Title:
ATTITUDE AND FLIGHT DIRECTOR DISPLAY APPARATUS UTILIZING A CATHODE-RAY TUBE HAVING A POLAR RASTER
United States Patent 3605083


Abstract:
Apparatus for providing a contact analog display on a cathode-ray tube representing the horizon, ground plane grid lines in perspective, and a runway or other flight path reference in perspective, by intensity modulating the electron beam of the cathode-ray tube while it traces out a spiral raster. Center reference indicia and flight director bars may be generated during the retrace time of the raster.



Inventors:
KRAMER MELVIN G
Application Number:
04/864815
Publication Date:
09/14/1971
Filing Date:
10/08/1969
Assignee:
SPERRY RAND CORP.
Primary Class:
Other Classes:
340/975, 348/116, 348/117
International Classes:
G01S19/15; (IPC1-7): G08G5/02
Field of Search:
178/6
View Patent Images:
US Patent References:
3037382Visual contact analog1962-06-05Aid et al.
2967263Simulated ground display1961-01-03Steinhauser
2581589Position indicating system1952-01-08Herbst



Primary Examiner:
Claffy, Kathleen H.
Assistant Examiner:
Black, Jan S.
Claims:
I claim

1. In attitude display apparatus for a movable craft,

2. In apparatus of the character recited in claim 1 including

3. In apparatus of the character recited in claim 1 including

4. In apparatus of the character recited in claim 1 including

5. In apparatus of the character recited in claim 1 including

6. In apparatus of the character recited in claim 2 including

7. In apparatus of the character recited in claim 1 in which said polar raster generating means includes square wave generating means for providing a square wave voltage signal,

8. In apparatus of the character recited in claim 7 including

9. In apparatus of the character recited in claim 2 in which said horizon and roll-generating means includes

10. In apparatus of the character recited in claim 9 in which said summing means responsive to said pitch attitude signals includes means for shifting said visible half of said raster up or down to present aircraft pitch attitude.

11. In apparatus of the character recited in claim 3 in which said runway perspective generating means includes

12. In apparatus of the character recited in claim 3 including

13. In apparatus of the character recited in claim 4 in which said ground plane perspective generating means includes

14. In apparatus of the character recited in claim 13 in which said ground plane perspective generating means includes

15. In apparatus of the character recited in claim 14 in which said ground plane perspective generating means includes

16. A contact-analog attitude display apparatus for navigable craft comprising:

17. The apparatus as set forth in claim 16 including further means responsive to the position and attitude of said craft for moving said polar raster origin relative to the visible area of said display media.

18. The apparatus as set forth in claim 16 further including means coupled with said modulating means and responsive to the roll attitude of the craft for generating two light and dark sectors of substantially 180° each representing sky and ground areas respectively, the demarcation line therebetween representing the earth's horizon.

19. The apparatus as set forth in claim 18 further including means coupled with said modulating means for rendering narrow, ray-shaped sectors on said dark sector visible whereby to create ground plane perspective lines.

20. The apparatus as set forth in claim 18 further including means responsive to the pitch attitude of the craft for displacing said polar raster origin relative to said visible area in accordance with the pitch attitude of the aircraft.

21. The apparatus as set forth in claim 18 further including means responsive to the pitch and heading attitude of the craft for displacing said polar raster origin relative to said visible area in accordance with the pitch and heading attitude of the craft.

22. The apparatus as set forth in claim 16 wherein said display media is a cathode ray tube, having deflection means and beam-producing means, wherein said visible area comprises the viewing surface of said tube, wherein said polar raster generating means comprises means coupled with said deflection means for spirally moving said beam with respect to the cathode-ray tube face and wherein said modulating means comprises means for intensity modulating said beam producing means.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to display apparatus for providing a contact-analog or realistic representation of the position and attitude of a moving craft as well as flight director command information.

2. Description of the Prior Art

Previous methods of presenting information on the face of a cathode ray tube are not well suited for an attitude display. The nature of a contact analog display particularly for aircraft requires that the image viewed must be in perspective with respect to the viewer and must remain in proper perspective for all attitudes of the aircraft. The most difficult problem is to maintain everything in proper perspective when the aircraft is rolling, pitching and/or changing heading. Other than complex electromechanical means, a digital computer in conjunction with a conventional horizontal raster scan, such as used in the usual television, might be considered utilizing the proper interface. However, the problem is extremely complex and requires the storage of the coordinates of every point and the calculation of every coordinate change to maintain the correct perspective for every change in aircraft attitude. In addition, the information must be renewed on the face of the tube at least 30 times per second to prevent flicker resulting in a procedure far more complex and expensive than that of the present invention.

SUMMARY OF THE INVENTION

The present invention provides an attitude and flight director display apparatus utilizing a cathode ray tube having a polar or spiral raster to provide a realistic or pictorial image representing the horizon, the ground plane, the runway or flight path, flight director functions and a reference indicia. This pictorial type of display is sometimes referred to as a contact-analog display. The use of the spiral or polar raster greatly simplifies the electronic generation of the pilot's scene in single point perspective, that is, a perspective scene having a single vanishing point. It also permits the image to remain in accurate and realistic perspective for all aircraft attitudes without the need for complex electromechanic apparatus, a digital computer, interface equipment or storage of any type. Further, refreshing of the image is automatic and need not be externally synchronized. It will be understood that the terms raster etc. are used in their broadest sense, i.e., the overall area of the display on which the image is reproduced by intensity modulation of CRT beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a display appearing on the face of a cathode ray tube in accordance with the present invention;

FIG. 2 is a schematic block diagram of a polar raster generator;

FIG. 3 is a schematic diagram partially in block form of a voltage controlled symmetrical limiter;

FIG. 4 a and b are detailed wiring diagrams of the polar raster generator;

FIG. 5 is a schematic block diagram of horizon and roll generator;

FIG. 6 is a detailed wiring diagram of the horizon and roll generator;

FIG. 7 is a graph showing the relationships of the waveforms with respect to FIG. 5;

FIG. 8 is a schematic block diagram showing pitch and roll attitude information provided to the display;

FIG. 9 shows how the display would look for an arbitrary pitch and roll input;

FIG. 10 shows the development of the runway display;

FIG. 11 is a graph showing symmetrical expansion of the pulse width τ about the time phase corresponding to μ;

FIG. 12 is a block schematic diagram of a portion of the runway generator;

FIG. 13 is a graph showing the relationships of the waveforms with respect to FIG. 12;

FIG. 14 is a display showing line generation where r is variable and δ is constant;

FIG. 15 is a display showing line generation where r is constant and δ is variable;

FIG. 16 is a graph and display showing the generation of a straight line on the polar raster;

FIG. 17 is a block schematic diagram of a portion of the runway generator;

FIG. 18 is a display of the runway image showing the inhibited areas;

FIG. 19 is a detailed wiring diagram of the runway perspective generator;

FIG. 20 is a display showing the intensified area forming a parallel ground plane grid line;

FIG. 21 is a block schematic diagram showing the parallel ground plane grid line generator; pp FIG. 22 is a graph showing the relationships of the waveforms with respect to FIG. 12;

FIGS. 23a and b are detailed wiring diagrams of the ground plane perspective generator;

FIG. 24 is a display showing the intensified area forming a converging ground plane grid line;

FIG. 25 is a block schematic diagram showing the converging ground plane grid line generator;

FIGS. 26a and b are displays showing the image for no roll input and with a roll input respectively;

FIG. 27 is a display showing the additional deflection necessary to rotate the raster center about the display center;

FIG. 28 is a block schematic diagram showing the x, y and φ coordinator;

FIG. 29 is a detailed wiring diagram of the x, y and φ coordinator;

FIG. 30 is a schematic diagram partially in block form of the absolute value circuit;

FIGS. 31a and b comprise a display of the flight director bars and the center reference cross and the deflection voltages respectively;

FIG. 32 is a schematic block diagram of the flight director bars and center reference circuit;

FIGS. 33a and b are detailed wiring diagrams of the flight director bars and center reference circuit; and

FIG. 34 is a composite schematic block diagram of the flight attitude and flight director display system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the present invention provides a contact analog flight attitude display 10 for an assumed flight situation which presents the following information on the face of a cathode ray tube (CRT) 11:

1. Horizon 12

2. Aircraft roll φ

3. Aircraft pitch φ

4. Runway 13 in a single point perspective with position and size variable (a, b, μ, τ, x)

5. Heading error x (when runway is not in use)

6. Ground plane grid 14 in perspective

7. Horizontal and vertical flight director bars 15 and 16 displaceable by amounts e and g respectively from the

8. horizontal and vertical positions of the center reference marker 17.

The letters recited above define the parameters of the display 10 which are variable.

The display 10 to be described utilizes a unique method of generating attitude information on the CRT 11. The display inputs are bipolar analog voltages corresponding to each of the parameters as shown in FIG. 1. The image change on the CRT 11 in response to an input is automatically maintained in perspective without the use of electromechanics, a digital computer, or any type of storage. The image is refreshed at about 60 times per second without any need for external synchronization. This simplification is possible by utilizing a polar or spiral raster scan in lieu of the conventional horizontal raster scan. With this approach, an intensity modulated beam starts from the center and spirals outward. After scanning the entire face of the CRT 11, the beam is brought back to the center and the scan repeated. The image is produced by modulation of the electron beam intensity of the polar scan, in a manner to be described. The advantage of this type of scan for producing a single vanishing point perspective display will become evident when it is explained how the information for the display 10 is generated.

The first thing which must be accomplished is to produce the polar raster. Let R, the distance from the center or origin of the polar raster to the beam position, and φ, the angle at which the beam is located, be the polar coordinates of the beam. Then, if it is assumed that the vertical (y) displacement from the center is

where E is the peak y deflection voltage

T is the period of the sweep

K is the deflection sensitivity and the horizontal (x) displacement from the center is

then the polar coordinates of the beam as a function of time are found by substituting equations (1) and (2) into equations (3) and (4).

R= x2 +y2 (3)

φ= tan-1 y/x (4)

The result is

φ=ωt (6)

From equations (5) and (6) it is evident that the beam will start from center and spiral outward. The number of curved lines of the sweep is given by

N=TIf (7)

where f is the frequency of the deflection voltages. The parameters are selected such that the increase in R for the period 1/f seconds is about one spot diameter for noninterlaced scanning. Alternatively, for interlaced scanning, on the first sweep, the beam traces out a spiral path with one spot size distance between adjacent lines. On the second sweep, the spiral path covers the area not covered by the first sweep. This results in double the resolution by the display 10 for the same number of lines per sweep.

As indicated above, a sin and cos voltage must be amplitude modulated by a ramp voltage in order to generate the polar raster. A block diagram of a polar raster generator 19 which accomplishes this is shown in FIG. 2. A square wave generator 20 provides a square wave to a voltage controlled symmetrical limiter 21. A ramp generator 22 is connected to a retrace monostable circuit 23 which in turn is connected to an input terminal of the ramp generator 23. The other output of the ramp generator 22 is connected to provide a ramp voltage to another input terminal of the limiter 21. The output of the limiter 21 is filtered in a low-pass filter 24 to provide a signal ey equal to E/Tt sinωt and via a 90° phase shift network 25, a signal ex is provided equal to E/Tt cosωt.

It will be appreciated that although only a few cycles are shown contained within the sweep period T, for purposes of simplicity, in fact, there are many in order to obtain a continuous raster. It is to be further understood that in practice the beam is blanked during retrace to the center and intensified as the beam moves out from the center.

The voltage-controlled symmetrical limiter 21 is shown schematically in FIG. 3 and includes a low impedance amplifier 30 adapted to receive a limit voltage and having its output connected to the input of an inverting amplifier 31. Oppositely-pole diodes 32 and 33 are connected to a relatively high impedance point on the signal path to be limited as represented by the lead 34. The other terminal of the diode 32 is connected to the junction of the amplifiers 30 and 31 while the other terminal of the diode 33 is connected to the output of the amplifier 31. If the positive excursion of Eout exceeds EA, the diode 32 conducts and thus limits Eout to EA. If Eout is less than EB, the diode 33 conducts and limits Eout to EB.

A detailed wiring schematic of the polar raster generator 19 is shown in FIG. 4. Throughout the specification, like reference numerals are used to indicate like elements or components. An interlaced polar raster scan is provided by a raster interlace circuit 26. This is accomplished by synchronizing the ramp generator 22 to the square wave generator 20 in such a way that the ramp start position is one-half cycle of the square wave generator output displaced every other sweep. The output of the retrace monostable 23 causes the flip-flop 27 of the raster interlace circuit 26 to change state at the end of each sweep. By the diode gates 28, the state of the flip-flop 27 determines which output of the square wave generator 20 is going to terminate the sweep ramp. Because the flip-flop 27 changes state after each sweep, the ramp is terminated by alternate half cycles of the square wave generator 20 thereby accomplishing the raster interlace.

Having generated the polar raster, the next step is to provide a method to intensity modulate the beam to produce the image as shown in FIG. 1. This will be explained in sequential steps. The first step is to generate the horizon 12. From equation (6) it is noted that the scan spot angle is the same as the angle of the deflection voltage. This means that for every cycle of the deflection voltage the spot moves 360° on the face of the CRT 11 in a spiral path. Thus, if the beam is intensified for precisely 180° out of each cycle of the deflection voltage, only one half of the raster would be visible. Which half of the raster is visible will depend upon the phase of the intensify pulse relative to the deflection voltage. As the phase changes, the intensified half of the raster will rotate about the center. This effect is exactly what is needed to present the aircraft roll information on the CRT 11. To obtain the horizon and aircraft roll attitude image, it is only necessary to intensify the beam for precisely 180° out of each cycle of the deflection voltage, and vary the phase of this intensify pulse to obtain the roll information. FIG. 5 is a block diagram of an horizon and roll generator 35.

A low-pass filter 36 is responsive to the square wave signal from the square wave generator 20 (shown in FIG. 2) to provide a sine wave A to a zero crossing detector 37. The zero crossing detector 37 is a differential amplifier with an input referenced to ground or zero potential which provides two pulses B from each cycle of input since a sine wave passes through zero twice for each cycle. A half period ramp generator 38 is synchronized by each of these pulses, thus producing two ramps C for each cycle applied to the zero crossing detector 37, i.e., a ramp for each half cycle. Both the zero crossing detector 37 and the half period ramp generator 38 are shown in detail in the wiring schematic of the horizon and roll generator of FIG. 6. As shown in FIG. 6, the half period ramp generator 38 consists of a constant current source 39 feeding a capacitor 40 and a discharge circuit 41, which is activated by the input synchronizing pulses B.

Referring again to FIG. 5, a voltage comparator 42 is responsive to the output of the half period ramp generator 38 and to a bipolar voltage D representative of the roll angle φ of the aircraft obtained from a vertical gyroscope or stable platform for example. The output pulses E of the voltage comparator 42 are connected to an input terminal of a bistable flip-flop 45 which has its other input terminal connected to receive the sync pulses B from the detector 37. The flip-flop 45 is connected to a buffer circuit 46 which in turn is connected to a video amplifier 50 shown in FIG. 8.

FIG. 7 shows the relationships of the various voltage waveforms of FIG. 5. As FIG. 7 reveals, two ramps are generated for each cycle of A which is of the same frequency and phase as one of the deflection voltages. This sawtooth is balanced to ground and compared to the roll voltage (D). When the two voltages are equal, an output pulse (E) is generated which toggles a flip-flop and produces a voltage (F). It should be noted that (F) is precisely 180° of (A), and with a phase shift φ proportional to the roll voltage (D). φ is the actual angle the horizon 12 is rotated about the center and corresponds to the roll angle of the aircraft.

The addition of pitch information can be incorporated by shifting the raster up or down by summing the pitch voltage and the vertical deflection voltage. FIG. 8 shows the block diagram of a display capable of providing pitch and roll attitude information. The square wave generator 20 is connected to the polar raster generator 19 and to the horizon and roll generator 35. The output of the polar raster generator 19 and a signal representative of the pitch angle θ of the aircraft from a vertical gyroscope or stable platform (not shown) are summed in a summing amplifier 47 which has its output connected to the Y-deflection amplifier 48 of the CRT 11. The output of the polar raster generator 19 is also directly connected to the X-deflection amplifier 49 of the CRT 11. The horizon and roll generator 35 has its output connected to the video amplifier 50 of the CRT 11.

FIG. 9 shows how the display would look for an arbitrary pitch and roll input. It should be noted that the raster area is larger than the actual viewing area of the CRT. This is necessary because for pitch inputs the raster is shifted up or down. The raster is also shifted horizontally for other inputs in a manner to be explained. If the raster was only large enough to cover the CRT viewing area, part of the image would be lost for vertical or horizontal shifts of the raster. This technique greatly simplifies the pitch presentation as well as runway location and heading presentation as will be explained.

The next step is to provide a method to generate the runway 13. A single vanishing point always located on the horizon 12 is required. FIG. 1 shows such a runway image 13 and defines the parameters a, b μ, τ, and x of the runway 13 which must be variable in order to control its position and size. In the illustrated embodiment of the invention, the above variable parameters a, b, μ, τ, and x are all derived from readily available measures of aircraft position and attitude relative to the runway, as defined by ILS beams and heading. Furthermore, it will be understood that the runway is illustrative of only one possible track reference and other track presentation may be employed, for example, a "path in the sky" type of presentation. Because a polar raster is used, this can be readily accomplished. It will be recalled that the horizon 12 was generated by intensifying the beam for one-half cycle of the deflection voltage or 180°. This resulted in the polar raster being visible for 180°. If the intensify pulse had been less than 180°, the visible portion of the raster would have been less, and pie-shaped. The angular width λ of this pie-shaped raster area is identical to the electrical angular width of the intensify pulse. In other words, if an intensify pulse whose width is equal to 15° of the sinusoidal deflection voltage is applied to the video amplifier 50, the resulting image on the CRT 11 will be a pie-shaped area with an angular width of 15°.

If the phase of the 15° width pulse is changed relative to the deflection voltage, the pie-shaped area will rotate about the center of the raster by an angle μ. This pie-shaped segment can be used to represent the basic image of the runway 13 shown in FIG. 10. It will be noted that the runway 13 will always converge at the horizon 12 regardless of how λ and μ are varied. Therefore, it will always be in perfect single point perspective. Where on the horizon the runway converges, relative to the center of the display, is adjusted by shifting the raster horizontally by an amount x. All that is left to complete the runway is to be able to show only a given section of the pie-shaped area as is shown in FIG. 1. This will be explained later. It appears then that the basic runway image shown in FIG. 10 can be generated by a voltage controlled phase shift circuit to adjust μ and a voltage controlled pulse width circuit to adjust τ. Of course, this must be locked in phase to the deflection voltage. One point which should be noted is that the pulse width τ must be centered with respect to the phase corresponding to the center of the runway. If it were not centered, a change in τ would also result in a change in μ and the other way around. The pulse width cannot simply be stretched, but must expand symmetrically about the time phase corresponding to μ as shown in FIG. 11.

FIG. 12 is a block diagram of a circuit capable of generating the runway image shown in FIG. 10 which allows μ and τ to be voltage controlled and independent. As shown in FIG. 12, the output of the horizon and roll generator buffer circuit 46 is applied to a half period ramp circuit 55 (generally of the type shown in FIG. 5) and to one input terminal of a NAND circuit 56. The output of the ramp circuit 55 is connected to respective input terminals of an inversion and summing circuit 57 and a summing circuit 58. A signal Kμ is applied to the other input terminals of the circuits 57 and 58. The inversion and summing circuit 57 is connected to a voltage comparator 59 which has its other input terminal responsive to the Kτ signal. The summing circuit 58 has its output connected to a voltage comparator 60 which has its other input terminal responsive to the Kτ signal. The outputs of the voltage comparators 59 and 60 are connected to respective input terminals of the NAND circuit 56. The remaining input terminal of the NAND circuit 56 is responsive to a runway inhibit pulse in a manner to be explained. The output of the NAND circuit 56 is applied to the video amplifier 50.

An understanding of the generation of the runway 13 can be attained by studying the block diagram of FIG. 12 and the waveforms of FIG. 13. The waveforms are for arbitrary values of τ and μ. If Kμ is allowed to decrease, waveform (C) shifts up and (D) shifts down with respect to ground. This causes the low width of (E) to increase and the high width of (F) to increase. This will shift the (G) pulse to the right without changing the width τ. If Kτ is allowed to decrease, the low width of (E) will increase and the high width of (F) to decrease. This will decrease the width τ without changing the position of the center of the pulse, i.e., μ remains the same.

To complete the runway generator 61, it is necessary to provide a means to show only a part of the pie-shaped area as shown in FIG. 1. This adjustment of the start a and finish b of the runway 13, will allow the length and distance from the horizon 12 to be controlled. To do this requires a method to produce a straight line on the polar raster. Assume that a voltage P is available which is the same as the y axis deflection voltage but shifted in phase by δ radius.

P=Et/T sin (ωt+δ) (8)

Expanding this provides

P=Et/T sinωt cosδ+Et/T cosωt sin δ (9)

Substituting equations (1) and (2) into equation (9) provides

P=ycosδ/K +X sinδ/k (10)

Solving this for y results in

y=-tanδX+P K/cosδ (11)

Equation (11) is an equation of a straight line but with the y intercept term a function of time since P is a function of time. If the beam is intensified each time P passed through a predetermined level, r, by means of suitable circuits, then equation (11) indicates that a straight line would be generated on the polar raster. The slope of this line is -tanδ and the y intercept or position can be placed anywhere on the display by detection and intensifying the beam for different constant values of r. This is shown in FIG. 14 for a constant δ. If δ is changed but we still intensify each time P passes through a fixed value r, a different family of straight lines is generated. This family is shown in FIG. 15.

FIG. 15 shows that as 67 is varied and r held constant, a family of straight lines, all tangent to a circle of radius Kr, is generated. It is clear then that a straight line of any slope or position can be generated on the polar raster by adjustment of the phase of P, (δ) and r as shown in FIG. 16 which shows the generation of a straight line of the polar raster. The line is generated by intensifying the beam each time P passes through a fixed value r. If the beam were intensified for all values of P greater than r, then the area enclosed by the maximum polar raster arc and the chord produced by the straight line would be seen.

Now that a method has been provided to produce a straight line or an area to the right or left of the straight line, two things can be accomplished. One, the runway generator 61 can be completed and, two, ground plane grid lines can be produced. First, the runway generator is completed by incorporating the circuits necessary to allow adjustment of the length and distance from the horizon 12, (see a and b of FIG. 1). This is done by inhibiting the output of the runway generator 61 in the areas of the raster where we do not want it to appear. The runway 13 must stay in perspective when the aircraft rolls so the inhibited areas must rotate and stay parallel to the horizon 12 during roll. This can be done by deriving P from the output of the roll generator 35. This forces δ to be locked to φ and causes the runway front and rear edges to stay parallel to the horizon 12 at all times. FIG. 17 is a block diagram of the circuit necessary to complete the runway generator 61. The output from the horizon and roll generator 35 of FIG. 5 and the signal from the voltage-controlled symmetrical limiter 21 of FIG. 2 are applied to respective input terminals of clipper diodes 65 which are connected via a low pass filter 66 to first and second voltage comparators 67 and 68 respectively. The output signal of the low pass filter is P. The voltage comparator 67 is also responsive to the signal 37 a" while the voltage comparator 68 is further responsive to the signal "b". The outputs of the voltage comparators 67 and 68 are connected to a negative logic OR circuit 69 which provides a runway inhibit pulse. The output of the circuit 69 goes to zero when either input goes to zero.

The output from the first voltage comparator 67 is a negative going pulse for the length of time that P is greater than a. From the second voltage comparator 68, the pulse appears for a time when P is greater than b. the result is that the runway generator output of FIG. 13 is inhibited in two areas of the polar raster as shown in FIG. 18.

As shown in the wiring diagram of FIG. 19 of the runway perspective generator 61, the circuit realization of the negative logic OR circuit 69 of FIG. 17 and the NAND circuit 56 of FIG. 12 combines both. The combined logic is shown in FIG. 19 as the runway generator output logic 70. When the junctions 71 and 72 are high (which corresponds to the waveforms of FIG. 13), the the collectors of the transistors 73 and 74 are low thus producing a low output to the video amplifier 50. If the junctions 75 and 76 are low, the collectors of the transistors 77 and 78 are high with the result that the output to the video amplifier 50 is held high regardless of the state of the junctions 71 or 72. The output is therefore inhibited by the signal at either junction 75 or 76 which is the desired result.

Of course, the runway image of FIG. 18 will rotate about the center of the raster during roll and still maintain its proper distance from the horizon 12 and its correct length. This would not be possible if it were not for the constant r, variable δ characteristic shown in FIG. 15.

The next thing to be explained is the generation of the ground plane perspective lines forming the grid 14 which converge at the center of the horizon 12 as shown in FIG. 1. It will be recalled that the runway 13 always converges to the center of the raster and not necessarily at the center of the display or viewing area (see FIG. 10). This requires that the ground plane lines do not necessarily converge at the center of the raster, but must move relative to the raster center such that they always converge on the point of the horizon which corresponds to the center of display for zero pitch. To accomplish this, advantage is taken on the constant δ, variable r characteristic shown in FIG. 14. A closer look at FIG. 1 will reveal that the ground plane grid 14 is made up of two constant width lines 80 and 81 parallel to the horizon and six lines 82-87 which converge to a point on the horizon 12. Each of the constant width parallel lines 80 and 81 is generated the same as the runway start and finish edges, a and b. The only difference is that the two areas are overlapped and the beam is intensified in the area created by the overlap. This is shown in FIG. 20.

FIG. 21 shows the block diagram for generating one of these lines. A signal from the output of the low pass filter 66 of the runway generator 61 is applied to a buffer circuit 90 which provides an output signal P to first and second voltage comparators 91 and 92 respectively. The comparator 91 is also responsive to a fixed voltage V1 while the comparator 92 is further responsive to a fixed voltage V2. The outputs of the comparators 92 and 92 are connected to a negative logic AND circuit 93 that in turn is connected to a negative logic OR circuit 94 which provides signals to the video amplifier 50 shown in FIG. 8. The negative logic OR circuit 94 is responsive to the inhibit signal from the negative logic OR circuit 69 of the runway generator 61.

The signals V1 and V2 are fixed voltages selected to position the intensified area and adjust its width. FIG. 22 shows the phase relationships of the voltages at points A, B and C of FIG. 21. Because the signal P is not of constant amplitude, the widths of the signals A and B start from zero and increase as the amplitude of the signal P increases. The pulse width of waveform C, however, remains constant. These line intensify pulses are inhibited by the runway pulses to maintain constant runway brightness. If this were not done, the runway 13 would appear brighter where the line crosses over since all of these pulses are summed at the video amplifier 50. To generate the two parallel lines of constant width 80 and 81, two of the circuits of FIG. 21 are necessary except that they share the buffer circuit 90 and output OR circuit 94 as shown in the detailed wiring schematic of the ground plane perspective generator 108 of FIG. 23 is also shared with the circuits of the converging ground grid lines 82-87.

As shown in FIG. 23, the negative logic OR circuit 94 is somewhat different in that a low voltage on the cathode of any of the diodes 94-102 coupled to any of the line circuits 80-87 respectively saturates the transistor 104 which turns off the transistor 105 thus tending to cause the collector of the transistor 105 to go to a low value. This is inhibited by the inhibit circuit 106 by turning on the transistor 107 which holds the collector of the transistor 105 high regardless of the other inputs.

The negative logic AND circuit 93 has two interconnected diodes 108 and 109. When both diode anode voltages are low, the cathode voltage is also low.

The converging grid lines 82-87 are generated in a similar manner. Two areas are made to overlap and the beam intensified in the resulting area as shown in FIG. 24.

As explained above, the raster is moved horizontally to position the runway 13, but the converging lines 82-87 must always converge on the point of the horizon 12 which corresponds to the center of the display for zero pitch. Looking at FIG. 24, this means that the intensified area, while maintaining its angular position with respect to the horizon line 12, must be capable of sliding either left or right but always converging on the horizon line. To do this, advantage is taken of the variable r, constant δ characteristic shown in FIG. 14. By changing r and holding δ constant, the intensified area of FIG. 24 can be translated to the left or right of the raster center. A block diagram for one converging ground plane grid line such as 82 is shown in FIG. 25. The waveforms at points A, B and C are identical to the waveforms shown in FIG. 22.

Comparing FIG. 21 and FIG. 25, it will be noted that the generation of a converging ground plane grid line differs in only two respects from the generation of a parallel ground plane grid line. The first is the phase shift networks 110 and 111 which establish the angular position of the intensified area relative to the horizon lien 12. The second, is the reference voltages C1 X and C2 X applied to the voltage comparators 112 and 113, respectively are not constant but proportional to the raster displacement from the center of the display. It is these voltages which maintain the converging lines at the proper point on the horizon. For the six converging lines 82-87 shown in FIG. 1, six of the circuits of FIG. 25 are used.

As explained above, to implement the pitch and runway position, the raster center is shifted relative to the center of the display as shown in FIGS. 9 and 24. However, because of the way the horizon and roll generator 35 function, the horizon 12 rotates about the center of the raster. For the image to stay in proper perspective, the image must rotate about the center of the display and not the center of the raster since the raster center is not at the center of the display for pitch and runway position inputs. To accomplish this, it is necessary to rotate the raster center about the center of the display when a roll input is received. FIG. 26a shows the image for no roll input. FIG. 26b shows that a roll input of φ requires the raster center to be rotated φ radians about the center of the display, thus maintaining the image at the center to remain at the center. To produce this rotation requires additional inputs to the x and Y-deflection circuits 49 and 48, respectively. The inputs necessary can be derived with the aid of FIG. 27.

From FIG. 27, it is seen that x1 is the additional x deflection input, and y1 is the additional y input necessary to rotate the raster center about the display center. Their relationship to φ , the roll angle, is from FIG. 27.

x1 =-x(1-cos φ) (12)

y1 =x sin φ (13)

With a pitch input θ (see FIG. 1), the additional inputs are found to be:

42 =-θ(1-cos φ) (14)

x2 =θ sin φ (15)

where it is understood that θ is the vertical or y displacement of the raster from the center. With pitch and roll inputs, all of the above inputs are necessary. To implement the inputs expressed by equations (12) through (15) would normally require two function generators, two two-quadrant, and two four-quadrant multipliers. It has been found that a linear approximation of the functions 1-cos φ and sine φ give an acceptable presentation and at the same time simplifies the implementation of the multipliers. Using a linear approximation, equations (12) through (15) become

x1 =-x K1 φ (16)

y1 =xK2 φ (17)

y2 =-θK1 φ (18) x2 =θK2 φ (19)

FIG. 28 is a block diagram of a coordination circuit 115 which performs the calculations and adds the results to the x and y deflection voltages. The outputs are given by

With proper selection of the constant, the image will rotate about the center of the display and not the center of the raster for x, θ and φ inputs.

Generation of the equations 16 through 19 will now be explained by referring to the circuit 115 shown in FIG. 28. The bipolar roll voltage φ is passed through an absolute value circuit 116 and this output modulates a pulse width modulator 117. This results in a duty cycle proportional to the absolute value of the roll voltage 100. The pulse width modulated signal operates a switch 118 or 119 which allows the x or θ voltage respectively to pass with the same duty cycle. The average value is extracted by respective low pass filters 120 and 121. The average value is proportional to the product of the absolute value of the roll voltage φ and x or pitch θ respectively. Thus, equations 16 and 18 are generated but with improper constants at this point. The constants are corrected by the x and y summing amplifiers 125 and 47 respectively. Equations 17 and 19 are now developed by changing the sign of equations 16 and 18 when the roll angle voltage polarity changes. This, in effect, removes the absolute value bars from equations 16 and 18 thereby producing equations 17 and 19 but again with improper constants which are corrected by the summing amplifiers 47 and 125. This is performed by the polarity detector 126, switches 127-130 and amplifiers 131 and 132. The polarity detector 126 determines which input, either the inverting or noninverting input, of the amplifier 131, 132 is fed the signal. Thus, when the roll voltage changes polarity, the polarity of the signal at the output of the amplifiers 131, 132 also changes. The circuit can be thought of as two four-quadrant multipliers generating equations 17 and 19 in which equations 16 and 18 are produced by an intermediate step. Last, all of the voltages are summed in the proper x and y summing amplifiers 125 and 47, respectively. This results in the complete x and y deflection voltages previously expressed by equations 20 and 21.

A detailed wiring diagram of the coordination circuit 115 is shown in FIG. 29. The absolute value circuit 116 comprises an inverting amplifier 135 as shown in FIG. 30 and two diodes 136 and 137. When Ein is positive, the output of the amplifier 135 is negative so the diode 137 is reversed biased and the diode 136 is forward-biased. The output then follows the input. When Ein is negative, the output of the amplifier 135 is positive with the diode 136 being reverse-biased and the diode 137 being forward-biased. The output then is of the same magnitude as the input but is positive. In other words, the magnitude of the output is the same as the magnitude of the input but always of the same polarity regardless of the polarity of the input.

To complete the description of the display, the center reference cross 17, and flight director bars 15 and 16 will not be discussed. Up to now, all of the image on the CRT 11 was generated by intensity modulation of a polar raster. The center reference cross 17 and the flight director bars 15 and 16 are not produced in this way. When the polar raster reaches its maximum diameter, it must be retracted or returned to the center so that it can be repeated. The raster is not immediately started but waits for a length of time to allow the filters to recover from the retrace transient. The length of time for this delay is about eight cycles of the deflection voltage. This time is controlled by the retrace monostable 23 (see FIG. 2) which inhibits the ramp generator 22. The time is about 5 percent of the sweep time. During this retrace time period, the video amplifier 50 and the deflection circuits 48 and 49 are idle and can be used to generate additional information on the CRT 11. This is when the center reference cross 17 and flight director bars 15 and 16 are placed on the CRT 11.

The center reference cross 17 and flight director bars 15 and 16 are shown in FIG. 31a and are generated by analog switching and logic which is synchronized to the raster retrace. The process requires four steps. Generation of:

1. Center reference horizontal line 140

2. Center reference vertical line 141

3. Flight director horizontal bar 15

4. Flight Director vertical bar 16

FIG. 31b shows the deflection voltages for the four steps. The sequence as shown is repeated each time the retrace occurs. The raster deflection voltages are removed from the deflection amplifiers 48 and 49 during the retrace time and the voltages shown in FIG. 31b are applied. The line 140 is produced by allowing two cycles of a small amplitude sine wave to deflect the beam in the x direction. At that time the y deflection voltage is zero. The line 141 is then formed by allowing two cycles of the same sine wave to deflect the beam in the y direction. Since the lines 140 and 141 are formed by sinusoidal voltages with no DC voltage present on the opposite axis, their intersection defines the center of the display for the images formed by the polar raster and for the flight director bars. The bar 15 is formed by allowing two cycles of a larger amplitude sine wave to deflect the beam in the x direction. The bar 15 is displaced vertically from the center by placing the flight director horizontal bar signal on the y axis. The bar 16 is formed by allowing two cycles of the same sine wave to deflect the beam in the y direction. The bar 16 is displaced horizontally from the center by placing the flight director vertical bar signal on the x axis. Two cycles for each of the lines was selected because eight cycles of the raster deflection voltage frequency was available during the retrace time period. FIG. 32 is a block diagram of the Flight Director Bars and Center Reference circuit 145. The sequence is initiated by the pulse from the retrace monostable circuit 23 in the polar raster generator 19 of FIG. 2. This pulse removes the raster deflection voltages ex and ey from the deflection circuit via analog switches 146 and 147, respectively. At the same time, it releases the counter 148 comprising three flip-flops 149, 151 and 152. The counter 148 is driven by the signal from the square wave generator 20 of FIG. 2 which also supplies the polar raster generator 19 and the horizon and roll generator 35. Decoding for the four steps is accomplished by the analog switch circuits 153-156 which also serve as "AND" gates, i.e., the switch allows the analog signal to pass when all of its bipolar digital signal inputs are negative. The change in the amplitude of the gated sine wave is accomplished by the discrete gain change circuit 157. The two monostable circuits 158, 159, the AND gate 160, and the OR gate 161 produce beam blanking when the beam moves from one line or bar to the next. At the termination of the retrace monostable pulse, the counter 148 is locked and the signals e x and ey are allowed to pass through the analog switches 146 and 147 respectively while all the other analog switches 153-156 are held open until the next retrace occurs. A detailed wiring schematic of the circuit 145 is shown in FIG. 33.

FIG. 34 is a block diagram of the complete system utilizing the circuits explained above. The polar raster generator 19 is connected to the horizon and roll generator 35 as well as to provide input signals to the x and y summing amplifiers 47 and 125, respectively and to the runway perspective generator 61. The horizon perspective generator 61 and is responsive to the roll signal φ. An output of the horizon and roll generator 35 is connected to the video summing amplifier 50. The runway perspective generator 61 is connected to the ground plane perspective generator 108 and is responsive to the signals μ, τ, a, b and those from the polar raster generator 19. The output of the runway perspective generator 61 is connected to an input terminal of the video summing amplifier 50. The ground plane perspective generator 108 is responsive to the x signal and is connected to another input terminal of the video-summing amplifier 50. The x, y and φ coordinator 115 is responsive to the θ, x and φ signals for providing output signals to the y and x summing amplifiers 47 and 125 respectively. The y summing amplifier 47 is further responsive to the θ signal directly and provides a summation signal to the flight director-bar circuit 145. The x summation amplifier 125 is further responsive to the x signal directly and provides a summation signal to the flight director-bar circuit 145. The circuit 145 is directly responsive to vertical and horizontal bar signals for providing output signals to the x and y deflection amplifiers 49 and 48 respectively. The operation of the CRT 11 in response to the aforementioned signals has been described above with respect to the individual circuits.