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US2213273A - Radio direction finding apparatus - Google Patents

Radio direction finding apparatus Download PDF

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Publication number
US2213273A
US2213273A US16919837A US2213273A US 2213273 A US2213273 A US 2213273A US 16919837 A US16919837 A US 16919837A US 2213273 A US2213273 A US 2213273A
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Prior art keywords
wave
modulating
signals
waves
means
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Earp Charles William
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International Standard Electric Corp (ISEC)
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International Standard Electric Corp (ISEC)
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S3/00Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
    • G01S3/02Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using radio waves
    • G01S3/14Systems for determining direction or deviation from predetermined direction
    • G01S3/143Systems for determining direction or deviation from predetermined direction by vectorial combination of signals derived from differently oriented antennae
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S1/00Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
    • G01S1/02Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S1/00Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
    • G01S1/72Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using ultrasonic, sonic or infrasonic waves
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S3/00Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
    • G01S3/02Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using radio waves
    • G01S3/14Systems for determining direction or deviation from predetermined direction

Description

Sept. 3, 1940. c. w. EARP RADIO DIRECTION FINDING APPARATUS Filed-Oct. 15, 1937 3 Sheets-Sheet 1 Sept 3, 1940- c. w. EARP RADIO DIRECTION FINDING APPARATUS Filed Oct. l5, 1937 3 Sheets-Sheet 2 Sept. 3, 1940. Q w EARP 2,213,273'

RADIO DIRECTION FINDING APPARATUS Filed 00t- 15. 1957 3 Sheets-Sheet 3 Patented Sept. 3, 1940 UNET Charles William Earp, London, England, assignor to international Standard Electric Corpora.-

tion, New York, N. Y.

Application ctober'15, 1937, Serial No. 169,198 in Great Britain February 23, 1937 Claims.

rihis invention relates to methods of and apparatus for giving a direct indication of the direction and/or polarisation of Waves.

An object of the invention is the provision of such a device which is more simple in construction and more eii'icient in operation than those heretofore in use.

According to one feature of the invention signals received by two or more wave collectors are modulated with different wave-forms and the resultant modulation envelopes and the original modulating Wave forms are used for operating an indicating device. The indicating device may comprise a cathode ray oscillograph or an electro-mechanical device such as a dynamometer.

According to a further feature of the invention three or more Wave collectors are provided and the signals picked up from at least two of them are modulated before all are combined, the modulation envelope of the combined signals being used together with the original modulating wave-forms for the determination 0f the direction and/or polarisation of the waves.

These and other features of the invention will be clear from the following description taken in conjunction with the accompanying drawings Figs. 1 to 4. of which show four embodiments of the invention.

Reference should rst be made t0 Fig. 1 which shows schematically the arrangement according to the invention in its most general form. An aerial system A.E.W. which gives optimum sensitivity for east-west signals picks up a signal the strength of which is proportional to cos 0 Where 0 is the bearing of the transmitting station measured anti-clockwise from east.

This signal (e cos 0) is applied to a modulating device M.E.W. where it is modulated by a Wave form f(t) generated by the modulating generator G.E.W. the output from the modulating device MEW. is be cos 0f(r'). A second aerial system A.N.S. which gives optimum sensitivity for north-south signals picks up a signal the strength of Which is proportional to sin 0, which is applied to a second modulating device M.N.S., where it is modulated by a different Wave form f' (t) by a modulating generator G.N.S. The output from the modulating device M.N.S. is ,ce sin l 0f (t). The outputs from the modulating devices M.E.W. and M.N.S. are combined in an amplifier A with the pick-up Ke from a non-directional antenna A.N.D. After detection in a detector D the output from D is the modulation envelope of the signal Ke produced by the mod- E STATES PATENT @FHQE ulated outputs from the modulating devices The modulation envelope is applied to an indicator I, of which two types are described later, to which is also applied the Wave forms fu) 5 and f(t).

envelope with fd) and f(t) in the indicator gives the. bearing of the transmitting station as will be described later.

Fig. 2 is, in principle similar to Fig. l, but the aerial systems A.E.W. and A.N.S. are each connected to a modulator ME, MW, and lVLN, lVLS of the balanced type giving true north-south and east-west symmetry of circuit. l

Fig. 3 shows a practical embodiment-of the invention. Heptode valves VE, VW, VN andr VS, respectively in balanced relation are employed in known manner as combined `modulators and low frequency oscillators, the inner elec trodes of the pairs of frequency oscillators.

valves constituting low The plates of all the four valves together with the output from a non-directional antenna A,N.D. are connected in parallel to an input of amplier A.

The resultant wave forms are indicated above the various portions of the amplier-detector circuit. In the ampliiier A, a symmetrically modulated high frequency wave is shown. AAfter detection and ltering in detector D, the low frequency wave is amplified in a low frequency amplier L F. and passed through a limiting de` vice VL which producesav square wave form, the vertical sides of which correspond to the crossing of the low frequency wave through Zero amplitude. The limiting device VL is of any suitable type and in the form shown comprises a valve provided with a high value grid leak so that positive pulses are sharply'limited by grid current and negative half cycles are limited by the bottom bend of the anode current grid vcltv age characteristic.

The peaky Wave form so produced gives an exact indication at every other instant at which the outputs from VE, VW, VN, VS total zero. It Will be noted that every successive instant is not indicated since half the the device VD.

peaks are suppressed by A comparison of the modulation v and the If, now, the wave form of one oscillator is applied to the X delector plates of a cathode ray oscillograph O, and the wave form of the other oscillator is applied to the Y deiiector plates, eaky wave form from the device VD is employed to control the brilliance of the spot, for example, by controlling the grid of the oscillograpii and causing the spot to appear only at the peaks, then a straight line from the centre oi the scale, pointing in the direction of the transmitting station, will be indicated on the oscillograph will now be demonstrated.

Referring again to Fig. 1, modulating generator G.E.W. produces a wave-form f(t). This wave form is applied as i(t) to the Y deector plates of the oscillograph causing a y or north deiiection of -f(t). Modulating generator CNS. has a wave form f(t) which is applied to the X deflector plates causing an x or east deection of f (t). The input to modulator is e cos 6 from antenna A.E.W. and fd) from generator G ELW. The output of this modulator is ,uercos 0f(t), the product of these components. Similarly the output of modulator l\/I.N.S. is c sin 0]"(t). The outputs from the modulators and lVLNS. are added together with the pick up Ke from the non-directional antenna A.'N.D. If Ke is greater than never to Zero) by ce cos 0f(t) -l-pe sin 0f(t) The instantaneous zero-amplitude of modulation of the detected and ltered signal, which is indicated at the input of tube VD in Fig. 3 by sharp peaks, occurs when ce` cos HIGH-lie sin 0f' (t) :0, i. e. when tan 6 j@ :c The y axis of the oscillograph is understood to be represented by a line on the surface of the oscillographA screen perpendicular to the Y deflector plates so that the y axis will represent the path traced by the oscillograph spot if an alternating potential is applied to the Y plates only. The :1: axis is then a similar line related to the X plates. Since the waves f, f', applied to the plates are different in wave form, for example in frequency, the pattern traced if the spot were effective at all times would vary so as to cover the face of the screen with a given pattern. Since the peaks occur at the time when the ratio w fu) and only at this time is a spot indicated on the oscillograph screen, it is clear that the screen will only be illuminated at points where the parwhich will satisfy the conditions as outlined with the structure of Fig. 3, is one in which 6=45. At the same time the peaks will occur to control the spot of light, a line will be indicated on the oscillograph at 45 to the x and y axes of the screen which corresponds to the direction of the source.

Thus, if the oscillograph operates only when the instantaneous modulation amplitude equals Zero, then the diagram will be a straight line pointing in the direction of the transmitter.

By the use of different modulating wave-forms the whole of the oscillograph screen is scanned and the direction is indicated by a line which extends to the edge of (or even beyond) the circular compass scale.

The particular method of selection of Zero amplitude of modulation shown in Fig. 3 picks out alternate zeros, and the line drawn on the oscillograph extends in one sense only from the centre of the screen, thus giving no ambiguity of sense of the signal.

If the non-directional signal Kc is omitted the envelope of the high frequency wave passed through the ampliiier A gives twice as many maxima and twice as many minima as for the case when a non-directional antenna is used and Ke is added. The production of peaks by the same method as before does not indicate zero amplitude from the modulators, but a mean amplitude. These peaks, however, may still be used to indicate direction on the oscillograph.Y In this case, the number of peaks being doubled, direction but not sense of the signal will be indicated.

As shown in Fig. 4 the coils of the low frequency oscillator modulator valves V.N.S. and V.E.W. may be connected as the ield coils FCS- FCi of a dynamo-meter indicating instrument. When the oscillator portions of V.N.S., VFW. operate at frequencies f1 and f2, then modulation of the signal is produced at each of these frequencies. 'This produces a field which varies with time since the potentials vary with time under control of f1 and f2. If, now, the output from detector D is taken to a search coil S suspended in the iield of the coils FCl-FCIL' each oscillator exerts a couple on the search coil S according to the strength of the modulation at fr (for one oscillator) and f2 (for the other oscillator). The total effective held upon the search coil, varies then not only with the impressed eld on the eld coils but also in proportion to the current in the search coil. Thus, the

direction is indicated by the resultant iield forces i in a manner analogous to that more fully explained in connection with Fig. 3. The result is that the search coil S takes up a position corresponding to the direction of the incoming signal.

It will be noted that in this arrangement the output from the detector D is applied directly to the search coil S. The deliberate distortion of the wave form employed for the oscillograph indication described with reference to Fig. 3 is not necessary. It will be noted that simple modulators are employed in Fig. ll. Although the symmetrical circuit as used in Fig. 3 is likely to give the best results in practice there is no theoretical reason why two simple modulators only should not be used. i

The invention is of course not limited to the precise arrangements described and many modications will readily occur to be versed in the art. The modulation of the signals is conveniently carried out by heptode valves, but any modulating device could be used including both electrical and mechanical methods. For electrical methods, metal rectifiers, diodes, triodes, tetrodes, pentodes, etc., are obvious solutions. The modulating wave forms are most conveniently provided by simple low frequency oscillators but any wave forms could be used. Moreover, Whilst two methods for the comparison of the modulation envelope with the modulating wave have been described numerous other methods could obviously readily be evolved.

The directional aerials A.E.'VV., ANS. may be loop aerials but larger structures could be used, such as are employed, for example, in Adcock diiection finding systems. Instead of the conventional north-south and east-west aerial systems four completely separate non-directional aerials may be used with individual associated modulators.

The aerials ABW. and ANS. are not necessarily disposed at right-angles. They have been described as being at right-angles only because it is more convenient to design an indicating device with controlling forces operating at right angles.' The loops could be arranged at any angle provided a corresponding disposition were given, for example, to the field coils FCL- FCLi in Fig. 4l.

Any type of amplier and detector may be used. The use of a single amplifier and detector is convenient but two separate amplifiers and detectors could be used, one for the north-south signal and the other for the east-west signal.

An exact balance of the two directive aerial systems is not necessary, nor an exact balance of the two modulators. A compensation for unbalance may be eiected by controls operating upon the amplitude of signals from one antenna system, sensitivity of a modulator, a relative sensitivity of north-south and east-west indications on the indicating device, or the amplitude of input to the indicator of either modulating wave forms. A Calibrating signal may be used vforI effecting a balance of the system.

Any site error or error due to unbalance in the indicating device may be compensated for as described in the preceding paragraph, by mechanical rotation or" either of the aerial systems, or by adjustment of direction of the eld coils in Fig. L or the deector plates of the oscillograph in Fig. 3. For example, the northsouth bearing is corrected on the indicating device by rotation of the east-west aerial particularly if this is a loop. Similarly the exact eastwest bearing is corrected by rotation of the north-south aerial. Any remaining errors which may be evident at NW, SVT, SE or NE may now be compensated by adjustment of one modulating wave-form input to the indicating device. When these adjustments have been made, a small octantal error, only, is possible.

The broad principle of the invention as dened by the appended claims is also applicable to systems for automatically indicating the polarisation of incoming electromagnetic waves, and for automatically indicating the direction of sound waves.

What is claimed is:

l. Apparatus for giving a direct indication of the direction of waves, comprising at least two wave collectors, means for producing two modulating waves of diierent wave form, means for modulating the signals received by one of said collectors with one of said modulating waves and the signals received bythe other of said collectors with the other of said modulating waves and for combining said modulated signals, and means controlled jointly by said combined modulated signals and said modulating Waves, and responsive to the relation between said combined modulated signals and said modulating waves for indicating the relative strengths of the signals received by said collectors.

2. A device for giving a direct indication oi the direction of incident electromagnetic waves, comprising two directional wave collectors the directions of maximum pick-up of which are arranged at an angle, means for producing two modulating waves of different wave forms, means for modulating the signals received by one of said wave collectors with one of said modulating waves and the signals received by the other of said wave collectors with the other of said modulating waves and for combining said modulated signals, and means controlled jointly by said combined modulated signals and said modulating waves, and responsive to the relation between said. modulated signals and said modulating waves for indicating the relative strengths of the signals received by said wave collectors.

3. Apparatus for giving a direct indication of the direction of incident electromagnetic waves, comprising two directional wave collectors the directions of maximum pick-up of which are arranged at an angle, means for producing two modulating waves of different wave form, means for modulatingthe signals received by one of said wave collectors with one of said modulating waves and the signals received by the other of said collectors with the signals of the other of said modulating waves and for combining said modulated signals, means for deriving a control signal from said combined modulated signals, a cathode ray oscillograph for producing Va spot of variable brilliancy and controllable position, means for controlling the brilliancy of said spot by said control signal, and means for controlling the position of said spot by said modulating waves.

4. Apparatus for giving a direct indication of the direction of incident electromagnetic waves, comprising at least two directional wave collectors the directions of maximum pick-up of which are arranged at an angle, means for producing two modulating waves of diiferent wave form, means for modulating the signals received by one of said wave collectors with one of said modulating waves and 4the signals received by the other of said wave collectors with the other of said modulating waves and for combining said modulated signals, means for deriving a control signal from said combined modulated signals, a dynamometer having field coils and a search coil movable relative thereto, means for energizing said search coil with said control signal, and means for energizing said ield coils with said modulating waves.

5. Apparatus as claimed in claim 2, further comprising a non-directional wave collector and means for combining the signals received by said non-directional collector with said modulated signals.

6. Apparatus as claimed in claim 2, wherein said means for modulating the signals received by one of said collectors with one of said modulating waves and the signals received by the other of said wave collectors with the other of said modulating waves and for combining said modulated signa-ls, comprises a balanced thermionic modulator for each of said signals to be modulated, and means for combining the outputs of said modulators.

7. Apparatus as claimed in claim 2, wherein said means for modulating the signals received by one of saidcollectors with one of said modulating waves and the signals received by the other of said wave collectors with the other of said modulating waves and for combining said modulated signals, comprises a thermionic valve: for each of said signals to be modulated, connections whereby each of said valves operates as a combined oscillator and modulator, and means for combining the outputs of said valves.

8. Apparatus as claimed in claim 3, wherein said means for deriving a control signal from said combined modulated signals comprises means for detecting said combined modulated signals, and means for producing from said detected signals a peaky wave form having peaks corresponding to alternate instants of zero amplitude of the envelope of the combined modulated signals.

9. Apparatus as claimed in claim 3, wherein said means for deriving a control signal from said combined modulated signals, comprise means for detecting said combined modulated signals, means for modifying said detected signais to produce a square wave form, means for converting said square wave form into a peaky wave form, and means for passing alternate ones only of said peaks.

l0. Apparatus for giving a direct indication of the direction of incident electromagnetic waves, comprising two directional wave collectors and a non-directional wave collector, means for producing two modulating waves of different frequency, means for modulating the signals received by one of said wave collectors with one of said modulating waves and the signals received by the other of said wave collectors with the other of said modulating waves and for combining said modulated signals, means for combining both of said modulated waves with each other and with the signals received by said nondirectional collector, and means responsive to the rela-tion between said combined signals and said modulating waves for indicating the relative strengths of the signals received by said directional wave collectors.

CHARLES WILLIAM EARP.

US2213273A 1936-11-06 1937-10-15 Radio direction finding apparatus Expired - Lifetime US2213273A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
GB207367X 1936-11-06
GB541237A GB490940A (en) 1936-11-06 1937-02-23 Improvements in or relating to radio direction finding apparatus

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US2215781A Expired - Lifetime US2215781A (en) 1936-11-06 1937-10-15 Radio direction finder and course indicating device
US2213273A Expired - Lifetime US2213273A (en) 1936-11-06 1937-10-15 Radio direction finding apparatus

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US2215781A Expired - Lifetime US2215781A (en) 1936-11-06 1937-10-15 Radio direction finder and course indicating device

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FR (1) FR828300A (en)
GB (1) GB484590A (en)
NL (2) NL49017C (en)

Cited By (22)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2415954A (en) * 1942-02-28 1947-02-18 Rca Corp Radio direction finding
US2415955A (en) * 1942-02-28 1947-02-18 Rca Corp Radio direction finding
US2419994A (en) * 1942-08-01 1947-05-06 Rca Corp Direction finding
US2422073A (en) * 1942-07-30 1947-06-10 Rca Corp Radio direction finder
US2450014A (en) * 1944-10-28 1948-09-28 Standard Telephones Cables Ltd Electronic goniometer for radio direction finders
US2468109A (en) * 1945-03-19 1949-04-26 Standard Telephones Cables Ltd Direction finder
US2475412A (en) * 1949-07-05 Torcheux
US2475612A (en) * 1945-06-27 1949-07-12 Paul G Hansel Direction finding system
US2476977A (en) * 1944-06-24 1949-07-26 Paul G Hansel Radio direction finder
US2485642A (en) * 1945-03-05 1949-10-25 Standard Telephones Cables Ltd Electronic switching system
US2494553A (en) * 1945-10-16 1950-01-17 Paul G Hansel Direction finding system
US2524768A (en) * 1944-12-16 1950-10-10 Standard Telephones Cables Ltd Radio direction finder
US2577443A (en) * 1946-12-12 1951-12-04 Hartford Nat Bank & Trust Co Radio beacon system
US2666192A (en) * 1944-07-27 1954-01-12 Frederick V Hunt Apparatus for determining the direction of underwater targets
US2883663A (en) * 1955-03-11 1959-04-21 William J Kessler Means for direction finding and ranging on atmospherics
US3007162A (en) * 1957-07-23 1961-10-31 Albert C Nolte Sr Airplane instrument approach and landing system
US3099009A (en) * 1960-02-05 1963-07-23 Collins Radio Co Automatic direction finder system
US20040032363A1 (en) * 2002-08-19 2004-02-19 Schantz Hans Gregory System and method for near-field electromagnetic ranging
US20050046608A1 (en) * 2002-08-19 2005-03-03 Q-Track, Inc. Near field electromagnetic positioning system and method
US20060132352A1 (en) * 2004-12-21 2006-06-22 Q-Track, Inc. Near field location system and method
US20060192709A1 (en) * 2002-08-19 2006-08-31 Q-Track, Inc. Low frequency asset tag tracking system and method
US8774837B2 (en) 2011-04-30 2014-07-08 John Anthony Wright Methods, systems and apparatuses of emergency vehicle locating and the disruption thereof

Cited By (26)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2475412A (en) * 1949-07-05 Torcheux
US2415955A (en) * 1942-02-28 1947-02-18 Rca Corp Radio direction finding
US2415954A (en) * 1942-02-28 1947-02-18 Rca Corp Radio direction finding
US2422073A (en) * 1942-07-30 1947-06-10 Rca Corp Radio direction finder
US2419994A (en) * 1942-08-01 1947-05-06 Rca Corp Direction finding
US2476977A (en) * 1944-06-24 1949-07-26 Paul G Hansel Radio direction finder
US2666192A (en) * 1944-07-27 1954-01-12 Frederick V Hunt Apparatus for determining the direction of underwater targets
US2450014A (en) * 1944-10-28 1948-09-28 Standard Telephones Cables Ltd Electronic goniometer for radio direction finders
US2524768A (en) * 1944-12-16 1950-10-10 Standard Telephones Cables Ltd Radio direction finder
US2485642A (en) * 1945-03-05 1949-10-25 Standard Telephones Cables Ltd Electronic switching system
US2468109A (en) * 1945-03-19 1949-04-26 Standard Telephones Cables Ltd Direction finder
US2475612A (en) * 1945-06-27 1949-07-12 Paul G Hansel Direction finding system
US2494553A (en) * 1945-10-16 1950-01-17 Paul G Hansel Direction finding system
US2577443A (en) * 1946-12-12 1951-12-04 Hartford Nat Bank & Trust Co Radio beacon system
US2883663A (en) * 1955-03-11 1959-04-21 William J Kessler Means for direction finding and ranging on atmospherics
US3007162A (en) * 1957-07-23 1961-10-31 Albert C Nolte Sr Airplane instrument approach and landing system
US3099009A (en) * 1960-02-05 1963-07-23 Collins Radio Co Automatic direction finder system
US20040032363A1 (en) * 2002-08-19 2004-02-19 Schantz Hans Gregory System and method for near-field electromagnetic ranging
US20050046608A1 (en) * 2002-08-19 2005-03-03 Q-Track, Inc. Near field electromagnetic positioning system and method
US6963301B2 (en) 2002-08-19 2005-11-08 G-Track Corporation System and method for near-field electromagnetic ranging
US20060192709A1 (en) * 2002-08-19 2006-08-31 Q-Track, Inc. Low frequency asset tag tracking system and method
US7298314B2 (en) 2002-08-19 2007-11-20 Q-Track Corporation Near field electromagnetic positioning system and method
US7414571B2 (en) 2002-08-19 2008-08-19 Q-Track Corporation Low frequency asset tag tracking system and method
US20060132352A1 (en) * 2004-12-21 2006-06-22 Q-Track, Inc. Near field location system and method
US7307595B2 (en) 2004-12-21 2007-12-11 Q-Track Corporation Near field location system and method
US8774837B2 (en) 2011-04-30 2014-07-08 John Anthony Wright Methods, systems and apparatuses of emergency vehicle locating and the disruption thereof

Also Published As

Publication number Publication date Type
NL50131C (en) grant
FR828300A (en) 1938-05-13 grant
NL49017C (en) grant
US2215781A (en) 1940-09-24 grant
GB484590A (en) 1938-05-06 application

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