turning first to fig1 a typical , ideal electronic flux gate compass produces two outputs having magnitudes v x and v y representing , respectively , in the absence of magnetic field interference , the magnitude of vector components of the earth &# 39 ; s magnetic field along the x and y axes , respectively , of a reference cartesian coordinate system 10 associated with the flux gate compass . vector components v x and v y add to form the compass output vector v m which represents the magnetic field . the angle θ m between the compass output vector v m and the x axis of the cartesian coordinate system 10 represents the angle of the flux gate compass with respect to magnetic north of the earth &# 39 ; s magnetic field . if the compass is rotated 360 degrees the compass output vector v m will trace out a locus of points 12 which forms a circle whose center is at the origin o of the cartesian coordinate system . the amplitude of the vector components v x and v y , corresponding to the respective orthogonal outputs of the flux gate compass , are shown as a function of the compass angle θ m in fig2 . in that figure the amplitude of a of those outputs may be positive or negative in magnitude . these figures assume that the flux gate compass is not in close proximity to magnetic material which would distort the local magnetic field , and that the circuitry of the compass is balanced . however , that is not ordinarily the case . it is difficult to construct a real magnetic flux gate compass whose circuits are adequately balanced within an acceptable range of tolerance and such a compass is ordinarily mounted in a vehicle which , in addition to other pieces of magnetic material near which the vehicle may be located , has its own magnetic field distortion characteristics which vary over time and circumstances . consequently , the idealized results shown in fig1 and 2 rarely occur . turning to fig3 assuming that the circuitry of the flux gate compass is balanced , distortions in the local magnetic field , ordinarily caused by hard magnetic material in the vehicle in which the flux gate compass is mounted , typically produce an offset vector v o which , in effect , produces a translated cartesian coordinate system 14 having axes x t and y t and an origin o t . the outputs of the flux gate compass comprise vector components v mx and v my , which add to form v m , but v m is not the earth &# 39 ; s magnetic field vector ; rather , it is the vector sum of the actual earth &# 39 ; s magnetic field vector v a and the offset vector v o . as the compass is rotated 360 degrees the distorted magnetic field produces a circular locus of points 16 which is centered about the translated coordinate system 14 rather than the reference coordinate system 10 , and is therefore misleading . thence , in the absence of correction , v m falsely suggests that the direction that the compass heading is represented by the angle θ m , whereas it is actually θ a . the true magnetic field vector is obtained by subtracting the offset vector v o from the output vector v m to obtain v a , which represents the actual direction θ a that the compass is headed with reference to magnetic north . the angle θ a is obtained by finding the arctangent of the ratio of the y t and x t components of v a , that is , arctangent v ay / v ax fig4 shows that , while the peak to peak amplitudes of the v mx and v my compass output components are not affected by the local distortion of the magnetic field , in the example given the average amplitude of the v mx component 18 shifts to a negative value , while the average amplitude of the v my component shifts to a positive value 20 . the vector diagram in fig5 shows that when , in addition to hard magnetic material interference , there is soft magnetic material interference or the electrical circuits of the flux gate compass are not balanced , the identification of the actual earth magnetic field intensity vector becomes more complex . in addition to the offset caused by hard magnetic material interference , the locus of points generated by the compass output is distorted to an elliptical , rather than a circular , shape by either soft magnetic interference or electrical circuit imbalance or both . to obtain the actual magnetic field intensity vector v a , v o must be subtracted from v m and the resultant vector must be normalized . fig6 shows that in this case the peak - to - peak amplitudes of v mx and v my differ . according to the present invention , these problems are corrected by the procedure described below . first , assuming that the flux gate compass is installed in a vehicle , the vehicle is rotated 360 degrees ; that is , driven in a closed loop within a predetermined area , at its initial location to obtain the maximum and minimum values for each of the x and y components of vector v m , that is : x max , x min , y max , y min . then , calibration coefficients are computed as follows : x off = the magnitude of the x axis component of v o , y off = the magnitude of the y axis component of v o , x scale = the x scaling factor by which the vector v c - v o is normalized , and y scale = the y scaling factor by which the vector v c - v o is normalized . x off = and y off are the coordinates of the center of the ellipse , i . e ., the coordinates with respect to reference coordinate system 10 of the origin o t of the translated coordinate system 14 . x scale and y scale are used to transform the ellipse back to a circle . by applying these coefficients to the flux gate compass outputs x and y before computing the heading of the vehicle in which the compass is mounted , magnetic interferences are eliminated and the &# 34 ; ideal &# 34 ; behavior of the compass is restored . they are applied as follows : x a = the calibrated x component of the compass output , and the above calibration procedure is done once at the initial installation and should remain valid if the vehicle &# 39 ; s magnetic signature does not change . in reality , the magnetic signature of the vehicle does alter over time ; that is , the ellipse shifts and changes its shape . therefore , the calibration procedure has a second part designed to provide continuous compass calibration by detecting the gradual movement of the ellipse , along with the change of its shape as the vehicle performs its daily routine . after the initial calibration , four points where the ellipse intercepts the translated axes x t and y t are established : ( x max , y off ), ( x min , y off ), ( x off , y max ), ( x off , y min ). as the vehicle travels from place to place , the x and y outputs from the compass are used along with the calibration coefficients ( x off , y off , x scale , y scale ) to produce the current heading θ m . in addition , when the output point ( x m , y m ) is close enough to any one of the four intercepts above using the following criteria : where k a and k 2 are empirically determined values that may or may not be equal , then the corresponding initial values of x max , x min , y max or y min are modified as follows : where k . sub . 1 and k . sub . 2 are empirically determined values that may or may not be equal , then the corresponding initial values of x . sub . max , x . sub . min , y . sub . max or y . sub . min are modified as follows : this is illustrated graphically in fig7 . it can be seen that x max , y max , x min and y min , represent the maximum values of the x m and y m outputs of the flux gate compass ( with respect to the reference coordinate system ) where they intersect x t and y t axes of the translated coordinate system 14 . in this calibration scheme , it is assumed that if the maximum and minimum measured values of x and y fall within a predetermined region around the current x max , y max , then a modification should be made . in fig7 rectangles 22 , 24 , 26 , and 28 represent such regions . region 22 is defined in the x direction by x max ± k 1 and y off ± k 2 . region 24 is defined by y max ± k 2 and x off ± k 1 . similarly , region 26 is defined by x min ± k 1 and y off ± k 2 . finally , region 28 is defined by y min ± k 2 and x off ± k 1 . determination of the appropriate constants for defining these areas is accomplished empirically . that is , experiments are performed to determine what constant lead to acceptably consistent , accurate , and repeatable results . once four new x max , y max , x min and y min values have been determined , new x off , y off , x scale and y scale values may be computed . preferably , that is done digitally as is represented by the weighted digital filter represented by fig8 where the constant c is a made a characteristic of that filter . such a filter can be realized by a computer program , as is commonly known in the art . ordinarily , the correction would be accomplished by a digital processor , as shown , for example , in the system of fig9 . in that system , a flux gate compass 22 and the speedometer 24 provide digitized inputs to a central processor 26 , having an operator interface , such as a keyboard 28 . the output from the central processor 26 is fed to a radio 30 which transmits speed and bearing to a central navigation monitoring station via antenna 32 . in such a system , the central processor 26 would include a computer program for performing the aforedescribed calibration technique , as shown in the flow chart of fig1 . that is , beginning at step 34 , the processor first inquires of the operator as to whether an initial compass calibration should be performed . if so , the operator drives the vehicle in a circle at a starting location . the processor reads the compass outputs x m and y m periodically , as indicated by step 36 . the maximum and minimum values of x are found in step 38 , and the maximum and minimum values of y are found in step 40 . at step 42 the operator has a choice of repeating the process or stopping and allowing the compass to be calibrated . once the initial calibration procedure is stopped , calibration coefficients x off , y off , x scale and y scale are computed in step 44 . these are used in step 46 to compute x a , y a and the heading θ a , as described above . meanwhile , the system returns to decision step 34 . in order for the system to work properly , an initial calibration must be performed at least once . thereafter , if the operator chooses at step 34 not to perform an initial calibration , the compass output values will nevertheless be read in step 48 . in step 50 , the system will determine whether the current values are within predetermined amounts of the current ( x max , y off ), ( x min , y off ), ( y max , x off ) and ( y min , x off ) intercepts . if so , then in step 52 , the corresponding maximum or minimum values are modified . if not , the modification step 52 is skipped . if the vehicle has completed a 360 degree loop , which is determined at step 54 , the new calibration coefficients are computed at step 44 . if not , the correct compass values continue to be determined using existing calibration coefficients and the entire process is started anew . the terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation , and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof , it being recognized that the scope of the invention is defined and limited only by the claims which follow .