[1]Um"ted States Patent Office 219752417 2,975,417 LONG RANGE RADIO NAVIGATTON SYSTEIM 5 Ben Alexander and Miortimer Rogoff, Nutley, and Malcolm C. Vosburgh, Montclair, N.J., gssignors to International Teleiihone and Telegr.-pit Cozporatiou, Nutley, N.J., a corporation of Maryland Ffled June 27, 1957, Ser. No. 668,525 10 11 Claims. (Cl. 343-106) This invention relates to radio navigation systems and 15 more particularly to long range type of radio navigation systems which yields bearing and distance information to a mobile craft from a ground b.-acon site. For long range navigation, particularly for guiding aircraft over routes crossing oceans, it is desirable that radio 20 beacon systems be provided which are reliable in operation so that scheduled travel may be maintained substantially all of the time. Due to long stretches of ocean between land bases, it is essential that the beacon transmissions of such systems have a range of from 1500 to 25 ZOOO miles. For the usual type of radio range beacons, operable over short or intermediate distances, it has generally been considered more suitable to use relatively short waves and broadband equipment. It has been proposed, however, that for reliability over long distances, 30 long range nav,.gation radio beacon systems be provided operating at relatively low frequencies and with relatively narrow bandwidths. Such a type of radio beacon system has been disclosed by way of example in U.S. Patent No. 2,524,765, issued to Henri G. Busignies and 35 Paul R. Adams, and U.S. Patent No. 2,510,065, issued to -H. G. Busi.-n,:es, P. R. Adams and R. 1. Colin. In such a system it has been shown that very high reliability with reasonable power consumption could be expected if the system operated in a low frequency region, for example, 40 around 100 kilocycles and utilized extremely narrow frequency bandwidths. In U.S. Patent No. 2,541,040 issued to R. 1. Colin, an improved version of this type of radio navigation system, now known as "Navaglobe," was provided which cbnsisted essentially of three t ransmitting 45 -,intennas arranged in an equilateral triangle and which were successively energized in pairs so that different di. rectional distribution of energies were produced in different angular sectors omnidirectionally about the beacon. Although extremely accurate azimuth indica- .50 tions were obtainable by using the Navaglobe signals, for relatively coarse indications, with an accuracy of l' o@- Z', the three transmitting antennas were spaced less than a half wavelength apart and successively energized cophasally in pairs with one of the antennas being ener- 55 gized to produce an omnidirectional pattern at the begi ning of each cycle to function as a synchronizing signal. When the signals were received aboard a mobile craft, the synchronizing signal served to control the distribution of the other received en,-rgies at the receiver so that 60 the successively received energies or bearing signals were applied to different windings of a three coil ratiometer. f-he ratiometer needle then assumed a position dep-.ndent tipon the resultant field in the three coils to provide the bearing of the craft with respect to the beaco-.i. 65 It has been recognized that an ideal system for position fixing is one which could provide at the craft both the distance and bearing indications with respect to a single ground station. By combining the advantages of the long 70 range bearing system known as Navaglobe with means which could provide long range distance measurements, Patented Mar. 14, 1961 2 a superior complete aerial navigation position fixing system is provided which has been generically termed "Navarlio." Such a "Navarho" system is disclosed in the copending application of M. Dishal and M. Rogoff, Serial No. 491,082, filed February 28, 1955, now Patent No. 2,924,820, issued February 9, 1960. In this copending application a radio navigation system is provided in which the transmitter transmits cyclically and successively a plurality of signals including a plurality of differently directed radiation pattern bearing signals and a synchronizing signal at a given frequency and phase. Each receiver carried by the mobile units includes means responsive to the bearing signals to provide azimuth indications, and by means of a source of stable reference signals such as obtainable from a crystal clock on board the craft and means for comparing the phase of said reference signals and the synchronizing signals from the beacon, a distance indication is obtained. While encouraging operations have been obtained ivith the Navarho system, the weakest factor appears to be a satisfactory crystal clock as a highly reliable reference signal source. An object of this invention, therefore, is to modify the Navarho system in a manner to eliminate the need of a crystal clock or the like on board the craft and yet provide a long range radio navigation system which is highly accurate in both the bearin@ and distance information obtainable on board the craft. Another object of the invention is to utilize the bearing information obtainable froni the Navarho beacon in conjunction with hyperbolic information obtainable from two beacons. Still another object is to provide an improved radio navi-ation system utilizing two or more beacon stations and a hyperbolic position determining device on board mobile craft. In order to provide long range Navarho navigation coverage throughout the important areas of the globe, a distribution of 28 to 30 Navarho beacon stations has been proposed, the separation between beacons being approximately 2000 miles. Since the range of the Navarho beacon has proven to exceed 2000 miles there is available beacon signals from at least two beacon stations for most of the areas covered and in others'signals from three or more beacon stations are available. One of the features of this invention makes use of the fact that signals froin at least two beacon stations are obtainable simultaneously in practically all areas. By means of receiving signals from two different beacons at a known distance separation, it is possible to obtain a differential distance of the mobile craft with respect to the two stations by comparing the phase of the carrier frequencies of the two stations after reductibn to a common frequency or by comparing the phase of a wave modulation imposed on the carriers. This establishes a hyperbolic line on which the craft is located with respect to the nearest of the tvio beacons. By obtaining the bearing of the craft with respect to the nearest beacon the position of the craft on the hyperbolic line is determined by the intersection of the bearing and hyperbolic lines. By means of a coordinate converter mechanism on board the craft the exact distance of the craft to the nearest beacon is obtained so that the craft may have continuous tracking in'Lormation for navigation. Another feature of the invention utilizes two hyperbolic lines determined froth information receivable from three beacons to obtain the location of the craft. By means of this embodiment the Navarho beacon bearing information is not essential although it may be used, if available, to confirm the tracking inforniation obtained from the hyperbolic indications. The above-mentioned and other features and objects of this invention wiU become more apparent by reference
[2]2,975,417 3 ,to the follov;ing description taken in - conjunction with the accompanying drawings, in which: Fig. 1 represents schematically and in block diagram -the general relation of the beacon system and a mobile .craft -carrying receiving equipment according to one enibodiment of this invention-, Fig. 2 i@llustrates the field. pattern distribution of one 'of the beacons; Fig@ 3 is a graphical representation of the signal switch,ing, cycle utilized in conjunction with the beacon field ,pattern iilustrated in Fig. 2; _ Fig. 4 is a schematic and block,diagram of the receiver equipment carried by ihe mobile craft. indica@ted in Fig. 1; Fig. 5 shows in perspective a cAm type bf c@oordinate converter mechanisms used in the receiver equipment of Fig. 4; Fig. 6 represents schematically and in, block diagram the general relation of a beacon system and hiobile craft ,capable of operating on hyperbo-lic; information obtaiii:able from three beacons; and Fig. 7 is a schematic and block diagram of the receiver equipmont carried by the mobile craft indicated in Fig. 6. Referring to Fig. 1, a radio navigaeion system in accordance with this invention is shown to comprise two ground beacon stations I and 2 spaced apart a distance D and a mobile receiver 3. While only two be.acons are shown, it will be understood that were a larger area shown in the drawing additional beacons would be included, the beac ons being distributed approximately 2000 miles apart so that aircraft flying over the area will be able to receive .signals simultaneously from two or more beacons. In @the North At-lantic area, for example, three or four Navarho stations may be received simultaneously, thus enlarging the number.of fixes that are possible by hyper,bolic lines. In the illustration of Fig. 1, beacon I is shown to be a typical Navarho beacon operating on a, carrier frequency f, while beacon 2 is shown to be a simple beacon transmitting a modulated carrier f2 for ideniification of the beacon station. The beacon 2 may be a Navarho type of beacon, should it be required for bearing information. According to the present invention, however, all beacons need not be of the Navarho type @since according to the present embodiment only one of the beacons from which sionals are received need be 6f the -Navarho type. _ The hyperbolic, lattices shown in Fig. I can be created 'from either measurements m8Lde on a coinmon modulation tone from the two stations, or by differential phase measurements made between the two received carrier @@ignals. As in the cas.- of Navarho distance measureri@ent, these two different @measurements result in a form of coarse and fine hyperbolic network. If a modulation frequency of 250 c.p.s. is used, the hyperbolic lines of position will recur every 340 miles al6ng the base line. This represents the extent of the am-biguity in the system. If carrier phase measurements are used, the lines of position will repeai every 8/10 of a mile, (@xhibitin- a much greater sensitivity of change with rbspect to motion of the observer. In the case of coarse phase measurement it is necessary to insert into the phase measuring servomechanism the known distance between the receiving equipment and the beacon from which distance is measured@ After this is done, 360' of differentia-I phase will be measured approximeitely every 0.8 mile and this will be converted into mile distance from thl. beacon@ In other words, th6 equipment will continuously track and display the correct distance from the beacon after this initial setting is made. All users of the Navarho systeni will be within the reception area of at least a pair of stations at a time. A to thirty station network over the globe assures this performance. Coverage from a pair of stations is assumed @to lie within a circle whose diameter is the ba8eline between the pair of stations. At all points within this circle the b@a-ring lin@es Will ititerse@ct hyper4 bolic lines at an angle between 45' and 90'. Ou the basis of this limitation, the twentyeight-thirty station networic proirides useful bearing and hyperbolic fl-,es within all important areas of world navigation. 5 Referring more particularly to beacon I s own in g. 1 and the -illustrations in Figs. 2 and 31 the principles of the N-,tvarho beacon wi-11 be described. The beacon is provided with three antennas 4, 5 and 6 located at the corners of an equilateral triangle@ Coupled, by means Of a 10 switch 7 is an R.-F. transmitter 8 which provides a carrier frequ-,ncy fl. The switch 7 applies the carrier frequency to the antennas in sequence as illustrated in Fig. 3. More specifically the basic transmission cycle is shown by curve ' 9, each one second period being divided into four pulses i5 10, 11, 12 and 13. The s-,vitching circuit applies one of the pulses, such as i'@3, to one of the antennas, such as 5, for synchronizing purposes. This is indicated in the tirne sequence by.curve 14. The first pulse 10 of the series is app ed to antennas 4 and 5 sim taneous t ere pro20 ducing a directive pattern, such as illustrated at 15. The pulse 11 is applied next iii sequence to antennas 5 and 6 thiis producing pattern 16. Pulse 12 is similarly appli--d to anlennas 4 and 6 in i-ts turn to produce pattern 17. Assuming that the receiving equipment 3 is in line with 25 @the pattern 16, as shown in Fig. 2, the pulse reception will be substantially as shown by curve 18 and are labeled A, B, C, and S, the si,-nal S being the synchronizing signal which is transmi.'Lted omnidirectionally by the antenna 5. The operation of the Navarho@ beacon as well as a form 30 of bearin- indicating rareiver is described in the aforementioned patent to R. 1. Colin, No. @2,541,240, aild in the aforementioned copending application of M. DishalM. Ro,-Off, Serial No. 491,082. Referring to '.qig. 4, the receiver equipment 3 is shown 35 to 21 is used iii conjunetion with the eference frequency f, of a slaved oscillator which is controlled by the second receiver 22, to establish the hyperbolic line 67 on which the craft is located. The second receiver 22 together 40 with its associated circuitry responds to the Na@,,arho bearing signals A ' B, and C, to establish the bearing or azimuth line 66 which intersects the hyperbolic I;ne 6/ at the location of the craft. With these coordinates determined, the voltages thereof are applied to a converter 25 45 to obtain the distance of the craft from the nearest beacon. The craft bearing and distance is displayed on i-@ldicators 26 and 75, 764 @ The Navarho portion of the receiver equipnient of Fig. 4 will now be described. Referring a.-ain to Fi.-. 2, 50 t@e magnitude of the transmitted beacon signals varies sinusoidally at constant radius around th,- fi.-ure-of-eight patterns for each antenna pair. Since each antenna pair axis is 120 degrees from the preceding pair, three sinusoids with 120 degrees separati6n are observed at constant 55 radius from the beacon station. This unique radiation pattern permits the use of a precisio-@l 3-phase resolver 30 at the receiver for bearing derivation. The receiver 22 is a known broadband receiver wherein the carrier f, is reduced by double conversion to 1000 60 c.p.s. and 75@O c.p.s. The synchronizing pulse S is fed t,o generator 31 to provide - gate pulses A', B', C', and S', in synchronism with the received pulses A, B, C, and S. These gate pulses are applied to a signal distributor 32 to selectively route the received bearing pulses to the proper 6. stator windings of the bearing: resolver 30. The voltage induced in the resolver rotor 33 is proportional to the sine of the rotor shaft angle. This voltage is applied to one stator winding of a square law detector 34. The detector 3@4 is a very sensitive 2-phase drag-cup motor 70 which is well known wherein its rotor displacement is proportional to the product of the applied voltages. The bearing pulses from the output of the receiver 22 are passed through an amplifiet 35 which imparts a 60-de,-ree phase shift and for pulse S eliminati.on blefore being ap75 plied 'to the quadraiure -windiiig of the drag-cup motor.
[3]2,975,417 5 The 60-degree phase difference produces optimum torque in the 2-phase drag-cup motor. If the bearing resolver rotor is initially set at the correct angle for the bearing signal ratio being received, then the detector rotor will be displaced iii one direction for the A and B pulses, but will be displaced in the opposite direction by the C pulse. The sum of the rotor motion during tL'@e A and B signals w,@'ll be equal but opposite to the motion durin-, the C signal. Hence the net displacement will be zero at the end of the pulse sequence. The rotor shaft error, zero in this example, is sensed by a servo 36 at the end of the ABC pulses, and corrects the bearing resolver rotor ar,.@le durin.- the S interval in a direction to correct the cor@iputed bearin.- angle. The action of the square IaAv detector-integrator is equivalent to vec@orial addition, and the resultant vector error is unaffected by equa@l amounts of noise power in each bearing pulse, The synchronizing gate generator 31 which is well know.i derives a one c.p.s. timing pulse by binary division from a 256 c.p.s. tuning fork oscillator, not shown. The timing pulse is coincident with the received carrier sync pulse; and if it is -iiot, shifting the phase of the tuning fork output will maintain coincidence. The A, B, and C gates are derived from the same binary divider. The flywheel action of this tuning fork oscillator allows maintenance of synchronization in the presence of heavy noise or temporary loss of signal. Since the ratio of the A, B, and C pulses is a measure of bearing, the automatic gain control for the receiver 2,2 adjusts receiver gain during the S interval only. In pi-inciple, the ABC carrier pulses are integrated to yield a D.C. volta-e proportional to signal strength. This is clone b--cause ol'L t-he nature of the transmitted pattern; the sum of the three received signals being virtually constant for all bearings frorn the transmitter. The fo.-egoing descridtion of receiver 22 and associated c-ircuitry comprises theNavaglobe portion of this invention, that is, the portion which provides for bearing inforiiiation at the craft by reception of beacon signals A, B, C, and S. The next portion to be described is the portion associated with the receiver 22 which provides a reference .-requency f, for phase comparison with the carrier frequency f2 of receiver 21. The frequencies of the crystal local oscillator 40 of receiver 22 are used to convert the irequency of slaved oscillator 41 to an I.F. frequency of i'000 c.p.s., for example. This conversion is accomplished by the action of niixers 40a and 40b. A carrier phase detector 42 provides a phase error detection at 1000 c.p.s. for application to the slaved oscillator 41 to lock t@e latter to the carrier frequency fl. The output of oscillator 41 is applied to the frequencysynthesizer 43 for derivation, by known division and single side-band techniques, of the local-oscillator frequencies for receiver 21 and a 1000 cycles per second reference frequency. The 1000 c.p.s. output of receiver 21 is compared in phase to 'che 1000 c.p.s. reference frequency in the carrier phase detector 44. This is done during the synchronizing period rf pulses by means of gate 45. The difference in phase detected is the phase difference between stations I and 2. The spa,-ing between hyperbolic lines on the base line 63 between stations is approximately equal to one-hal@f the wavelen,-th of the average of the carrier frequencies. For carriers in the 100 kc. band this is approximately eight-tenths of a n-iile. The shaft position information of the carrier phase resolver 46 associated with servo 47 and gear train 48 is coupled to a coordinate converter 25 through a synchrotransmitter 49. The coordinate converter also receives bearing inilormation f.-om the receiver 22 and its associa',ed bearing deriving circuitry. This unit converts baseline distance to actual radial distance from the; nearest beacon station such as station 1. Thus, bearing and radial distance with respect to station 1 are displayed at the indicators 26 and 75, 76. 6 It is sometimes desirable to obtain a coarse indication of distance for an initial setting of the indicators. For this purpose the circuit is provided with means to detect a relatively low frequency modulating signal with which the carrier frequencies are modulated, the low frequency modulation being in the order of 250 c.p.s. This coarse distance indication is provided by measuring the modulation phase difference. The output of the carrier phase detector 44 contains a modulating frequency component 10 such as 250 c.p.s. which is compared in phase at detector 50 with the modulation frequency output of carrier phase detector 42 to yield modulation phase difference, Modulation phase resolver 51 shifts the modulation phase of carrier f2 to equal the modulation phase of carrier fl. 15 Resolver 51 is driven by null servo 50a via gear train 50b in response to a shaft output from phase detector 50. The number of degrees of resolver rotation is a measure of distance on station-to-station baseline, with coarse spacing between hyperbolic lines of 324 miles, for a modu20 lation frequency of 250 c.p.s. The coordinate converter yields the radial distance along the actual bearing from station 1. Operation of the coarse distance indication is obtained by clutch 52 coupjina the shaft information of modulation phase resolver 51 into the coordinate convert25 er 25. When clutch 52 is engaged, clutch 53 is disengaged. This coarse indication may be made initially and then followed by a fine indication by using the carrier frequency comparison. The coordinate converter 25 may be an electronic con30 verter or preferably it may comprise a cam arrangement. In Fig. 5 a cam 60 is shown which is controlled by the bearing and carrier phase difference informations. The bearin,@ angle information controls the axial position of the cam while the carrier phase difference controls the 35 rotational position of the cam. In Fig. 5, the forward end 61 of the cam represents zero bearing with respect to the base line between stations I and 2, Fig. 1, and the portion 62 represents one-half of the distance D between the stations. In Fig. 1, 63 is the base line while line 4o 64 is the half way mark. 65 represents the location of the craft and as shown is at the intersection of radial bearing line 66 and the hyperbolic line 67. Since the distance of the hyperbola 67 at the base line is represented by the radius of circle 68, the cam must make a 45 correctio.1 proportional to the size of the bearing an,-le 0 measured from the base line. This cam variation is represented by the increasing length of the radial portion 62 as one progresses axially of the cam. At the far end of the cam the length of this radial portion is increased to 50 1.414 D 2 The cam is supported on a shaft 70 and is controlled in its rotational position by motor 71 in accordance with the 55 carrier frequency phase difference. The carriage 72 for the cam unit is caused to move axially by motor 73 is controlled by the bearing information. The distance indicator is shown to be a simple calibrated dial and pointer arrangement 75, 76. The pointer 60 76 is carried by a pin 77 which is rotatab supporte at 78 and controlled by follower 79 which en-a- s the . e curved surface 80 of cam 60. Fixes can be obtained in Navarho by the intersection of two hyperbolic lines of position. To do this requires 65 a minimum of three stations within the reception area of a receiving equipment. In many areas on a worldwide network of 28 to 30 stations it is possible to receive signals from three or more stations simultaneously; in other areas only two stations can be used. If it is desir70 able to add an auxiliary station in an area, it can be done with a relatively simple installation. This type of auxiliary station need only transmit its signals from a single antenna. Since no bearing information will be radiated it is not necessary to form directional patterns at the 75 ground stat4on. The signals that are transmitted will be
[4]2,975,417 7 a pair of stabilized frequencies that tkie separated by a frequeii I difference equal to the system modulation frecy qu,,cy. The normal time sequence of Navarho si-,nals will be used at this station; that is, the distance signals will be transmitted at a time when other stations, in the network are also transmitting distance information. The site requirements for such a stdtion are modest since only a single transmitting antenna need beaccommodated. Therefore, these stations can be installed in locations where it would not be possible to set up a complete Navarho bearin.@ plus distance transmitter. Many islands, for example, which could not cont@in an entire Navarho installation can easily accommodate this simple distanceonl@ facility. . Referring to Fig. 6, an area is shown containing three transmitting stations 101, 102 and 103, all of which may be of the Navarho type or one or two may be of the auxiliary type@ T-.he length of base lines lo@4 and 105 between stations are predetermined in the order of 2000 miles. The craft location is indicated at 106, near station 101. The nearest intersecting hyperbolic lines of the two sets of stations, station 101 being common to both, are indicated at 107 and 108. The carrier frequencies F,,, Fb a d F@ n , 6f these stations are preferably modulated with a frequency in the order of 250 c.p.s. so that coarse fixing may be obtain d as well as fine fixing. In Fig. 7 the receiver equipment of the -craft is shown. The carrier and modulation si,-nals from the three stations 101, 102 and 103, are used to establish the intersection of the t,,vo hyperbolic lines 107 and 108, for p!osition determination within their mutual co-verage area. The frequency Fa of the nearest station 101 is chosen as the reference for phase comparison. Carrier phase difference is measured between F,, and Fb, and F,, and F, at a commonfrequency,of 1000 c.p.s. derived from each carrier by double freq uency conversion. This places the aircraft on a hyperbolic line of given phase difference for each p-air of stations. The intersection of these two linesof-position indicates present positilon,. The spacings between adjacent hyperbolic lines at the base line are approximately one-half the, average wavelen,-th of the two received carriers, which is -ap-prox a mile. The 250 c@p.S. modulati6n tfrom carh station is synciironously detected to provide coarse hyperbolic linesof-position which establishes initial position if not known. The,coarse spacings Of hyperbrolic lines are equal to onehalf the wave-length at 250 c.p.s. or 324 miles. @ In the receiver equipmeni in Fig. 7, the receiver IIIL is tuned to receive the carrier F@,, from station 10 1, receiver 112 receives carrier Fb of station 102 and receiver 113 receives carrier F,, of statio@n- 103. The slaved oscillator 114 is lotked t6 a ca@riei F. by phak comparison at I 000 c.p.s. in ph@se dete6tor 115. Mixers 114a and 114b serve to b6at dowti the @ frequency output frorn oscillator 114 in two steps to 1000 cp.s. for phase coinparison with the output of recei-@er 111. The Fa-n-,Linus-250 c.p.s. carrier is also available at the output of receiver III as 750 c.p.s. This is also applied to the phase detector 115 to produce a 250 c.p.s. output for modulation phase difference measurement. The output of the slaved osoillator 114 is fed to frequency synthesizer 116 which produces by divisiori and mixing, "local-oscillator" frequencies for receivers 112 and 113 a-nd a 1000 c.p.s. "F,,," reference freque,n@y at 117. The- 1000 c.p;.s. "F,," reference is phase-compared to the "Fb" 1000 c.p.s. outp ut of. recei@er 112 at deiector 118 for F,,-@-Fb c;irri6r phase differeiice, a-@id phase comparecl tb "Fc" 1000 c.p.s. output of receiver 103 at detector 119 for F a-F c carrier phase difference. Gates 112a and 113a coupling the I ooo c.p.s. oiltputs from receiv&rs 112 aiad 113 to detectors 118 and'li9, respecfively, are c6ntiolled by signals iii time- coincidence with -"D" lyulses so that phlase c,&mp'arisons of 1000 c.p.s@ sig-@nals in detectors 119@ a-nd 119- a're m@.do during@ the syn'chroiftizitig interv@ls. - Thd 4s@bcitted cartitt ph@8e difeight-tenths of ference resolvers 120 and III insert the necessary phase shifts so as to make the 1000 c.p.s. "r-'," reference cophasal with the "Fb" and "F,," 1000 c.p.s. signals, rcspectively by the actions of null servo 120a driving resolver 120 via gear train 120b in response to the output of detector 119 and niill servo 121a drivin.- resolver 121 via gear train 121b in Tesponse, to the, output of detector 119. The shaft positions of resolvers 120 and 121 are synchrotransmitted at 122 and 123 respectively to a dual micro10 second indicator 125 which reads microseconds of titne difference @directly. 'W@.th these indications the navigator of the craft can easily plot the position of the craft. For coarse indication, the carrier phase difference detectors 118 and 119 also provide '250 c.p.s. modulation 15 phase output signals at 126 and 127 which are compared at 128 to t@he "F,," 250 c.p.s. mbdulation phase. The modulation phase difference resolver 129 inserts the proper phase angle in the "F@" 250 c.p.s. signal so as to make it co-phasal with the "Fb" or "F," 250 c.p.s. signal in the 20 modulation phase difference detector 128. This is accomplished by the action of servo 129a driving resolver 129 via gear train 129b in Tesponse to the output of detertor 128. The resolver shaft position is transmitted at 130 to the coarse microseconds indicator 1311, which 25 reads coarse microseconds difference. This display can be switched at 132 for either the "Fa"-"Fb" or "Fa"-"Fc" 250 c.p.s. phase difference. In considering all the possible tises of Navarh@o deseribed above, some general conclusions may be drawn 30 as to the utility of this system. Navarho is well suited for modem air navigation. Of all the systerns proposed, it is the only one that has successfully demonstrated the capability of providing fixing in terms of bearing and distance from a point on the ground. These pblar coor35 dinates are compatible with the common system Vortac and Tacan, short ran,- aids now being im-plemented ge , thioughout the world. Since it generates polar coordinate@s, it can share a p,osition c,omputer or an auto-navigator with these sh6rt range aids. The pilot or navigator 40 can read his position by means of a common system with the same display instruments. In its hype@rbolic form, wherein distance is measured on a differential basis, the conversion from hyperbolic lines uf positio-n to radial distances is done intemally b@ the receiver equipment with45 out knowledge or assistance required by the user. This is attributable to the fact that bearing can be measured from every Navarho ground station. Navarho, has already demonstrated its ability to measure distances at exiire-mely long ranges from the ground stations. The ex50 tension of one-way -distance measurement by means of an airborn frequency standard to differential distances measurement is a simple one and a6tually results in less com@licated equ@ipment in the aircraft. The long effective ran-ge of the Navarho system means 55 that a pair of Navarho ground stations c-an provide navigation in areas of the world where it is impossible to site the larger nuriib6r of ground stations required for short range aids. Moreover, in this era of high speed flight, the fact thdt the service area of Navarho is so extensive 60 means that it is not necessary to shift from one ground referonc6 point to another many times during a flight as would be required by shorte@ range aids. Navarho can be 'used en rdute with acc-iiracy compatible, with the needs of traffic control. In almost all cases, a flight will begin 65 and end with guidance froma short range distance-bearing or rho-theta navigation system. The transition from terminal area to en route c6ntrol can be accomplished in the airpl-ane by merely shifting from Tacan data to Navarho data. A flight then progresses with Navarho. position 70 fixing, with position known to an ac,curacy of a few minutes flying tinle thr6ughout this phase of the journey. Termination of the flight is then. made under the guidance of -Tacan in the destination a@ea. Iii those ca . es where it is desirable to compute position 75 as accurately -as p@ossible, the u@e of three Navarho ground
[5]9 gtations will provide complete hyperbolic position fixing. These three stations, with their 2000-mile baselines, provide greater sensitivity of measurement. The hyperbolic contours within the service area never diverge to more than 50% of their baseline separation. This is equivalent to a sensitivity of no greater than 11/2 miles per 360' of differential phase in any one measurement. With this degree of sensitivity equipment errors completely vanish from system accuracy considerations, and the only remaining uncertainties are those due to the vagaries of propagation. To insure high accuracy, it will be necessary to make first-order corrections to observed delay readings. These will apply to those portions of the coverage area where 'the modes of propagation are well defined. In other words, it is possible to assume that ground wave signals dominate the areas close to a transmitter and sky wave m,odes are the predominant signals further out. By making such corrections, it will be possible to predict mode delay differences to within a few microseconds, thereby retaining at least one mile accuracy in the position fix. While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims. We
