning circuit 22 is describe d in U.S. Patent No. 2,915,74 8 issued Decemb er 1, 1959. The output condii ctor 26 is conne cted to an oscilla tor 27 which is adjust able over a wide range in an,app roxim ately lin.-ar mann er under the contro l of the voltag e 65 level existing on conduct or 26. The oscillator has such frequenc y range - that when its otitput modulat es the incoming Doppler si,-nal the lower sideband can be made to have a frequenc y of 36 ke. p.s. The oscillatgr output 28 is applied as @the second or heterody ning input of 70 modulat or 11. The comp onents so far descri bed i@iclu de essent ial@@ parts of one form of freque ncy tracke r loop. In the operat ion 3,077,594 DOPPLER,@ NAVIGATOR LAIND-WATER CO-P@REICTOR M!chael W. McKay, Tarrytowi3, and Joseph Reed, New Rochelle, N.Y., assignors to General Pr-,ecision, Inc., a cor@i)oration of Delaware Filed Jan. 25, 1960, Ser. No. 4,466 9 Claims. (Cl. 343-8) This invention relates to microwave Doppler rada@r 'mstruments, and particularly to apparatus for providing automatic correction over the error introduced in such an airborne instrument when passin.- over a shore line. Doppler navigation instruments for a-ircraft employ several narrow beams of microwave radiation. These beams are directed obliquely toward the earth and, by measurement of the Doppler frequency chan-es of their echoes, the aircraffs -round track directio@l and sp,-ed can be found. The aircraft's speed may change, chan.-ing the rec,-ived Doppler frequency information. That portion of the receiver which measures the Doppler frequency must therefore be adapted to lock to the incomin.- frequency and to track it as it chan.-es. This componen, of the receiver is termed the frequency tracker, and can be constructed to measure the center freqlency of the received Doppler spectrum with an accuracy of 0.1%. The calibration constant of the frequency tracler is the ratio of the ipeasured Doppler center frequ-.ncy to the aircraft speed. For a given instrument as install-,d this rat@o is almost exactly constant whep --qyin,@ at any speed over any kind of land, at a given angle of beam incidence. However, vihen flying over waler the calibration constant is, in general, different by an amount which is of the order of 1%. Since this error is ten times the possible accuracy of the frequency traclcer, control of the error beco-mes important. The exact amount of this error in a given microwave beam depends principally on the angle of incidence of the beam at the water, and on the sea state, or roughness and shape of the water surface. When the an.-le of incidenee is fixed by the design, only the nature of the water surface affects the amolint of error. As an example of a microwave beam used in such Doppler navigation instrumenlls, a b.-am of microwave energy having a width in the direction aloithe ground tra.-k of, say, 4', is directed downward and forward frora the aircraft toward the ground track at an incident angle of 28'. The amplitude distribtition across this beam is Gaussian to a first approximation. The relation between the Doi)pler frequency in the echo and the angle of incidence, ip, is 2V fd=- sin VI (1) x in which fd is the Doppler frequency, V is tl-le aircraft velocity parallel to the ground track relative to the earth and X is the wavelength of the microwave beam. Since the path from the aircraft to the earth varies throughout the beam, being longer at the larger angles of incidence, the return signal is sli.-htly weaker at these larger a-.igles than at the smaller angles. This results in th,- echo frequency spectrum, plotted as an approximately Gaussian curve with power densities as ordinates and frequencies as abscissac, becoming skewed with the maximum moved toward the lower frequencies. This skewing occurs in all conventional Doppler radar instruments of the kind described, but is negligible in amount over land. It has been found that the reflection of a microwave beam normal to a calm water surface is almost cornplete, with little scattering. As the water surface becomes rougher and less specular for the wavelengths of interest,
[2]3,077,594 3 of this loop, a signal spec@'Lrum applied at conductor 12 has, for example, an approximately Gaussian form with a midfrequency;of 14 kc. p.s. and a width between 3 db points of 12% !of the midfrequency. The range of input center frequency is, ty@ic@Eilly, from 1.3 kc. ps. 5 to 24.kc. p.s., being proportional to the:aircraft speed. When Lhe input isignal center,frequency is 14 kc.:p.s., the average frequency iof the oscillator 27 should be 50 .,kc. p.s. except for loop error. However, the .25 c.p.s. .modulation of the direct current control of the oscilla10 -tor produces an excursion of the oseillatur frequency of -6%, or between 47 kc. p.s.. and 53 -kc.:p.s., so that the difference sideband applied to the filter -13@ -.also moves in frequency, dwelling at 33 ke. p.s. during one half-cycle of the 25 c.ps. period -and at 39 ke. @p.s. during the other 15 ,.half-cycle. Thus, when this movin,g differencesideband ,is exactly centered. about 3.6 kc. -p.s' the output of the fil,ter, at 3 6 @ ke. -p.s., has exactly.the same -amplitude during both - half-cycles, but when the sideband is @ inexactly centered the filter output varies in am plitude at the 25 20 cycle rate. This 25 c-ycle com ent, when 1,t -exists,, is Pon ampl-itude detected at detector. 17 to form a direct current err.,or signal eFT zit resistor .-19. The integr@al of this -signal:at conductor 23 is use@d to. readjust the - Dscillator 27 in such direction las to reduce the error. signal- eF.T to zero, 25 whereupon.the integrator 21 maintains the oscillator at i,ts then existing output frequency, the mean of which is exactly equal to the D(>ppler signal median frequency plus 3 6 kc. ps. The 25 c.p.s. pwver supply at terminal 19 preferably 30 has a square waveform, and such a waveform will be tassumed in descr@ibing the -operation of this - invention. A second power isppply at 125 c..P.s; is provided at terminal 29 which may have any waveshape, bu@t a sinusoidal form will be assumed,for purposes of discussion herein. The 35 tw,o power inputs are combined without intermodulation in an adder 3,1 to form a composite alternat@ing current supply in conductor 24. A 125 c.pis. phase detector 3,7, is p-hase-referenced b y the power ,Wpply energy impressed on termin-al 29 and 40 is additionally slipplied with a signal derived from the ,output,of.detector 14. The output of the phase detector 32,is gated at a 25 c.p@s. rate by a -ate 36. This.gate is controlled by the 25 -c.p.s. power imposed through conductor 37 in such a way that it is conductive during@POsi- 45 tive half-cycles of the 25 c.ps. square wave, and @nonconductive durin.-,the negative half7cycles. T-he gate 36,output is smoothed in a,low-pass filter 38@, and is then applied to one end terminal 41 of a multiplier consisting of a potentiometer 39. The other input, of the 50 niultiplier consists of a mechanical connection 42 moving t'ic slider 43. This me--hanical connection 42 connects the slider 43 to the shaft output of a pc)sition servomechanism 44 which is in turn @actuated b-y the @local osciflator output applied to conduct<)r 28. The servomechanism 55 44 is designed to position the shaft .42 at an angular de,flection which is proportional to the average,frequency, .f,, of the signal in conductor 29. The inertia of the servomechanism smooths the local oscil@lator frequency to prodtir-e a constant shaft position. 60 A servomechanism such as disclosed in U.S. @Patent No. 2,584,866 may @advanta ,geously be used for-this purpose. T-he potential -on sl@ider @43 'is applied to an - glgebraic 2adding -device which inr-ludes the several resistors, 46, 47 65 aiid 19, all connected to the input conductor 48 of the integrating aniplifier 21. This common form of -adding device is described.in the Radiation Laboratory.Series, Volume No. 21, ent-itled Electronic Ins@Lruments, on 70 page 33. The resistor 47 is connected to a potentiometer 49 having:a pdsitive potential source connect.ed t@o its terminal 51'. Since the signal -,applied. from slider 43 is -always 4 subtracting therefrom a qu!antity determined by the manual adjustment,of the knob 52. In the operation of the circuit of FIGURE 1, let it be assumed -that the Doppler signal input applied to conductor 12 is derived fr<)m a land echo. Its frequency spectrum is roughly Gaussian in form as indicated in FIGURE 2 at 53. When, on the other hand, the signal is derived from a water echo it is reduced in amplitude with the returns at frequencies bel<)w the median frequency, fn, being reduced less than are those above f,,,, resulting in the skewed shape 54. The center of area of form 54 is then lower in frequency than frequency f... When the curve 54, FIGURE.2,-is inereased7in-amplitude !as.is in fact effected by an automatic gain control circuit @precedin.- @or contained within the frequency tracker, so that the average -or peak ami)litudes are equal, -the lower-frequency side of curve 54.has less slppe than that of curve 53 wliile the higher7frequency side has a steeper sl<)pe., This is shown in FIGURE 3, in )@hich tve 4, URE 2, -amplified da hed curve - 54' is the cu 15 FIG to a peak level dqual to -that of the spectrum 53. Now @assume, as before, that the ii@put -Doppler signal at conductor 12, FIGURE 1, has-a-center frequency of ke. @p.s. 'The oscillator 27 has@lan:aver,age frequency of 50 k@c. ps. 'modulated at 25 c.p.s. vvith an excur@ion of --3 kc. p.s. so that the two lower sideband spectra applied to the filter 13 are ias shown in FIGURE 4. ihe two sidebands alternated at 25 r-.p.s. are @designated by the reference @ characters 58 and 59 w'nile the line . 6,1 represents the Mter frequenpy, the,lopp-bein.- exartly balanced. Instead of visulalizing the two Gaussian @spectra, 58 and 59, of FIGURE 4, as existing alternately on a 25 c.p.s. time-shared basis, it is convenient to draw a single spectrum, 62,,FIGURE 5, with two!-filter.positions 63 and 64 occqpied alternately in time by the filter transmission band at the 25 c.p.s. rate. In other words the sarne relations can be @visualized either from the standpoint of two spectra alternating in,frequency displacement -about a fixed frequency filter or as a single fixed -frequency spectrum with filter frequency -varying in its position with respect to the single .@pectrum. The local oscillator 27, FIGURE 1, is actually -frequency modulated at 25 and 125 cps. rates simultaneously, so that there actually exists themodulating signal depicted in FIGURE 6. Therefore the 200-cycl,e-wide band pass positions 63 and 64, FIGURE 5 are in effect moved back . and forth, each . during its %0, second time, over,the excursion caused by the @125 c.p.s. modulation and shown as heavy lines 68 and 69. .As,drawn, the graph,62.ls symmetrical, and the amplitudes traced on the ordinate scale by the excursions 68:and 69 -are identical and shown as an amplitude variation 7.1. This is the case when the return is @ from overland operation. Howev,-r, when the gr4ph 62 is, skewed.due to water.reflection, as shown to an exaggerated degree in FIGURE 7, the excursion on the -steep slope produces the amplitude variation 72 while the excursion on the lesser slope produces -only the variation.73. When the spectrum is @symmetrical and the speed changes, blit the frequency tra@cker loop has not had time to change the local oscillator frequency, the filter frequency 61, FIGURE 4,is no,lor)ger at the crossover point of forms of 59 and 58. Dqpicted as@in.FIGURE 5, the, average power de.Wities of the graph portions @68 - and 69 are :no longer -equal, and -a 25 c;ps. error signal is thereby produced. This sil-nal -.is iormed,at:the,-output of detcctor 14 and is similar to that @ shown -by -the graph of FIGURE 6. The 25 c.p.s. error:signal is applied to the 25 c.p@s. phase detcetor 17, causing @the @loop balance itself by adjusting the os-cillator 27. At the same time, the signal is applied to the:125 c.p.s. phase detector 32 and phase detection there produces a direct potential!in conductor 74 which,:for@overland op@np,gative. -the i)otentiometer -49 constitutes a @device for 75 eration, has the - Sameamplitude during both hal-ves of
[3]8,077,594 5 the 25 c.p.s. period, this amplitude being proportional to the amplitude variation 71, FIGURE 5. Half of this poten'lial is applied through the gate 36 and filter 38 to the multiplier terminal 41. One of the characteristizs of a Doppler echo signal 5 spectrl,m is that its widtli is proportional to its center frequency, and because of this the slopes of its sides are inversely proportional to its center frequency. The corrective, or land-water, signal developed in this circuit is a measure of the difi'erence in slope of one of the Dop- io pler signal spectruri sides, this difference in slope being caused by the ske,,vness of the spe.trum caused by overwater operation. It therefore is necessary to remove the chan-e in slope caused by aircraft speed change which otherwise mask the effect sought. This is done by multi- i5 plying the voltage, E, applied by the filter 33 to terminal 41, by a factor representing the aircraft speed S, this factor S bein.- the amount of angular deflection of the shaft 42. That is, 20 EkS=y (2) in wbich k is the scale factor of the servo@nechanism 44. The broduct y is a constant eL in overland operation because of the inverse relationship between E and S. 25 It is necessary in overland operation to reduce to zero the e@'fect of the constant eL at the input 48 of the integrator 21, so that in such operation the accuracy of the main frequency tracker loop will be unimpaired. This is done by adding to conductor 48 a potential equal 30 in amount and opposite in sign to the potent;al eL- In order to do this, during overland operaion the knob 52 - is adjusted to a potential c, such that the total potential e,,, appli.-d from these two sources to the inte.-rator is zero. That is, 35 eL-c=ec=O (3) -ihe only error potential applied to the in,egrator is then that applied through resistor 19 from the main loop phase detector 17. 40 When the spectrum is skewed as shown in FIGURE 7 for operation over water, the demodulated otil-put of detector 14 is as shown in FIGURE 8. During one halfcycle the 125-cycle compone-@it 67 has large amplitude -because the ampli' ude variation 72, FIGURE 7, is large, and during the remaiiider of the cycle 66, FIGURE 8, 45 the 125-cycle component is small because the variation 73 FIGURE 7, is srnall. When phase detected at deteetor ' 32, FIGURE 1, the output in conductor 74 is as ghovin in FIGURE 9, varying at the 25 c.p.s. rate. The -25 c.p.s. .-ate 36, FIGURE 1, selects the higher amplitude 50 parts of the graph of FIGURE 9, since it is conductive only during these half-cyc'es and is otherwise - non-conductive. The tesultiri.6 signal is smootlied by the lowpass filter 38 and is applied to the multiplier 39. Since thd combined outputs of resistors 46 and 47 have been 55 adjusted for zero output with the lower i-@iput from the Iz@nd signal, they iiow have oth@-r than zero output. This @CO.Istitutes an error output e.. That is (4) 60 in which e, -s the signal applied, in overwater operation, to resistor 46. The ne.-ative potential constituting signal e,, is combined in thd zidding circait with the niain loop si.-nal eFT from resistor 19, to form a co-,nposite error signal e, which is applied to the integrator 21. The Op- 65 eration of the main loop now causi-@ig the error signal e to become or approach zero, the output eFT Of phase detector 17 is found to assume some value other than zero a-@id equal, in fact, to e,, but of opposite si,@n. It follows that at null over water the avera-e freqiiency of 70 the oscillator 27 is slightly different from what it would be without the operation of the correction loop includinresistor 46. The direct potential ouii)ut oj' the instrument at conductor 23 is correspordingly differerit. -ihis difference caused by the correction loop is of such sign 75 6 and amount that the resulting output potential at conductor 23, over water, is the same as it would be in overland operation. That is, the outputs over land and over water are substantially identical. From the above it will be apparent that the apparatus of the inventor operates to measure the slope of a portion of th-. Doppler spectrum and from this measurement develoi)s an error signal which corrects for the calibration error introduced when operating over water areas. It will be apparent to one skilled in the art that the measurement of Doppier spectrum slope may also be used as an indication that the craft is passing over the shore line. Also in overwater operation the amount of Doppler spectrum slope may be utilized to indicate the sea state of the water beneath the aircraft. Still more generally, the apparatus may b-. employed to detect and to correct for any asymmetry in the Doppler frequency spectrum as applied to the f.-eqency tracker, from whatever cause arising. What is
