Abstract:
A velocity measuring system utilizes a pair of magnetic sensors each  moun in a displaced location with respect to the other on a vehicle whereby such sensors passing over the same stationary base location at different times provide geomagnetic field signals to a correlator circuit for producing a value for the ground speed of the vehicle.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to velocity determination by correlation of geomagnetic fields and more particularly to two magnetic sensors mounted in displaced locations on a moving vehicle each providing geomagnetic field related signals to a correlator for determining the vehicle ground speed. 
     In the field of velocity measurement, it has been the general practice to employ Doppler velocity sensor devices to provide velocity data relative to ground or water for navigation and flight control purposes. Although such devices serve the purposes, they have not proved entirely satisfactory under all conditions of service. Difficulties have been experienced such as measurement errors and frequent loss of signals when flying over water, poor dynamic response when measuring changes in velocity, and the compromising of covert activity due to detection of radiated energy. Additionally, although electromagnetic log devices used exclusively on ships and submarines provide an independent source of own ship velocity reference for damping SINS SCHELUR type oscillations, difficulties such as increased sensitivity to hydrodynamic effect, water currents, and altitude are encountered when measuring velocity with respect to water mass. 
     SUMMARY OF THE INVENTION 
     The general purpose of this invention is to provide a velocity measuring system which embraces all the advantages from similarly employed DOPPLER velocity and electromagnetic devices but possesses none of the described disadvantages. To attain this, the present invention utilizes a unique arrangement of two magnetic sensors mounted in a displaced location with respect to each other on a moving vehicle whereby each sensor passing over the same stationary base location at different times provides geomagnetic field signals to a correlator for producing a value for the ground speed of the vehicle. An object of the present invention is to provide a system having the capability of determining the ground speed of an aircraft, a ship, or submarine for navigation and fire control purposes. Another object is to provide a system for determining vehicle ground speed globally, regardless of weather conditions. A further object of the invention is to provide a system for determining vehicle ground speed without the radiation of energy therefrom and resultant compromising of vehicle covertness. Still another object is to provide a system for determining vehicle ground speed that is not susceptible to jamming. Still another object is to provide a system for determining vehicle ground speed without knowledge of actual magnetic fields or mapped storage thereof. Yet another object of the present invention is to provide a system for determining vehicle ground speed which is not affected by temporal or spatial variations in magnetic field. 
     Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the side view of an aircraft showing the displaced locations of a pair of magnetic sensors employed in accordance with the present invention; 
     FIG. 2 is a block diagram of a preferred embodiment of the invention; 
     FIG. 3 is a drawing of a stationary sample of a geomagnetic field signal A(t) from a first sensor and a delayed geomagnetic field signal B(t) from a second magnetic sensor of the invention as shown in FIG. 2; and 
     FIG. 4 is a cross correlation function indicating the measure of similarity between signals A(t) and B(t) of the invention as shown in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1, an aircraft 10 having magnetic sensors 12a and 12b positioned a longitudinal distance D apart from each other on aircraft 10. The magnetic sensors 12a and 12b are mounted apart so that each passes over the same-earthed-based location at different times. Earthed-based locations such as location 14 of the terrain shown in FIG. 1 provide geomagnetic fields FMt 1  and FMt 2 , consisting essentially of an internal field Fi generated in the core of the earth and anomalous fields Fa arising from inhomogeneities in the outer earth layer. Correlation of the two signals FMt 1  and FMt 2  as received from the earthed-based location 14 produces a value for the ground speed of aircraft 10. 
     Referring to FIG. 2, magnetic sensors 12a and 12b are similar magnetometers suitable for airborne applications such as a flux gate, proton precession, or optical absorption magnetometers having the capabilities of providing respective sensitivities in the range of 0.01 to 1.0 gamma. Magnetic sensor 12a mounted in a forward area of aircraft 10 receives magnetic field FM at time t 1  and provides an analog signal A t  indicative thereof. Sensor 12b mounted in an aft area of the aircraft at a fixed longitudinal distance d from sensor 12a receives the magnetic field FM at time t 2  and provides an analog signal B t  indicative thereof. Analog to digital converters 14a and 14b are connected to receive instantaneous values of signals A t  and B t  and a convert signal C for respectively providing in each conversion period T digitized signals A n  and B n  comprising values thereof in incremental form. Signals A n  and B n  are data samples of the input signals FMt 1  and FMt 2  respectively and are each comprised of twelve parallel bits. A controller 13 is connected to provide a shift signal S comprising a pulse having a repetition rate of 100 hertz and convert signal C having a repetition rate of 50 hz. Storage registers 15a and b are connected to receive signals A n  and B n  respectively for providing storage thereof until the succeeding convert signal C is received. Registers 15a and b are refreshed with new input data with each signal C. A shift register 16 is connected to receive signal A n , move it to the right as a function of signal S and to provide signal A n-s . Signal A n-s  is indicative of signal A n  delayed by a period t equivalent to the number of shifts that have occurred prior to achieving maximum correlation. A multiplier circuit 20 of conventional design is connected to receive signals A n-s  and B n  for providing the product signal A n-s  B n . An averager circuit 22 of conventional design is connected to receive product signal A n-s  B n  to provide a signal D indicative of the similarity or cross-correlation of signals A n-s  and B n . Signal D is mathematically stated as: ##EQU1## 
     An envelope detector circuit 24 of conventional design is connected to receive signal A n-s  B n  and to provide a signal E indicative of the envelope or amplitude level of signal A n-s  B n . A threshold and peak detector circuit 26 of conventional design is connected to receive signal E and signal D and to provide signal F indicative of the maximum similarity or maximum correlation of geomagnetic signals FMt 1  and FMt 2 . A microprocessor 28 of conventional design for storing and utilizing a statistical filter program of conventional design is connected to receive signals F and S for providing signal G indicative of the ground velocity of the moving aircraft. As a result of noise and randomness of the input signals FMt 1  and FMt 2 , signal F is caused to vary necessitating the statistical filter program to determine by statistical computations the true maximum similarity of signals FMt 1  and FMt 2 . Within microprocessor 28 the number of shifts, which occur between two successive convert signals C are determined by a process of counting and storing shift signals S. The number of shift signals S corresponding with true maximum similarity signal F from the statistical filter is the true delay period between signal FMt 1  and FMt 2 . The true delay period is divided into a stored constant d equal to the physical distance between sensor 12a and 12b resulting in the true velocity therefore ##EQU2## 
     In summary, operation of the present invention is explained with reference to FIGS. 1-4. The magnetometers 12a and 12b, are mounted in the fore and aft positions of aircraft 10 at the center maneuver axis of the aircraft&#39;s magnetic centroids to minimize magnetic noise. During normal flight the magnetic sensor 12a at a time t 1  receives a terrain generated magnetic anomaly providing analog signal A t . At a later time t 2 , depending on the speed of the aircraft, sensor 12b receives the magnetic anomaly providing analog signal B t . Signal A t  and B t  are converted to digitized signals A n  and B n  respectively. Signal A n  is delayed through a shift register 16 prior to being provided to multiplier circuit 20 whereas signal B n  is directly applied to multiplier circuit 20 20. The output of multiplier 20 is averaged by averager circuit 22 and envelope detected by envelope detector 24. At the peak of the detected envelope the average signal F is provided to microprocessor 28 for processing by a statistical filter program residing therein. Microprocessor 28 also receives signal S from controller 13 and provides signal G to display the ground velocity of the moving aircraft 10. 
     Therefore, some of the many advantages of the present invention should now be apparent. A magnetic velocity measuring system comprising a pair of magnetic sensors separated by a fixed distance and a signal correlator to measure the time difference between the signals from the sensors provides velocity of the true ground speed of the aircraft. The system provides global all weather speed over ground of the moving aircraft. No radiation is emitted from the system and therefore it is covert and unjammable. The system utilizes magnetic gradiometers. There is no requirement for prior knowledge of actual magnetic fields and no map storage. The system is not affected by temporal or spatial variations in magnetic field. The present invention is reliable and easily adapted for use in combination with other magnetic systems presently used on military aircraft. 
     Obviously, other embodiments and modifications of the present invention will readily come to those ordinarily skilled in the art having the benefits of those teachings presented in the foregoing teachings. It is therefore to be understood that various changes in the details, materials, steps, and arrangement of parts, which have been described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principal and scope of the invention as expressed in the appended claims.