colour for air traffic control displays

Colour for air traffic control displays Linda Reynolds

Air traffic controllers of the future will be working with full- colour raster-scan displays. This paper describes an approach to the use of colour whereby the objects on the display are assigned to a series of conceptual layers which are in turn represented as visual layers. Background map features are shown as opaque infills, overlaid with transparent infills for overlapping areas; alphanumeric labels in the foreground are shown in black, each with an attached infill to ensure good legibility and effective colour coding. Colour palettes are provided for each layer so that the display designer has flexibility but can be confident that the display will be free of colour illusions and ambiguities. The resulting displays can be used in normal office lighting. The work was undertaken for the Chief Scientist’s Division of the National Air Traffic Services (part of the UK Civil Aviation Authority).

Keywords: colour, transparency, maps, air traffic control

Until relatively recently the displays used by air traffic controllers have been in monochrome, usually green lines and symbols on a black background. With the advent of high- resolution colour displays, some air traffic control (ATC) centres now display lines and symbols in a range of colours against a black or uniformly dark background. So far the approach has been, essentially, to apply colour to the monochrome picture, but given the possibility of using infilled, coloured backgrounds, the question of how colour might best be used to facilitate the controller’s task becomes much more complex. A completely new approach was required.

Although there has been research on the selection of optimum colour sets for displays, much of it relates to the use of coloured lines and symbols on a black background, and to the use of colour for coding (e.g. Carter and Carter’, Laycock’, Gill and Trigg’, de Corte4). There has been relatively little work on more complex displays that incorporate infilled background colour (e.g. Spiker et c~l.~, Kaster and Widdel’). Furthermore, most of the work has

Linda Reynolds & Associates, 27 Spencer Road, Chiswick, London W4 3SS, UK Paper received 15 February 1994; revised 3 May 1994

1994 Civil Aviation Authority

been carried out by engineers, psychophysicists, psychol- ogists and human factors specialists, with little or no input from graphic designers. The UK Civil Aviation Authority felt that such an input, when based on a knowledge of colour perception, could provide the new approach that was needed.

It was hoped that the judicious use of colour would provide a number of advantages, as compared with existing monochrome displays, while at the same time avoiding colour illusions and colour ambiguities. Performance benefits from-the use of colour are notoriously difficult to demonstrate, but colour displays are undoubtedly popular with users and can improve motivation7-“. The following additional benefits were also sought:

l A reduction in the controller’s information processing load by visually emphasizing the most important data and thus directing attention to them.

l The possibility of adding information not currently available on ATC displays, without significantly increasing the visual complexity of the display.

l The creation of a visually more restful display. l The ability to use the display at normal office lighting

levels.

The initial brief was to develop experimental full-colour radar displays in static form and to produce guidelines for the use of colour in ATC displays”. An early stage of the work has recently been reviewed by Hopkin”. This paper describes the thinking behind the experimental displays. The work is still in progress; the colour palettes described here may be subject to fine tuning and more colours may be added to some of them, but the underlying principles will remain unchanged.

VISUAL AND CONCEPTUAL LAYERS

Visual layering

The typical ATC display consists of static background map data and dynamic foreground data relating to the aircraft.

0141-9382/94/040215-l 1 0 1994 Butterworth-Helnemann Ltd Displays Volume 15 Number 4 1994 215

Colour for air traffic control displays: L Reynolds

On a monochrome display the map data have much the same visual importance as the dynamic foreground data, as may be seen from Figure I. As far as the controller is concerned, the dynamic data are the most important items on the display; the map data need to be there for reference only. It was felt, therefore, that one of the main uses of colour should be to separate out the various objects on the display and to give them appropriate emphasis by creating a series of visual layers. This, it was hoped, would reduce the visual complexity of the display and allow additional objects to be displayed where necessary.

Ordnance survey maps provide a superb example of visual layering. The more important objects, such as roads and town names, seem to lie on a plane closer to the user, while ancillary objects such as contours, forests, etc., seem to lie on a plane farther away. This is achieved by the subtle use of colour. The aim was to create a comparable effect on ATC displays by first grouping the displayed objects into a series of conceptual layers and then representing these layers visually using colour.

In addition, it was required that the use of colour should create a more restful display picture that could be used in normal office lighting, and that colour coding should be available in the foreground layers.

Seven conceptual layers

Taking the London Traffic Manoeuvring Area (TMA) as an example, the CAA listed the objects on a typical display and identified seven conceptual layers on which those objects might fall. They were as follows.

1 Background areas: These are map elements such as land and sea, controlled airspace, sectors, danger areas, control zones, and so on.

2 Background detail: These are also map elements. They would include inherently linear objects such as range rings and airway centre-lines, and symbols for features such as airports and reporting points.

3 Low foreground: This would consist primarily of dynamic radar data relating to aircraft not of immediate interest to the controller. For example, aircraft currently being handled by another controller might be assigned to this layer.

4 High foreground: This would consist primarily of dynamic radar data representing normal air traffic which the controller is currently handling.

5 Low level alert: This layer would be used for dynamic radar data representing aircraft that are not in an emergency situation of any kind, but which the controller needs to be especially aware of for some reason. A Royal Flight, for example, might come into this category.

6 Emergencies and conjlict alert: This layer would be used for aircraft in an emergency situation, such as an engine failure or a hijack, or to indicate aircraft that would no longer have the required minimum horizontal and vertical separation if they were to continue on their present courses.

7 77ze cursor: This must be clearly visible a;all times, so it was assigned to the top layer.

The aim, then, was to use colour to represent these seven conceptual layers as seven visual layers.

THE NEED FOR A NEW APPROACH TO THE USE OF COLOUR ON ATC DISPLAYS

The subtle visual layering required cannot be achieved simply by adding colour to the symbols and lines typically shown on a monochrome display.

If small objects such as letters, numbers and lines are to be accurately discriminated from one another, the colours will need to be relatively highly saturated. This will limit the number of colours that can be used for colour coding, and will severely reduce the chances of achieving any kind of visual layering. Furthermore, on a CRT there will inevitably be differences in the luminance, and hence in the perceived brightness, of these highly saturated colours. A fully saturated yellow, for, example, will have a higher luminance and appear brighter than a fully saturated red. This will result in differences in visual prominence (conspicuity), which will further limit the usefulness of the available colours.

Highly saturated colours on a black background will also cause other difficulties. Different wavelengths of light are refracted by different amounts as they enter the eyes (chromatic aberration), so re-accommodation may be necessary as the viewer looks from one highly saturated colour to another. This could be fatiguing for older users’“. The phenomenon of chromostereopsis may also occur, causing highly saturated reds and blues to appear to lie in different planes in relation to the surface of the screen”.

In addition to these problems, there are further disadvantages associated with the conventional black background. Such displays will usually require subdued ambient lighting to reduce the effect of reflections on the screen. Where the user’s task also involves printed materials these will probably need to be separately illuminated. The resulting luminance difference between page and screen is likely to cause changes in pupil size as the user glances from one to the other, and this will result in changes in depth of focus’“.

These difficulties have been overcome in the experimental displays described here (see Figure 2). The background is of medium luminance, resulting in displays that can be used comfortably in normal office lighting. Figure 2 shows the same data as Figure I, but the relative importance of the various objects displayed is now immedi- ately apparent. The medium-luminance, low-saturation background colours allow scope for the use of higher luminances and saturations in the foreground, and the range of usable foreground colours is sufficient to allow for colour coding within each of several levels of emphasis.

The selection of colours for the experimental displays followed logically from the advantages that were sought from the use of colour for ATC, considered in relation to some basic perceptual principles underlying the effective use of colour.

216 Displays Volume 15 Number 4 1994


Figure 1 An example of a typical monochrome ATC display. The boundary lines represent controlled airspace, the London Traffic Manoeuvring Area, danger areas, control zones and a military corridor. The data blocks give the callsign of the aircraft and its flight level in hundreds of feet. (Note: the figures on this page were printed directly to 35 mm slide film from a Macintosh computer.)

Figure 2 The map data are exactly the same as those in Figure 1. The data-block colours might indicate the following: darker grey (layer 3) for aircraft outside the sector of interest, lighter grey (layer 4) for aircraft currently within the sector and under control, buff (layer 4) for aircraft entering or leaving the sector. blue (layer 5) for a low-level alert, and red (layer 6) for a Conflict Alert

SOME BASIC COLOUR PRINCIPLES

Before describing the experimental displays in detail, it may be helpful to summarize some of the perceptual principles on which the selection of colours was based. The effects of the use of colour on the conspicuity, legibility and discriminability of displayed objects are of particular relevance.

Conspicuity

The aim was to create a series of visual layers by means of variations in conspicuity, or visual prominence (see WoodI and Taylor” for a discussion of the principles involved in creating visual layers on maps). Objects differing in conspicuity will appear to differ in emphasis or importance. Conspicuity depends on a number of factors, including the following:

??Luminance contrast: The luminance contrast between

Figure 3 A larger section of the monochrome map

Figure 4 The map data from Figure 3. shown aa colour inflls instead of boundary lines. Danger areas are shown in transparent red, control zones in yellow, military corridors in green, airways in white and the London Traffic Manoeuvring Area in a second layer of white

a displayed object and its background is the most important factor affecting conspicuity. The greater the luminance contrast, the greater the conspicuity of an object.

??Hue: Chromatic aberration causes reds to appear to advance towards the viewer and blues to recede. This enhances the conspicuity of saturated reds.

??Saturation: Against a low-saturation background. high- saturation colours will be more conspicuous than low- saturation colours.

??Size: If several objects of different sizes are displayed in the same colour against the same background, the largest object will be the most conspicuous.

??Number of other objects in the same colour: If a number of identical objects are coded using two colours of approximately equal conspicuity , the objects in the colour that occurs least often will appear to dominate the display. They will be seen as a group or ‘figure’ against the ‘ground’ or background of the more frequently occurring colour (see Taylor” for a discussion of perceptual grouping).

Displays Volume 15 Number 4 1994 217


Legibility

The legibility of text and other small symbols depends largely on the luminance contrast between the text and its background. A contrast in hue and/or saturation alone is not sufficient because the human visual system relies mainly on luminance differences rather than hue differences for the detection of edges between differently coloured areas; luminance differences are also needed to trigger the focusing mechanism”. Edges between contiguous colours of equal luminance will appear blurred and will be unpleasant to look at.

The direction of contrast will also affect legibility. Small, bright images on a dark background appear to spread so that letters or numbers may appear to fill in or run together. This phenomenon is known as irradiation16. Small, dark images on a light background may appear thinner than the same size bright images on a dark background, but there is less likelihood of critical detail becoming illegible.

Discriminability

The ability to discriminate accurately between colours on a display is of particular importance when colour is used for coding. For this purpose, each colour must be correctly identified even when the full colour set is not available for comparison. The following factors were borne in mind in selecting colours for coding (see Silverstein” for a review of factors affecting colour discrimination):

??The greater the area of colour, the greater the ease of discrimination. Solid blocks of colour will therefore be more easily discriminable than differently coloured alphanumeric characters.

??Colours for coding should be as different as possible in hue.

??Differences in luminance as well as in hue will improve colour discrimination. However, differences in luminance will result in differences in conspicuity and hence in perceived importance. This may not be desirable if the categories to be coded differ only in kind and not in importance. In relation to ATC displays, the concern was that there should be differences in conspicuity between layers, but that differences in conspicuity within layers should be minimized. Colours in a set within a given layer were therefore of approximately equal luminance.

??High-saturation colours are more easily discriminable from one another than low-saturation colours. However, highly saturated colours can introduce a number of undesirable perceptual artefacts, as described by Walraven13. The use of such colours was therefore confined to layers 5 and 6, used for alerts. Colours used for coding on layers 3 and 4 were of medium saturation, but were judged to be adequately discriminable given that the colour always appears as a block rather than in the form of small alphanumeric symbols.

The effect of background colour on the perception of small areas of foreground colour must also be considered. Bright,

highly saturated backgrounds can cause a shift in the perceived hue, luminance and saturation of small foreground areas18. It is important to avoid such colour contrast illusions if colour is to be used for coding in the foreground.

THE MODELLING ENVIRONMENT

The experimental static displays were developed using an Apple Macintosh computer and a Radius 20in colour monitor. The white point and gamma of the monitor, as specified by the manufacturer, were 9300 and 1.8, respect- ively. The drawing package used was Aldus FreeHand. This was particularly suitable because it allows for the organization of the image as a series of layers which can be turned on and off at will.

The colours were specified in FreeHand using RGB (red/green/blue) values. The output of each of the three guns is expressed as a percentage, so R90/G9O/BlO would be a slightly desaturated yellow in which the red gun was set at 90 % of its full output, the green gun also at 90 % , and the blue gun at 10%. This method of specification has the disadvantage of allowing only very approximate duplication of colours from one monitor to another and from one computer system to another. Ultimately, therefore, the recommended colours will be specified in terms of CIE (Commission Intemationale de 1’Eclairage) 1931 xyY values so that they can be exactly reproduced on multiple monitors or on other systems.

In the selection of colour sets, differences and similarities between colours in terms of hue, saturation and luminance were judged by eye on the basis of experience. For practical purposes, brightness and luminance were assumed to be perfectly correlated, as suggested by Travis”. The relationships between the recommended colours will be checked by converting the CIE 193 1 xyY values to L*u*v* values within the perceptually uniform 1976 CIE LUV colour space, and minor adjustments will be made where appropriate.

THE CONCEPT OF COLOUR PALETTES

The kinds of objects that need to be shown on an ATC display will depend on the job that the controller is doing. For example, displays typically used for controlling aircraft taking off from or landing at a major airport will differ from displays used for controlling aircraft that are en route across Britain. Displays for transoceanic routes will differ yet again.

It would therefore not be feasible to attempt to specify a single set of colours that would meet every need, nor to specify a different set for each application. Instead, the aim was to provide a toolkit that the display designer could use to build up a display for a particular application. This toolkit comprises a series of palettes of colours, one for each visual layer. The display designer would first sort the objects to be displayed into layers, then choose colours from the appropriate palettes. By using colours only from


Colour for air traffic control displays. L Reynolds

these palettes, the designer could expect to gain the benefits of colour without introducing any illusions or ambiguities.

LAYER 1: BACKGROUND AREAS

General principles

In the past, map features such as danger areas, control zones, airways and sectors have had to be represented by boundary lines, as shown in Figure 3. Multiple overlapping boundary lines create visual clutter and can be confusing, particularly when the scale is such that larger objects are not visible in their entirety. The aim, therefore, was to use infilled areas of colour to reduce clutter and ambiguity. The result is shown in Figure 4, which contains the same information as Figure 3.

Inhlled colour will also avoid the disadvantages of the conventional black background. Increasing the luminance of the background reduces the effect of reflections, which means that displays can be used in normal office lighting. Where the task involves printed materials as well as the CRT, there is less likelihood of differences in overall luminance causing the pupil size to change substantially whenever the controller looks from one information source to the other14. The higher the luminance of the display, the smaller the pupil size and the greater the depth of focuszo. Increased luminance may also provide a more effective stimulus for accommodation*‘. A higher-luminance background has the additional advantage of transmitting more light to the eye, thereby improving both visual acuity and colour discrimination”. Enhanced colour discrimination increases the range of colours that can be used.

The choice of background infill colours was determined by the following considerations:

??The background should not dominate the display. Back- ground map data should be clearly visible if needed for reference, but they should not distract attention from foreground data in higher layers.

??Background colours need to be sufficiently different from one another for a clear boundary line to be visible between contiguous areas, but they do not necessarily need to be uniquely identifiable. Air traffic controllers will already be familiar with the map data shown on the display and will be relying on context rather than colour cues for their recognition.

??Background colours should not be so bright or so saturated that they distort the appearance of small areas of foreground colour superimposed on them.

a Background colours should be sufficiently close in luminance and low in saturation to prevent the formation of noticeable after-images. After-images resulting from areas of highly saturated colour will temporarily interfere with normal colour perception and could result in errors in identifying small areas of foreground colour.

For these reasons, colours of moderate luminance and low saturation were selected for use as background infills.

Brighter and more saturated colours were held in reserve for use in higher layers.

Two kinds of infill

One of the difficulties in representing areas on ATC radar maps is that objects can overlap one another. For example, the coastline may be straddled by a danger area, which may lie partly under an area of controlled airspace. Super- imposed on this there may be a sector boundary. If all the boundaries are shown as lines there is a risk of confusion, but if they are all shown as solid infills, some of the boundaries will be obscured by overlying objects. Two kinds of infills were therefore defined: opaque and transparent. Opaque infills are intended to be used for areas that are mutually exclusive (e.g. land and sea), and for areas that will never need to have other boundary lines running beneath them (e.g. danger areas on a map where the coastline is not required). Transparent infills are for areas that may be required to overlap or overlay other features. Airways, for example, may overlap the coastline, danger areas and control zones. Figure 5 shows the Southend area, where the opaque colours used for land and sea are overlaid by four transparent colours representing danger areas, control zones, controlled airspace, and the London TMA (Traffic Manoeuvring Area). All boundaries are never- theless clear and unambiguous.

The opaque infill palette The opaque infill colours are illustrated in Figure 6. There are three sets of the same six hues, each set having a different luminance. The luminance differences between the sets are just sufficient to give a clear boundary when colours from adjacent sets appear contiguously on the screen, but not so great as to distract attention from the foreground data. The colours within each set have approximately the same perceived brightness when viewed against a medium luminance grey background.

Bach set of hues was created from a neutral grey by adding just sufficient colour to enable hues in the same set to be distinguished from one another when contiguous. The red, blue and green were created by increasing the output from one gun at a time, and the buff and blue/green by increasing the output from two guns at a time. The aim was to select hues that were as different as possible, and which would at the same time be visually pleasing when used at higher levels of saturation and luminance on other layers. The suitability of the hues for representing map data was also considered. For example, if opaque colours were used to make a distinction between land and sea, the sea might reasonably be represented by either blue, blue/green or grey, and the land by red, buff, or green.

The designer of a display for a particular ATC application would choose one of the colours from set 2 for the largest area of the map. (This set has the optimal background luminance for the transparent overlay colours.) Colours from set 1 and set 3 would then be chosen for other non-overlapping areas. (The same hue could be chosen from each set, but this may lead to ambiguities if white or



Figure 5 An enlargement of the Southend area, showing four layers of transparency. Note that the data blocks are fully opaque in this example. (Note: the figures on this page were printed direct to 35 mm slide film from a Macintosh computer)

Figure 6 The layer 1 palettes, for opaque and transparent background infills

Figure 7 The data-block infill colours from the palettes for layers 3-6

black transparent infills are subsequently overlaid.) In the examples shown in Figures 2,4 and 5, the red from

set 2 has been used for land and the blue from set 3 for sea.

Metelli23.26.27 argues that the perception of transparency comes about through colour scission, which is the reverse of colour fusion, or mixing. The mixture of the transparent colour and the background colour is known as the stimulus colour. This is perceptually split into two scission colours, one of them being assigned to the transparent layer and the other to the background layer. For example, if a green lawn is viewed through the reflection on a window of a red book lying on the windowsill, the stimulus or mixture colour is in fact a yellowish green. This can be seen by careful inspection, but the more usual perception is of a green surface seen through a transparent red layer. The yellow stimulus colour is scissioned into red and green. The transparent intill palette

The hues of the six transparent infills (see Figure 6) were Assuming that colour scission is the reverse of colour selected to coordinate with those of the opaque infills, and fusion, Metelli23*26 gives a formula (based on Talbot’s law also for their potential associative value. The saturation of of colour fusion) for predicting the reflectance of an achrom- each transparent infill is just sufficient to give a clearly atic colour overlaid by an achromatic transparent layer:

discriminable hue when overlaid on any opaque or other transparent infill. The luminance of each was chosen so as to give sufficient but not excessive contrast with underlying opaque or transparent infills. Some of the transparent infills are darker than the opaques (black, red and blue), and some are lighter (white, green, ochre).

Each transparent infill is applied at a specified level of transparency, expressed as a percentage. The level of transparency is just low enough to create a clearly discriminable colour difference at the boundary between the transparent overlay and the underlying infills.

In Figures 2, 4 and 5, red is used at 80% transparency for danger areas, ochre at 80% for control zones, green at 80% for inactive military corridors, white at 95% for airways, and another layer of white at 95 % for the London TMA.

Method of generating transparent infills FreeHand does not generate transparent overlays automatically, so it was necessary to find a means of calculating the gun values that would result when one or more transparent infills were superimposed on an opaque infill. The method used may be of interest.

Transparent colour mixtures can be generated on the basis of either additive mixing (as when coloured lights are mixed) or subtractive mixing (as when pigments are mixed). There has been some discussion in the literature as to which of these colour mixing principles gives the most reliable perception of transparency. Beck22 argues that subtractive mixing relates most closely to everyday experience, as when coloured surfaces are viewed through coloured glass. Metelli23 favours additive mixing. It could be argued that this relates to the everyday experience of viewing coloured surfaces through coloured reflections on clear glass. Da POSES found that the perception of transparency was more reliably achieved with additive mixtures (where the primaries are red, green and blue and the addition of colours results in progressively lighter mixtures), and this would seem to be the more appropriate for CRT displays. There seems to be general agreement that figural effects also play a part in the perception of transparency, though Beck25 attributes more importance to them than does Metelli.


Colour for air traffic control displays. L Reynolds

p = eta + (1 - CY)~

where p is the reflectance of the mixture colour, a is the reflectance of the underlying colour, r is the reflectance of the transparent colour, and CY is the percentage transparency required (expressed in the range 0, 1).

Beck er a1.28 argue that the perception of transparency is likely to be dependent on perceived lightness rather than reflectance or luminance values and that a psychophysical function would need to be introduced into the equation in order to predict quantitative judgements of transparency. However, the formula appears to give a good qualitative impression of transparency and it is recommended by Foley et a1.29 for the generation of transparent colours on CRTs. It was applied to the colours used in this study by carrying out a separate calculation for each of the three guns, using the RGB percentage values. Although this gives only a rough approximation to true reflectance or luminance values, the resulting transparency effects were found to be satisfactory in relation to the application described here.

LAYER 2: BACKGROUND DETAIL

General principles

Objects on this layer include the following:

??Small symbols (such as the squares used to denote airports and the triangles used for reporting points in Figure 2).

??Linear objects (such as range rings and airway centre- lines).

??Boundary lines for emphasis, or for areas that are too small or too numerous to be shown as background infills (see Figure 2).

These objects need to be visible against the background infills, but they must not compete with the foreground data.

The palette

A range of greys is recommended for both lines and symbols, relying on luminance contrast with the background to give adequate visibility. The reasons for this choice are as follows:

??The use of highly saturated colours would be intrusive and would reduce the number of colours available for use in higher layers.

??For use with lines and small symbols it would be difficult or impossible to arrive at a set of low-saturation colours that would be visible against all possible background colours and that could be discriminated from one another.

??The fewer colours there are in the background, the greater will be the impact of colours used in the foreground.

??The use of two or possibly three different luminances in combination with different line thicknesses will be

sufficient .to code the number of different kinds of line likely to be required. Different line patterns could also be used if necessary.

??Symbols can be coded by shape, and also by the use of luminance differences if necessary. The main concern was that symbols in layer 2 should not compete with those used in layers 3-6 to represent aircraft positions. (Because of the infilled background, solid rather than open symbols are recommended. They will be more visible and more easily discriminable from one another.)

The optimum luminance values for the greys used in layer 2 will depend on the designer’s choice of background infills for a particular application. A display that incorporates two or three layers of transparent white covering a large area of the background will require a different range of greys from those that would be appropriate for a display with one or no layer of white. The designer must therefore select greys that have sufficient contrast with the background to be visible but not intrusive, and sufficient contrast with one another to be discriminable. Relatively subtle judgements are required in selecting the appropriate luminance values and these can only be made in relation to a known combination of background colours.

The choice of line thicknesses and symbol sizes must also be taken into account when selecting luminance values. For example, if two lines of different thicknesses are intended to have the same conspicuity and both are lighter than the background, the thinner line will need to have a higher luminance than the thicker one. Similarly, if a boundary line and a symbol are intended to have the same conspicuity, the symbol may need to have a higher luminance than the line.

LAYERS 3 AND 4: LOW AND HIGH FOREGROUND

General principles

The low foreground and high foreground layers contain dynamic radar data, typically:

??A target symbol indicating the position of each aircraft. It may be a requirement that the symbol should indicate the type of radar return associated with an aircraft.

??A data block (or label) associated with each target symbol. The data block usually shows the callsign of the aircraft and its current flight level, but there may be more information than this.

Associated with each target symbol there may also be:

??A ‘history’, usually shown as a trail of slashes or dots, which shows what the position of the aircraft was on previous screen updates. Histories were an unavoidable feature of early radar displays, but with the raster scan displays currently in use they could be omitted. Never- theless, histories are considered to be of value because



they show when an aircraft is turning, and the spacing of the slashes or dots gives an indication of speed.

??A speed vector, usually shown as a solid line, which indicates where the aircraft will be in a given number of minutes’ time. The length of the vector gives an indication of the aircraft’s speed.

??A leader line, linking the aircraft symbol with its data block.

The purpose of layers 3 and 4 is to provide two levels of emphasis for this kind of data. There must also be provision for the colour-coding of differences in kind within layers.

Data on these layers must stand out clearly from the background infills and background detail, but there must be further scope for emphasis in layers 5 and 6, the alerting layers. All data blocks must be optimally legible.

Treatment of data blocks

The data block characters are shown in black for two reasons:

??Black characters on a light background are more legible than light characters on a dark background (as discussed under ‘Legibility’ above).

??The perception of white or other light-coloured characters may be distorted by the hue of their immediate background, even where this is of relatively low saturation. Black characters will not be affected in this way.

Each data block has its own infill (see Figure 2). The reasons for this are as follows.

??Legibility: For good legibility there must be sufficient luminance contrast between the data-block characters and their immediate background. Black characters superimposed directly onto the layer 1 background infills specified here will be legible, but legibility will not be optimal and it will vary depending on the luminance of the area on which the characters fall. The use of data- block infills gives control over legibility and ensures that it is consistent.

??Colour coding: If colour coding is required in the foreground, the colour can be applied to the data-block infills. This will ensure good legibility, which could not be guaranteed if differently coloured characters were superimposed onto the layer 1 background infills. The larger area of colour provided by the data-block infills (as compared with coloured characters) will improve colour discrimination and allow the use of a wider range of colours than would otherwise be possible. The coding will also be more conspicuous than it would be with coloured characters.

The palettes

The palettes for layers 3 and 4 are shown in Figure 7. The

colours have been chosen on the assumption that they will be used as data-block infills, rather than for the data-block alphanumerics themselves.

The same six hues (including grey) are provided at two different luminances, those for layer 4 having a higher luminance and hence greater conspicuity than those for layer 3. All of the colours are of relatively low saturation, but because they have a higher luminance than the background map they stand out clearly from it. Both sets of colours give good legibility with black alphanumerics.

The two levels of conspicuity and the six different hues can be used for colour coding, in whatever way the display designer feels is most appropriate for the application in question. Because the same six hues are available on both layer 3 and layer 4, the same coding by hue can be carried through both layers if necessary.

The hues within each layer, having approximately the same luminance and saturation and hence the same conspicuity, are intended to be used primarily to indicate differences in kind rather than differences in importance or emphasis. Categorical differences are best coded by hue because hue differences are not generally interpreted as implying relative values”. However, if the majority of data block infills on a given layer are in the same hue, with only a small number in a different hue, the small number will stand out clearly as a group or ‘figure’ against the predominant hue.

Possible uses of the two levels of conspicuity might include distinguishing between aircraft that the controller is currently handling and those that are not his or her direct responsibility, or distinguishing between aircraft that are above or below a selected flight level. Coding by hue might be used to distinguish various stages in the passage of aircraft into, through, and out of a sector, or to signal specific kinds of interaction between the controller and the aircraft.

It is not envisaged that all of the hues available on layers 3 and 4 will be needed in any given application; the intention was to provide the display designer with choice. It is recommended that the number of different hues in use at any one time on these layers should be kept to a minimum, and that grey should be the standard hue. The greater the number of colours in use on layers 3 and 4, the less will be the impact of the alerting colours on layers 5 and 6. The strength of the grouping effect of colour within layers 3 and 4 will also be diminished as the number of different hues increases.

The general principles of colour coding are summarized in References 9, 10, 19 and 30-32. Possible uses of colour for coding in ATC displays are discussed at length in References 14, 17, 20 and 33.

Data-block overlap

In a busy ATC sector, controllers have to cope with the problem of data-block overlap. On today’s displays the alphanumeric characters overlay one another and one or all of the data blocks involved may become totally or partially indecipherable for a short period. It is often at such times


. that *the controller most need6 to be able to read the data blocks. Controllers point out that because they have been continuously monitoring the progress of the aircraft involved, they can usually decipher enough to identify the aircraft and their flight levels. However, the situation is not ideal.

A possible option is to move one or more of the labels involved in an overlap, either automatically or by controller intervention. Controllers tend to dislike the unexpected spatial re-arrangement of labels, so a solution involving controller intervention is likely to be preferred.

In an attempt to alleviate the problem of overlap, two options are suggested for the presentation of data blocks.

Data-block injills opaque to text on underlying data blocks: The data-block infills are opaque, such that where two or more data blocks overlap, the infill attached to the upper- most block will partially obscure the alphanumerics on the lower one (see Figure 5). With this option, one data block will be legible in its entirety. The other data blocks involved in the overlap could successively be brought to the top by clicking on a projecting corner of a data block, by clicking on an aircraft symbol, or by holding down a key or mouse button to cause the blocks to come to the top cyclically.

Rules would be needed to determine the stacking order of data blocks in an overlap situation. Layer 6 data blocks would always be at the top of the stack, followed by those on layers 5, 4 and 3. Within layers, the most helpful stacking order would need to be determined on the basis of experience.

Opaque infills have a dark grey border. This ensures that there is no ambiguity as to which characters belong to which data block. Data-block infills transparent to text on underlying data blocks: In some situations, controllers feel that they would be losing data with opaque data blocks. The alternative, therefore, is for all overlapping data-block characters to be superimposed on top of the overlapping infills, as in Figure 2. The alphanumeric data available are then exactly the same as on present displays, but the advantages of the data block infills are retained.

Fonts

It is important that the font or fonts used should be maximally legible so that character sizes can be kept to a minimum. The smaller the character size, the smaller the data blocks and the less severe the overlap problem.

Most of the fonts that are readily available for use with systems such as X-Windows were not designed specifically for screen viewing. They were designed simply to mimic on screen the appearance of the documents that might be printed from the system. As a result, the type fonts are often confined to faces such as Courier, Times and Helvetica. These were not designed with the physical characteristics of the raster scan display in mind.

What is needed is a font or fonts designed specifically for viewing on screen as dark-on-light images. For legibility,

Coiour for air traffic control displays L Reynolds

bit-mapped fonts would be preferable to the vector-based fonts currently in use because they can be designed pixel by pixel to suit the visual characteristics of the display. Further improvements could be made by the use of fonts having more than one level of pixel luminance (greyscale fonts). For air traffic control purposes, a range of three character sizes, preferably in two weights, would be needed. There may also be a requirement for two distinct type faces, possibly a monospaced (single-width) face and a propor- tionally spaced (multiple-width) face, or a serifed and a sans serif face. The feasibility of designing and imple- menting fonts to meet these requirements is currently being investigated.

Aircraft symbols

The symbols representing the aircraft must always overlay any data blocks with which they coincide, so that they are never hidden.

With an infilled map background, aircraft symbols will be more conspicuous if they are solid shapes, rather than the open shapes generally used at present. Black is the preferred colour because it will always be clearly visible against both the map background and the data block infills on layers 3, 4, 5 and 6.

A circle is the preferred shape because trail dots and speed vectors attach more neatly to it than to a square, diamond or triangle. A solid black circle would therefore be the simplest symbol (see Figure 5). The optimum size will depend on the level of conspicuity required for the symbols in relation to the data blocks. A black circle infilled with white would give still greater conspicuity, as shown in Figure 2.

If there is a need to encode additional information within the aircraft symbol, such as the type of radar return, possibilities include the following:

??A solid black circle with a diagonal, vertical or horizontal slash in white. This, however, tends to reduce conspicuity somewhat.

??A solid black circle for the most frequently occurring category, and other solid shapes, such as the square, triangle and diamond, for less frequently occurring categories.

??A black circle with a white or other coloured centre. The colours would need to be highly saturated, however. Layer 5 colours not required for alerting could possibly be used.

These possibilities are still being investigated.

Trail dots

Trail dots are required as an option on the ATC displays of the future because they give a good indication of the speed and direction of the aircraft. The number of dots required may vary from five to nine or more, depending on the application.



Black will give the appropriate level of conspicuity for trail dots, but the dots then tend to interfere with data-block text. However, as an option, the dots could be made to run beneath any data-block infills with which they coincide. Alternatively, white trail dots could be made to run beneath the data-block characters but above the data block infills. The dots may then become too conspicuous overall, however. The optimum presentation of trail dots is still under investigation.

LAYERS 5 AND 6: LOW LEVEL ALERTS, EMERGENCIES AND CONFLICT ALERT

General principles

Layers 5 and 6 contain dynamic radar data, as described for layers 3 and 4.

Layer 5 is intended to provide a range of colours for low level alerts, indicating aircraft that need to be watched closely, but that are not in an emergency situation of any kind. Colours on this layer need to be more conspicuous than those on layers 3 and 4.

Layer 6 is for various kinds of emergencies, and for Conflict Alert. In a Conflict Alert, the system alerts the controller to the fact that if something is not done, two or more aircraft will be closer than is permitted by the rules governing the minimum separation between aircraft. Colours on this layer should have a high level of conspicuity and should be readily distinguishable from those on layer 5.

Data-block infills for layers 5 and 6 are always opaque and will always lie on top of any data-block infills on lower layers which they may overlap. This guarantees legibility of the entire data block for any aircraft involved in an alert.

The palettes

The layer 5 and layer 6 palettes are shown in Figure 7. The layer 5 colours are more saturated than those on layers 3 and 4. and because of this they have greater conspicuity (note the blue data block in Figure 2). The layer 6 colours are from the red end of the spectrum and are more conspicuous still. This is largely because red wavelengths are brought to a focus short of the retina, so the eye has to accommodate as if for a closer object in order to obtain a sharp image. Reds therefore appear to advance towards the viewer. Red also has a strong association with warning and danger and it would be a mistake to try and work against this.

Red is inevitably a dark colour on screen, however, because only one of the three electron guns is active. White type will therefore give better legibility than black. The addition of a white border gives still greater conspicuity (see Figure 2). Black type could be used with a lighter red, but this would mean using a less saturated red (i.e. a pink).

LAYER 7: THE CURSOR

The cursor must be easy to find at all times. A black shape with a white infill gives high conspicuity against all backgrounds. The ideal shape has not yet been determined, but it must give sufficient precision in pointing to allow accurate selection of objects on the display.

FUTURE WORK

The colour displays described here have now been implemented on a dynamic demonstrator developed at the Defence Research Agency at Malvern. The work so far has been shown to groups of air traffic controllers who are representative of particular ATC applications, to ensure that the colour palettes satisfy their requirements. The palettes will shortly be incorporated in an ATC simulator system at the Air Traffic Control Evaluation Unit at Hurn airport. Controllers will be able to use the displays to carry out normal ATC tasks in a realistic operational setting, thus enabling a thorough evaluation of the colour guidelines to be carried out.

Meanwhile, work continues on the use of colour for tabular data and for the windowed environment as a whole. Problems of colour measurement, monitor calibration, ambient lighting and font design are also under investigation.

CONCLUSIONS

To summarize, the aim has been to develop a set of guidelines for the use of colour in ATC displays, based on sound scientific principles. Colour is used not only for coding, but also to produce a ‘cleaner’ information display, a more restful display, and a better working environment. If the guidelines are followed, the ATC application designer can be confident that problems arising from the use of colour will be minimized, but at the same time the colour palettes offer a great deal of freedom and flexibility. The palettes are not intended to be a once-and-for-all- time solution to a particular problem, but rather to provide a basic toolkit that can be used for a wide range of applications.

ACKNOWLEDGEMENTS

The work described here was undertaken for the Chief Scientist’s Division of the National Air Traffic Services (part of the UK Civil Aviation Authority). It was carried out while the author was employed as a Research Fellow in Graphic Design at the Royal College of Art, London.

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GLOSSARY

Brightness The perception associated with luminance. However, the perceived brightness of a colour is also affected by a number of other factors such as the brightness of surrounding colours and the state of adaptation of the eye.

Hue The perceived attribute of a colour which is described by terms such as red, green, blue, yellow, etc. The hue is determined by the dominant wavelength or wavelengths, i.e. the wavelengths most heavily represented in the colour.

Lightness The extent to which a surface appears to be light or dark as compared with a surface that appears to be white. Light- ness is the approximate perceptual correlate of reflectance.

Luminance An objective measure of the effective intensity of light emitted from (or reflected by) a surface.

Reflectance The proportion of incident light reflected by a surface.

Saturation The perception associated with the purity of a colour in terms of the wavelengths represented in that colour. Saturated colours consist of a narrow band of wavelengths and appear vivid; desaturated colours contain many other wavelengths as well as the dominant wavelength and appear muted.


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