EP0208781A1

EP0208781A1 - Visual approach slope indicating system

Info

Publication number: EP0208781A1
Application number: EP19860901575
Authority: EP
Inventors: Michael L. O'brien
Original assignee: Brien Michael L O
Current assignee: Brien Michael L O
Priority date: 1985-01-16
Filing date: 1986-01-15
Publication date: 1987-01-21
Also published as: AU5459086A; WO1986004437A1

Abstract

Un système visuel d'indication de l'inclinaison d'approche d'aéronefs comprend deux sources de lumière (12 et 16) sensiblement planes et circulaires montées l'une au-dessous de l'autre près d'une zone d'atterrissage. Les sources de lumière (12 et 16) se situent sur des plans horizontaux verticalement écartés, et leur centre géométrique se situe le long d'une ligne verticale imaginaire commune. Les sources de lumière (12 et 16) projetant deux images distinctes contre un aéronef qui s'approche pour atterrir, quelle que soit la direction d'approche de la zone d'atterrissage. La relation spatiale entre les deux images fournit une base pour déterminer la position verticale de l'aéronef par rapport à un angle spécifique d'approche (A). Les sources de lumière (12 et 16) sont montées sur une structure verticale commune de support (20) enfoncée dans le sol. Le système comprend un mécanisme d'ajustement (24) pour ajuster verticalement la position de la source inférieure de lumière (16) par rapport à la source supérieure de lumière (12) et par conséquent leur écartement vertical (S), en modifiant ainsi l'angle spécifique d'approche (A) communiqué au pilote par la relation spatiale entre les deux images.An aircraft approach tilt indication system includes two light sources (12 and 16) substantially planar and circular mounted one below the other near a landing zone. The light sources (12 and 16) are located on vertically spaced horizontal planes, and their geometric center is located along a common imaginary vertical line. The light sources (12 and 16) projecting two distinct images against an approaching aircraft to land, regardless of the approach direction of the landing zone. The spatial relationship between the two images provides a basis for determining the vertical position of the aircraft relative to a specific angle of approach (A). The light sources (12 and 16) are mounted on a common vertical support structure (20) sunk into the ground. The system includes an adjustment mechanism (24) for vertically adjusting the position of the lower light source (16) relative to the upper light source (12) and therefore their vertical spacing (S), thereby changing the 'specific approach angle (A) communicated to the pilot by the spatial relationship between the two images.

Description

VISUAL APPROACH SLOPE INDICATING SYSTEM

The present invention relates generally to external, visual aircraft guidance systems and more particularly to visual approach slope indicator

("VASI") systems for aircraft, especially helicopters.

BACKGROUND OF THE INVENTION Virtually all commercial airports require landing airplanes to approach the runway from a specified angle of approach (or angle of descent) . The required angle of approach ("approach angle") varies from airport to airport, depending upon the surrounding terrain and any potential obstructions near the flight path, and may even vary for different runways of a single airport. Such airports typically use a visual approach slope indicator (VASI) system to provide the pilot with information about the plane's approach angle during conditions of poor visibility as, for example, at night when the plane's altitude, distance from the runway and rate of descent are difficult to judge visually.

The most common if not only known VASI system includes two light units, one elevated above the other, located near the beginning and just off to the side of the runway. Each light unit has a powerful red light and white light aimed in the direction of an incoming plane. A set of louvers or blinds is mounted in front of each light at a preset angle relative to a horizontal plane. The blinds of the four lights are preset at appropriate angles such that usually only one light from the first light unit and one light from the second unit is visible to the pilot. The single visible light from the one light unit appears above the single visible light from the other light unit. Further, the angles of each set of blinds are preset such that the pilot sees a red light over a white light when the plane is aligned with the required approach angle, white light over a white light when the plane's altitude is above the required approach angle and therefore too high, and red light over a red light when the plane's altitude is below the required approach angle and therefore too low.

Although the foregoing system is basically unidirectional, this is not a problem because the runway determines the only direction from which the plane can approach the runway. (A VASI system typically would be located at each end of the runway.) The foregoing VASI system is powered by electricity. Despite the ability of helicopters to hover, unlike airplanes, the strongly preferred method of landing a helicopter is to maintain forward airspeed as the landing pad is approached, stopping the forward airspeed only when the helicopter is several feet directly above the landing pad and can be maneuvered slowly downwardly to the ground. Thus, the helicopter's landing approach is much like that of an airplane in that it maintains a desired approach angle to the landing pad. During conditions of poor visibility, most helicopter landing areas are lighted by perimeter lights. Although a pilot preferably should land the helicopter into the wind, some landing areas require an approach from a particular direction because of surrounding obstacles, such as buildings, power lines and the like. Such landing areas typically have a triangular landing "pad" of distinguishing color centrally located within the landing area. A large "H" is painted within the triangular pad such that one vertex of the triangle is above the "H". The pilot approaches the landing pad from a direction perpendicular to the side of the triangle opposite such vertex.

Occasionally, the foregoing VASI system is provided adjacent the landing pad to provide the pilot with visual information about the helicopter's position relative to the required flight path, as determined by the specified approach angle. As with airplane runways, such system is useful only if the helicopter approaches the landing pad from the direction in which the VASI system is aimed. Landings from different directions are possible only if multiple VASI systems are spaced about the perimeter of the landing pad. Sometimes, a flashing "lead-in" light is provided, typically about 1/4 mile from the landing pad on a building, to guide the pilot along a flight path that will bring the helicopter into visual contact with the VASI system and in general alignment with the required approach direction to the landing area. T e foregoing VASI system has several disadvantages. First, it is unsuited for remote locations where electrical power is not readily available. Even if such system were to be solar- or battery-powered, its reliability would be suspect because of possible dust and moisture contamination, lack of sunlight for recharging, and circuit or other failure due to severe weather conditions. Even a system electrically powered by a portable generator would be subject to power failure due to fire or other causes. Moreover, continual maintenance would be required. Reliability is an important concern because the main reason for employing a VASI system in remote locations is to permit emergency evacuations at night or during other conditions of poor visibility, whether y airplane or helicopter. Second, with respect to helicopter landings in remote and other areas where multi-directional landing approaches are possible because of the absence of nearby obstacles, the foregoing VASI system is impractical because it is one-directional. This is highly undesirable because it is much preferable to land the helicopter into the wind which could be blowing from any direction.

Third, the foregoing VASI system provides no visual clue as to how far the aircraft's position deviates from the required approach angle.

Accordingly, there is a need for an improved VASI system that is highly reliable, effective, suited for use in remote, rugged areas having severe weather conditions, virtually maintenance free, and not unidirectional.

SUMMARY OF THE INVENTION The present invention is a new VASI system that is particularly well suited for use in helicopter landings. The foregoing advantages are achieved by providing a first light source adjacent a landing area and a second light source located proximate to and below the first light source. Both light sources cooperate to project two separate images to an aircraft approaching the landing area to land, the spatial relationship of the two images providing a basis from which to determine the position of the aircraft relative to a desired flight path, as determined by a specified approach angle.

In a preferred embodiment, the light sources each comprise substantially planar light sources having a substantially circular configuration and lying substantially in horizontal, vertically spaced planes, with their geometric centers lying along a common vertical line. A mounting means supports the light sources in this relationship. Adjustment means is provided to vary the vertical spacing between the first and second light sources, and hence the image observed from any given distant point. It is therefore an object of the present invention to provide an improved VASI system that assists a pilot when landing in following a specified flight path and approach angle under conditions of poor visibility, regardless of the direction from which the landing area is approached.

Another object of the invention is to provide a VASI system as aforesaid that is reliable and effective, even in remote, rugged areas subject to severe weather conditions. A further object of the invention is to provide a system, as aforesaid, which has an effective, self-contained and long lasting power source that requires minimal maintenance.

Still another object of the present invention is to provide a system, as aforesaid, which tells the pilot not only whether the aircraft's present altitude is above or below the desired flight path but roughly how much such altitude deviates from such flight path. A further object of the present invention is to provide a system, as aforesaid, that is easily adjustable to change the desired approach angle signaled to landing aircraft.

Other objects and advantages of the invention will become apparent from the following detailed description and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 is a top plan view of a visual approach slope indicator system in accordance with the present invention.

Fig. 2 is a side view of the system of Fig. 1. Fig. 3 is an enlarged view taken along line

3-3 of Fig. 1.

Fig. 4 is a sectional view taken along line 4-4 of Fig. 3.

Fig. 5 is a diagrammatic view illustrating the visual image perceived by the pilot of an aircraft aligned with a desired approach angle.

Fig. 6 is a diagrammatic view illustrating the visual image perceived by the pilot of an aircraft positioned above a desired approach angle. Fig. 7 is a diagrammatic view illustrating the visual image perceived by the pilot of an aircraft positioned below a desired approach angle.

Fig. 8 is a diagrammatic view similar to Fig. 5 illustrating the visual image by the pilot of an aircraft aligned with a desired approach angle that is less than the approach angle of Fig. 5.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT System and Apparatus The present invention is a visual approach slope indicator (VASI) system for aircraft that is particularly well suited for use by helicopters. The system includes two substantially planar light sources, an upper light source 12 and lower light source 16, mounted one above the other proximate a landing area. In the case of a landing area for helicopters, the system is located near the perimeter of the landing pad. In the case of a landing area for airplanes, the system is located at the beginning of the runway just off to one side. Upper light source 12 and lower light source 16 lie substantially in horizontal, vertically spaced planes, with their geometric centers lying substantially along a common imaginary vertical line. The light sources project two distinct visual images to an aircraft approaching the landing area to land, regardless of the direction from which the landing area is approached. The spatial relationship of the two images provides a basis from which to determine the aircraft's vertical position relative to a descending flight path corresponding to a specified "angle of approach" or "angle of descent" to the landing area.

The term "angle of approach" or "approach angle" refers to an angle between an imaginary line corresponding to the aircraft's descending flight path (which presumably is substantially linear) and an imaginary horizontal line corresponding to ground level. In determining the correct flight path for a landing aircraft, the flight path is determined by the desired or required approach angle with its vertex at the landing area or, in the case of a long runway, at the beginning of the runway. Thus, a desired approach angle is determined with reference to the point where the aircraft should initially contact the ground. The system includes a mounting means or mounting structure 20 having one end implanted in the ground for mounting the light sources. It also includes a height adjustment means or adjustment mechanism 24 for vertically adjusting the position of the upper light source relative to the lower light source and hence the vertical spacing therebetween, thereby to change the required approach angle signaled to the pilot by the spatial relationship of the two visual images. For reasons to be explained later, when the vertical spacing between the light sources is correctly set, the approach angle corresponds to the angle "A" (Fig. 2) formed between the horizontal plane of light source 16 and an imaginary line "X" (Fig. 2) extending from the rearwardmost point of light source 16 to the forwardmost point of light source 12. Because of the system's close proximity to the landing area, line X, if extended, closely corresponds to the desired descending flight path of the aircraft.

The upper light source is mounted by bearing means for free rotation on mounting structure 20. The orientation of the upper light source is determined by a wind vane 28, rotatable with the upper light source, which responds to measurable wind by moving to a position directly downwind of the mounting structure. As shown in Fig. 1, when the wind blows from the ^• direction "W", vane 28 assumes the orientation- shown. Thus,, a helicopter pilot can use the vane to determine wind direction and preferably approach the landing pad from a direction "D" (Fig. 2) which follows the tail of the vane and hence faces the wind. Although the approach direction "D" for a helicopter preferably varies depending upon the direction of the wind, the approach direction of an airplane is always aligned with the runway. Thus, in the case of airplanes, the wind vane simply indicates the direction of the prevailing wind at the landing area.

For remote areas, the upper and lower light sources are both preferably comprised of discrete, cylindrical self-powered light units 30 connected together by a connecting means. It has been found that a light unit 30 comprised of a sealed container means filled with a radioactive gas, such as the self-luminous ISOLITE (TM) tritium light wands manufactured by Safety Light Corp., Bloomsburg, Pennsylvania, works well. Such ISOLITE light wand

(Model 2001) provides a phosphor coating on the inside of the sealed container means to interreact with the

3 tritium ( H) contained therein and requires no other power source.

Other types of light sources, such as those which are battery-powered, solar-powered or electrically-powered, will also suffice and may even be preferred, as where commercial electricity is readily available. It is not essential that the light source be comprised of multiple discrete light units, although such construction works well with tritium light wands because of their commercial availability in that form. With reference to Figs. 3 and 4, the connecting means serves to connect the individual light units 30 of the lower light source end to end to form a closed geometric figure, preferably a toroid. Because the thickness of each light source is relatively small when compared to its diameter, it is perceived from a distance as being two-dimensional and hence can be considered to be substantially circular. The connecting means serves to mount the light units in a way that permits individual replacement. The connected light units are supported by a circular support rim 32b having circumferentially spaced, upwardly projecting flanges 36.

The connecting means includes a hollow threaded bolt 40, locking nut 42 threadably received on bolt 40, threaded shank 44 rigidly secured at one end to flange 36 and threadably engaged at its other end by a threaded bore of bolt 40, and lock nut 46 threadably received on shank 44. The foregoing parts serve to threadably secure one end of each light unit 30 to a first flange 36 by taking advantage of a threaded bore in such end. The connecting means further includes a stub 48 rigidly secured to a second flange 36, which loosely extends into a threaded bore in the opposite end of the light unit. Stub 48 passes axially through a bore of a shock absorbing element 50 positioned between flange 36 and unit 30. Element 50 serves to avoid metal to metal contact between the flange and light unit and to resist axial rotation of the light unit.

An individual light unit 30 may be removed from its mounting on stub 48 and bolt 40 by loosening lock nuts 42 and 46 and unscrewing bolt 40 from its threaded engagement with the light unit. Once bolt 40 clears the threaded bore of the light unit, it is continuously threaded onto shank 44 as far as necessary to provide sufficient clearance for the light unit to be slid axially off stub 48.

The light units of the upper light source are connected together by a connecting means and mounted to a support rim 32a in the same manner as the lower light source.

The only difference between the upper and lower light sources is that the diameter of the upper light source preferably is less than the diameter of the lower light source to reduce the cost of the system and slightly reduce the amount of interference or shadow which the upper light source casts on the lower light source. Even so, light sources of equal diameter work well. For helicopter landings, light source diameters of about 7-8 feet for the lower light source and about 4-5 feet for the upper light sources work well. For airplane landings, larger diameters or light sources with more intense light signals would be required.

In addition to mounting structure 20, the mounting means includes radial support elements 54a, 54b (Fig. 1) for supporting the upper and lower light sources. Support elements 54a are rigidly secured, such as by welding, at one end to support rim 32a and at the other end to a vertical support member 58. Similarly, support elements 54b are rigidly secured at one end to support rim 32b and at the other end to a vertical support post 60. Thus, support rims 32a, 32b and support elements 54a, 54b together comprise a light source supporting means.

Support member 58 and support post 60 together comprise mounting structure 20. Support post 60 is hollow to permit support member 58 to be coaxially received within its bore.

The height adjustment means includes a crank 64 mounted to post 60 and suitable conventional gearing means (not shown) for translating rotary motion of crank 64 to rotary motion of support member 58. For example, the gearing means could comprise three bevel gears, one mounted to member 58, a second mounted to crank 64, and a third coupling the two. The height adjustment means also includes a threaded nut (not shown) rigidly and coaxially secured to support post 60, which engages and supports a threaded portion of the support member's outer surface. Thus, rotary motion of the support member about its vertical axis (imparted by crank 64) causes the support member to move either up or down relative to support post 60, depending upon the direction of rotation. This causes the vertical spacing "S" (Fig. 2) of the upper and lower light sources to change which in turn varies the angle A corresponding to the desired approach angle for reasons later apparent.

The bearing means (not shown) rotatably supporting the upper light source preferably comprises a conventional ball bearing in internal bearing contact with support member 58 and external bearing - 12 -

contact with a cup or boss rigidly secured, such as by welding, to support elements 54a. Operation and Theory

The theory and operation of the present invention is best illustrated with reference to Figs. 5-8. The described VASI system projects two distinct images to the pilot of an aircraft. The spatial relationship of the images provides a basis for determining the vertical position of the aircraft relative to a flight path corresponding to a predetermined desired or required approach angle. Thus, the pilot can determine whether the aircraft is above, below or aligned with the desired flight path, as determined by the specified approach angle. With reference to Fig. 5, "A" represents the specified approach angle (or descent angle) for the „_ aircraft, as predetermined in view of various considerations, such as surrounding neighborhoods- subject to noise, surrounding terrain, surrounding ° flight obstructions, and aircraft aerodynamics. The vertical spacing "S" between the upper and lower light sources is preset in accordance with the following formula, such that an imaginary line extending from the rearwardmost point of the lower light source to the forwardmost point of the upper light source (and hence passing through the mounting structure 20) forms an angle "A" with a horizontal plane:

S = (P_T + D?) Tan A 2 0 where D, and D are the respective diameters of the first and second light sources and "A" is the specified approach angle.

Thus, in the example of Fig. 5, when the aircraft intersects (or is aligned with) the flight 5 path corresponding to the specified approach angle, as where the pilot's line of observation (designated by the line "0" and dashed lines parallel thereto) forms an angle of observation equivalent to the specified approach angle "A", the pilot observes two distinct ellipses, one above the other, contacting each other along at least part of one side (as illustrated in Fig. 5) . As long as the two elliptical images maintain this spatial relationship (as viewed by the pilot) , the aircraft will descend precisely along the desired flight path and approach angle to the landing area.

In the example of Fig. 6, the desired approach angle is still "A" and hence the vertical spacing of the upper and lower light sources remains unchanged. However, the aircraft's observation angle "B" is greater than the desired approach angle "A", indicating that the aircraft is above the desired flight path and that its approach is either too steep or on a course that will miss the landing area. Stated differently, the aircraft needs to lose altitude at a sufficient rate to intersect with the desired flight path and then maintain the required approach angle to the landing area. In this situation, the dual images projected by the two light sources appear as two vertically overlapping ellipses. In the example of Fig. 7, the desired angle of approach is still "A" and the vertical spacing between the light sources still "S". However, the aircraft's observation angle "C" is less than the approach angle "A", indicating that the aircraft is below the desired flight path and that its approach is either too shallow or on a course that will miss the landing area. Thus, the aircraft needs to gain altitude (or at least lose altitude at a rate less than that of the desired flight path) to intersect with the desired flight path. In this situation, the dual images projected by the light sources appear as two vertically spaced ellipses.

In the example of Fig. 8, the aircraft is properly aligned with, or at least intersecting, the desired flight path. However, the specified angle of approach has been decreased from "A" (Figs. 5, 6, 7) to "A* " by reducing the vertical spacing between the light sources. Thus, the pilot still sees two ellipses, one above the other, contacting each other along a portion of one side, although the ellipses appear "flatter" because of the shallower observation angle. If the pilot follows the rate of descent prescribed by the specified approach angle, the two ellipses will maintain the same spatial relationship. However, if the aircraft deviates from the prescribed rate of descent, the images will overlap or become spaced apart, indicating that the aircraft has deviated from the desired flight path and must undergo a flight correction. it will be apparent from the foregoing that the desired approach angle can be increased simply by increasing the spacing between the two light sources, as determined by the foregoing formula. Conversely, the desired approach angle can be decreased by decreasing the vertical spacing between the two light sources.

It will also be appreciated that the present invention provides the pilot not only with information about the aircraft's altitude (or vertical position) relative to the desired flight path, as determined by the specified approach angle, but also with a rough estimate of the amount of vertical deviation from the desired flight path. The amount of vertical overlap or spacing of the two elliptical images roughly indicates how much the aircraft's present course deviates from the desired flight path. The present invention is particularly well suited for heliports and other helicopter landing sites because it provides approach angle information to all incoming helicopters, regardless of the helicopter's approach direction. Thus, the pilot can always land the helicopter facing the wind, in the absence of other constraints.

It will be appreciated that the principles of the present invention are applicable to a wide variety of light sources of different geometric shapes if the aircraft must approach the landing area from one particular direction, as in the case of an airplane, or a few limited directions. This is true because the spatial relationship of virtually any two distinct geometric shapes vertically aligned, when viewed from a distance, can provide a visual reference for determining the aircraft's present position relative to a desired flight path from at least one approach direction. For example, two vertically spaced light sources having a "square" configuration could provide consistent flight path information to an aircraft approaching from any of four approach directions, each perpendicular to one side of the square. However, light sources which are not geometrically symmetric about all axes will project at least two different images when viewed from a locus of points having the same altitude and distance from such light sources, making omni-directional approaches impractical and possibly unsafe. For unidirectional landings, the present invention can be used to tell the pilot roughly when the aircraft is a certain distance from the landing area by giving one or both of the light sources an elliptical or rectangular shape, with the shape's length or major axis extending parallel to the aircraft's direction of approach. The size, shape and spacing of the light sources can be calculated such that for a given approach angle, the elliptical or rectangular light sources will appear as a perfect circle or square when the aircraft is a predetermined distance (e.g., 3/4 mile) from the landing area.

It is not essential that the light sources lie in parallel horizontal planes. For example, for unidirectional landings the light sources can be "squares" lying in respective parallel vertical planes extending perpendicularly to and off to one side of the runway. The first light source is forward of and has its lowermost side above the second light source. The approach angle is defined by a line extending from the uppermost side of the second light source to the lowermost side of the first light source.

It will be apparent from the foregoing that the present invention is ideally suited for use in remote, environmentally hostile locations because it requires minimal maintenance, is reliable and effective, and has a long lasting power source

(tritium light wands) that is not subject to power failure, lack of sunlight and the like. Yet, in less severe environments, or where reliable commerical power is available, the system works equally well with other types of light sources.

Also, the system provides reliable, accurate approach information to aircraft approaching the landing area from any direction, making it ideally suited for use by helicopters. It provides the pilot with information as to not only the aircraft's position relative to a desired flight path but roughly how much the aircraft's position deviates from such flight path as well.

Having illustrated and described the principles of my invention with reference to one preferred embodiment, it should be apparent to those persons skilled in the art that such invention may be modified in arrangement and detail without departing from such principles. I claim as my invention all such modifications as come within the true spirit and scope of the following claims.

Claims

I CLAIM:

1. A visual approach slope indicator system for aircraft comprising: a first light source located adjacent a landing area; a second light source located proximate to and below said first light source; said first and second light sources cooperating to project two distinct images to an aircraft approaching the landing area to land, the spatial relationship of said two images providing a basis from which to determine the position of the aircraft relative to a desired flight path corresponding to a predetermined angle of approach to the landing area.

2. A system according to claim 1 including mounting means for supporting said first and second light sources such that a pilot of the aircraft perceives the two images as being substantially vertically aligned.

3. A system according to claim 2 wherein said mounting means supports said first light source such that it appears vertically higher than said second light source from a distance.

4. A system according to claim 1 including mounting means for supporting said first and second light sources, said mounting means supporting said first light source such that it appears vertically higher than said second light source from a distance.

5. A system according to claim 4 wherein said first and second light sources each have a substantially circular configuration.

6. A system according to claim 5 wherein said mounting means supports said first and second light sources such that they lie in separate horizontal, vertically-spaced planes.

7. A system according to claim 6 wherein said second light source has a diameter that exceeds the diameter of the first light source.

8. A system according to claim 6 wherein said first and second light sources are supported such that a vertical line passes substantially through the geometric centers of both said first and second light sources.

9. A method of assisting an aircraft to approach a landing area along a desired flight path corresponding to a predetermined approach angle comprising the steps of: positioning proximate the landing area a pair of light sources which project two distinct vertically aligned images to an aircraft approaching the landing area to land; and presetting the spatial relationship and relative orientation of the light sources such that from the aircraft's point of observation the two images appear: a) vertically overlapping when the aircraft's position is above the desired flight path; b) contacting one another along one side when the aircraft's position is aligned with the desired flight path; and c) vertically spaced apart when the aircraft's position is below the desired flight path.

10. A visual approach slope indicator system for aircraft comprising: a substantially planar first light source? a substantially planar second light source; mounting means for supporting said first and second light sources such that they lie in substantially horizontal, vertically spaced planes with said first light source above said second light source.

11. A system according to claim 10 including adjustment means for varying the spacing between said light sources to provide a predetermined spacing therebetween, such that for a predetermined angle of approach said light sources project two distinct vertically aligned images perceived by the pilot of an aircraft within visual range as: a) contacting one another along one side when the aircraft is aligned with the predetermined approach angle; b) being vertically spaced apart when the aircraft is below the predetermined approach angle; and c) being vertically overlapping when the aircraft is above the approach angle.

12..^' A system according to claim 10 wherein said first and second light sources each define a closed geometrical figure.

13. A system according to claim 12 wherein said first and second light sources have a substantially circular configuration, said mounting means supporting said first and second light sources such that their geometrical centers lie substantially along a common vertical line.

14. A system according to claim 13 wherein said first light source has a diameter less than the diameter of said second light source.

15. A system according to claim 13 wherein said first and second light sources each include multiple discrete light units, and further including connecting means connecting said light units together to form said geometrical figures.

16. A system according to claim 15 wherein each said light source unit comprises a sealed container means filled with a radioactive gas and internally coated with a phosphor, whereby each said light source unit is self-luminous.

17. A system according to claim 16 wherein said radioactive gas is tritium.

18. A system according to claim 13 including adjustment means for adjusting the vertical spacing between said first and second light sources by vertically moving at least one of said first and second light sources relative to the other.

19. A system according to claim 18 wherein said mounting means includes bearing means for rotatably supporting said first light source, and further including vane means radially aligned with the axis of rotation of said first light source for rotating said first light source in response to changes in wind direction.

20. A system according to claim 12 wherein said first and second light sources each include multiple discrete light units, and further including connecting means connecting said light units together to form said geometrical figures, said connecting means including hollow threaded means for threadably engaging said light unit and threaded rod means for threadably engaging a threaded bore of said threaded means, said threaded means being secured to a rigid support structure.

21. A visual approach slope indicator system for aircraft comprising: a first light source; a second light source proximate to and spaced from said first light source; and means for supporting said first and second light sources; said first and second light sources cooperating to project two distinct images in every direction; said light sources having a geometric shape and spatial relationship such that the two distinct images, when viewed from any locus of points substantially equidistant from said first and second light sources and lying in a common horizontal plane, appear to have the same geometric shape, size and spatial relationship.

22. A system according to claim 21 wherein said first and second light sources each have a substantially circular configuration and lie substantially in a horizontal plane, the geometric centers of the first and second light sources lying along a common vertical line.

23. A system according to claim 22 wherein • said second light source has a diameter that exceeds the diameter of the first 'light source.

24. A system according to claim 21 including adjusting means for vertically moving at least one of said first and second light sources relative to the other to vary the vertical spacing therebetween, whereby such spacing can be calculated for a desired approach angle such that the images projected by said first and second light sources will be perceived by the pilot of an aircraft within visual range as: a) contacting one another along one side when the aircraft is aligned with the approach angle; b) being vertically spaced apart when the aircraft is below the approach angle; and c) being vertically overlapping when the aircraft is above the approach angle.

25. A system according to claim 24 wherein said first and second light sources each have a substantially circular configuration and lie substantially in a horizontal plane, the geometric centers of the first and second light sources lying along a common vertical line.

26. In a guidance system for an operator-controlled object traveling toward a predetermined destination point and having a desired approach path leading to such destination point, the system comprising: first light source means for projecting a first closed geometric image observable by the operator; second light source means for projecting a second closed geometric image observable by the operator; the spatial relationship of said first and second closed geometric images being determined by the position of the object relative to the desired approach path, and providing the operator with an observable reference showing any deviation of the object from the desired approach path.

27. The guidance system of claim 26 wherein said first and second geometric images, as viewed by the observer, abut one another along an edge thereof when the path of travel of the object intersects the desired approach path.

28. The guidance system of claim 27, further comprising: means for causing said first and second geometric images, as viewed by the observer, to overlap one another when the object's angle of approach to the destination point is steeper than the angle of approach associated with the desired approach path; and means for causing said first and second geometric images to appear to the observer to be in spaced, non-enclosing relationship to one another when the object's angle of approach to the destination point is less steep than the angle of approach associated with the desired approach path.

29. The system of claim 28 wherein said geometric images appear to the observer to be in substantially vertical alignment one above the other.

30. The system of claim 29 wherein said first geometric image appears vertically higher than said second geometric image as viewed by the observer.

31. The system of claim 30 wherein said geometric images are ellipses.

32. The system of claim 31 wherein the major diameter of the bottom ellipse is greater than the major diameter of the top ellipse.

33. The system of claim 32 wherein the ellipses are circles.

34. in a guidance system for an operator-controlled aircraft traveling toward a predetermined landing destination and having a desired approach path leading to such destination, the system comprising: means for projecting visual images of first and second closed geometric configurations to the operator, each geometric configuration having an outer edge; and means for causing the outer edges of the geometric configurations to appear to the operator to be in substantially abutting relationship when the aircraft's path of travel at least intersects the desired approach path, thereby to provide the operator with an observable reference showing any deviation of the object from the desired approach path.

35. The system of claim 34 further comprising: means for overlapping the first and second geometric configurations, as viewed by the observer, when the aircraft's path of travel to the destination is steeper than the desired approach path; and means for causing the first and second geometric configurations to appear in vertically spaced, nonintersecting relationship to one another, as viewed by the observer, when the aircraft's path of travel to the destination is less steep than the desired approach path.