Introduction
52. The most widely used navigation aid for low level VMC operations is the topographical map.
Such a map allows very accurate pinpoints to be obtained, and also presents information about the aircraft's position in relation to its surroundings in a relatively easily assimilated way. However, the use (in small aircraft cockpits) of conventional maps covering large areas presents handling problems therefore moving and electronic map systems have been devised to overcome these difficulties.
53. Early systems used strips of paper maps wound on rollers with an overlying cursor to indicate position where the rollers and cursor were driven by outputs from a doppler or radio navigation aid.
The majority of current systems use maps projected from a film strip, and tend to be driven by an inertial or other automatic navigation system. However, progress is being made in the realm of electronically produced maps, and these may become more prevalent in the future.
Projected Moving Maps
54. The most usual technique for displaying a moving map in low-level, high speed aircraft is the projected map display (PMD) driven by an inertial or mixed inertial navigation system. The map is reproduced onto 35 mm film and back-projected using conventional optics to give a bright image on a translucent screen.
55. The screen is designed to concentrate the image luminance within a limited field of view, matched to the observer's eye, in order to increase the resistance to strong ambient light. A typical system is illustrated in Fig 12-10, where it will be seen that the screen has three layers. The first (inner) layer is a Fresnel lens which converts the light cone from the projection lens into a light
cylinder in the plane of the operator's eye datum. The image is formed on the second layer which is designed to minimize hot spots towards the centre, and image degradation towards the circumference. The third (outer) layer is a polarizing filter which eliminates reflections from both inside and outside the PMD which might otherwise obscure the image.
56. In a typical PMD, the map is photographed in
segments onto 35mm film, and a coverage of up to 4 million square miles at a scale of 1:500,000 can be reproduced on a 20 metre strip. In practice some area coverage will usually be sacrificed in order to have a selection of map scales available, and there may also be check lists and terminal charts included. In normal operation the change over from one frame to the next is automatic and is usually accomplished in under three seconds. The life of the film strip tends to be limited by the currency of the map, fading, or wear and tear.
Fig12-10: Schematic of Projected Map Display (PMD)
57. The accuracy of a PMD is governed by the accuracy of the driving navigation system, and by manufacturing tolerances in the electro-mechanical projection system. Errors due to map scale and convergency limitations are reduced to relatively insignificant levels by automatically applying a correction to the map drive system, or by applying a correcting distortion during the photographic process. Typical values for the overall accuracy of the system are ¼ nm on a 1:500,000 map and 50 metres on a 1:50,000 map.
58. In a PMD the scale change is accomplished by increasing magnification rather than by changing the map. The fairly complex electro-mechanical system is one of the drawbacks of current PMDs, and although reasonable levels of reliability have been achieved, the use of many moving parts leads to mechanical wear and failure. It is this aspect which is largely driving the development of purely electronic alternatives.
59. The other disadvantage of the technique is that the film strip is not produced at squadron or station level and cannot be easily amended to reflect latest changes, or to incorporate up-to-date tactical information.
Combined Displays
60. The PMD overcomes the problem of handling paper maps in a small cockpit. However, the equipment uses considerable cockpit space for just a single function, and the current trend is to combine the moving map with the display from other electronic systems, principally radar. Two techniques are used:
(a) Ported CRT. The ported CRT is a conventional CRT with a transparent window in the envelope through which the film image can be optically back-projected on to the phosphor surface. The phosphor must be
selected so that it can be used both as a back-projection screen and for writing the electron beam.
Since the electron gun and the projector cannot both be on the optical axis of the system, one of the images must be distorted to allow them both to be correctly harmonized for simultaneous viewing. A simplified diagram of a ported CRT system is illustrated in Fig 12-11.
(b) Optically Combined Display. The optically combined display combines the optical and combined display is shown in Fig 12-12.
Fig 12-12: Arrangement of an Optically Combined Display
Fig 12-11: Combined CRT and Projected Display Using a Ported CRT
61. Both techniques are capable of over-writing the topographical map with cursive symbology thus allowing routes, danger areas, and tactical information to be added or amended.
Electronically Displayed Maps
62. Combined display systems overcome some of the problems of keeping the map up to date, but the problems inherent in an electro-mechanical device remain. A preferred solution is to replace the projected map by an electronically derived version in one of two following forms:
(a) Digitized (or raster) Maps. Most of the displayed maps are digitized versions of the familiar paper originals. An electronic scanner is used to scan the paper map in a raster fashion either as a complete entity or as separate overlays. For example, the air information, including airfields, restricted airspace and obstructions, can be scanned separately from the base topographical map, to facilitate more efficient updating. Digitized maps, which are also referred to as ‘raster’ maps, have the advantage that the source data in the form of paper maps is already available at the scales and coverage required. Disadvantages include the fact that a digitized map is expensive in terms of digital storage requirements and also that complex algorithms are required to process and reduce the data for display on small CRTs while still retaining the required detail.
(b) Digital (or Vector) Maps. Digital maps are those that contain separate feature types as data sets in a digital data base. Features such as coastlines, contours, railways, roads, woodland etc are constructed in vector format and combined on the display to provide a usable map. Digital (or vector) maps are more efficient in terms of data storage requirements and they offer great flexibility in what is displayed on the screen - unwanted data sets can be de-cluttered from the display as required. The major disadvantage of the digital map concept is that, currently, only limited data base production has taken place.
63. Both types of maps allow superimposition of latitude/longitude grids and other mission data.
Mission data can be produced in a ground-based facility and transferred to the aircraft at initialization or inserted manually by the aircrew at any stage throughout the mission.
64. Various data storage techniques have been investigated. These include holographic, magnetic, bubble, optical disc and random access memory (RAM). Both optical disc and RAM have been used in airborne applications for data storage. Each can now provide sufficient data storage for a high speed ground attack mission and can be manufactured with adequate ruggedness to survive the harsh airborne environment.
CHAPTER 13