fig1 is a block diagram of the invention and various aircraft systems . shown in fig1 are display 10 , symbol generator 11 , air data computer 12 , navigation system 13 , mode panel 14 and radio altimeter 15 . in the preferred embodiment display 10 is a head - up display as shown . elements of display 10 include overhead unit 10a , brightness control 10b and combiner 1oc ( also generically referred to as display screen 10c ). the invention however , is not limited to head - up displays . the invention is adaptable to essentially any display including , but not limited to helmet mounted displays , crts displays , lcd displays and plasma displays . all of the above listed components are known in the art and will be discussed only briefly here . the invention consists of a new way to use these known components in a new way to provide a new navigation display . symbol generator 11 generates the symbology commands which control what is displayed by display 10 . in order to generate the symbology commands , symbol generator 11 communicates with various aircraft systems 12 - 15 to obtain the necessary data . air data computer 12 provides barometric altitude . navigation system 13 provides glide slope deviation and localizer deviation . mode panel 14 provides glide path angle ( gpa ), runway altitude , runway heading and course . radio altimeter 15 provides radio altitude . it is understood that many alternate embodiments are envisioned by the invention . particularly it is envisioned that equivalent data may be obtained from other aircraft systems including , but not limited to satellite landing systems , gps , microwave landing systems , inertial reference systems , flight management systems and the like . fig2 illustrates the invention in level flight as embodied on a head - up display . shown are horizon line 20 , centerline 21 , aircraft symbol 22 and lateral deviation marks 23 . horizon line 20 is indicative of the earth horizon and is known in the art . centerline 21 extends toward vanishing point 24 which is at or near horizon line 20 . vanishing point 24 is indicative of the selected course of the aircraft . centerline 21 is shown embodied as a perspective view where centerline 21 narrows as it extends toward vanishing point 24 . centerline 21 is drawn on the right side of aircraft symbol 22 thereby communicating that the aircraft is left of the desired course . as discussed above , centerline 21 is representative of a course centerline drawn on the earth and therefore is an intuitive navigation display . aircraft symbol 22 is a standard aircraft reference symbol . symbol 22 remains in essentially a fixed position on display screen loc as is standard in the art . aircraft symbol 22 does , however , rotate during banking maneuvers . lateral deviation marks 23 are located a predetermined distance from aircraft symbol 22 . deviation marks provide a reference scale for indicating either an angular or distance deviation from centerline 21 . in the preferred embodiment of an ils approach , deviation marks 23 represent one degree and two degrees of deviation from either side of the localizer signal . during cruise the deviation marks represent 1000 feet and 2000 feet of lateral displacement from the course . deviation marks 23 are shown in the preferred perspective view . in this embodiment deviation marks 23 are depicted as portions of deviation lines extending toward vanishing point 24 . to reduce clutter only the portions of the deviation lines adjacent the aircraft symbol 22 are displayed . during the approach phase of flight deviation marks are scaled to be representative of marks drawn on the earth . for example , at high altitudes the deviation marks 23 appear shorter and closer to aircraft symbol 22 . as altitude decreases the deviation marks appear to lengthen and spread out from the aircraft symbol 22 . fig3 illustrates the invention during a right banking turn maneuver . during a banking maneuver horizon line 20 tilts thereby simulating the view of the horizon during a banking maneuver . centerline 21 , aircraft symbol 22 and deviation marks 23 are each referenced to horizon line 20 and therefore tilt along with horizon line 20 . fig4 illustrates the invention approaching a runway 40 . aircraft symbol 22 remains left of centerline 21 thereby providing an intuitive indication that the aircraft is left of the desired approach path . runway symbol 40 extends toward vanishing point 24 in the same manner as centerline 21 and deviation marks 23 . runway symbol 40 differs from centerline 21 and deviation marks 23 in that the size and position of runway 40 is indicative of the location of the actual runway . it is instructive to point out that perspective depiction of runway symbol 40 is known in the art . discussion of how to draw and implement such a symbol can be found in the prior art . one reference which discusses these techniques is an article by richard s . bray , associate , nasa ames research center , entitled &# 34 ; a nasa - ames head - up display , dated jan . 10 , 1994 . it is apparent to those skilled in the art that the invention &# 39 ; s centerline 21 and deviation marks 23 symbols are implemented using the same techniques that are used in the prior art to implement perspective runway symbols such as runway 40 . modifications are made to the prior art techniques to compensate for length , width and vertical and lateral displacement of the centerline 21 and deviation marks 23 . an example of such computations is provided in fig6 a through 6b discussed below . fig5 illustrates the invention embodied in a head - up display approaching a runway . in this figure the aircraft is exactly on the approach path as indicated by aircraft symbol 22 located exactly on centerline 21 . the aircraft is 940 feet above runway altitude as indicated by radio altimeter 50 . the aircraft is also slightly above the glide slope as indicated by glide slope indicator 51 . also in this figure is pitch scale 52 . fig6 a through 6d illustrate the preferred embodiment for computing and implementing the invention . those skilled in the art can readily adapt these calculations or use substantially similar calculations for other embodiments including embodiments using satellite landing systems such as the global positioning system ( gps ) and the like . fig6 a illustrates the distance to localizer calculations used in the preferred embodiment . this calculation is straight forward using basic trigonometry as shown in the figure . the glide path angle ( gpa ) for the particular runway approach is either entered by the pilot or retrieved from a database . runway length is also either entered by the pilot or retrieved from a data base . glide slope deviation ( gs dev ) is available from the ils receiver . adding ( or subtracting ) the glide slope deviation yields the actual approach path angle of the aircraft . altitude ( h ) is available from other aircraft systems such as the air data computer or radio altitude . in the preferred embodiment , barometric altitude is used when the aircraft is more than 500 feet above the runway . a blend of barometric altitude and radio altitude is used from 500 feet to 100 feet . finally , radio altitude is used below 100 feet . fig6 b illustrates the lateral displacement distance calculations used in the preferred embodiment . localizer deviation ( loc dev ) is available from the ils receiver . the lateral displacement ( y ) is easily computed as : fig6 c illustrates the inverse slope calculations used in the preferred embodiment . calculation of inverse slope ( θ ) is straight forward as shown in the figure using the results of the previous computations . fig6 d illustrates the centerline and deviation mark coordinate calculations used in the preferred embodiment . as discussed above , these are similar to the calculations used in the prior art to implement a perspective view runway . given the coordinates of the vanishing point and the inverse slope ( computed above ), the coordinates of the centerline are easily computed using the equations shown in the figure . the depression angle top point ( datp ) and depression angle bottom point ( dabp ) are predetermined values and specify how far below the horizon line the symbol is positioned . in the preferred embodiment ( illustrated in fig5 ) the depression angles for the symbols are as follows : ______________________________________symbol datp dabp______________________________________runway 1 deg . gpacenterline gpa + 1 deg . 11 degs . deviation marks gpa + 3 . 5 deg . gpa + 4 degs . ______________________________________ the coordinates for the lateral deviation marks are computed in a similar manner except that lateral displacement is computed relative to the aircraft &# 39 ; s position . in the preferred embodiment the marks are positioned at one and two degrees of localizer deviation on each side of the aircraft . variable width ( i . e . perspective view ) centerlines and deviation marks are achieved by adding ( and subtracting ) a correction factor to the inverse slope prior to calculating the x , y coordinates . in the preferred embodiment , however , computations for the width of the deviation marks is simplified by computing coordinates for a single line for each deviation mark and merely adding ( and subtracting ) a fixed correction factor to each coordinate to yield coordinates for the marks . this also has the desirable effect of fixed width deviation marks . the method of implementing the invention follows from the description above . first , navigational or positional signals are received . typically these are ils , mls , gps , vor signals or the like . the navigational signals are interpreted yielding aircraft position signals or data indicative of aircraft position relative to a desired course . using the aircraft position signals to compute a centerline signal representative of a desired course centerline which is conformal to the earth ( or virtual earth ) surface and displaying a centerline symbol representative of the computed centerline signal on a display device . the method is enhanced by the addition of steps to display lateral deviation marks . this requires the steps of using the aircraft position signals to compute deviation mark signals representative of altitude dependent lateral deviation marks and displaying deviation mark symbols representative of the deviation mark signals on the display device . the method is yet further enhanced by steps to include perspective views of the centerline and deviation mark symbols as discussed in the above specification . this description has been for descriptive purposes only and is not intended to limit the scope of the invention . those skilled in the art recognize numerous alternate embodiments of the invention which deviate from the described embodiment but still perform the same work in substantially the same way to achieve substantially the same result and are therefore equivalent to the invention . alternate embodiments envisioned include , but are not limited to , using new electronic display devices which may be developed in the future and using the invention with various aircraft systems and navigation systems including gps . it is clear from the foregoing that the present invention represents a new and useful display symbology and apparatus for displaying information on electronic displays and the like .