Real time video perspective digital map display method

Claims

What is claimed:

1. A method for storing and retrieving digital information in a system producing a perspective display, comprising the steps of:

storing digital data representing a two-dimensional pattern of elevation information in addressable memory locations of a memory;

generating signals representing the parameters of the desired perspective display including the height and location of the viewing point and the angle of the display with respect to the two-dimensional pattern;

reading elevation data from selected addressable memory locations of said memory through the generation of address signals and applying said address signals to said memory; and

transforming the address of each elevation data read out of said memory from the address of that point in the two-dimensional pattern to the address of a respective pixel position in the perspective display.

2. A method according to claim 1, wherein said digital data is stored in said memory with said pattern having a fixed orientation and wherein said reading step includes selectively generating said address signals so as to read out pattern information in a direction which may be selectively different from said fixed orientation.

3. A method according to claim 2, wherein said transforming step includes generating a signal .DELTA.E representing the difference in height between the viewing point of the perspective display and each elevation point in said elevation pattern, generating a signal R representing the distance parallel to said fixed orientation between each elevation point and the viewing point in said two-dimensional elevational pattern, and generating a signal .DELTA.W representing the distance between each elevation point and a line through the viewing point parallel to said fixed orientation.

4. A method according to claim 3, wherein said transforming step further includes generating transformed addresses a,b for each pixel according to the relationship: ##EQU9## where .alpha. is the angle of the perspective display with respect to said two-dimensional elevational pattern and K is a constant.

5. A method for storing and reading out digital information use in generating a perspective display of terrain over which a vehicle is moving on the basis of digital data representing a two-dimensional elevation terrain pattern which is addressable in relation to the coordinate position of each point in said terrain pattern, comprising the steps of:

storing said elevation data representing at least a portion of the terrain pattern of the terrain over which the vehicle is passing in memory;

providing signals representing the angle of heading of said vehicle with respect to a fixed geographical orientation, the angle of the perspective display with respect to said two-dimensional elevation terrain pattern, and the elevation and current position of said vehicle with respect to said terrain;

reading elevation data out of said memory through the generation of address signals and the application of said address signals to said memory; and

transforming, in response to said provided signals, the address of each point read out of said memory from the address of the point in the two-dimensional terrain pattern to the address of a respective pixel position in said perspective display.

6. A method according to claim 5, wherein said transforming step includes generating a signal .DELTA.E representing the difference in elevation between the vehicle and each elevation point in said terrain pattern, generating a signal R representing the distance parallel to the vehicle heading between each elevation point position and the vehicle position in said terrain pattern, and generating a signal .DELTA.W representing the distance between each elevation point and a line through the vehicle position parallel to the vehicle heading in said terrain pattern.

7. A method according to claim 6, wherein said transforming step includes further generating transformed addresses a,b for each point according to the relationships ##EQU10## where .alpha. is the angle of the perspective display with respect to said two-dimensional elevational terrain pattern and K is a constant.

8. A method according to claim 5, wherein said terrain pattern of data is stored in said memory with a fixed geographical orientation, and said reading step includes selectively generating said address signals so as to read out pattern information in a direction which may be selectively different from said fixed geographical orientation.

9. A method according to claim 5, wherein said terrain pattern of data is stored in said memory with a north-up orientation, wherein said reading step includes selectively generating said address signals so as to scan said elevation data in said memory either in a north-south or an east-west direction depending on which is closest to the direction of heading of the vehicle with respect to said terrain.

10. A method according to claim 5, wherein said reading step includes means for selectively generating said address signals so as to scan an area of said memory in successive lines which extend from a line through the point position corresponding to the position of the vehicle in the general direction of heading of the vehicle with respect to the terrain.

11. A method according to claim 5, wherein said reading step includes selectively generating said address signals so as to scan a generally diamond-shaped area of said memory, said area being positioned within said memory with one edge thereof being substantially transverse to the direction of vehicle heading with respect to said terrain pattern and the scanning of said area being in the direction parallel or transverse to the geographical orientation of said data in the scene memory, whichever makes the smallest angle with said vehicle heading.

12. A method for generating a perspective display of terrain over which a vehicle is moving, comprising the steps of:

storing, in memory, digital data representing a two-dimensional elevation terrain pattern which is addressable in relation to the coordinate position of each elevation point with respect to a selected geographical orientation;

providing signals representing the heading angle of said vehicle with respect to said selected geographical orientation, the angle of the desired perspective display with respect to said two-dimensional elevation terrain pattern, and the elevation and current position of said vehicle with respect to said terrain;

reading-out of memory, in response to said provided signals, elevation data for respective ones of said elevation points through the generation of address signals and the application of said address signals to said memory;

in response to said provided signals, transforming the address of each point read out of said scene memory from the address of that point in memory to the address of a respective pixel position in said perspective display;

storing the elevation data read out of said memory in a matrix of memory positions in a display memory, which form storage locations corresponding to said transformed addresses; and

displaying the elevation data stored in said display memory to produce said perspective display.

13. A method according to claim 12, further including

storing the data read out of said memory including the transformed address of each elevation in storage locations of a buffer storage corresponding to the storage locations occupied by said elevation data in said memory; and

transferring said data from said buffer storage to storage locations in said display memory in accordance with said transformed address.

14. A method according to claim 13, wherein said reading-out step includes selectively generating said address signals so as to scan a generally diamond-shaped area of said memory along lines substantially parallel or transverse to said selected geographical orientation depending on which makes the smallest angle with the vehicle heading, said area being positioned within said memory with one edge thereof transverse to said vehicle heading.

15. A method according to claim 13, wherein said reading-out step includes selectively generating said address signals so as to scan a generally diamond-shaped area of said memory along parallel lines substantially in the direction of the heading of the vehicle.

16. A method according to claim 15, wherein said buffer storage includes a column buffer memory having a plurality of columns for storing respective lines of data read out of said memory by said reading-out step in positions corresponding to the locations of said data in said memory.

17. A method according to claim 16, wherein each column in said column buffer memory has a capacity to store data associated with at least the number of points which occupy the diagonal of said generally diamond-shaped scan area of said memory, and further including the steps of storing the address locations for each column of said column buffer memory at which data begins and ends and, in response to said address locations storing step, reading plural data from adjacent columns of said column buffer memory.

18. A method according to claim 16, wherein said transferring step includes reading out plural data including the transformed addresses thereof in groups from adjacent columns of said column buffer memory by scanning said columns, in response to the data read out of said column buffer memory, generating additional elevation data representing data occupying positions extending along lines between data points of each group in accordance with the transformed addresses thereof, and transmitting said data and said additional data in said display memory.

19. A method according to claim 18, wherein said transferring step includes inhibiting, in response to the elevation data of each pixel to be transferred to said display memory, such transfer in connection with any pixel along a given column in said matrix of memory positions of said display memory which has an elevation value which is less than the elevation value of a pixel from the same column which has already been transferred to said display memory.

20. A method according to claim 19, wherein said inhibiting step includes storing in a column max memory an elevation value for each column of data stored in said display memory comparing the elevation value for each point read out of said column buffer memory and for each point generated by said additional data generating step with the elevation value stored in the column max memory for that column in which the data is to be written into said display memory, in response to said comparing step, storing, in the column max memory, the elevation value of the point when that value is greater than the value previously stored and inhibiting transfer of said data to said display memory when the elevation value thereof is less than that previously stored.

21. A method according to claim 18, wherein said additional data generating step includes storing the transformed addresses of two adjacent points, incrementing one of said addresses in first and second coordinate directions toward said other address by calculated stepping values, generating an additional data value at each incremented step, and controlling the storing of transformed address of the next points read out of said column buffer memory when the one address has been incremented until it corresponds to said other address.

22. A method according to claim 21, wherein said storing step includes storing the transformed addresses of four adjacent points read out of said column buffer memory, and said incrementing step includes comparing said addresses in predetermined combinations of pairs and incrementing said addresses in said predetermined combinations of pairs only when said comparison thereof indicates that the one address of the pair has a selected relationship to the other address.

23. A method according to claim 22, wherein said comparing step includes determining whether the line extending between the pair of points will face in a predetermined direction with respect to the orientation of data in said display memory.

24. A method according to claim 13, wherein said transferring step includes writing into said display memory along successive scanning lines only those data values which represent a different value of slope from the previous data value stored in said display memory on the same scanning line.

25. A method according to claim 24, wherein said displaying step includes reading pixel data out of said display memory, including the step of generating pixels for storage locations containing zero information in accordance with the value of the last pixel read out along the same scanning line.

26. A method according to claim 25, wherein said display memory includes first and second display memories, and said further including the step of writing pixel data into one of said first and second display memories while pixel data is being read out of the other.

27. A method for storing and reading out digital information for use in generating a real-time perspective display of terrain over which a vehicle is moving on the basis of stored digital data representing a two-dimensional elevational terrain pattern and signals indicating the instantaneous heading, geographic location and elevation of the vehicle and the desired angle of view of the resultant perspective display, comprising the steps of:

storing in a scene memory said digital data in addressable coordinate positions related to a selected geographic orientation of the terrain;

in response to signals representing heading, geographic location and angle of view, reading data out of said scene memory through the generation of address signals to address a selected area of said scene memory on the basis of the heading and geographic location of the vehicle and the desired angle of view with respect to the terrain represented by said digital data;

in response to signals representing heading, elevation, geographic location and angle of view, transforming the address of each data value read out of said scene memory from the address of its associated point in the two-dimensional terrain pattern to the address of a respective pixel in said perspective display; and

in response to the transformed addresses, generating a real-time perspective display of said terrain.

28. A method according to claim 27, wherein said transforming step includes varying said transformed addresses in response to selected variations in the signal representing the desired angle of view.

29. A method according to claim 27, wherein said transforming step includes generating a signal .DELTA.E representing the difference in elevation between the vehicle and each point in said terrain pattern, generating a signal R representing the distance parallel to the vehicle heading between each point position and the vehicle position in said terrain pattern, and generating a signal .DELTA.W representing the distance between each point and a line through the vehicle position parallel to the vehicle heading in said terrain pattern.

30. A method according to claim 29, wherein digital data stored in said scene memory represents, for each point in the terrain, an elevation value E.sub.i equal to the actual elevation of that point reduced by a base elevation E.sub.b, and said step of generating the signal .DELTA.E includes multiplying the elevation value E.sub.i read from said scene memory by a scale factor S.sub.f and adding to the result of said multiplication the base elevation E.sub.b, and subtracting the output of said multiplying step from the signal representing the elevation of said vehicle.

31. A method according to claim 29, wherein said transforming step includes generating transformed addresses a,b for each point according to the relationships ##EQU11## where .alpha. is the desired angle of view with respect to the horizontal and K is a constant.

32. A method according to claim 27, wherein said reading-out step includes selectively generating said address signals so as to scan a generally diamond-shaped area of said scene memory, said area being positioned within said scene memory with one edge thereof being substantially transverse to the direction of vehicle heading with respect to said terrain pattern and the scanning of said area being in the direction parallel or transverse to the geographical orientation of said data in the scene memory, whichever makes the smallest angle with said vehicle heading.

33. A method according to claim 27, wherein said displaying step includes providing a column buffer memory having a plurality of columns for storing respective lines of data, including both elevation data and the transformed address of each point read out of said scene memory by said read-out means, in positions corresponding to the locations of said points in said scene memory, in response to groups of the data read out of said column buffer memory, generating additional data representing the elevation and addresses of points extending along lines between the points of each group in accordance with the transformed address thereof, and storing in a display memory, the elevation data of points processed by said step of generating additional data and points read out of said column buffer memory at locations corresponding to the transformed addresses of said points.

34. A method according to claim 33, wherein said displaying step further includes transferring data into said display memory by scanning said display memory in successive lines beginning with pixel data representing the bottom of the display, the storage locations in each line of pixel data in the display memory forming columns with the corresponding positions in the other lines.

35. A method according to claim 34, wherein said transferring step includes inhibiting, in response to the elevational data of each point to be transferred to said display memory, such transfer in connection with any pixel along a given column of said display memory which has an elevational value which is less than the elevational value of a pixel of the same column which has already been transferred to said display memory.

36. A method according to claim 35, wherein said inhibiting step includes storing, in a column max memory, an elevation value for each column of data stored in said display memory, comparing the elevation value for each point read out of said column buffer memory and for each point generated by said additional data generating step with the elevation value stored in the column max memory for that column in which the pixel is to be written into said display memory, in response to said comparing step, storing, in the column max memory, the elevation value of the pixel when that value is greater than the value previously stored and inhibiting transfer of said pixel value to said display memory when the elevation value thereof is less than that previously stored.

37. A method according to claim 33, wherein said additional data generating step includes storing the transformed addresses of two adjacent points, incrementing one of said addresses in first and second coordinate directions toward said other address by calculated stepping values, generating an additional data value at each incremented step, and controlling the storage of transformed addresses of the next points read out of said column buffer memory when the one address has been incremented until it corresponds to said other address.

38. A method according to claim 37, wherein said storing step stores the transformed addresses of four adjacent points read out of said column buffer memory, and said incrementing step includes comparing said addresses in predetermined combinations of pairs, and incrementing said addresses in said predetermined combinations of pairs only when the comparison thereof indicates that the one address of the pair has a selected relationship to the other address.

39. A method according to claim 38, wherein said comparing step includes determining whether the line extending between the pair of points will face in a predetermined direction with respect to the orientation of data in said display memory.

40. A method according to claim 33, wherein said displaying step further includes generating, with respect to each data value read out of said column buffer memory and each data value generated by said additional data generating step, the slope at the point of the terrain represented by that data value on the basis of the data values around that point, and transferring said slope data into said display memory at addresses provided by said additional data generating step in successive lines.

41. A method according to claim 40, wherein said slope generating step includes comparing each value of slope which is generated with the slope value previously generated for the preceding point, and in response to said comparing step, controlling the storage in said display memory only that data which represents a change in slope on a given scanning line in said display memory.

42. A method according to claim 41, wherein said display step includes reading pixel data out of said display memory, including generating pixels for storage locations containing zeros in accordance with the value of the last non-zero pixel read out along the same scanning line.

FIELD OF THE INVENTION

The present invention relates in general to information display systems, and more particularly, to a method for use in a digital system for producing a real-time visual display in perspective of the terrain over which an aircraft is passing on the basis of display data reconstruction from compressed digital data and aircraft position, heading, and attitude information received from the on-board navigational computer of the aircraft.

BACKGROUND OF THE INVENTION

The proper control of an aircraft in all phases of its flight is based to a large extend upon the ability of the pilot to visually observe the terrain over which the aircraft is passing. In this regard, instrumentation, such as radar systems, and altimeters in combination with the use of accurate terrain maps aid the pilot in the flight of the aircraft; however, there are numerous conditions of flight which require actual observation of the terrain by the pilot to ensure proper navigation of the aircraft. For example, in cases of low altitude flying and landing of the aircraft under conditions which require quick reaction in the guiding of the aircraft over terrain which may provide rapidly changing contours and other obstacles to flight, the use of instruments alone is often unsatisfactory.

Accordingly, various systems have been proposed heretofore, including radar scanning systems and systems using preprocessed films of terrain over which an aircraft is to pass for providing to the pilot a display which simulates that which he would visualize if he were to actually view the terrain over which the aircraft is passing. One of the most recent developments in the area of moving map displays is a system for the dynamic display of terrain data which is stored as compressed data in digital form and which may be viewed on a cathode ray tube display in the form of a moving map that is automatically oriented under the control of the aircraft's navigational computer system to the instantaneous position of the aircraft with a heading-up disposition. Such a system is disclosed in copending U.S. application Ser. No. 224,742, filed Jan. 13, 1981, abandoned, and continued as Ser. No. 671,179, entitled "Digital Map Generator and Display System", in the name of Paul B. Beckwith, Jr., and assigned to the same assignee as the present application.

The system disclosed in the above-mentioned copending application provides a topographical two-dimensional real-time display of the terrain over which the aircraft is passing, and a slope-shading technique incorporated into the system provides to the display an apparent three-dimensional effect similar to that provided by a relief map. This is accomplished by reading compressed terrain data from a cassette tape in a controlled manner based on the instantaneous geographical location of the aircraft as provided by the aircraft navigational computer system, reconstructing the compressed data by suitable processing and writing the reconstructed data into a scene memory with a north-up orientation. A read control circuit then controls the read-out of data from the scene memory with a heading-up orientation to provide a real-time display of the terrain over which the aircraft is passing. A symbol at the center of display position depicts the location of the aircraft with respect to the terrain, permitting the pilot to navigate the aircraft even under conditions of poor visibility. However, the display provided by this system is in the form of a moving map rather than a true perspective display of the terrain as it would appear to the pilot through the window of the aircraft. Thus, the system disclosed in the copending application provides an arrangement for an indirect guidance of the aircraft over the terrain in that guidance is based upon the relative position of the symbol of the aircraft at the center of display position with respect to the moving map, rather than an arrangement for direct control of the aircraft with respect to the terrain on the basis of a three-dimensional display corresponding to the scene as it would actually appear to the pilot through the window of the aircraft.

Three-dimensional displays of terrain have been provided heretofore for aircraft guidance and flight simulation; however, such displays have been primarily simulated displays including only general characteristics of the terrain, such as an aircraft runway for aiding in aircraft landing and the like. Other systems for providing more detailed display of terrain data have been based on systems using preprocessed films of terrain. Unfortunately, such systems have not been entirely satisfactory in that they are often quite complex and are not capable of providing the detail insofar as elevation and cultural data is concerned which is required by the pilot of the aircraft for proper guidance. Such systems also are incapable of providing three-dimensional displays which correspond directly to a scene as might be observed through the window of the aircraft and, with the exception of the system disclosed in the above-mentioned copending Beckwith application, are incapable of providing a real-time display of terrain data taken into consideration changing altitude, heading and aircraft attitude.

BRIEF DESCRIPTION OF THE INVENTION

The present invention proposes a method for use in a system for the dynamic display of terrain data which is stored in digital form and which may be viewed on a cathode ray tube display in the form of a moving map, similar to that disclosed in the above-mentioned copending Beckwith application, but represents an improvement over that system by provision of a perspective processor circuit for processing the data to produce a three-dimensional display of terrain equivalent to that which would appear to the pilot by direct observation of the terrain from the aircraft, if that were possible. This perspective processing circuit would replace the shades of gray processing circuit and slope-shading circuit provided in the previously-disclosed system to enable the provision of an actual three-dimensional display, rather than the topographical display in base relief appearing as a simulated three-dimensional display in the previously-disclosed system of the copending application.

The perspective processing circuit of the present invention includes a read control which calculates a starting point for the scanning of data in the scene memory on the basis of the heading of the aircraft. Contrary to the system disclosed in the above-mentioned copending Beckwith application, the read control need not reorient the data read from the scene memory in accordance with the aircraft heading, since this will be automatically accomplished during perspective processing. However, in order to facilitate such perspective processing and the subsequent display, it is desirable to read out the data in the scene memory in accordance with the direction of heading of the aircraft. On the other hand, since the data is stored in the scene memory with a north-up orientation, scanning of that data is most easily effected either in a north-south direction or east-west direction. Thus, depending upon which of these two directions the heading is closest to, the starting point for scanning is selected along with the scanning direction to ensure that scanning will take place in a direction closest to the direction of heading of the aircraft.

The addresses of the elevation data read out of the scene memory representing points in the two-dimensional scene of the terrain are then transformed to relocate the points to positions where they would appear in a perspective scene of the terrain. Thus, each point in the two-dimensional scene is transformed to its new location in the perspective scene to be displayed on the viewing screen, and in the process, the data is automatically oriented with a heading-up disposition. The transformed points are then stored in a speed buffer for further processing by sun angle and line writing logic prior to being stored in a display memory from which data is read out to the display screen. Since data in the display memory represents one-to-one data to be displayed on the CRT, this data will be referred to as pixels (picture elements) in terms of its storage in the display memory for transfer to the CRT display.

One of the additional features of the present invention resides in the sun angle logic and line writing logic which operates on the transformed points to fill in shades of gray between the relocated points. The shades of gray (i.e. intensity of the CRT) are generated by the sun angle logic based upon the slope of the terrain at a given point rather than upon the absolute elevation of the point. By scanning rows of display data in a left-to-right direction, it becomes necessary to store in the display memory only the left-facing boundaries between changes in elevation. In this way, less data needs to be stored in the display memory, increasing the operation speed of the system by avoiding the processing of nonessential data. By simply filling in between boundaries as data is read out of the display memory to the display screen, the processing time of the system and the complexity thereof can be greatly reduced.

These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of an exemplary embodiment of the present invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a display system incorporating the features of the present invention for providing a moving map display of terrain in perspective;

FIG. 2A is a diagram of the data format as provided in the data base of the system illustrated in FIG. 1;

FIG. 2B is a diagram of the coordinate system used in the compression and storage of terrain data in the system of FIG. 1;

FIG. 3 is a schematic representation of the scene memory map;

FIG. 4 is a schematic block diagram of the perspective processing circuit;

FIG. 5 is a schematic diagram illustrating the starting point calculation and scanning as applied to the scene memory;

FIG. 6 is a schematic block diagram of the X-scanning portion of the scan control for the scene memory;

FIGS. 7a-7h are schematic diagrams which illustrate the various scanning directions and selected starting points based on aircraft heading;

FIG. 8 is a schematic diagram of the relationship of viewing screen angle to the resultant display;

FIG. 9 is a schematic diagram illustrating the starting point calculation for a vertical viewing screen angle as applied to the scene memory;

FIG. 10 is a schematic diagram illustrating the geometry in obtaining an address in the scene memory in conjunction with perspective processing;

FIGS. 11a through 11e are schematic illustrations of data point patterns showing the perspective processing technique of the present invention;

FIG. 12 is a schematic diagram of the perspective transform circuit;

FIG. 13 is a schematic diagram illustrating the storing of transformed data from the scene memory in the column buffers;

FIG. 14 is a schematic diagram of a point pattern used in describing the line writing logic;

FIGS. 15 through 19 are schematic diagrams of the line writing logic circuit and column max. memory;

FIG. 20 is a schematic diagram of a block of points for use in explaining the operation of the sun angle logic;

FIG. 21 is a schematic block diagram of the details of the sun angle logic circuit;

FIG. 22 is a schematic diagram of the read and fill logic circuit; and

FIG. 23 is a schematic diagram showing an example of the data stored in a portion of the display memory.

DETAILED DESCRIPTION OF THE INVENTION

One of the basic problems faced by systems which attempt to provide a real-time visual display of terrain on the basis of stored digital information relates to the ability to store sufficient information to provide all of the elevation features for a significant area of terrain over which the aircraft might wish to travel without limitation to a single predetermined flight path. In this regard, efficient digital terrain data storage is absolutely essential to a reduction of the capacity of the data base memory required for each aircraft if the on-board system is to be reduced to a practical size. The basic objective in this regard is to provide a practical aircraft operating range on a single large cassette tape that can be mission updated. This may be accomplished by utilizing a transformed compression approach which serves to convert the spatial elevation points to the frequency domain.

The compression and storage of terrain elevation data may be based on the Defense Mapping Agency data base which provides elevations on 12.5 meter grid points. Overall, the terrain elevation data can be compressed within 12.5 km square areas, which are submultiple of the 100 km square used on the transverse mercator projection military maps. Thus, the data base will be addressed on the basis of 16 bit X and Y coordinate words, each of which provide 3 bits for the 100 km identification, 3 bits for the 12.5 km identification, 3 bits for the 1.56 km identification and 7 bits for identification of the individual 12.5 meter grid points.

One of the largest capacity mass storage systems available for mobile applications is a cassette tape unit, which is easily capable of providing storage capacities of up to 12 megabits in a single tape. With this in mind, if it is assumed that one-third of the storage capacity of the tape is reserved for non-elevation data, then eight megabits are available for elevation data storage. Conventional grid elevation data, stored as eight bits of data for each 12.5 m grid point, will use the available eight megabits in the form of a square area with 12.5 km per side. The discrete cosine transform compressed data approach may then use the available eight megabits to store a square area of approximately 140 km per side. Thus, it is quite apparent that all of the flight mission data can be stored on a single tape providing all of the information relating to a significantly-large area of terrain.

FIG. 1 is a basic block diagram of a system into which the present invention is incorporated for the dynamic display in perspective of terrain data including elevational information for use in the navigation of an aircraft along a predetermined flight path under control of a navigation computer 100 which is connected to the system via interface 90. Prior to flight operation, a cassette tape which stores the properly-formatted mission data is loaded into the cassette unit 10. The mission data, which includes compressed elevation grid data, is stored on the cassette tape in blocks organized according to their coordinate location in a format such as shown in FIG. 2A. In this regard, the header associated with each block will include the X and Y coordinate addresses of the block made up of three 100 km I.D. bits and three 12.5 km I.D. bits, as seen in FIG. 2B.

The output of the cassette unit 10 is applied through a cassette tape control unit 15 to an intermediate memory 20. Since the latency time of the cassette unit 10 (the difference in time between data requisition and data acquisition) can be as much as several seconds, which is clearly beyond the instantaneous response required in the system, the cassette tape unit 10 is not used as the primary source for acquiring data processing. Rather, the intermediate memory 20 is provided as the primary data source and the cassette unit 10 supplies data in compressed form to the intermediate memory 20 as required under control of the tape control unit 15.

The cassette unit 10 will be accessed relative to translatory movement of the aircraft and the maximum cassette tape access rate, under control of the memory