PRINCIPLE OF OPERATION OF COMMON TYPES OF DISPLAYS USED IN MODERN AIRCRAFT
The visual display is the interface between the man and the system, and in some cases, between the system and the man with the touch screens.
1. GENERAL:
Image synthetizer: it transforms the electric signals into light signals on the screen.
Image processing: it ensures the processing of the radar image signal and information from sub-systems.
Generation of symbols: this unit provides to the operator the elements of suuplementary representations making it possible to improve the reading on the screen and understanding of the system.
Adjustment and power supplies: this stage regulates the light and the x-ray on the screen and provides the various power supplies to the synthetizer.
Display commands: they ensure to the operator the choice of the parameters to use, the information selection and the functions to switch to.
The visual display function gathers a set of means that allow an operator to use, in the form of light signals perceptible by the eye, information coming from systems like the radar.
1.1. Various types of synthesizer:
1.1.1 Cathode ray tube (C.R.T.)
The Cathode ray tube allows the visual display of an electric signal on a fluorescent screen struck by an electron beam.
1.1.2. Plasma screen:
This screen uses the principle of gas ionization, producer of photons.
1.1.3. Liquid crystal display:
The screen with liquid crystals uses the electrochemical phenomena.
1.1.4. The screen with light emitting diodes:
Screen consisting of matrix of light emitting diodes which, subjected to an electric field, produce light. This light is of animating colors according to semi conductor material used.
1.2. Classification of the visual display systems:
1.2.1. Classification according to the type of synthesizer:
Screens with continuous structure: whole or part of the screen is scanned by an uninterrupted moving electron beam from one point to another.
The cathode ray tubes have this structure.
Screens with discrete structure: the screen is composed of a whole system of elements that needs to be addressed by co-ordinates matrix in rows and columns.
The plasma, liquid crystals and L.E.D. screens have this structure.
1.2.2. Classification according to the type of scanning:
Oscilloscope type: the displacement of a light point is seen with respect to a horizontal axis, the signal to be observed is transcribed in vertical displacement of the spot.
Panoramic type: displacement of the scanning point following 2 axes proportional to the movements of the sensor, the signal to be observed is transcribed by the light of the point.
Television type: the scanning point is moved very quickly on all the surface of the screen, by moving it in horizontal lines.
All these lines make up a field.
Rider type: the light point is moved from point to point using a discrete displacement (jump).
2. CATHODE RAY TUBES (C.R.T.):
If new techniques of image reproduction made their appearance, the cathode ray tube still remains largely used in the visual indicator mechanisms/means.
2.1. The electrostatic C.R.T.:
2.1.1. Composition:
2.1.2. Description:
The tube:
The tube is made up of glass and there is a high vacuum of 10 -5 to 10 -4 Pa inside it.
The cathode:
A heating filament brings the cathode to a temperature higher than 800°C. This will allow the cathode to emit a cloud of electrons. To increase the flow of electrons, it is impregnated with a mixture of oxide.
The wehnelt:
The applied voltage to the wehnelt is strongly negative and lower than that of the cathode. While varying this voltage the flow of electrons can be controlled and can even be cancelled by a voltage lower than U cut This " cut off " voltage corresponds to the voltage which allows the electron beam to be turned off (in electricity two negative elements are repelled, here these two elements are the electron and the wehnelt).
Concentrating (focusing) anode:
It is a succession of concentric cylindrical electrodes which are at higher potentials that makes the beam accelerates. It concentrates the beam so that it arrives on the screen in a point about some tenth of millimetre called the spot. The action on these various voltages constitutes the focus control.
Accelerating anode:
The accelerating anode is at a higher potential than the concentrating(focussing) anode. It allows an axial field to be applied, which accelerates the beam, but without exceeding the limit so as to allow the deflection plates to operate. The beam is monocinetic because the electrons move at the same speed.
Horizontal and vertical deflectors:
They are plates that are laid out as the plates of a capacitor. They receive the voltages used to move the beam which create a moving trace. In this way we will obtain a screen scanning.
Post accelerating anode:
This circular anode is close to the screen. The applied voltage on this anode is positive and very high (a few thousands of volts). It increases the kinetic energy of the beam to obtain a very luminous inscription(spot).
Phosphorescent screen:
The glass screen is covered with a phosphorescent matter varying according to the contrast and the color of the inscription. The electron beam strikes the screen and emits visible photons for the human eye.
2.2. The hybrid C.R.T.:
2.2.1. Presentation:
In their immense majority, whether on television, on the computer screens or the displays installed on aircraft, the C.R.T. are of the hybrid type. The guiding principle of the hybrid C.R.T. does not differ from the electrostatic C.R.T.
The only very significant difference lies in the deflectors.
The electrons deflection is carried out in an electromagnetic way, via coils which act on the magnetic component of the field.
2.2.2. Advantages:
The name of the various types of C.R.T. rises from their focusing and deflection devices:
electrostatic C.R.T.: electrostatic concentration and deflection,
electromagnetic C.R.T.: electromagnetic focusing and deflection (very rare),
hybrid C.R.T.: electrostatic focusing and electromagnetic deflection.
a) The electrostatic device:
Advantage: It is light and compact, because it uses a simple movement of the annular electrodes for the focussing and the deflection plates.
Disadvantage: It allows weak angles of deflection thus the screens are of small diameters.
b) The electromagnetic device:
Advantage: It allows significant angles of deflection and thus of the screens are of large diameters.
Disadvantage: It uses heavy and bulky coils. Placed in the neck of the tube, long, thin and fragile, a heavy coil can cause a rupture of this very delicate element.
This is why the hybrid C.R.T. is the best compromise because it combines the main advantages of the two devices. ; i.e. electrostatic focusing and electromagnetic deflection.
This is why hybrid C.R.T. screens are mostly used.
2.3. The polychrome C.R.T.:
2.3.1. Presentation:
See below: Principle of color selection of a mask tube.
2.3.2. Elementary principle of three-colour process:
To reproduce the totality of the colors, we use the additive synthesis of three colors known as primary. By mixing the three colours equally we can have eight combinations of bases.
Three primary colors, red, green and blue.
Cyan = blue + green.
Yellow = red + green.
Magenta = blue + red.
White = red + green + blue.
Blackness, absence of colors.
See below: Composition of colors.
2.4. Problems with conventional visual displays:
The C.R.T. induces from its design, constraints or limitations in its use:
2.4.1. The image format:
The overall dimension of a C.R.T. is measured not only by its surface area of screen, but more especially by its depth; because of the electron gun, the depth of the tube increases proportionally with the size of the screen. The weight is also a limiting factor: The mass of the glass envelope, which makes up the tube is far from being negligible. Thus we cannot reasonably consider screens of very large size.
2.4.2. Duration of the spot:
It is limited by material which forms the luminophores of the screen. They emit the photons since they are excited by the electrons which are projected at very high speed. The observed luminescence is the result of two phenomena:
The fluorescence during the excitation period and
the phosphorescence or contrast which begins at the end of excitation and lasts for some time, and which can vary according to the luminophore used.
2.4.3. Image resolution:
It is above all determined by the size of the luminophores used; the smaller they are, the better is the image definition. But we cannot decrease their diameter without any more lost in luminosity, we already saw that it was limited by the duration of the spot.
2.4.4. Brightness of the spot:
The main factor affecting the luminosity of a screen is the luminous environment in which it must be operated. According to these conditions, the displays will be controlled by some characteristics from the luminosity and contrast point of views. However, the C.R.T. currently used in radar so much are limited in this field that we must adapt the environment according to the tube, and not vice-versa. This is why we install anti-light on these indicators.
2.4.5. Symbology:
There should be no free time to write a symbol, except if we want an information loss, which of course should not be considered.
2.4.6. Conclusion:
To be aware of these series of defects, it can be wondered why this type of visual display has been chosen.
In fact, the answer is rather simple: there was nothing better!
This fact has lead to the research of solutions, and we will now see the main trends.
2.5. Improvement of the outlay, the duration and the resolution:
These improvements can be made in a drastic way: by changing the screen!
The development of new technologies makes it possible today to use the essential element: the C.R.T.
This new equipment, the plasma screens, liquid crystals or L.E.D., allows most of the problems previously mentioned to be solved. However, if there are strong reasons (financial, for example...) force to preserve a CRT, improvements with the guiding principle can be introduced.
They relate mainly to the brightness of the spot, which is the main handicap of the C.R.T.
2.6. Improvement of the brightness and the contrast:
The luminous environment of the C.R.T. is strongly influenced by the lack of brightness of the spot.
To improve it, and thus developing screens known as "day time", i.e. usable under normal luminosity conditions, two channels are possible and complementary:
to improve the brightness itself, i.e. the fluorescence of the luminophores,
to improve contrast, i.e. the duration of the spot once the excitation has stopped.
2.6.1. Brightness improvement:
To increase the brightness, two actions are possible:
by increasing the kinetic energy of the electrons. The impact on the electroluminescent layer is then stronger, thus raising the level of brightness of the image. We notice the increase in kinetic energy when acting on the accelerating anode, giving a higher speed to the electrons. However, this causes a divergence of the E - beam which becomes less fine
by increasing the beam focus. We improve the smoothness thus, the electronic density and consequently the brightness. However an increase in the el focusing electrodes' length is required thus resulting in an increase in the length of the tube.
2.6.2. Contrast improvement:
Contrast contributes indirectly in the improvement of the brightness: if the duration of the spot increases, the average luminance also increases.
Several solutions are possible:
increasing the phosphorus persistence: the duration of the spot increases, but if the spot persistence increases, the more it is difficult to erase;
maintaining the image: it is the function of Tubes of Image Maintenance (T.I.M.) (or of the digital video), but these screens have less contrast;
regenerating rapidly the image at higher frequencies than retinal persistence: it is the principle of the screen scanning. This type of scanning is currently privileged because there is a strong increase in contrast and thus a substantial increase in the brightness.
2.6.3. Conclusion:
If actually the C.R.T. remains the most employed device, its numerous defects, in especially the brightness, has lead in the research of more effective solutions. Either by improving the guiding principle, or by using another type of screen.
3. DOT MATRIX SCREENS:
All these equipment do not use the principle of the thermoelectronic effect of the C.R.T, which was widely used in the past.
They contain a structure made up of rows and columns forming a matrix. The intersection of each row with each column determines a point which can be addressed individually.
The matrix and addressing principle of this equipment remain similar. Only the technology being used differs, primarily because of the electro-optical material, which are in solid, gaseous or liquid state. Let us study each one of them in detail.
3.1. L.E.D. screens:
These screens are often used for control boxes.
This type of screen has considerable advantages:
no high voltages and X-rays,
high speed of image change,
reduced cost.
On the other hand, there are two significant disadvantages:
a disproportionate electrical consumption as compared to other screens,
poor resolution.
3.2. Plasma screens:
3.2.1. Presentation:
The plasma screens have considerable advantages:
Excellent contrast.
Can be operated within wide range of temperature (-25°C with +75°C).
Absence of high voltages and X-rays,
Wide angle of vision as compared to a C.R.T.
Very robust as they can withstand high levels of shocks and vibrations,
Small size and weight,
Lower power consumption.
But they have however a major disadvantage:
Very high cost.
See below: Structure of a color plasma panel.
3.2.2. Operation:
The electro-optical material is the neon gas.
Each point on the screen consists of a cell containing gas. Subjected to a rather high voltage (160 V), this gas ionizes and becomes a plasma (mixture of ions and free electrons) which emits a visible radiation.
An alternating voltage is supplied to the rows and the columns forming the matrix, such that the potential difference between rows and columns is lower than the energizing voltage of the cells: it is the standby voltage (100 V, 50Hz).
To light up a cell and thus ionize the gas, one only needs to apply an energizing impulse (+ 160 V) between a given row and a given column. A memory effect then occurs. The alternating standby voltage is then enough to maintain the gas in the plasma state.
To switch off the cell, a (+ 40 V) impulse is applied.
Thus, the image on a plasma screen does not need to be continuously refreshed; once displayed, it is maintained until the erase command or replaced by another image.
3.2.3. Color displays:
Color is obtained by placing luminophores in the panel and by exciting them with ultraviolet radiation discharge of the gas.
3.3. Liquid crystal display (LCD):
3.3.1. Presentation:
The visual displays installed in the cockpits of aircrafts used until now as image generators, the traditional cathode ray tube which is on its way to be replaced by liquid crystal displays (LCD).
The LCD brings a reduction in size, weight and power consumption. Lower power, means lower operating temperatures and thus better reliability.
See below: LCD screens in a modern aircraft.
The LCD screens have moreover one major advantage in the visual displays of cockpits: they do not fade when they are subjected to an intense luminous environment and can thus ensure a better contrast than the cathode ray tubes. Its disadvantages, are a high cost and a limited angle of vision.
3.3.2. Operation:
The liquid crystals are made up of solid molecules bathing in a fluid. These molecules have the property to be able to be orientated when subjected to an electric field.
The liquid crystals are locked up between two glass blades which support the transparent electrodes generating the electric field.
These crystals naturally adopt a helicoid formation between the two glass blades, spaced in such a way that the helix turns exactly 90°. Each glass blade consists of a polarizer.
The orientation of the polarizers depends on the desired operating mode:
Crossed polarizers, the screen is normally white, i.e. transparent if no voltage is applied,
Parallel polarizers, the screen is normally black.
Let us study the type of screen particularly used on the Boeing 777:
In the absence of an electric field, the polarized light by the first filter crosses the layer of liquid crystal while being guided by the bending of the helix. Having swiveled 90°, it cannot cross the second filter: the cell is opaque.
When a voltage is applied to the electrodes, all the molecules take the same orientation and the helix disappears. The light, which does not undergo any more rotation, preserves the orientation imposed by the first polarizer and crosses the second: the cell is then transparent.
For an intermediate voltage, the helix formed by the liquid crystal is not destroyed but the molecules start to be aligned according to the field. Therefore only part of the light is transmitted.
Nuances (shades) of colors are thus generated.
See below: Principle of operation of the LCD screen .
For liquid crystals screens, two constructions are used:
Screens with passive matrix,
Screens with active matrix.
a) Screens with passive matrix:
The liquid crystals are located between two glass plates equipped with transparent conductive tubes, laid out vertically and horizontally, making up the matrix.
In this configuration the crystals have the disadvantage of being rather slow in positioning themselves in an optimal way.
b) Screens with active matrix:
They were conceived to correct the preceding defect. A transistor is positioned on each row – column intersection, and this greatly accelerates the orientation of the crystals.
However, the number of transparent transistors (307200 for a matrix of 640 X 480 points) makes this solution very expensive.
3.3.3. Lighting:
The light source located at the back of the LCD cells generally consists of fluorescent tubes. The number and the location of the tubes are defined in order to maintain sufficient lighting in the event that one of the tubes fails.
A diffuser makes it possible to uniformly distribute the light on all the surface of the screen.
A function controls the luminosity of the fluorescent tubes according to the pilot intensity control and the information resulting from the photoelectric cells. These cells make it possible to adapt the luminosity of the screen according to the luminous environment.
See below: Configuration of the LCD screen.
3.3.4. Color displays:
Colors are obtained by applying red, green and blue color filters.
See below: Various types of filters .
Band type: it corresponds to the worse definition. It was used in the first apparatuses.
Diagonal type: the improvement is obvious, the colors are different horizontally and vertically. There remains however a risk of banding of oblique colors.
Triangular type: the color elements are placed like bricks in the construction of a house. These screens are the most expensive but the most powerful ones.
4. RADAR SCANNING:
4.1. Oscilloscope type scanning:
This visual display employed at the origins of the radar, but not very practical from an operational point of view, is hardly used, only for auxiliary means. Its study enables us to study the other types of scanning more easily.
4.1.1. Presentation:
Distance, or time information since there is a relation between time and the distance, is materialized by the echo abscissa. The amplitude information is indicated as an ordinate.
The principle of this type of indicator is very similar to that of an oscilloscope.
4.1.2. Horizontal deflection without vertical deflection:
The applications of the blue curve on the horizontal deflector allow the spot to move from the left to the right. When the current is minimum the spot is on the left and when the current is maximum the spot is on the right. If this phenomenon is repeated several times at the adequate frequencies by having the red curve for the vertical deflection, a horizontal line is obtained on the above screen. Indeed if the frequency is too low it is a moving point that can be seen.
4.1.3. Horizontal deflection with vertical deflection:
With the deflection of the spot from the left to the right and the green curve applied to the vertical deflector, a green trace on the above screen is obtained. The principle of the vertical deflection is identical to that of the horizontal deflection
Indeed with a minimum current the spot deviates downwards and with a maximum current the spot deviates upwards.
4.2. Panoramic type scanning (Plan Position Indicator - PPI):
4.2.1. Presentation:
This is the way the general public sees a radar indicator. The two vital data which it shows are the layer (or the azimuth) and the echoes distances.
The radar is located at the center of the screen; the layer is read on graduated circles all around the screen. The distance is proportional to the distance of the spot compared to the center of the screen. The further the spot is from the center, the more the echo distance is.
A third information is given, although less significant in most cases, it is the amplitude of the echo which is the result of the brightness of the spot. The displacement of the scanning indicates the movements of the antenna.
This indicator is most frequently used; this is why we will study it in more detail.
4.2.2. Principle of realization of a turning PPI scanning:
Actually the rotating trace that is displayed is an illusion due to the properties of the human eye.The scanning in fact consists of a very great number of radii traced from the center to the periphery of the tube.
The association of these radii extremely close gives the impression of a rotating scanning.
Before developing the creation of this rotation, let us study the development of the radii or the vector scanning.
4.2.2.1. Creation of a vector scanning:
As we saw during the study of the hybrid CRT, the displacement of the spot will be carried out by electromagnetic deflection, due to coils. A set of coils ensures the deflection from south to north, and another one from east to west.
Thus the trace should be moved, while playing on the currents circulating in the coils.
To create a horizontal rectilinear beam, it is then enough to apply current in the coil. When applied to the east-west coils, the current will cause horizontal displacements as represented here.
When applied to south–north coils, the same current will cause vertical displacements.
Consequently, we can easily conceive that by combining the two displacements we will create an oblique trace. Thus, to obtain a 120° radius, we will have:
To re-create the displacement of the antenna, it will be enough for us to control the screen scanning by the movements of the antenna.
4.2.2.2. Image chain:
The image consisting of the radar echo passes through the wehnelt which modulates the intensity of the E - beam.
With this rough image, some information such as the distance indicator can be added and gives to the operator a better perception of the situation.
All this information (rough image, indicators, etc.) will be summed and applied to the wehnelt to work out the total image which will be formed on the screen.
Let us take the example of the radius displayed on the previous screen.
5. SCREEN SCANNING.
Screen scanning was developed, based on the characteristics of the human eye: its limitations and its features. This made it possible to better adapt the visual display mechanisms.
5.1. Properties of the human eye:
5.1.1. Retinal persistence:
The excitation of the retina, caused by a luminous stimulation, does not disappear instantaneously when the stimulation ceases. This implies that if two images follow one another quickly, the feeling on the eye is equivalent to only one image during the time of the two images.
If succession frequencies of the images are too weak, the eye perceives an unpleasant flicker of the image. Refreshing the image will thus have to be carried out with frequencies higher than this limit, which is of 15 Hz approximately.
5.1.2. Mean separator:
It characterizes the distance separating two light cells when the eye cannot distinguish them individually any more. This minimum distance is an obvious function of the distance observer – source, this is why, in practice, one defines mean separator as a minimal angle below which the eye cannot separate the two points any more. This angle is approximately 1 minute of arc (1 ').
5.1.3. The field of vision:
The field of vision of an average observer is 40 degrees in the horizontal plane and 30 degrees in the vertical plane.
The width of the image is thus equal to 4/3 of its height.
It is this ratio which is used for television screens.
5.2. Principle of the scanning field:
The scanning field depends first of all on the retinal persistence of the eye: a fast succession of still frames gives to the eye the impression of continuity. If each image differs slightly from the preceding one and if the frequencies of renewal are higher than 15 Hz, the eye does not perceive discontinuity and “sees”a continuous motion.
Each point of the screen, also known as pixel, must be analyzed in order to obtain the most faithful possible image. It is not however possible to carry out this analysis permanently. We would need a pixel data raster.
This is why an analysis and a consecutive reproduction of the image are necessary where all the points on the screen will be analyzed successively. Each image is recreated by moving the spot of the electron beam following the horizontal rasters, from left to right and from top to bottom of the screen.
These rasters, equally spaced, make up the field of the image, from where comes the name of scanning field.
The horizontal scanning, allowing the creation of the rasters, is called raster scanning.
The vertical scanning, making it possible to shift from one rasters to the next, is called field scanning.
Scanning starts at the upper left corner of the screen to the upper right corner following a slightly tilted line. The spot is then switched off to proceed to the flyback. The spot turns over to the left of the screen. The spot where the electrons impacts the screen then starts to move again on a new raster and so on, until it reaches the bottom of the screen. The field is then complete.
The spot is then returned to the top left of the screen to sweep a new field.
5.3. Calculation of the signal magnitude:
5.3.1. Calculations of the number of rasters:
The number of rasters will be calculated in such a way that the observer sees the image clearly and without distinguishing the rasters from each other. It is related to three general properties:
mean separator of the eye,
the type screen,
the optimal distance of vision which is related to the size of the screen.
It is indeed estimated that an observer has the best perception and best visual comfort when he is placed at a distance away from a screen three to five times the length of the diagonal of the screen.
Thus, for a screen with a 4/3 format, the diagonal of the screen will be 5/3 of its height:
If the observer is placed at a distance of four times the diagonal that is 20/3 H.
We have:
The eye is able to distinguish between two details separated by 1 ' of arc. To have an image without raster separation, it will thus be necessary to have more than 515 rasters on the screen.
In Europe, the television standards PAL and SECAM consist of 625 rasters.
5.3.2. Calculations of the field frequencies:
The field corresponds to an image. To calculate the field frequencies results in the calculation of the number of images per second.
A human eye, as described previously, is able to distinguish two images if the variation of presentation is lower than 1/15th of a second. In order not to distinguish the "jump" between two images, the refreshing frequencies will have to be higher than 15 Hz.
In Europe, the fixed standard of the image frequency is at 25 Hz.
5.3.3. Calculations of the raster frequency:
The scanning frequency of the rasters is equal to the product of the number of images per second and the number of rasters per image.
Thus, in Europe: 625 X 25 = 15 625 Hz. In fact, the total duration of a raster will be of 1/15625 = 64 µs.
The system, as described previously, if it functions well, presents however a defect.
Even by presenting 25 images per second (frequencies theoretically more than sufficient), there is an unpleasant and tiring phenomenon for the eye. At the end of 1/25th second, the luminosity of the first pixel already started to weaken when it is brutally brought back to its maximum brightness. This produced a flutter of the image.
This is why another type of image scanning was worked out and it is frame scanning.
5.4. The frame scanning:
In this type of scanning, the image is cut into two halves.
Each half–image (also known as a field) consisting of either even rows of raster or odd rows of raster.
If the European standard is taken into consideration to reconstitute the 25 images per second, one will thus have field frequencies of 50 Hz: 25 even fields inserted between 25 odd fields.
Two half images are thus formed which give an image without flicker, each field passing very quickly in front of the eye.
5.5. Command signals:
5.5.1. Synchronization signal:
To function without risk of image deformation, signals are provided to the monitor which will enable it to detect the changes in the raster and the field. In addition, they allow extinguishing of the spot for the return of the scanning.
the synchro raster: it indicates to the monitor the raster signal change and is represented in the form of impulses.
the field synchro: it indicates to the monitor the field signal change . To ensure a perfect interline spacing the synchro rasters must be maintained even during the field synchro.
5.5.2. Code signal for luminosity or luminance:
Luminance is analyzed and translated using an electric signal whose amplitude is proportional to the luminosity of the pixel. This signal is sent on the wehnelt tube with the following convention:
maximum level: brightness level, a spot with the maximum brightness,
minimal level: black level , black spot.
6. THE DIRECTED BEAM SCAN:
6.1. Introduction:
As the visual display systems keep changing, significant efforts were devoted to the improvement of the situation's perception.
Indeed, whatever the system and how much it is automated, the pilot still remains the only one to decide.
In order to facilitate the decision-making as much as possible, it is essential that any ambiguity be avoided about the source of the presented information. The use of indicators, carets or symbols ensures a much faster and precise comprehension of the situation.
However, in the conventional visual displays, PPI scanning does not allow many elements to be displayed. This scanning is permanent, whether information is present. Thus, a precious time is wasted as it could have been used for other tasks. Consequently, the idea of developing a type of scanning, which would be carried out only when information must be present came.
The basic principle for this type of scanning is very simple. Rather than of scanning the whole screen, it goes directly to where the information must be displayed. Consequently, there is a considerable time saving which could be used to trace greater number of symbols.
6.2. Symbols positioning:
The exact location of the symbol layout could be given automatically by the system according to the detections or received information.
This designation is recognized by the system in the form of co-ordinates. The screen is indeed cut out according to an X Y grid, which allows a precise positioning.
The co-ordinates are converted into currents supplying the deflection coils. Thus the spot is directed towards the indicated point.
6.3. Symbols layout:
All the symbols, which can be used by the system (sometimes several hundreds), are stored in the memory. The spot being positioned, the system removes from the memory the symbol to be registered. According to the visual display, two types of layout can be used.
6.3.1. Vectorial layout:
The symbol is traced in the form of lines or curves.
Example: symbol layout of "=“
This layout will be carried out in three steps.
1st step: the laying out of the higher bar is an horizontal displacement corresponding to the length of the segment. The tube is switched on.
2nd step: the tube is switched off. We move horizontally to return to the initial value and vertically so as to be positioned at the same level as the second segment.
3rd step: The tube is switched on again. The vertical position is maintained and we move horizontally along the length of the segment.
The memorizing of the symbol corresponds in fact to these various displacements and times that the tube is switched on or off.
This type of directed beam scan makes it possible to register a very large variety of symbols; the only restriction is the size of the storage memory.
6.3.2. Point by point layout:
This type of layout does not allow a very large variety of symbols since they are all defined according to the same matrix (in general 5 X 7), but its advantage is simplicity.
The displacements will always be the same whatever the symbol to be registered; only times of switching the tube on and off will be different.
From the initial position on the screen (X0,Y0), the displacement of the spot is increased by a value of Δy (representing the difference between two points) whether the spot is switched on or not. The value is brought back to Y 0 and X 0 is increased by Δx so that the spot is shifted by one column and the process is started again until the symbol has been written completely.
Here, the memorizing of the symbol corresponds only to the when the tube is switched on or off, since all displacements are carried out according to the same matrix whatever the symbol, from where the simplicity of the process is.
Presenting the symbols to the crew definitely facilitates the evaluation of the situation. On the new aircraft generation, the flight has been divided into phases where the information is presented when they are normally requested by the crew.
7. DIGITIZATION:
7.1. Introduction:
Originally, the digitization was mainly used for radar image by the PPI scanning or by screen scanning on a CRT. Nowadays, the screens with pixel matrix structure also impose the digitization of the images whatever the scanning being used.
The digitization of the video allows a very small contrast improvement by memorizing the image and by presenting it as long as it does not move. Moreover some processes authorize a better perception of the situation by the crew.
These reasons explain the increasingly frequent use of the digital video.
The video, digitized and memorized, is redistributed several times at frequencies higher than the retinal persistence frequencies, which is approximately of 15 Hz.
The video is renewed by emptying and by loading the memory with the new data.
To be able to memorize the video, first of all it should be transformed so as to be easily stored.
7.2. Digitization of the video:
7.2.1. Components of the screen:
The digitization of the video is based on the fact that the screen is considered as lines.
Each line is broken up in" n” elementary points called pixels.
A screen is composed of “p” lines, each one divided into “n " pixels. All the lines are called field.
Thus this field consists of p x n pixels.
This field will be renewed with frequencies much higher than the retinal persistence frequencies of 15 Hz, that is 50 to 60 Hz.
7.2.2. Sampling and balance:
The analogical video from the receiver is transformed in an Analogue to Digital converter (ADC).
The video is sample at a rate such that: a sample = a pixel= a cell.
The balance principle is that each video sample is associated to a level known as the weight. Each weight shows a level of luminosity: In the following example “0” corresponds to the black and “3”to maximum brightness. Each weight is shown in binary.
We will have four possible levels and thus a code of luminosity of two bits (L=2). If we want to increase the number of levels, it will be necessary to increase the number of code bits (L=3 or 4) and thus the complexity of the system.
At the end, each point on the screen, having a line and column known as co-ordinates, receives a code of luminosity.
See below: .Digitization of a video line
7.3. Arrangement in memory:
The bits representing the luminosity are arranged in the memory at the sampling rate. Each line in our example, loaded in two stacks of type FIFO (First In First Out). This implies that the necessary memory capabilities depend on the number of lines, the number of points per line and the number of code bits of luminosity.
To obtain the desired contrast, the one line video is loaded once, then recirculated as many times as necessary before the appearance of the refreshed data of this line. That means, as long as the data about a line has not been modified, this line will be presented on the screen in the same way.
At the input of the FIFO register, a command allows new data to be entered or the old ones to be recirculated. This command is controlled by the change of line information. To be displayed, the video is injected on the wehnelt of the tube. This is done by a Digital Analogue Converter (DAC) which transforms the digital video into analogue.
Before this last operation, some processes can be carried out on the digital video.
7.4. Digital processing of the signal:
The signal can be processed in three ways:
to improve the quality of the image,
to improve the resolution of the image,
to improve the way the image is presented.
7.4.1. Quality improvement:
The aim of this process is to eliminate the possible remaining interference. It is based on an autocorrelation of the signal.
This correlation is carried out:
on a line by comparing three adjacent cells,
by the same principle on a group of three contiguous lines.
The principle of the correlation is as follows:
The weight of the central cell's image is compared to the weight of the adjacent cells,
if the two adjacent cells have the same weight, the weight of the central cell is increased up to the weight of the contiguous cells, otherwise nothing is changed.
For example:
7.4.2. Resolution improvement:
This artificial process multiplies at the memory output the number of lines by two. For that, two contiguous lines are considered and the average of the video's weight cell by cell is made.
Thus a new line is obtained where each cell has for weight the average of the contiguous cells and it is inserted between the two existing lines.
For example:
7.4.3. Presentation improvement:
Several possibilities exist:
a) Indicators and symbols:
At the location of a distance indicator or azimuth, the video level is increased to an immediate heavier weight and the symbol appears in intensified brightness.
For example:
b) Outline function.
Some echoes reflect very strongly the radar waves (reliefs, clouds, close echoes...). In order to avoid the saturation of the screen and tiring the sight of the operator by large and very brilliant echoes, only the outlines of these echoes whose level is increased to the maximum brightness are represented.
The cells located inside the delimited area are decreased to 0.
For example:
