AIRCRAFT TERRAIN WARNING SYSTEM

Abstract: In order to provide the pilots with a display or a map of the terrain below the aircraft, a display is generated on a cockpit display, such as a weather radar, of the contours of the terrain below the aircraft. The terrain contour display is generated from aircraft position information, a terrain data base, and a terrain awareness envelope defined by awareness boundaries. Terrain contours are depicted with the highest terrain within the envelope colored red, high intermediate terrain contours in yellow with varying dot densities, low intermediate terrain contours in green with varying dot densities, and the lowest terrain in black. The green contours can be overwritten with a low density yellow based on a pre-caution envelope which extend certain of the awareness boundaries to provide notice to a pilot of upcoming cautions.

Description: AIRCRAFT TERRAIN WARNING SYSTEM Cross-Referenceto Related Application This application claims priority to Provisional Patent Application titled Premature Descent into Terrain Visual Awareness Enhancement to EGPWS, filed December 23, 1997, and from US Application serial number 08/509,642 filed July 31, 1995, each of which is incorporated herein by reference. Technical Field The invention relates to field of aircraft teirain advisory systems and more particularly to such systems that provide a cockpit display of terrain, and including a pre-caution display for advance warning of terrain. Background of the Invention Various systems have been developed that provide warnings and advisory indications of hazardous flight conditions. Among such systems are systems generally known as ground proximity warning systems (GPWS) which monitor the flight conditions of an aircraft and provide a warning if flight conditions are such that inadvertent contact with terrain is imminent. Among the flight conditions normally monitored by such systems are radio altitude and rate, barometric altitude and rate, air speed, flap and gear positions. These parameters are monitored and an advisory signal and/or warning signal is generated when the relationship between the parameters is such that terrain impact is likely to occur. Typical examples of such systems arc disclosed in U.S. Patent Nos. 3,715,718; 3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221; 3,958,219; 3,925,751 ; 3,934,222; 4,060,793; 4,030,065; 4,215,334; and 4,319,218, all assigned to the same assignee as the assignee ofthe present invention and hereby incorporated by reference. While the above-referenced systems do provide advisory and warning signals in the event of proj mity to terrain, the warnings generated by such systems are based solely upon flight conditions of the aircraft and do not provide any navigational information nor a visual indication of the terrain below the aircraft. Consequently, the sensitivity of such systems must be adjusted to provide adequate warnings when a hazardous flight condition exists without generating false or spurious warnings. However, such an adjustment can result in a compromise that may still result in nuisance warnings over terrain unique to particular geographic areas and shorter than desired warning times in yet other geographic areas. Several attempts have been made to improve upon such ground projcimity warning systems utilizing ground-based navigational information. For example, U.S. Patent Nos. 4,567,483; 4,646,244; 4,675,823; and 4,914,436 all disclose ground proximity warning systems which monitor the position of the aircraft relative to stored terrain data in order to provide modified ground projdmity warnings. However, the utility of such systems is limited. For ejcample, the systems disclosed in U.S. Patent Nos. 4,567,483 and 4,914,436 disclose ground proxmity warning systems which utilize navigational data to modify predetermined warning envelopes surrounding certain particular airports. U.S. Patent Nos. 4,646,244 and 4,675,823 disclose terrain advisory systems which utttize various ground-based navigational inputs and stored terrain data to provide various ground proximity warning systems based on the position of the aircraft. However, as with the systems described above, none of these systems provide the pilot with navigational information or an indication of the terrain below the aircraft. Recently an enhanced ground projdmity warning system (EGPWS) which includes a terrain advisory and warning system has been developed. In this system data stored in a terrain data base is used in conjunction with aircraft position infoimation to generate a visual terrain advisories and warning on a cockpit display, such as weather radar. An object of the EGPWS is to provide the crew with a terrain awareness system which provides increased warning times to the pilot of an aircraft of a hazardous flight condition while minimizing nuisance warnings. An e^mple of a EGPWS is described in U.S. patent application Serial No. 08/509,642 filed on July 31,1995 and which is assigned to the assignee of this application. However, the terrain display feature of the EGPWS provides at best only a simple contoured display of terrain relative to the altitude of the aircraft. Further, when the aircraft is more than 2000 feet above the highest terrain within the range of the display, no terrain features are shown. However, there are some circumstances in which it would be desirable to provide the pilots with a contoured display of terrain below the aircraft when the aircraft is not in an immediate hazard condition. For example, it would be useful to be able to provide the flight crew with a means of assessing the potential terrain conflicts which might occur if a rapid descent is initiated from the aircraft's normal flight path. A loss of cabin pressure,. -for instance, might require the crew to initiate an emergency decent and it would be very useful if the crew had a map of the terrain below the aircraft. This would be particularly important when the aircraft is traversing mountainous terrain. Since the terrain display of the EGPWS normally does not display terrain more than 2000 feet below the aircraft this system is not normally available for planning a rapid decent from a relatively high altitude. From a pilot's point of view it is desirable to know from the background terrain depiction whether a caution alert will occur before the actual caution alert happens. This is particularly desirable when the aircraft is descending, as for landing. Summai of the Invention An enhanced aircraft terrain awareness system, also known as an Enhanced Ground Pro.ximity Warning System ("EGPWS"), is disclosed. The system has a processor configured to evaluate terrain over which an aircraft will can fly. The processor is further configured to generate cautions for terrain features which will come within a first predetermined range of the aircraft, based on a current position and course of the aircraft. The processor is yet further configured to generate pre-cautions a predetermined distance, i.e., a "precaution distance", beyond the first predetermined range. The pre-cautions for a given terrain feature are generated at an earlier point in time than cautions for the given terrain feature. This provides a low-level caution to the pilot of upcoming normal level cautions. More specifically, at least a portion of the predetermined precaution distance is generated based on a descent rate of the aircraft. The processor of the terrain awareness system can be further configured to generate warnings (versus cautions) for terrain features which will come within a second predetermined range of the aircraft, based on a current position and course of the aircraft. Warnings are considered to be- a higher priority awareness alert than are cautions. The second predetermined range is less than the first predetermined range. That is, the second range is a subset of the first range. The outer boundaries of the second range are generally closer to the aircraft than are the outer boundaries of the first range. Preferably, the cautions and warnings of the aircraft terrain awareness system are displayed on a visual display representative of a plan view of the terrain over which the aircraft can fly. More preferably, the cautions are displayed in a first color having a first color density, and the warnings are displayed in a second color. Most preferably, the visual display further displays the pre-cautions in the first color, but at a second color density. The second color density is preferably less than the first color density. Preferably, the first predetermined range comprises a forward-looking component and a downward-looking component. The predetermined precaution distance for the downward looking component is generated based on a descent rate of the aircraft. The predetermined precaution distance for the forward- looking component is generated based on a preselected percentage of the forward-looking component. More specifically, the aircraft terrain awareness system comprises a navigational system configured to determine latitude, longitude, altitude, ground- track, and ground speed of an aircraft employing the system. The navigational system generates signals in response to the listed parameters. The system further comprises: a terrain database comprising altitude data for terrain over which the aircraft can fly; a processor configured to identify from the terrain database altitude data within a range of the aircraft; a look-ahead warning generator; and a warning device. The look-ahead warning generator is configured to jeceive the altitude identified by the processor and generate an altitude data subset of terrain altitudes within a first determined zone around the aircraft. The warning generator is further configured to receive the signals generated by the navigational system, wherein the warning generator processes the altitude data subset, as well as the signals generated by the navigational system, to identify terrain which can intrude the first predetermined zone. The warning generator generates a caution signal in response to identified terrain which can intrude the first predetermined zone. The warning device is configured to receive the caution signal and generate a prima.ry warning in response thereto. The primary warning is accorded a first priority. The warning generator is further configured to generate an incremental data subset of terrain altitudes a determined incremental range beyond the first determined zone. The warning generator processes the incremental data subset to identify terrain which will intrude the incremental range, and generate a pre- caution signal in response to thereto. The warning device is further configured to receive the pre-caution signal and generate a secondary warning in response thereto, the secondary warning being accorded a second priority. The second priority is a lower priority than the first priority. The first determined zone around the aircraft comprises a look-ahead distance in front of the aircraft and a look-down distance below the aircraft. The determined incremental range comprises an incremental look-ahead distance beyond the look-ahead distance. The determined incremental range further comprises an offset distance beyond the look-down distance. A method for generating terrain feature caution notifications for an aircraft which can fly over a terrain is also disclosed. The method comprises defining a terrain advisory envelope in a region projdmate the aircraft, such that when terrain features intrude within the terrain advisory envelope a caution signal detectable by a pilot of the aircraft is generated. The method further comprises defining a pre-caution envelope extending beyond at least a portion of the terrain advisory envelope, such that when terrain features intrude within the terrain advisory envelope a pre-caution signal detectable by the pilot is generated. The caution signal is configured to be more easily detectable by the pilot than the pre-caution signal. Such can be accomplished by using a low-density variation of the caution alert color on a visual display. The method can further comprise providing altitude data for terrain over which the aircraft can fly, comparing the altitude data immediately below the terrain advisory envelope and the pre-caution envelope against an altitude of the aircraft, and identifying terrain features which intrude the terrain advisory envelope and the pre-caution envelope. Thereafter caution signals are generated when terrain features intrude the terrain advisory envelope, and the pre-caution signals are generated when terrain features intrude the pre-caution envelope. The present invention further provides a cockpit display of terrain to a pilot. The invention assists the pilot in determining what background terrain visible on the display is likely to provoke an alert of a ha.zardous condition. Brief Description of the Drawings FIGS. 1A and IB represent a block diagram of a terrain advisory and warning system; FIG. 2 is a diagram of the world broken down into various latitude segments and longitude segments in accordance with the present invention; FIG. 3 is a graphical representation of the various memory map and resolution sequencing in accordance with the present invention; FIG. 4A is an exemplary representation of the digital header and subsquare mask word for identifying geographical areas in accordance with the present invention; FIG. 4B is a terrain map alternative to FIG. 3; FIG. 5 is a graphical illustration of the LOOK-AHEAD distance assuming the aircraft turns at a 30° angle; FIG. 6 is a graphical representation of a DH terrain floor boundary which forms the bases for a terrain advisory signal and a terrain warning signal; FIG. 7 is a graphical illustration of a terrain advisory signal for an aircraft relative to the terrain and an airport for a condition when the aircraft flight path angle is less than a first predetermined reference plane or datum; FIG. 8 is similar to FIG. 7 but for a condition when the aircraft flight path angle is greater than the first predetermined reference plane; FIG. 9 is a graphical illustration of a terrain warning signal for an aircraft relative to the terrain and an airport for a condition when the aircraft flight path angle is greater than a second reference plane or datum; FIG. 10 is similar to FIG. 9 but for when the flight path angle of the aircraft is less than the second reference plane or datum; FIG. 11 is a graphical illustration of a cut-off angle correction boundary for a level flight condition in accordance with the present invention; FIG. 12 is similar to FIG. 11 but for a condition when the flight path angle of the aircraft is greater than a predetermined reference plane or datum; FIG. 13 is similar to FIG. 11 but for a condition when the flight path angle of the aircraft is less than a predetermined reference plane or datum and also illustrates a BETA sink rate enhancement boundary; FIG. 14 is a graphical illustration of the terrain advisory and superimposed terrain warning signals for an aircraft relative to the terrain for a condition when the aircraft flight path angle is less than a first reference plane or datum; FIG. 15 is similar to FIG. 14 but for a condition when the flight path angle is between a first datum and a second datum; FIG. 16 is similar to FIG. 14 but for a condition when the flight path angle is less than the second datum; FIG. 17 is a graphical illustration of LOOK-AHEAD /LOOK-DOWN terrain advisory and warning boundaries for a condition when the aircraft is descending; FIG. 18 is similar to FIG. 17 but for a condition when an aircraft is climbing; FIG. 19 is a graphical illustration of LOOK-UP terrain advisory and warning boundaries in accordance with the present invention; FIG. 20 is a graphical illustration of an aircraft during a pull-up maneuver; FIG. 21 is an exemplary block diagram for a system for generating a signal representative of the altitude loss due to pilot reaction time ALPT, as well as altitude loss due to a pull-up maneuver ALPU; FIG. 22 is a graphical illustration of an alternative methodology for generating a cut-off altitude boundary; FIG. 23 is a block diagram of the display system; FIG. 24 is a functional diagram of the display data format in accordance with the ARINC 708/453 standard for a weather radar display; FIG. 25 is a plan view of a background terrain display illustrating the variable density dot pattern; FIG. 26 is similar to FIG. 25 but additionally illustrates the terrain threat indications; FIG. 26A is similar to FIG. 26 but additionally illustrates the pre-caution terrain threat indications. FIG. 27 is a block diagram illustrating the configuration of a flash ROM for a terrain data base and a RAM used with the present invention; FIG. 28 illustrates a wedding-cake configuration for a RAM utilized in the present invention; FIG. 29 is a diagram of the file format of the files in the terrain data base in accordance with the present invention; FIG. 30 illustrates a terrain map of the terrain data base in accordance with the present invention illustrating the highest elevation in each of the cells within the map; FIG. 31 is similar to FIG. 30 and illustrates a method for correcting various cells in the vicinity of a runway; FIG. 32 is a simplified elevational view of certain data illustrated in FIG. 31; FIG. 33 is a diagram of a display illustrating the weather radar sweep spokes and the determination of the range increments; FIG. 34 is a block diagram of the display system in accordance with the present invention illustrating the memory configuration and the data flow; FIG. 34A is a block diagram of a configuration alternative to that shown in FIG. 34; FIG. 35 is a diagram of a terrain map illustrating the determination of the look-ahead vector array; FIG. 36 is a diagram of a display illustrating a radar sweep spoke and a method for determining the range increments of the radar sweep spoke; FIG. 37 is a terrain map illustrating the computation of the hazardous terrain; FIG. 38 is a diagram of a terrain map superimposed on a display screen illustrating the sweep range and the determination of the incremental range increments; FIG. 39 is a diagram of a pixel map of the display; FIG. 40 is a diagram of the different fractal patterns used to provide the variable density display of non-hazardous display indications; FIG. 41 is a diagram illustrating the determination of the X and Y increments of the display screen; FIG. 42 is a diagram of the method for superimposing the fractal patterns on the displayed pixel map; Fig 43 is an illustration of a terrain data format in accordance with the present invention; Fig. 44 is illustrative of steps to build the Terrain database from the original data in accordance with the present invention; Fig. 45 is illustrative of steps to build the Terrain database from the original data in accordance with the present invention. Fig. 46 is a block diagram of a combined terrain advisory and warning system and a terrain information system according to the invention; Fig. 47 is a flowchart illustrating the logic of the terrain contour map generated by the terrain information system of Fig. 46; Fig. 48 is diagram of a cockpit display illustrating a terrain contour map generated by the terrain information system of Fig.46; Fig. 49 is a terrain depiction according to one embodiment of the present invention; Fig. 50 is a side elevation view of a pre-caution extended background envelope according to one embodiment of the present invention; and Fig. 51 is a plan view of a pre-caution background envelope according to one embodiment of the present invention. Fig. 52 is a flowchart of the pre-caution e.xtended envelope algorithm. Detailed Description of the Preferred Embodiments Introduction Since the preferred embodiment of the terrain information system or Peaks Mode utilizes many of the elements of a terrain warning and advisory system, including for example the terrain data base and the cockpit display, this description will first describe in detail the operation of the terrain advisory and warning system. The terrain advisory and warning system is described in connection with Figs. 1-45 and the terrain information system of the invention is described in connection with Figs. 46-49. Terrain Advisory and Warning System Referring to FIG. 1, a terrain advisory and warning system or terrain awareness system (TAS), generally identified with the reference number 20, is illustrated. As will be discussed in more detail below, the TAS 20 utilizes inputs from a satellite-based navigational system, such as global positioning system (GPS) 22 (longitude, latitude, altitude, groundtrack, ground speed), and/or an FMS/IRS navigational system which may be updated by the GPS and/or DME/DME, a terrain data base 24, an aiφort data base 26 and corrected barometric altitude, for example from a barometric altimeter 28 to provide a LOOK-AHEAD warning system which provides relatively longer warning times while minimizing nuisance warnings. Since the TAS 20 does not require a radio altimeter input, the system can be used with certain aircraft, such as commuter aircraft, which are not normally equipped with a radio altimeter. The current longitude and latitude of the aircraft from the GPS 22 are applied to an Airport and Terrain Search Algorithm, indicated by a block 29, which includes location search logic for determining the terrain data, as well as the airport data surrounding the aircraft. Such search logic is described in detail in U.S. Patent Nos. 4,675,823 and 4,914,436 assigned to the same assignee as the present invention and hereby inco.rporated by reference. The GPS inputs, along with terrain and airport data surrounding the aircraft from the search algorithm, indicated by the block 29, are applied to the LOOK-AHEAD warning generator 30, which provides both terrain advisory and terrain warning signals based upon the position and projected flight path of the aircraft. The LOOK-AHEAD warning generator 30 may provide both an aural warning by way of a voice warning generator 32 and speaker 34, and/or a visual warning by way of a map or a display 36, as discussed below. GLOBAL POSITIONING SYSTEM The primary positional information for the aircraft is provided by the GPS 22, such as disclosed in U.S. Patent Nos. 4,894,655; 4,903,212; 4,912,645; 4,954,959; 5,155,688; 5,257,195; 5,265,025; 5,293,163; 5,293,318; and 5,337,242, all hereby incorporated by reference. The GPS 22 includes a GPS receiver 37 and a combination GPS receiver and monitor 38. The positional inputs from the GPS 22 are processed by a dead reckoning algorithm, indicated by the block 40, for ej ample as discussed in detail in U.S. Patent No. 5,257,195, to compensate for temporary outages of the GPS 22, such as loss or masking of satellites. The dead reckoning algorithm can be replaced by an additional navigational system, such as an FMS or an IRS, which can be used during GPS outages. A differential GPS receiver 42 can also be used to provide an indication of a GPS altitude. As long as the differential GPS information is received and four more satellites are visible, the differential GPS information from the differential GPS receiver 42 is sufficiently accurate for the TAS 20. If the differential GPS receiver 42 is not available, a compound altitude signal may be generated by the block 44 and connected to the LOOK-AHEAD warning generator 30 by way of a single-pole, double-throw switch 46, under the control of a signal "DIFF GPS AVAILABLE", which indicates when the differential GPS receiver 42 is available. In a first mode when differential GPS information is available, the switch 46 connects the GPS altitude signal from the GPS 22 to the LOOK-AHEAD warning generator 30. In this position, the GPS altitude signal is generated either from the GPS 22 or from the differential GPS receiver 42, as discussed above. When a differential GPS receiver 42 is not available, a compound altitude signal is provided by the block 44 to the LOOK-AHEAD warning generator 30. During this condition, the switch 46, under the control of a "DIFF GPS AVAILABLE" signal, connects the block 44 to the LOOK- AHEAD warning generator 30 to enable the compound altitude signal to be provided to the LOOK-AHEAD warning generator 30. Within the Differential GPC Receiver 42, the barometric altitude is compared with the GPS altitude to generate a compound altitude signal which could be used to limit the maximum difference between the GPS output altitude signal and the barometric altitude to the maximum expected GPS altitude error. Such a configuration would reduce the effect of erroneous pressure correction settings on the barometric altimeter 28. TERRAIN AND AIRPORT DATA BASES As mentioned above, the aiφort data and terrain data are separated into two different data bases 26 and 24, respectively. Such a configuration allows either of the data bases 24 or 26 to be updated without the need to update the other. The airport data base 26 contains various types of information relating to aiφorts, such as runway midpoint coordinates, runway length, runway heading, runway elevation and aiφort/runway designation data. Additional data relating to obstacles along the approach path (i.e. high-rise hotels adjacent Heathrow Aiφort in England) and a nominal final approach slope could also be included in the aiφort data base 26. A separate obstacle data base may also be provided, structured similar to the aiφort data base. In order for the TAS 20 to be useful, all aiφorts, for example, which commuter planes without radio altimeters can land, would have to be included in the aiφort data base 26. In areas where the aiφort data base 26 is not complete, the search algorithm 29 would indicate NO DATA. Since the aiφort data base 26 is organized in a similar fashion as the terrain data base 24 as described below, additional aiφort data could be added in a piecewise fashion when available. The estimated size of the aiφort data base 26 is contemplated to be relatively small, compared to the terrain data base 24, for example less than 200 kilobytes depending on the amount of data stored per aiφort. The terrain data base 24 may require up to 40 megabytes of storage space, which makes it compatible with available flash erasable programmable read only memory (ROM) devices, such as a flash EEPROM, or mini hard disk drives, which could take advantage of known data compression techniques to reduce the amount of storage space required and also promote easy data retrieval by the search algorithm 29. An important aspect of the invention is that the terrain data base 24 is structured to provide varying resolutions of terrain data as a function of the topography of the terrain, as well as distance to aiφorts. For ejcample, a relatively high resolution can be provided close to the aiφort on the order of 1/4 to 1/8 nautical miles and a medium resolution, for example 1/2 to 1 nautical miles within a 30-mile radius of the aiφort. Outside of the 30-mile radius from the aiφort, a coarser resolution is sufficient as will be described below. FIGS. 2 through 4 illustrate the organization of the terrain data base. Referring first to FIG. 2, the world is divided into a plurality of latitude bands 50, for e.xample each about 4° wide. Each latitude band 50 is then divided into a plurality of longitudinal segments 52, which are about 4° wide around the equator, such that each longitudinal segment 52 is about 256 x 256 nautical miles. In order to maintain a relatively constant segment size, the number of longitudinal segments 52 per latitude band 50 are reduced closer to the poles. Since the number of latitude bands 50 and the number of longitudinal segments 52 per latitude band 50 is fixed, determination of the particular segment corresponding to the current aircraft position is readily determined. For example, the aircraft current latitude X is used to determine the latitude band number. Next, the number of longitudinal segments 52 associated with that band 50 is determined either by way of a LOOK-UP table or by calculation. Once the number of longitudinal segments 52 in the particular latitude band 50 of interest is determined, the current longitudinal position Y of the aircraft can be used to readily determine the longitudinal segment number. The minimum data associated with each longitudinal segment 52 corresponds to the highest altitude within that segment. As mentioned above, each of the segments 52 is approximately 256 x 256 nautical miles. As shown in FIG. 3, these longitudinal segments 52 can be broken down into various subsquares to provide varying levels of resolution. For ejcample, each segment 52 may be broken down into a plurality of subsquares 54, each subsquare 54 being 64 x 64 nautical miles to provide a very coarse resolution. The subsquares 54, can, in turn, be further subdivided into a number of subsquares 56, for example, each 16 x 16 nautical miles, to provide a coarse resolution. These subsquares 56, in turn, can be subdivided into a plurality of subsquares 58, for e.xample, each 4 x 4 nautical miles, to provide a. -medium resolution. The subsquares 58 can also be broken down into a plurality of subsquares 60, for example l l nautical miles, to provide a fine resolution. In the vicinity around aiφorts, it may even be desirable to break down the subsquares 60 down into smaller subsquares 62 to provide even finer resolution, for e.xample 1/4 x 1/4 nautical miles. As shown in FIG. 4A, the minimum data associated with each longitudinal segment 52 consists of a header 65 which includes a multiple data byte 66 which includes the reference altitude which corresponds to the highest altitude for all the subsquares within the segment, which assumes that the highest elevation in a square is representative of the entire square elevation. The multiple data byte 66 may also include a flag to indicate when no further subdividing is required for certain geographical areas. For example, for segments representing the ocean, all subsquares would have the same maximum altitude and thus no further subdividing would be required. In order to enable the data base to be created and updated on a piecewise basis, the multiple data byte 66 may also contain a flag bit or code indicating that data corresponding to further subdivisions does not e.xist for a particular segment 52 containing a code indicating that no map data exists in the segment. For geographical areas, such as mountainous areas and areas in the vicinity of an aiφort, the longitudinal segments 52 are subdivided as discussed above. In such a situation, the stored data would include a 2-byte mask word 68, which points to the subsquares having different altitudes. For escample, as shown in FIG. 4, the 2-byte mask word 68 is illustrated with a pointer in the box 15 within the subsquare 54 within the longitudinal segment 52, which represents a different altitude which points to the next finer layer of resolution as discussed above. The subsquares each include a header 70 including a mask as described above to enable a pointer for each subsquare having a different altitude to point to the next finer layer until the finest resolution level desire is reached as shown. The data base stnicture provides for flexibility as to resolution required and can be constructed on a piecewise basis at different times. As such, the structure contains the entire framework necessary to add new data or modify existing data without changing any of the software. The size of the data base is proportional to the resolution and the size of the area to be covered. In order to cover the entire earth's surface, about 149 million square nautical miles, only about 2,500 to 3,500 longitudinal segments 52 are required. Thus, the overhead for the minimum header sizes is relatively small. Alternatively, the world may be divided into a plurality of, for e.xample, 1° x 1° map files or cells 51 as shown in FIG. 4B. The various map files 51 may be formed with variable resolution. More particularly, each of the map files may be subdivided into a plurality of subcells 53 with the highest terrain elevation in each subcell indicated, with the resolution of all of the subcells within a particular map file being of the same resolution. As indicated above, the maps are generally 1° x 1°, which may be combined for coarser resolution (i.e. 2° x 2° or 10° x 10°). The size of the subcells 53 is varied to provide varying degrees of resolution. For example, for relatively high resolution, the size of the subcells 53 may be selected as either 15 x 15 arc seconds or 30 x 30 arc seconds. The size of the subcells 53 may also be varied as a function of the terrain in particular geographic areas. For example, the size of the subcells 53 may be selected as 30 x 60 arc seconds at relatively far north longitudes and as 30 x 120 arc seconds at longitudes even further north. The size - of the subcells 53 may also be varied as a function of the distance to the nearest aiφort. For ejcample, for distances greater than 64 miles from an aiφort, the size of the subcells may be selected as 60 x 60 arc seconds while at distances greater than, for example, 128 miles from the aiφort, size of the subcell 53 may be selected as 120 x 120 arc seconds. For a course resolution, for example for use during a cruise mode as discussed below, the size of subcells may be selected as 5 x 5 arc minutes. As discussed below and as illustrated in FIG. 27, the data base may be compressed by a compression algorithm and stored in a flash read only memory (ROM), configured as a wedding cake as illustrated in FIG. 28. Various compression algorithms are known and suitable for this application. The compression algorithm does not form a part of the present invention. In order to simplify computation, each of the map files includes a header 55, which includes, inter alia, the resolution of the particular map file 51 as well as the location of one or more corners, for example, the northwest and southeast corners. The headers 55 as well as the map files 51 are discussed in more detail below and illustrated in more detail in FIG. 29. For certain terrain, for ejcample, terrain adjacent a runway, the resolution of the map files 51 (i.e. relatively large size of the subcells 53) may result in errors in runway elevation. More particularly, since each subcell 53 contains the highest elevation within the subcell 53, certain errors relating to runway elevations can occur, depending on the resolution of the map file 51. Correction of the elevations adjacent the runways is discussed in more detail below in conjunction with FIGS. 30-33. Systematic errors may also be introduced by the maps, which are digitized, for ejample, by the Defense Mapping Agency, using known averaging algorithms, which are known to fill in valleys and cut-off the tops of shaφ peaks of the terrain. LOOK-AHEAD WARNING GENERATOR As mentioned above, the LOOK-AHEAD warning generator 30 generates both a terrain advisory signal and a terrain warning signal based upon the position and trajectory of the aircraft relative to stored terrain data. There are two aspects of the terrain advisory and terrain warning signals: LOOK-AHEAD distance/direction; and terrain threat boundaries. LOOK-AHEAD DISTANCE/DIRECTION The LOOK-AHEAD direction for detecting threatening terrain is along the groundtrack of the aircraft. In order to prevent nuisance warnings, the LOOK- AHEAD distance is limited, as will be discussed below. Otherwise, potentially threatening terrain along the current flight path of the aircraft relatively far from its current position could produce nuisance warnings. Two different LOOK-AHEAD distances (LAD) are utilized. The first LAD is used for a terrain advisory signal (also referred to as a yellow alert LAD). A second LAD is used for terrain warning signals which require immediate evasive action (also referred to as a red-alert LAD). The LAD for a terrain advisory condition is considered first in determining the LAD because it is assumed that the pilot could make a 30° bank turn at any time at a turning radius R. The LAD is equal to the product of the speed of aircraft and the total LOOK-AHEAD time. As shown in FIG. 5, the total LOOK-AHEAD time is equal to the sum of the LOOK-AHEAD time T j for a single turning radius R; the LOOK-AHEAD time T for terrain clearance at the top of the turn (i.e. point 73) plus a predetermined reaction T 3 . The terrain clearance at the top 73 of the turn is provided to prevent inadvertent terrain contact as a result of the turn. This terrain clearance may be selected as a fixed distance or may be made equal to the turning radius R. As shown in equation (1), the turning radius R is proportional to the square of the speed of the aircraft and inversely proportional to the bank angle TG (ROLL). (1) R = V 2 /G x TG (Roll) For a 30° bank angle, the turning radius R in nautical miles (nM) as a function of the speed in knots is given by equation (2). (2) R = 0.000025284*V 2 The LOOK-AHEAD time T j for a single turning radius is given by equation (3). (3) T j = R/V Substituting R from equation (1) into equation (3) yields the LOOK- AHEAD time T j for a single turn radius as shown in equation (4) as a function of the speed of the aircraft and the bank angle. (4) T = V/G x TG (Roll) TABLE 1 provides various LOOK-AHEAD times T j at various turning radii and ground speeds. TABLE 1 By assuming that the fixed terrain clearance at the top 73 of the turn is equal to one turning radius R, the total LOOK-AHEAD time for two turn radii (i.e. T j + T 2 ) is simply twice the time for a single turning radius. Thus, the total LOOK-AHEAD time is 2 * T j plus the predetermined reaction time T 3 , for example 10 seconds, as given by equation (5). (5) T TOTAL = 2 * T j + T 3 By substituting R from equation (2) into equation (4), the LOOK-AHEAD time for a single turn radius is given by equation (6). (6) T, = 0.000025284 * V Thus, if it is assumed that the fixed clearance X at the top of the turn 73 is also equal to the turning radius R, the total LOOK-AHEAD time for two turning radii is given by equation (7) below, simply twice the value determined in equation (6). (7) T, + T 2 = 2 * 0.000025284 * V For a predetermined reaction time T 3 , for example 10 seconds, the LOOK-AHEAD distance (LAD) in nautical miles for a terrain advisory signal can be determined simply by multiplying the speed of the aircraft (V) by the total time T TOTAL as shown in equation (8). (8) LAD = V * (0.0000278 + 2 * 0.000252854 * V) + fixed distance * K, where K is a constant, for e.xample 0. However, in order to provide optimal conditions during a terrain advisory condition, the LAD .is limited with an upper limit and a lower limit. The lower limit may be a configurable amount, for example, either 0.75, 1 or 1-1 2 nautical miles at relatively low speeds (i.e. speeds less than 150 .knots) and limited to, for example, 4 nautical miles at speeds, for ejcample, greater than 250 knots. The LAD may also be limited to a fixed amount regardless of the speed when the distance to the runway is less than a predetermined amount, for example 2 nautical miles, except when the aircraft altitude is greater than 3,000 feet, relative to the runway. The terrain warning LAD is given by equation (9) below. (9) LAD = k, * LAD (terrain for LOOK-DOWN/LOOK-AHEAD advisoi indication), k 2 * LAD (terrain LOOK-DOWN/LOOK-AHEAD warning), k 3 * LAD (terrain LOOK-UP advisory), where k, = 1.5, except when the LAD is limited at its lower limit, in which case k j = 1, k 3 = 1 and where k 2 = 0.5, k 3 = 2. TERRAIN THREAT BOUNDARIES The terrain threat boundaries include a terrain floor boundary, terrain advisory boundaries (yellow alert), as well as terrain warning boundaries (red alert) and are provided along the groundtrack of the aircraft. TERRAIN FLOOR BOUNDARY The terrain floor boundary is the basis for the terrain threat boundaries and is similar to the terrain floor developed for the GPWS. The terrain floor relates to a distance DH below the aircraft and is proportional to the distance to the closest .runway to prevent nuisance warnings when the aircraft is taking off and landing, while providing adequate protection in other modes of operation. As illustrated in equation (10), the terrain floor boundary below the aircraft is essentially based upon providing 100 feet clearance per nautical mile from the runway, limited to 800 feet. (10) DH = 100(ft/nM) * (distance to .runway threshold - an offset) + 100 (ft/nM) * (distance to .runway center - 12 nautical miles) Equation (10) is shown graphically in FIG. 6. Referring to FIG. 6, the horizontal ajds represents the distance from the runway, while the vertical ajάs represents the DH terrain floor boundary beneath the aircraft. The first segment 71 represents the runway length, while the point 72 represents the runway center. After a small offset 74, the next segment 78, which starts at 0 feet, slopes at 100 feet per nautical mile distance from the runway threshold, identified by the point 80, up to a majdmum of 500 feet. The 500 foot ma.ximum continues along a segment 82 until the distance to the runway center 72 is a predetermined distance D, for example 12 nautical miles. The next segment 84 slopes up at a 100 feet per nautical mile distance until the distance rises 300 feet from the segment 82 for a total of 800 feet. The terrain floor DH boundary beneath the aircraft is limited such that the segment 78 begins at 0 and the segment 82 never goes above a predetermined maximum, for example 500 feet. In addition, the terrain floor DH boundary is also limited so that the segment 84 never goes below the vertical height of the segment 82 and rises no more than 300 feet relative to the segment 82. During conditions when the aircraft is 3,000 feet above the terrain, the terrain floor DH boundary is maintained at the upper limit of 800 feet. TERRAIN ADVISORY BOUNDARIES Two terrain advisory boundaries are shown graphically in FIGS. 7 and 8. FIG. 7 represents a condition when the aircraft is descending while FIG. 8 represents a condition when the aircraft is ascending. As will be discussed in more detail below, the terrain advisory boundaries are based upon the relationship between the flight path angle g and a first configurable datum, THETA1. Referring first to FIG. 7, the terrain advisory (yellow alert) boundaries are shown. The first segment of the terrain advisory boundary, identified with the reference numeral 92, corresponds to the DH terrain floor boundary. As mentioned above, the DH terrain floor boundary is a function of the distance from the runway 94. In order to determine the bottom segment 100 of the terrain advisory boundary, it is necessary to determine whether the flight path angle g is more or less than a configurable datum, identified as THETA1. As shown and described herein, THETAl is set for 0°. However, it should be clear that other angles for THETAl are within the broad scope of the present invention. For the condition illustrated in FIG. 7, the flight path angle g is less than THETAl. Thus, the segment 100 extends from the DH terrain floor boundary at the angle THETAl to the LOOK-AHEAD distance for a terrain advisory. The final segment 102 is generally parallel to the segment 92 and extends in a vertical direction along the LAD for the terrain advisory boundary. If the flight path angle g is greater than THETAl as shown in FIG 8., then different terrain advisory boundaries are provided. The segment 92 will be the same as shown in FIG. 7 and described above. However, when the flight path angle g is greater than THETAl as shown in FIG. 8, a bottom segment 106 is foπned by emending a line segment from the bottom of the segment 92, which represents the DH terrain floor, along a direction parallel with the flight path angle g up to the LAD. A vertical segment 108 extends from the bottom segment 106 along the LAD to form the terrain advisory boundaries. Thus, as shown, if the aiφlane is climbing, only terrain which penetrates the upward-sloping terrain advisory boundaries will cause an advisory signal, such that if the aircraft clears the terrain by a safe margin, no advisory signal is given, even if the terrain ahead is at or above the present aircraft altitude. However, if terrain penetrates the terrain advisory boundaries, an aural warning, such as "CAUTION, TERRAIN" aural advisory may be given. In addition, visual warnings may be provided. TERRAIN WARNING BOUNDARIES The terrain warning boundaries are shown in FIGS. 9 and 10 and, in general, indicate to the pilot of an aircraft conditions when evasive action is required to avoid terrain contact. The terrain warning boundaries are based on the relationship of the flight path angle of the aircraft relative to a second 26 configurable datum, THETA2. The datum THETA2 may be selected with an upslope of, for example, 6°, which is equal to the average climb capability of airliners. The datum THETA2 could be modified, taking into consideration aircraft type, configuration, altitude and time for takeoff. However, because of the longer LOOK-AHEAD distance as discussed above for the terrain warning boundaries, a terrain warning could occur before a terrain advisory indication for extreme terrain conditions. Referring to FIGS. 9 and 10, two different terrain warning boundaries are illustrated. In particular, FIG. 9 illustrates a condition when the flight path angle g is greater than the second datum THETA2. FIG. 10 illustrates a condition when the flight path angle g is less than the second datum THETA2. In both conditions, the first segment 114 of the terrain warning boundary is extended below the aircraft for a distance 1 2 of the DH terrain floor, which, as mentioned above, is a function of the distance of the aircraft from a runway. As mentioned above, the terrain warning boundaries are dependent on the relationship between the flight path angle g of the aircraft and the datum THETA2. Referring first to FIG. 9, since the flight path g is greater than the second datum THETA2, the bottom segment 118 of the terrain warning boundary is foπned by extending a line from the segment 114 at an angle equal to the flight path angle g up to the LOOK-AHEAD distance for a terrain warning as discussed above. A vertical segment 120 extends from the lower segment 118 along the LAD for a terrain warning, forming the terrain warning boundaries illustrated in FIG. 9. If the flight path angle of the aircraft is less than the slope of the second datum THETA2 as shown in FIG. 10, a bottom segment 122 extends from the segment 114 up to the LOOK-AHEAD distance at an angle equal to the flight path angle g. A segment 124 ejctends upwardly along the LAD from the segment 122. If any terrain along the groundtrack of the aircraft out to the LAD for the terrain warning penetrates the envelope, aural and/or visual terrain ahead warnings may be given. CUT-OFF BOUNDARIES In order to avoid spurious warnings when the aircraft overflies a ridge at relatively low altitudes, the warning boundaries may include cut-off boundaries, for example, as illustrated in FIGS. 11, 12 and 13. Without the cut-off boundaries, warnings would be given, although the terrain is virtually below the aircraft and no terrain is visible ahead. Referring first to FIG. 11, the cut-off boundary 126 begins at a predetermined cut-off offset 128 below the aircraft and extends in a direction in front of the aircraft at a predetermined envelope cut- off angle 130. The envelope cut-off angle 130 is equal to the flight path angle g plus a configurable predetermined cut-off angle, described and illustrated as -6°. For level flight as shown in FIG. 11, the cut-off boundary 126 ejctends from the cut-off offset 128 in the direction of the envelope cut-off angle 130 toward the front of the aircraft to a point 132 where it intersects a terrain advisory boundaiy or terrain warning boundary, identified with the reference numeral 134. For level flight, as shown in FIG. 11, the flight path angle g is zero. Thus, the cut-off boundary 126 illustrated in FIG. 11 will extend from the cut-off offset 128 along an angle equal to the cut-off angle, which, as mentioned above, is selected as -6° for illustration. As mentioned above, the cut-off boundary 126 e.xtends from the cut-off offset 128 to the point 132 where it intersects the terrain advisory boundary 134 discussed above. The warning boundary is then selected to be the highest of the terrain advisory boundary 134 and the envelope cut-off boundary 126. Thus, for the example illustrated in FIG. 11, the terrain advisory boundary would consist of the cut-off boundary 126 up to the point 132, where the envelope cut-off boundary 124 intersects the warning envelope 126. From the point 132 forward, the normal terrain advisory boundary 134, corresponding, for e.xample, to a THETAl slope, is utilized. Thus, if either a terrain advisory boundary or terrain warning boundary is below the cut-off boundary 126, the cut-off boundary 126 becomes the new boundary for the advisory or warning signal. The cut-off angle 130 is limited such that if the flight path angle g is a relatively high climb angle, the cut-off angle 130 does not exceed a predetermined limit, for example a configurable limit between 0 to 6° as will be illustrated in FIG. 12. In particular, referring to FIG. 12, the flight path angle g is shown at a relatively high climb rate (i.e. g = 9°). An unlimited cut-off envelope angle 130 is illustrated by the partial segment 136. As shown, this segment 136 is 6° below the flight path angle g. However, in order to decrease the sensitivity of the system 20 during conditions when the flight path angle is relatively large, for example 9°, indicative of a fairly steep climb, the cut-off envelope angle 130 is limited. In the example illustrated in FIG. 12, the cut-off angle limit is selected, for example, to be 1.4°. Thus, at ilight path angles g greater than 7.4°, the cut-off angle will result in an envelope cut-off boundaiy 140, which is more than 6° below the flight path angle g. As shown, since the cut-off angle limit was selected as 1.4°, the cut-off angle will be limited to 1.4° anytime the flight path angle is greater than 7.4°, even though the cut-off angle will be equal to or more than 6° less than the flight path angle g. Thus, for the conditions illustrated in FIG. 12, the segment 140 begins at the cut-off offset 128 and continues along a line that is 1.4° from the horizon until it intersects the terrain advisory or terrain warning boundary 142 at the point 144. The actual advisory or warning boundary will be formed by the limited cut-off boundary 140 up to the point 144 and will continue with the normal warning boundary 142 beyond the point 144. FIG. 13 illustrates a situation when the aircraft is descending and thus the flight path angle g is less than 0. During conditions when the flight path angle g is less than the normal descent angle, the warning boundary is lowered past the cut-off boundary for high sink-rate situations in order to prevent nuisance warnings during step-down approaches. In particular, a BETA sink rate enhancement is provided and is effective anytime the flight path g is below a predetermined configurable descent bias GBBIAS, for example, -4°. During such conditions, an angle BETA is added to the terrain advisery datum (THETAl) as indicated below. In particular, when the flight path angle g is greater than the descent bias angle GBBIAS or the flight path angle g is less than zero, BETA is selected as 0. When the flight path angle g is greater than zero, BETA is given by equation (11). (11) BETA = g - THETAl; for g > 0 When the flight path angle g is less than GBBIAS, the value for BETA is selected in accordance with equation (12) below. (12) BETA = K * (g - GBBIAS), where k = 0.5, g = flight path angle and GBBIAS is selected as -4°. FIG. 13 illustrates a condition when the flight path angle g is -8°. During this condition, the cut-off angle 130 is selected to be 6° below the flight path angle g and thus defines an envelope cut-off boundary 150 that extends from the cut-off offset 128 along a line which is -14° from the horizontal axis. Since the flight path angle g is less than GBBIAS, the angle BETA is added to the normal warning boundary according to equation (12) above. Thus, for a flight path angle of -8°, the BETA angle will be 2°. Thus, the terrain advisory or warning boundary, indicated by the boundary 151, instead of ej tending along the THETAl slope, will extend along an angle which is 2° below the THETAl slope to define the enhanced boundary 152. The warning cut-off bounda.ry 152 thus will eΛend to the point 156 where it intersects the normal terrain advisory or terrain warning boundary 152. Since the cut-off angle boundary 150 is higher than boundary 152 up to the point 156, the cut-off boundary 150 will be utilized as the warning boundary up to the point 156. Beyond point 156, the BETA-enhanced warning boundary 152 will define the warning envelope. FIGS. 14, 15 and 16 illustrate the composite terrain advisory and terrain warning boundaries for different conditions. The cross-hatched area illustrates terrain in the vicinity of the aircraft. Three conditions are illustrated. In particular, FIG. 14 represents a condition when the flight path angle g of the aircraft is less than the first datum THETAl. FIG. 15 represents a condition when the flight path angle g is between the first datum THETAl and the second datum THETA2. FIG. 16 represents a condition when the flight path angle g is greater than the second datum THETA2. The terrain advisory boundaries are indicated in solid line while the terrain warning boundaries are illustrated by dashed lines. Referring first to FIG. 14, the terrain advisory boundaries include a boundary 155, which extends from the aircraft to the DH terrain floor. Noπnally, the lower terrain advisory boundary 157 would ejctend from the segment 155 along an angle equal to the THETAl slope. However, in this situation, the flight path angle g is illustrated to be greater than the GBBIAS angle; therefore, a BETA-enhanced boundary 159, as discussed above, is utilized which e.xtends along an angle that is equal to the THETAl angle plus the BETA angle, up to the LOOK-AHEAD distance for a terrain advisory. A vertical boundary 158 extends from the first datum THETAl vertically upwardly along the LOOK-AHEAD distance for a terrain advisory. Since the cut-off boundary 157 is higher than the terrain advisory boundary 157 up to the point 169 where the two boundaries intersect, the cut-off boundary 157 becomes the terrain advisory boundary up to the point 169. Beyond the point 169, the boundary 159 is utilized as the warning boundary. The terrain warning boundary includes a boundary 162 extending to a point equal to 1/2 of the terrain floor as discussed above. A boundary 165 extends from the boundary 162 at the THETA2 slope. A vertical boundary 167 extends from the boundary 165 along the LOOK-AHEAD distance for a terrain warning as discussed above. FIG. 15 illustrates a situation when the flight path angle g is greater than the THETAl angle and less than or equal to the THETA2 angle. During this condition, the boundary for the terrain warning extends along a segment 180, which is at 1/2 of the DH terrain floor. A sloping boundary 184 extends from the segment 180 to the LOOK-AHEAD distance for a terrain warning (point 186). From there, a vertical boundary 188 extends vertically upwardly to define the terrain warning boundaries. The terrain advisoiy boundary includes a boundaiy 185 which extends downwardly to the terrain floor, i.e. point 187. From point 187, the terrain advisory boundary extends along a segment 189 at the flight path angle g up to a point 191, the LOOK-AHEAD distance for a terrain advisory. A vertical segment 194 extends along the LAD from the boundary 189 at the point 191. In this illustration, an envelope cut-off boundary 193 cuts off a portion of the terrain advisory indication boundary since the flight path angle g of the aircraft is greater than a predetermined limit. Thus, the portions of the warning boundary 189 below the cut-off angle boundary 193 are ignored; and the segment 193 becomes the effective terrain advisory indication up to the point 195, where the cut-off boundary 193 intersects the terrain advisory indication boundary 189. After the point 195, the boundaiy 189 will be the effective warning boundary. FIG. 16 illustrates a situation when the flight path angle g is greater than THETA2. In this situation, a terrain advisory boundary 200 extends from the aircraft to the boundary terrain floor. A lower boundary 202 eΛends at the flight path angle g from the boundary 200 up to the LOOK-AHEAD distance for a terrain advisory. A vertical terrain advisory boundary 204 eΛends upwardly along the LOOK-AHEAD distance for the terrain advisory. Since the flight path angle g is illustrated to the greater than GBBIAS, a segment 211, the cut-off boundary, defines the effective warning boundary up to a point 213 where the terrain advisory boundary intersects the cut-off warning boundary. After that point, segment 202, which is higher, becomes the warning boundary. The terrain warning boundary extends along a segment 208 down to a point of 1/2 the terrain floor. A lower boundary 210 ejctends along an angle equal to flight path angle g since it is greater than the THETA2 slope. A vertical terrain warning boundary 212 extends upwardly from the boundary 210 along the LOOK-AHEAD distance for a terrain warning. In order to provide additional sensitivity of the warning system, terrain advisory signals within a predetermined portion of the LAD for a terrain advisory, for ejcample 1/2 LAD, may be treated as terrain warnings. Thus, as shown in FIGS. 14, 15 and 16, vertical boundaries 161, 181 and 201, respectively, are shown, for example, at 1/2 LAD. Based upon the flight path angles shown, these segments define effective terrain advisory envelopes 166, 182 and 203. ALTERNATIVE TERRAIN THREAT BOUNDARIES Alternative terrain threat boundaries are illustrated in FIGS. 17-22. Similar to the terrain threat boundary discussed above and illustrated in FIGS. 5-16, the alternative terrain threat boundaries include a terrain floor boundary, terrain advisory boundaries (yellow alert) and terrain warning boundaries (red alert). In the alternative embodiment, the advisory and warning boundaries are further divided into two parts, a LOOK-AHEAD/LOOK-DOWN boundary for detecting terrain ahead or below the aircraft (FIGS. 17 and 18) and a LOOKUP boundary for detecting precipitous high terrain ahead of the aircraft which may be difficult to clear (FIG. 19). LOOK-AHEAD/LOOK-DOWN TERRAIN ADVISORY AND WARNING BOUNDARIES As mentioned above, the LOOK-AHEAD/LOOK-DOWN boundaries are illustrated in FIGS. 17 and 18. Referring first to FIG. 17, LOOK- AHEAD/LOOK-DOWN advisoiy and warning boundaries are illustrated for a condition when the aircraft is descending (i.e. g < 0). During such a configuration, the first segment of the LOOK-AHEAD/LOOK-DOWN terrain advisory boundary, identified with the reference numeral 300, corresponds to the DH terrain floor boundary. As discussed above, the DH terrain floor boundary is a function of the aircraft from the runway. In order to determine the bottom segment 302 of the LOOK-AHEAD/LOOK-DOWN terrain advisory boundary, the flight path angle g is compared with a configurable datum, THETAl, for example 0°. During descent conditions, the flight path angle g will thus be less than THETAl. Thus, the LOOK-AHEAD/LOOK-DOWN terrain advisory boundary segment will extend from the DH terrain floor boundary segment 300 along the angle THETAl to the look-ahead distance for a terrain advisory (LAD). The final segment 304 extends vertically upwardly from the segment 302 along the LAD. The LOOK-AHEAD/LOOK-DOWN terrain advisory boundary may also be modified by the BETA sink rate enhancement as discussed above. In this embodiment, the BETA sink rate enhancement ensures that an advisory indication always precedes a warning indication when the aircraft descends into or on top of terrain. The BETA sink rate enhancement is determined as a function of the flight path angle g and two (2) configurable constants KBETA and GBIAS. The BETA sink rate enhancement BETAl for a LOOK-AHEAD /LOOK-DOWN terrain adviscry is provided in equation (13) below for a condition when the aircraft is descending. (13) BETAl = KBETA * (g - GBIAS), where GBIAS is a configurable constant, selected for example, to be zero (0) and KBETA is also a configurable constant selected, for example, to be 0.5. Referring to FIG. 17, the BETA sink rate enhancement BETAl for the LOOK-AHEAD/LOOK-DOWN terrain advisory boundary provides an advisory warning at a distance less than 1 2 LAD, unlike the terrain advisory boundary described above). More particularly, for the values defined above for the configurable constants KBETA and GBIAS, the BETA sink rate enhancement BETAl results in a segment 306 which extends from the DH terrain floor segments 300 at an angle equal to g/2 up to 1 2 of the LAD. Beyond 1 2 LAD, a segment 308 ejctends at the angle THETAl to a distance equal to the LAD. A vertical segment 310 extends along the LAD to connect the segments 308 to the segment 304. As discussed above, a cut-off boundary may be provided to reduce nuisance warnings. Referring to FIG. 17, the cut-off boundary is identified with the reference numeral 312. The cut-off boundary 312 extends from a vertical distance 314 below the aircraft along a cut-off angle up to the point of intersection 316 with the terrain advisory boundary. For distances less than the intersection 316, the cut-off boundary 312 forms the terrain advisory .boundary. For distances beyond the point of intersection 316, the boundaries 306 and 308 form the terrain advisory boundaries. The terrain warning boundary includes the segment 300 extending from the aircraft along the DH terrain floor. A bottom segment 318 connects to the segment 300 and ejctends along a BETA sink rate enhancement angle BETA2. The BETA sink rate enhancement angle is determined as a function of the flight path angle g and a configurable constant KBETA2 as well as the constant GBIAS discussed above and provided in equation (14) below. (14) BETA2 = KBETA2 * (GAMMA - GBIAS), where GBIAS is a configurable constant selected, for example, to be 0 and KBETA2 is also a configurable constant selected, for e.xample, to be 025. For such values of the constants KBETA2 and GBIAS, the BETA enhancement angle KBETA2 will be 1/4 * g. Thus, the segment 318 eΛends from the segment 300 at an angle equal to 1/4 * g up to 1/2 the LAD. A vertical segment 320 extends along a distance equal to 1/2 * LAD from the segment 318 to define the terrain warning boundaiy. Similar to the above, the terrain warning boundaries are also limited by the cut-off boundary 312. Thus, the cut-off boundary 312 forms the terrain warning boundary up to a point 322, where the cut-off boundary 312 intersects the lower terrain warning boundary 318. At distances beyond the point of intersection 322, the segment 318 forms the lower terrain warning boundary up to a distance equal to 1/2 of the LAD. The terrain advisory and terrain warning boundaries for a condition when the aircraft is climbing (i.e. g > 0) is illustrated in FIG. 18. During such a condition, the BETA sink rate enhancement angles BETAl and BETA2 are set to a configurable constant, for example, zero (0). The terrain advisory boundary during a climbing condition is formed by emending a vertical segment 324 from the aircraft for a distance below the aircraft equal to the DH terrain floor. During a climbing condition, a segment 326 is extended from the segment 324 to the LAD at an angle equal to the flight path angle g. At a point 328 where the segment 326 intersects a position equal to 1/2 of the LAD, a vertical segment 330 is extended up from the segment 326, foπning a first vertical boundary for the terrain advisory condition. The line segment 326 from the point 328 to the LAD forms the lower terrain advisory boundary while a line segment 332 extending vertically upward from the line segment 326 along the LAD forms a second vertical boundary. For the exemplary condition illustrated, a cut-off boundary 334 does not intersect the terrain advisory boundaries. Thus, the terrain advisory boundaries for the exemplary condition illustrated is formed by the segments 330 and 332 and that portion of the line segment 326 between the line segments 330 and 332. The terrain warning boundaries for a condition when the aircraft is climbing includes the vertical segment 324 (FIG. 18) which extends from the aircraft to vertical distance equal to the DH terrain floor below the aircraft forming a first vertical boundary. For a condition when the aircraft is climbing, the line segment 326 extends from the segment 324 at the flight path angle g to form the lower terrain warning boundary. The vertical segment 330 at a distance equal to 1/2 of the LAD forms the second vertical terrain warning boundary. The cut-off boundary 334 limits a portion of the terrain warning boundary. In particular, the cut-off boundary 334 forms the lower terrain warning boundary up to a point 340, where the cut-off boundary 334 intersects the line segment 326. Beyond the point 340, a portion 342 of the line segment 340 forms the balance of the lower terrain warning boundary up to a distance equal to 1/2 of the LAD. LOOK-UP TERRAIN ADVISORY AND WARNING BOUNDARIES The LOOK-UP terrain advisory and terrain warning boundaries are illustrated in FIG. 19. As will be discussed in more detail below, the LOOK- UP terrain advisory and warning boundaries start at altitudes DHYEL2 and DHRED2, respectively, below the aircraft. These altitudes DHYEL2 and DHRED2 are modulated by the instantaneous sink rate (i.e. vertical speed, HDOT of the aircraft). The amount of modulation is equal to the estimated altitude loss for a pull-up maneuver, for example, at 1/4G (i.e. 8 ft/sec 2 ) at the present sink rate. The altitudes DHRED2 and DHYEL2 are dependent upon the altitude loss during a pull-up maneuver (A1PU) and the altitude loss due to reaction time (ALRT) discussed below. The estimated altitude loss due to a pull-up maneuver ALPU is best understood with reference to FIG. 20, wherein the vertical axis relates to altitude in feet and the horizontal ajάs relates to time in seconds. The trajectory of the aircraft from a time shortly before pull-up is initiated to a time when the aircraft has recovered is shown and identified with the reference numeral 344. Assuming an advisory or warning indication is generated at time T, at a point 346 along the trajectory 344, the altitude loss due to the reaction time of the pilot (ALRT) is given by equation (15) below. (15) ALRT = HDOT * T R , where HDOT equals the vertical acceleration of the aircraft in feet/sec and T R equals the total reaction time of the pilot in seconds. Assuming a pull-up maneuver is initiated at time T 2 , the altitude loss due to the pull-up maneuver ALPU may be determined by integrating the vertical velocity HDOT with respect to time as set forth in equation (16) below. (16) HDOT(t) = a * t + HDOT 0 , where "a" equals the pull-up acceleration and HDOT 0 is a constant. Integrating both sides of equation (16) yields the altitude loss as a function of time H(t) as provided in equation (17) below. ; (17) H(t) = 1/2 * a * t 2 + HDOT 0 t Assuming a constant acceleration during the pull-up maneuver, the time t until vertical speed reaches zero (point 348) is given by equation (18). (18) t = -HDOT 0 /a Substituting equation (18) into equation (17) yields equation (19). (19) ALPU = -(HDOT 0 ) 2 /(2 * a) Equation (19) thus represents the altitude loss during the pull-up maneuver. An exemplary block diagram for generating the signals ALTR and ALPU is illustrated in FIG. 21. In particular, a signal representative of the vertical velocity of the aircraft HDOT, available, for example, from a barometric altimeter rate circuit (not shown), is applied to a filter 350 in order to reduce nuisance warnings due to turbulence. The filter 350 may be selected with a transfer function of l/(TAHDOT * S + 1); where TAHDOT is equal to one second. The output of the filter 350 is a signal HDOTf, which represents the filtered instantaneous vertical speed; positive during climbing and negative during descent. In order to generate the altitude loss due to reaction time signal ALTR, the signal HDOTf is applied to a multiplier 352. Assuming a pilot reaction time Tr, for example, of 5 seconds, a constant 354 equal to 5 seconds is applied to another input of the multiplier 352. The output of the multiplier 352 represents the signal ALTR, which is positive when HDOTf is negative and set to zero if the signal HDOTf is positive, which represents a climbing condition. More particularly, the signal HDOTf is applied to a comparator 356 and compared with a reference value, for example, zero. If the comparator 356 indicates that the signal HDOTf is negative, the signal HDOTf is applied to the multiplier 352. During climbing conditions, the signal HDOTf will be positive. During such conditions, the comparator 356 will apply a zero to the multiplier 352. The altitude loss due to the pull-up maneuver signal ALPU is developed by a square device 358, a divider 360 and a multiplier 362. The filtered instantaneous vertical speed signal HDOTf is applied to the square device 358. Assuming a constant acceleration during the pull-up maneuver, for example, equal to 8 feet/ sec 2 (0.25 G), a constant is applied to the multiplier 362 to generate the signal 2a. This signal, 2a, is applied to the divider 360 along with the output of the square device 350. The output of the divider 360 is a signal 7 (HDOT f) /2a, which represents the altitude loss during a pull-up maneuver signal ALPU. These signals ALRT and ALPU are used to modulate the distance below the aircraft where terrain advisory and terrain warning boundaries begin during a LOOK-UP mode of operation. More particularly, during such a mode of operation, DH terrain floor segment of the terrain advisory boundary DHYEL2 is modulated by the signals ALRT and ALPU while the DH terrain floor segment of the terrain warning boundary DHRED2 is modulated by the signal ALPU as indicated in equations (20) and (21), respectively. (20) DHYEL2 = 3/4 * DH + ALRT + ALPU (21) DHRED2 = 1/2 * D + ALPU, where DH represents the terrain floor as discussed above. Thus, referring to FIG. 19, the LOOK-UP terrain advisory warning begins at a point 364 below the aircraft; equal to DHYEL2. If the flight path angle g is less than a configurable datum THETA2, a terrain advisory boundary 366 e.xtends from the point 364 to the advisory LAD at an angle equal to THETA2. Should the flight path angle g be greater than THETA2, the lower advisory boundary, identified with the reference numeral 368, will extend from the point 364 at an angle equal to the flight path angle g. Similarly, the LOOK-UP terrain warning boundary begins at point 370 below the aircraft; equal to DHRED2. If the flight path angle g is less than the angle THETA2, a warning boundary 372 extends from the point 370 at angle THETA2 to the warning LAD. Should the flight path angle g be greater than THETA2, a warning boundary 374 will extend at an angle equal to the flight path angle g between the point 370 and the warning LAD. CUT-OFF ALTITUDE The cut-off altitude is an altitude relative to the nearest runway elevation; set at, for ejcample, 500 feet. Altitudes below the cut-of altitude are not displayed and are ignored by the terrain advisory and terrain warning computations. The use of a cut-off altitude declutters the display discussed below around aiφorts, especially during a final approach when the aircraft approaches the ground. A second advantage is that nuisance warnings on a final approach, due to altitude errors, terrain data base resolution and accuracy errors are minimized. However, the use of a cut-off altitude during certain conditions, such as an approach to an ahport on a bluff (i.e. Paine Field) at a relatively low altitude or even at an altitude below the aiφort altitude, may compromise system perfo.rmance. More particularly, during such conditions, the use of a cut-off altitude may prevent a terrain warning from being generated because the threatening terrain may be below the cut-off altitude. In order to account of such situations, the system selects the lower of two cut-off altitudes; a nearest runway cut-off altitude (NRCA) and a cut-off altitude relative to aircraft (CARA). The NRCA is a fixed cut-off altitude, relative to the nearest runway. The CARA is an altitude below the instantaneous aircraft altitude (ACA) by an amount essentially equivalent to DH terrain floor boundary for an advisory condition, which, as discussed above, is a function of the aircraft distance to the nearest runway. Equations (22) and (23) below set forth the NRCA and CARA. As mentioned above, the absolute cut-off altitude (ACOA) is the lower of the NRCA and CARA as set forth in equation (24). (22) NRCA = COH + RE, where COH relates to the cut-off height and is a fixed configurable value, initially set between 400 feet and 500 feet; and RE equals the mnway elevation. (23) CARA = ACA - DH - DHO, where ACA is the instantaneous aircraft altitude; DH is the terrain floor which is proportional to the distance to the runway as discussed above; and DHO is a configurable bias, set to, for encamp le, 50 feet. (24) ACOA = lower of CARA, NRCA, except where DH DHl, in which case ACOA is always equal to NRCA. DHl is a point at which the ACOA is forced to be equal to NRCA independent of the aircraft altitude. The point DHl is related to COH, DH and DHO such that on a nominal three (3) degree approach slope, CARA is equal to NRCA when the aircraft is at a distance equal to a distance DHl from the aiφort, as illustrated in TABLE 2 below. TABLE 2 The point DHl forces the cut-off altitude (COH) above the runway whenever the aircraft is close to the runway to ensure robustness against nuisance warnings caused by altitude errors and terrain data base resolution margins to disable the terrain advisory and warning indications when the aircraft is within the aircraft perimeter. There are trade-offs between nuisance warnings and legitimate warnings. In particular, the lower COH, the closer the advisory and warning indications are given, making the system more nuisance prone. As indicated above, for a COH of 400, terrain advisory and warning indications are effectively disabled when the aircraft is closer than 1.5 n miles from the aiφort runway. FIG. 22 illustrates the operation of the alternative cut-off altitude boundaries. In particular, FIG. 22 illustrates a condition when the COH is set to 300 feet with DHl equal to 50 feet. The cut-off altitude substantially for area from the runway, for ejcample, greater than 4 nautical miles, is 300 feet, as indicated by the segment 378 when the glide slope angle is less than a predetermined angle, for e.xample, 3°. As the aircraft gets closer to the runway, the COH is lowered, as illustrated by the segment 388, until the aircraft is within one (1) nautical mile of the .runway, at which point the COH is forced to be 300 feet, which effectively disables any terrain advisory and warning indications within the aircraft is closer than one (1) nautical mile from the runway, as represented by the segment 382. During a condition when the aircraft is on a, for example, 3° glide slope angle, the ACOA is forced to be the NRCA. As shown, the NRCA is illustrated by the segment 382. DISPLAY SYSTEM A display system, generally identified with the reference numeral 400, is illustrated in FIGS. 23-42. The display system 400 is used to provide a visual indication of the terrain advisory and terrain warning indications discussed above as a function of the current position of the aircraft. Background terrain information is also provided which provides an indication of significant terrain relative to the current position of the aircraft. In order to declutter the display, background information may be displayed in teπns of predetermined dot patterns whose density varies as a function of the elevation of the terrain relative to the altitude of the aircraft. Terrain advisory and terrain warning indications may be displayed in solid colors, such as yellow and red, relative to the groundtrack of the aircraft. An important aspect of the invention is that the terrain background information as well as the terrain advisory and warning indications may be displayed on a navigational or weather display, normally existing within an aircraft, which reduces the cost of the system and obviates the need for e.xtensive modifications to existing displays, such as a navigational and weather radar type display. In particular, the terrain data is converted to a weather radar format in accordance with the standard ARINC 708/453 for digital buses on civil aircraft. The terrain data, masked as weather data, can then be easily displayed on an easting navigational or a dedicated weather radar display that conforms with the ARINC 708/453 serial interface standard. Referring to FIG. 23, the display system 400 includes a weather radar display or navigational display 36 connected to a control bus 404 conforming to an ARINC 429 standard and a terrain data bus switch 406. The terrain data bus switch 406 enables selection between terrain data and weather data for display. More particularly, the data bus switch 406 includes a common pole 408 connected to the display 36 by way of a serial display bus 410. One contact 412 on the data bus switch 406 is connected to a serial weather data bus 414 and, in turn, to a weather radar R/T unit 416, which transmits weather radar data over the weather data bus 414 in conformance with the ARINC 708/453 standard. An antenna 418 provides weather radar data to the weather radar R T unit 416. The weather radar R/T unit 416 is also connected to the control bus 404, which is used for range and mode data. In accordance with an important aspect of the invention, the terrain advisory and terrain warning indications, as well as the background information discussed above, are converted to "RHO/THETA" foimat. The converted terrain data is then applied to a terrain data serial bus 420, which conforms to the ARINC 453 standard, and connected to a contact 422 on the data bus switch 406 to enable selective display of either weather radar or terrain data on the display 36. The system for converting the terrain advisory and warning indications as well as background terrain data to an ARINC 708/453 standard serial format is indicated as a functional block 424 in FIG. 23 and is described in more detail below. By converting such data into an ARINC 708/453 standard serial format, the data can be displayed on an existing display 36, which confoπns to the ARINC 453 standard without the need for relatively complex and expensive modifications to the display's symbol generator. Referring to FIG. 24, each word 425, in accordance with the ARINC 453/708 standard, is 1600 bits long and represents one spoke or radial 426 of the display 36. Normally, 512 spokes or radials 426 are transmitted for each complete sweep of the display 36. At a transmission rate of, for example, 1 megahertz (MHz), each word 425 takes about 1.6 milliseconds (msec) to transmit. For 512 spokes, one complete sweep will thus take about 4 seconds. The 1600-bit words 425 include a header 428 and 512 range bins 430. The header 428 includes the control data on the control bus 404 and also includes a 12-bit code representing the antenna scan angle; the angle of the spoke relative to the aircraft heading equivalent to an indicator UP direction. The range bins 430 cover the distance from the antenna 418 to the ma.ximum selected. Each of the 512 range bins includes three intensity bits, encoded into colors as indicated in TABLE 3 below. TABLE 3 As will be discussed in more detail below, the display system 400 is capable of displaying background terrain information as well as terrain threat indications as a function of the current position of the aircraft. The threat detection algorithm is run along the groundtrack of the aircraft. Thus, terrain threat indications are typically displayed along the groundtrack while background terrain is displayed relative to the heading of the aircraft. Referring to FIG. 25, the terrain background information is shown on the display 36. As will be discussed in more detail below, the elevation of the highest terrain relative to the altitude of the aircraft is shown as a series of dot patterns whose density varies as a function of the distance between the aircraft and the terrain. For example, a relatively dense dot pattern 432 may be used to indicate terrain that is, for example, 500 feet or less below the aircraft. A medium dense dot pattern 434 may be used to represent terrain that is 1000 feet or less below the aircraft, while a lightly dotted pattern 436 may be used to indicate terrain 2000 feet or less below the aircraft. In order to declutter the display 402, terrain more than, for example, 2000 feet below the aircraft, is not shown. The dots may be displayed in one of the .colors indicated in TABLE 3 above, for example, yellow (amber). In addition, in the vicinity of an aiφort, a runway graph 438, for example, in green, may also be provided. The apparent display resolution may be increased by using more than one color along with the variable density dot patterns, for example, as shown in TABLE 4 below. TABLE 4 The display of a terrain threat indication is illustrated in FIG. 26. As shown, the terrain threat indication may be displayed contemporaneously with the background terrain information. Terrain advisory and terrain warning .indications are displayed in solid shapes 440 and 442, respectively, for ejcample, "squares"; the displayed terrain map cells which represent a threat painted solid yellow or red. More particularly, at an aspect ratio for the display of, for example, 3 x 4 (vertical to horizontal), the terrain cells will appear "square", particularly at relatively low-range settings. Colors are used to distinguish between terrain advisory and terrain warning indications. For example, red may be used to represent a terrain warning indication 442 while yellow or amber is used to represent a terrain advisory indication 440. By using colored shapes for terrain threat indications and dot patterns of variable density for the terrain background infoπnation, clutter of the display 402 is minimized. The terrain data from the terrain data base 24, for example, as illustrated in FIG. 4B, is used to compute the terrain threat indications as well as background information for the display 36. As discussed above, the terrain data base 24 may be stored in a flash ROM 444 (FIG. 27) for convenience of updates. In order to facilitate update of the display 36 as a function of the position of the aircraft, terrain data from the flash ROM 444 is transferred to a random access memory (RAM) A, identified with the reference numeral 446. As will be discussed in more detail below, the RAM A is mapped with terrain data from the flash ROM 444, centered about the current position of the aircraft, as illustrated in FIG. 27. In order to provide a relatively large display range, for e.xample, 160 nautical miles, while minimizing processing time as well as the size of the RAM A, the RAM A is configured as a "wedding cake" as generally illustrated in FIG. 28 and configured such that the highest terrain resolution is near the position of the aircraft and the terrain farthest from the aircraft position has the lowest resolution. For example, the RAM A may be provided with four (4) layers. As shown, the instantaneous aircraft position is shown generally centered in layer 1, having the highest resolution. The resolution of layer 2 lies between the resolution of layer 3 and layer 1. Layer 4 (not shown) is provided with a relatively coarse resolution for use in a cruise mode. Such a variable resolution display, configured as a layered wedding cake, minimizes the memory size of RAM A as well as the processing time. In order to improve the performance of the system, the flash ROM 444 may also be configured similar to a "wedding cake", as generally illustrated in FIG. 27. As discussed above, the terrain data base 24 may be provided with variable resolution with a relatively fine resolution of, for ejcample, 30 x 20 arc sec (i.e. 1 2 x 1/2 arc min). The relatively coarse resolution of layers 2 and 3 can be foπned by combining finer resolution maps. With such a resolution, the configuration of RAM A may be selected in accordance with TABLE 5. TABLE 5 Each layer is generally centered relative to the aircraft position as shown in FIG. 28 at the time of the update of RAM A. Each layer of the "wedding cake" has its own update boundary. For example, layer 1 of the RAM A includes an update boundary 448. As the aircraft passes through the update boundary 448, the RAM A is updated with new terrain data from the flash ROM 444 (FIG. 27) as will be discussed in more detail below. Because of the refresh rates required for the display 36, the terrain data may be stored in files alternative to the format illustrated in FIG. 4A. For example, the file format of the terrain data may include an index file or header 55, stored in one or more 128 K byte storage blocks and a plurality of data files 51, as generally shown in FIG. 29. As shown, each index file 55 may include pointers to specific data files 51, the length of the map file, as well as the position of one or more of the corner boundaries of the file 51, for ejcample, northwest and southeast corners of each map file 51, which facilitates updates of the display 36 as a function of the current aircraft position. The index file 55 may contain information regarding the length of the map file 51, a start of file or altitude offset, the resolution of the map file 51, and whether and how the file 51 is compressed to enable use of various compression algorithms. The data files 51 may include a file header 454 as well as data blocks 456 which, as shown, can be grouped according to resolution size. The file header 454 indicates various information about the file. The format of the header 454 may be structured as follows: <MAJOR VERSION BYTE XMINOR VERSION BYTE XFILE STATUS BYTE XATTRIB BYTE > <FILE NAME (8 CHARACTERS) XEXTENSION (4 CHARACTERS) > <FILE LENGTH LONGWORD > <TIME STAMP LONGWORD > <CRC LONGWORD > <SPME LONGWORD > The major and minor version bytes relate to the revision status of the data in the data files 51. The minor version is set as the . inverse of the byte value of the major version such that a default erase will indicate a zero version. The file status byte 457 provides various infoimation regarding the status of the file as discussed below, while the attribute byte is reserved. The file name, extension file length, and spare data are self-e.xplanatory. The time stamp relates to the operational time of the system (i.e. length of time in use) while the CRC data relates to the data files 51. The status byte may be used to provide the following information regarding the data blocks 456, for example, as set forth in TABLE 6 below. TABLE 6 The EMPTY status (FF) is set by erasing the data blocks 456, while the DOWNLOADING status (FE) is set as part of the image. The VALID status (FO) is set when the file is loaded. Lastly, the READY TO ERASE (EO) is set by the file system for obsolete blocks. The file structure may also be as described in Appendix 1. As indicated below, various corrections of the data files 51 may be required in areas near runways. FIG. 30 illustrates a portion of the terrain data base 24 around a particular aiφort, Boeing Field, with a resolution of, for ejcample, 1 2 x 1/2 minute subcells 455, shown with a runway 456 drawn in. The elevations, as discussed above, are selected to be the highest elevations within the cells 455, rounded up to the next highest resolution increment, for example 100 feet. While such a system provides useful and conservative terrain data for use with the terrain data base 24, such a system in the vicinity of an aiφort can cause the elevations of the runway 456 to be too high or too low. For e.xa.mple, the elevation of the .runway 456 at Boeing Field is known to be between 17 and 15 feet (indicated as "17/15 feet); rounded off to zero (0) feet. FIG. 30 shows the elevation of runway 456 to be 200/300 feet, for example, by the averaging algorithm discussed above. Such errors in the runway elevation can degrade system performance, and, in particular, cause improper performance of the terrain advisory and terrain warning cut-off boundaries below 500 feet, for e.xample, as illustrated in FIG. 22. In order to solve this problem, the actual elevation of the runway 456, rounded to the nearest resolution, is used for cells 455 around a perimeter of the runway 456, as generally shown in FIG. 31. In particular, a first perimeter, illustrated by the dashed box 458, is selected to be about ±1 cell (i.e. 1/2 minute) from the centerline of the runway 456. The elevations in all of the cells 455 intersected by the dashed box 458 are set to the actual runway elevation and rounded to the nearest resolution. For Boeing Field, the elevation for such cells is 0 feet. Consideration is also given for a narrow approach and a wide approach. The dashed box 460 represents a wide approach while the dashed box 462 represents a narrow approach. The cells 464 and 466, intersected by the wide approach box 460, are set on the actual runway elevation,. rounded to 0 feet. Inside the approach perimeter, as indicated by the arc 467, the elevation for the cells is selected to be either the runway elevation or the DH terrain floor (as discussed above). For example, if the corner of the cell 468, closest to the runway 456, is 1.386 nM from the runway 456, the DH terrain floor will be 86 feet; rounded to nearest 100 feet = 100 feet. Thus, the elevation of cell 468 is selected as 100 feet. FIG. 32 illustrates the differences between the elevations in selected cells 468, 470 and 472 before and after correction. Before correction, the elevation of cells 468, 470 and 472 was 300 feet. After correction, the elevation of cells 470 and 472 are corrected to the runway elevation and rounded to the nearest resolution (i.e. 0 feet). The cell 468 is corrected to the DH terrain floor elevation of 100 feet. FIG. 33 illustrates another situation where the elevation of cells, due to the rounding-off method discussed above, can result in a situation where the cell elevations adjacent a runway, represented as a staircase 474, are below the runway elevation. The correction method, discussed above, can also be used to correct the various cells around the runway 456 up to the runway elevation in a manner as generally discussed above. FIG. 34 represents a simplified block diagram for implementation for the display system 400 in accordance with the present invention. The display system 400 may include a microprocessor, for example, an Intel type 80486, 25 MHz type microprocessor ("486") and an Analog Devices type digital signal processor (DSP). The DSP is primarily used for calculating the RHO/THETA conversions to off load the 486. The display system 400 may have a normal mode and a cruise mode. In a normal mode, the display system 400 displays terrain out to, for example, 100 nautical miles, while in cruise mode out to about 320 nautical miles. In the cruise mode, it is contemplated that the background terrain display levels can be lowered by the pilot such that terrain, for example 10,000 feet or 20,000 feet below the aircraft, can be displayed. Alternatively, the display level could automatically be adjusted to the highest terrain within the selected display range; the value of this level set forth in amber, for example. The 486 runs a Tile picker" routine which searches the terrain data base 24 for terrain data covering the area relative to the current position of the aircraft. For example, in a normal mode, the file picker routine searches the terrain data base 24 for terrain data covering an area approximately 256 x 256 minutes relative to the current position of the aircraft. As indicated above, the terrain data base 24 includes index files 55 (FIGS. 4B and 29) which may include the northwest and southeast corners of each map file 51 in order to facilitate the search. A list of files covering the area is assembled and maintained. As indicated above, the index file 55 also includes information as to whether and how the data files 51 are compressed. The files may be compressed/decompressed by a modified Huffman code, or techniques known to those of ordinary skill in the art. If the data files 51 are compressed, the files are decompressed in the background because of the relatively long decompression time and stored in a decompressed file RAM (DFR) 476, for example 256 words. The decompressed files are then transferred to RAM A, identified with the reference number 446. On power-up, one or two files around the aircraft are decompressed and displayed to allow immediate display of a limited range, relative to the position of the aircraft. As more terrain data files are decompressed, the additional range is displayed. The RAM A may be configured in a plurality of cells 479 (FIG. 34), representative of a terrain area 15 arc seconds x 15 arc seconds, with true north appearing at the top of RAM A as shown. Each cell 479 is two bytes wide and includes data regarding the highest altitude in the cell as well as the identity of each cell 479. As such, the RAM A is addressable by X and Y coordinates and thus represents a map with an altitude in each cell 479. The identity of the cell may be used to distinguish between terrain data, for ejample, from the terrain data base 24 and obstacle data. As discussed above, in some applications, an obstacle data base may be utilized. As indicated above, the RAM A may be configured as a wedding cake with various layers, as generally shown in FIG. 28 with the top layer (i.e. layer 1) having the highest resolution (i.e. 1/2 x 1/2 minute cells). Terrain data is loaded into the RAM A such that the aircraft is roughly centered in the top layer, where the terrain advisory and warning detection algorithms are run. Once the aircraft moves outside the update boundary 448 (FIG. 28), the top layer of the RAM A is reloaded with terrain data with the aircraft centered. The lower layers may also be provided with update boundaries, which enable the lower layers to be updated independently and less frequently than the top layer. As discussed above and illustrated in FIG. 27, the flash ROM 444 for the terrain data base 24 may also be configured as a wedding cake as illustrated in FIG. 28. In such an implementation, the highest resolution is provided near the aiφorts while the coarser resolution is provided away from the aiφorts as generally shown in FIG. 27. For example, as illustrated in FIG. 34A, the terrain data base 24 may be configured with 1° x 1°, 2° x 2° or 10° x 10° map files. Some of the map files may overlap, similar to a "wedding cake." As shown, the terrain maps closest to aiφorts are provided with the highest resolution, for example 15 x 15 arc seconds. Maps further away from the aiφort may be provided with resolutions of, for example, l .-x 1 minute or 2 x 2 minutes while a coarse resolution (i.e. 5' x 5' or 10' x 10') may be provided even further from the aiφort. An upload routine may be used to upsample or downsample terrain or obstacle data into the RAM A from the DFR RAM 476 as shown in FIG. 34. Such a routine is well within the ordinary skill in the art. The top three layers (i.e. layers 1, 2 and 3) of the RAM A (FIG. 28) are updated from the map file source from the flash ROM 444 for the terrain data base 24 having the highest resolution, except for the 5 x 5 minute layer, which is updated only from a 5 x 5 minute file source to reduce processing time. Alternatively, as shown in FIG. 34A, an additional RAM A*, identified with the reference numeral 477, may be provided and structured similar to the RAM A 446, forming a "ping-pong" memory arrangement. In this embodiment, one terrain data file 51 is decompressed one file at a time and loaded into the DFR RAM 476 and then upsampled to either the RAM A or RAM A*. For example, while the RAM A is being loaded with additional decompressed files, the terrain threat and display algorithms are executed from the RAM A*. Once the RAM A is updated, the RAM A is then used for the computations while the other RAM A* is updated with new data from the DFR RAM 476 to account for changes in the aircraft position. The 5 x 5 minute resolution layer is primarily intended for use in a cruise mode to provide a look-ahead range of about 320 nM. As discussed above and illustrated in FIG. 29, each file includes an index file 55 which contains data relative to the resolution of the terrain data in a particular map file 51. Such information can be used to vary the resolution of the map files as a function of longitude and latitude position. For example, the latitude resolutions of the map files can be decreased at higher latitudes in a northward direction. Moreover, longitude resolution in the RAM A can be varied and provided with, for example, a one degree latitude hysteresis to prevent flipping back and forth as an aircraft cruises northwardly. Once the terrain data from the terrain data base 24 is loaded into the RAM A, the terrain advisory and warning algorithms ("detection algorithms") are run once per second as discussed below. The terrain threat algorithms discussed above are based on a single vector along the groundtrack. The single vector provides acceptable results because the relatively crude resolution of the terrain data base 24 is based upon the highest elevation per cell and the natural noisiness of the groundtrack angle and position information. However, the single vector approach does not take into account various errors, due to, for e.xample, the terrain data base 24, GPS longitude and latitude errors as well as the published track angle accuracy. As such, the single vector along the groundtrack may be substituted with an array of vectors, as generally shown in FIG. 35, which takes such errors into account. In particular, the groundtrack vector is identified with the reference numeral 482 originating at the instantaneous aircraft position X 0 , Y 0 . A pair of vectors 484 and 486 are disposed at a configurable distance D 0 p p . for example 0 nM along a segment 488, peφendicular to the vector 482 at points X j , Y j and X r , Y r A pair of outside vectors 490 and 492, originating at points X j , Y j and X r , Y r , respectively, are directed at a configurable angle DALPHA' relative to the vectors 484 and 486. The configurable angle D ALPHA may be selected as 3 degrees, equivalent to the groundtrack accuracy of the GPS. The coordinates X j Y j and X r , Y r for the vectors 490 and 492, respectively, are determined. More particularly, the incremental X and Y vectors, DX Q FF an d DYoFF' are determined as set forth in equations (25) and (26) as set forth below. (25) DX OFF = {D OFF /COS(LAT)} * COS(GROUNDTRACK), where D 0F p is a configurable distance between X 0 , Y 0 and X j , Y } , for e.xample 0.2 nm and the factor l/COS(LAT) is used to convert DX 0F p to nautical miles when DX 0FF , DY 0 FF S m minutes and D 0 FF ΪS m nautical miles. (26) DY OFF = D 0FF * SIN(GROUNDTRACK) The coordinates X r , Y r can be determined in a similar manner. The threat detection algorithms are initiated by calculating the range increments DELTA X NS and DELTA Y NS along a north-south coordinate system along the groundtrack. Referring to FIG. 36, each spoke 426 has 512 size range bins as discussed above, or 256 double range bins. In order to convert from an X-Y coordinate system to the north-south coordinate system, the range increments DELTA X^s, DELTA Y NS are determined in accordance with equations (27) and (28). (The term 1/COS (LATITUDE) is used to convert from arc minutes to nautical miles.) (27) DELTA X NS = DETECTION STEP SIZE * SIN (GROUNDTRACK) * 1/COS (LATITUDE) (28) DELTA Y NS = DETECTION STEP SIZE * COS (GROUNDTRACK) After the range increments DELTA X NS and DELTA Y NS are determined, the threat detection algorithm computation is initiated at the instantaneous position X 0 , Y 0 of the aircraft (FIG. 37). The position is then incremented by the range increments DELTA X NS and DELTA Y NS and the range is incremented by a configurable step size, for e.xample 1/8 unit, as discussed above. After the new coordinates are determined, the current terrain altitude at the new coordinate is determined. The terrain detection algorithms, as discussed above, are then computed for the current advisory LAD, as indicated by the arc 494, defining a yellow threat detection vector 496. In order to provide additional warning time for a terrain warning, the terrain warning indication can be computed out to a distance equal to 1.5 * LAD, as indicated by the arc 498 to define a red threat detection vector 500. The above steps are repeated for the look-ahead vector arrays 490 and 492 (FIG. 35). The entire computation is done at a rate of once per second. Whenever the threat detection algorithm detects terrain along the look- ahead vector arrays 490 and 492 (FIG. 35), the offending terrain is painted in an e.xpanded cone around the groundtrack as shown in FIG. 37. In particular, the expanded cone includes a plurality of vectors, spaced, for example, 4° apart (configurable value), for ±90° (configurable value) on each side of the groundtrack. For an expanded terrain warning, the cone may be expanded out distances, for example as set forth in TABLE 7 below. TABLE 7 (For an e.xpansion factor of 2) The terrain threat is painted on the display 36 by calculating the starting vector angle DISPA as set forth in equation (29) when a terrain threat along the groundtrack has been detected. (29) DISPA = GROUNDTRACK + 30 degrees The range increments are then determined as set forth in equations (30) and (31). (30) DELTA X NS = DISPLAY STEP SIZE * SIN (DISPA) * 1/COS (LATITUDE) (31) DELTA Y NS = DISPLAY STEP SIZE * COS (DISPA), where the display step size is a configurable constant, for example 1 2 unit. After the range increments DELTA X NS and DELTA Y NS are determined, the instantaneous position of the aircraft X 0 , Y 0 is incremented by the same and the range is incremented by the step size. The altitude is then determined for the current position on the terrain map. Subsequently, the threat detection algorithms are run for the distances as discussed above. Should a threat be detected, a color is loaded into a RAM B, identified with the reference numeral 504 (FIG. 34). At the end of the yellow and red threat vectors 496 and 500, the vector angle DISPA is incremented by 4 degrees with the above steps being repeated until all vectors are done. In addition to the terrain detection and display algorithms, a background terrain algorithm is also executed relative to the terrain data in RAM A (FIG. 34) once every screen update, about 4 seconds. The background terrain algorithm simply compares the current altitude of the aircraft with the various terrain altitude data in the RAM A. Any of the dot patterns 432, 434 and 436 (FIG. 25) resulting from the computation are stored in the RAM B (FIG. 34) organized in a wedding-cake configuration (FIG. 28), similar to the RAM A (FIG. 34). In addition, the RAM B is organized in single byte cells containing the color/dot pattern as discussed above. The closest runway may also be stored in the RAM B as either a circular or square pattern in, for es ample, a green color. The color/dot pattern in the RAM B (FIG. 34) is transferred to the DSP memory RAM B*, identified with the reference numeral 506, once per screen update (i.e. 4 sec) and synchronized to the DSP screen painting period. If a threat is detected as discussed above, a new RAM B image is transferred to the DSP RAM B* 206 immediately. The image transfer time typically fits between two spokes. If not, the DSP will send an old spoke stored in its spoke memory 508, 510 twice. The DSP then performs the RHO/THETA conversion on the data in RAM B* 506 using the updated aircraft position, selected range and heading data from the 486, as discussed below. Changes in the selected range are implemented on the fly (i.e. the next spoke) without restarting the sweep. As discussed above, the ARINC 708/453 weather radar format consists of 512 spokes covering a configurable scan range of typically ±90 degrees and a range selected, for example, on a weather data control panel. For ARINC systems, each spoke is calculated twice, once for the left selected range and once for the right selected range. The spokes are then transmitted by way of a serial port 512, interleaved left spoke and right spoke to . the bus switch 406 (FIG. 23) by way of a pair of drivers 514 and 516 (FIG. 34). The resolution of the scan conversion is 256 range bins; each range bin actually being a double range bin per the ARINC 708/453 standard format. Such a resolution is sufficient for the dot patterns 518 illustrated in FIGS. 25 and 26. The RHO THETA conversion is best understood with reference to FIG. 38. In particular, once per sweep (i.e. every 4 seconds) the range increment Rl is determined. The range increment is the selected range divided by 256. After the range increment is determined, the starting spoke angle is determined (i.e., the aircraft true heading plus 90 degrees). Lastly, the incremental spoke angle DELTA ALPHA is determined. The incremental spoke angle DELTA ALPHA is the sweep range divided by 512; the total number of spokes per sweep. Once per spoke (i.e. 512 times per 4 second sweep), the incremental range coordinates DELTA X ns , DELTA Y ns are computed and the spoke angle is incremented by DELTA ALPHA, as discussed above. The incremental range increment coordinates X ns and Y ns are computed in accordance with equations (32) and (33), respectively. (32) DELTA X^ = Rl * SIN (SPOKE ANGLE) * 1/COS (LATITUDE) (33) DELTA Y ns = Rl * COS (SPOKE ANGLE) Once per range increment (i.e. 512 x 256 times per 4 second sweep), the instantaneous aircraft position X 0 , Y 0 is incremented by DELTA X ns and DELTA Y ns . The color/pattern for the new position is looked up and output to the display 36. These steps are repeated for the rest of the spoke. With the use of the DSP, two computations can be done in parallel. In particular, the color/pattern of each range bin including the conversion of the longitude to nautical miles (i.e. multiplication by 1/COS (LAT)). With reference to FIG. 38, this computation is performed relative to the true north map coordinate system with the aircraft true heading pointing up and the spoke angles referenced to it. The second computation addresses each display indicator pixel in order to paint the required dot pattern, relative to the display screen coordinate system. The generation of dot pattern is illustrated in FIGS. 39-42. In general, the scan conversion is simulated by overlaying each RHO/THETA range bin over a calculated indicator pixel map which includes corrections for display aspect ratio using fractal 8 x 8 dot patterns. The fractal patterns corresponding to the variable density dot patterns 432, 434 and 436 (FIG. 25) are illustrated in FIG. 40. The fractal pattern size is selected to be 8 x 8 pixels. The densest pattern 432 includes 1/2, or 32, of the 64 pixels filled. In the medium density pattern 434, 16, or 1/4, of the pixels are filled. In the least dense pattern 4 of 64, or 1/16, of the pixels are filled. Referring to FIG. 39, the display screen is divided into a pixel map, consisting of, for example, 256 x 256 pixels 520. As mentioned above, the 8 x 8 fractal patterns are overlaid onto the pixel map to provide the variable density dot patterns 432, 434 and 436 illustrated in FIG. 25. The computation of the dot patterns is best understood with reference to FIGS. 39, 41 and 42. Using an X-Y coordinate system, each spoke is determined at the bottom center of the display 36. For - a 256 x 256 pixel display, the starting point corresponds to 120, 0. Ne.xt, the incremental X and Y coordinates relative to the scan angle ALPHA are determined according to equations (34) and (35). (34) DELTA X = Rl * SIN (ALPHA) (35) DELTA Y = Rl * COS (ALPHA), where Rl = a double range increment and ALPHA = the scan angle. These increments are added to the starting point 128, 0. After each increment, the selected fractal pattern is looked up in the RAM B * to determine whether to send the last range increment as either block or colored. Referring to FIG. 42, the increments DELTA X and DELTA Y are applied to a pair of adders 522 and 524, which, in turn, are applied to an X- counter 526 and a Y-counter 528. As discussed above, the X counter 526 starts at 128 and counts up if DELTA X > 0. If DELTA X < 0, the X-counter counts down, indicating a pixel on the left (FIG. 39) of the starting position. Since the starting pixel is at the bottom of the display, the Y-counter 528 starts at zero and always counts up. A range counter 528 counts up to 256; the maximum number of double range bins. The range counter 528 is driven by an increment clock such that the pattern/color is painted once per range increment. The output indicates whether the pixel is to be painted left black. As discussed above, the background terrain algorithm determines the particular fractal pattern to be used. This data is applied to the DSP to enable the pixels on the pixel map (FIG. 29) to be painted or left blank. In particular, the three least significant bits from the X counter 526 and Y counter 528 are used to select the fractal pattern as shown. The scan conversion simulation is performed in parallel with the RHO/THETA conversion which determines the color/dot pattern of the range bin to be transmitted. The color/dot pattern provided by the RHO/THETA conversion is then overlaid with the fractal dot pattern to determine whether the range bin color should be black or painted to provide the proper dot pattern on the display. 66 Terrain Information System According to invention, a terrain information system or "Peaks" mode is available to provide a contoured display of terrain below the aircraft. The display provides the flight crew with a means of assessing the potential terrain conflicts which might occur if a rapid descent is initiated from the aircraft's normal flight path. For ejcample, a loss of cabin pressure may require the crew to initiate an emergency decent and it would be very useful if the crew had a map of the terrain below the aircraft. This would be particularly important when the aircraft is traversing mountainous terrain. Since the terrain display 402 of the terrain advisory and warning system, as described above, normally does not display terrain more than 2000 feet below the aircraft, this system would generally not be available for planning a rapid decent from a relatively high altitude. Thus, the invention provides a system whereby a pilot can select a display of the terrain below and ahead of the aircraft regardless of the aircraft's current altitude. In another embodiment of the invention, the Peaks mode can be implemented such that this mode is the normal or default display when the terrain advisory and warning system is not displayed. Fig 46 provides in block diagram form an illustration of the major components of the terrain information system as indicated by the reference numeral 530. The embodiment of the terrain information system 530 shown in Fig 46 is an enhanced version of the terrain advisory and warning display system 400 as described above in connection with Figs 1A, IB and 23. A block 532 represents the logic for both the terrain warning LOOK-AHEAD warning generator 30 of Fig. 1A and the terrain information system 530. One method of selecting the Peaks mode is to provide a cockpit switch 533 in order to permit the crew to manually select a depiction of the contours of the terrain below the aircraft on the display 402. In one embodiment of the switch 533, one push would select the terrain advisory and warning display and a second push would select the Peaks mode. A third push would deselect both modes. In addition or alternatively, as indicated above, the Peaks mode can also be selected automatically such that it is the default display on the display 402. The preferred embodiment provides for the automatic switching from the Peaks mode to the terrain advisory and warning mode. For ejample in the situation where aircraft approaches close enough to the terrain to generate the terrain advisory or warning while in the Peaks mode, the terrain warning generator 30 will automatically switch the display 402 to the appropriate terrain advisory or warning. The terrain information system 530 operates by scanning the terrain database 24 in the region which is defined by the range selection of the terrain display 402. The terrain display algorithm, which is described in detail in connection with Fig. 47 below, locates the highest and the lowest points of the terrain within the range of the scan and, using these values as bounds, divides the intermediate terrain elevations into n layers or terrain contours. In the preferred embodiment of the invention, there will normally .be up to six layers (0-5) displayed on the display 402. These layers are depicted on the display 402 as shown in Table 1 below. Table 1 In one embodiment of the invention, the terrain layers intermediate between the lowest and the highest layer, as indicated in the second column, are displayed in varying densities of green. In this embodiment, the intermediate terrain layers are always colored green. The highest or peak elevations are displayed as a solid red or, for some applications, a red with a fill pattern of 75% . The lowest elevation are not displayed which will result in the lowest elevation being appearing in black on the weather radar CRT or EFIS display 402. In another embodiment of the invention a distinction is made on the display 402 based on whether or not the aircraft is above or below the terrain. In this case, as indicated in the third column, the intermediate terrain layers above the altitude of the aircraft will be displayed in varying fill densities of yellow. Since the lowest and highest terrain levels can be quite small in horizontal eΛent, for example sharp peaks, these layers can be smeared. In this case the surrounding cells in the database 24 are also depicted on the display 402 as either black or red as appropriate. It should be noted that in aircraft where a color display such as the radar or EFIS 402 is not available, a monochrome display can be used. In this case, the terrain contours would be displayed in the varying fill patterns described above but, to enhance the distinctiveness of the highest layer, the highest layer can be made to blink on and off at a predetermined rate. There also may be some cases where it would be desirable to have the highest layer of the color display blink on and off instead of smearing or perhaps even in combination with smearing. An example of a Peaks mode display 534 on the weather radar display 402 is provided in Figure 48. When the Peaks mode is selected, a legend TEAKS" 536 is generated on the cockpit display 402 along with a numerical display 538 indicating the highest terrain elevation and a numerical display 540 of the lowest terrain elevation. The numerical displays 538 and 540 can also be combined and displayed in terms of hundreds of feet such as 215/59. The range of the terrain display 534 is also selectable and is shown at 542 on the top right of the display 402. One or more range rings 544 and 546 can also be included in the terrain contour display 534 to serve as an indication of the range of portions of the terrain from the aircraft which in turn is depicted as a triangle 548 in the lower center of the terrain display 534. In order to provide a more intuitive display of the terrain contours, the intervals between the various layers displayed on the terrain display 534 are varied. In the preferred embodiment of the invention, the elevations of the tops of the layers are determined by the relations set forth in table 2 below. In table 2, Δ represents the difference between the highest elevation h MAX and the lowest elevation h MIN depicted on the terrain display 534. Table 2 As indicated in Table 2 above, the top of layer 0 will encompass all the terrain elevation cells that are within 200 feet of the minimum terrain elevation. In certain circumstances such as when the terrain is relatively flat, this constant can be reduced to a lower value such as 100 feet in order to provide a more detailed view of the terrain. Preferably, in order to avoid cluttering of the display when the terrain is relatively flat, the layers of the contour display 534 are successively eliminated, beginning with layer 5, when the value of A is less than 600 feet. For essentially flat terrain, only layer 1 having a fill pattern of 12.5% or alternatively, a completely black display will be provided on the terrain display 534. In the preferred embodiment, the top of layer 4 is determined by the relation h MIN + 95% A - 100 feet so as to insure that there is always some red displayed on the terrain display because there will be very little difference in elevation between layer 5 and layer 4. As can be seen from this relation, when the terrain is very flat, the terrain display 534 will be all black. It will also be appreciated that in the preferred embodiment, there is very little difference in the elevation of the top of the lowest layer 0 and the top of the next layer 1. The advantage of this approach is that it can provide a display of the contours of rivers and shorelines which can be very useful to pilots with respect to geographical orientation. Figure 47 is a flowchart generally illustrating the logic executed by the terrain information generator portion or peaks mode of the block 532 shown in Fig. 46. The first step in the peaks mode, as indicated at a block 550, the display range 542 is selected. Next, as shown at a pair of blocks 552 and 554, the values h MAX and h MIN are calculated for each cell in the terrain database 24 within the display range 542. Then, as indicated at a decision block 556, if the difference A between Ϊ-MAX and h MIN is less than a predetermined value, for example 100 feet, then a "flat terrain" flag is set at a block 558 otherwise it is reset at a block 560. At this point as indicated by a block 562, an inte.rval between contour levels is calculated and a determination is made at a decision block 564 as to whether the interval is less than a predetermined minimum. If the contour interval is less than the minimum interval, then the value for the interval is set equal to the minimum as indicated at a block 566. After the above computations have been performed, the peaks mode logic 532 starts a scan of each terrain cell within the display range as shown by a block 568. First, as indicated by a decision block 570, a determination is made as to whether the cell elevation is close to h MAχ . If it is, and if the flat_terrain flag is set, determined at a decision block 572, the logic generates, as shown at a pair blocks 574 and 576, a signal causing the display 402 indicated at a block 578, to draw the cell in green with a 12.5% fill pattern. Alternatively, if the flat_terrain flag is set, for example if A is less than 100, the display of each cell can be left black. Returning to the decision block 572 if the flat_terrain flag is not set, then, as indicated by a pair of blocks 580 and 582, the color of the cell is set to red with a 100% fill pattern. In one embodiment of the invention, after a determination at the point 570 that the elevation is not close to h MA , the aircraft altitude is compared to the cell elevation as shown at a block 584. If the aircraft is below the cell elevation, the cell color is set to yellow, indicated by a block 586 otherwise, the cell is set green as indicated by a block 588. However, in the preferred embodiment, the intermediate cell colors will always be set. to green. It should be noted, that if the cell elevation is equal to or below the lowest layer 0, no color will be generated for that cell. After the cell color has been set at 586 or 588, the cell elevation is scaled and the cell fill pattern selected as shown at a pair of blocks 590 and 592. Preferably, this scaling and selection of cell fill patterns will be in accordance with the values set forth in Tables 1 and 2 above. The process described above is then repeated for each cell in the display range as indicated by a decision block 594. PRE-CAUTION EXTENDED ENVELOPE The background terrain display described above displays the terrain elevation as bands of different colors and dot densities depending on the terrain elevation relative to the aircraft. These bands are generally 1000 feet apart and elevations below the aircraft are depicted as green (safe), while elevations at or above the aircraft (or which can become at or above the aircraft based on the current flight path of the aircraft) are depicted as yellow (caution) and red, as shown in Fig. 49. From a pilot's point of view it is desirable to know from the background terrain depiction whether a caution alert will occur before the actual caution alert happens. For instance, if the projected course goes through "yellow" terrain the pilot would know that a caution alert will occur shortly if he continues on the present course. The caution algorithm is fairly complicated and not only a function of the terrain elevation relative of the aircraft but also a function of flight path angle, speed and distance from the runway. The background terrain rules are relatively simple and the intent of the background terrain is to provide raw elevation data of the terrain ahead of the aircraft. Modifying the simple background terrain rules, however, would likely destroy the raw data nature of this display. A compromise is to override the simple background terrain rules in the area covered by the look down caution algorithm which is limited to about 4 nautical miles majdmum along the groundtrack of the aircraft at a width of about 1/4 of a nautical mile. One solution is a pre-caution emended background envelope as shown in Fig. 50. This envelope follows the caution algorithm envelope but it includes a small offset altitude as well as an altitude lead-time when descending towards terrain. The lead time preferably applies for descending vertical speeds. In addition, the look ahead distance is emended by a predetermined amount, about 20% , to ensure that potential dangerous terrain can be painted yellow before a caution occurs. Offset and lead time values have to be chosen such that they don't cause yellow terrain depiction on normal approaches. Some normal approaches have very low terrain clearances, such as Windsor Locks, CT. Figure 51 shows a plan view of the maximum area which can be affected by this modification. The equations for extending the caution envelope are given as equation (36) for the look-ahead distance, and equation (37) for the offset for the look-down distance. (36) DELTALAD = LADFACTOR * LAD wherein LADFACTOR >0 and typically <1, and is selected to give a preferred look ahead envelope; typically the LADFACTOR is approximately equal to 0.2. (37) OFFSET = ALTO + | SINKRATE (mps) | * LEADTIME (sec) wherein ALTO is a fixed amount greater than 0 and less than DH (terrain floor), and preferably is about 6 meters (20 feet) for .small civil aircraft. LEADTIME is selected based on a desired pre-caution envelope size. The BETA sink rate enhancement, described above, provides a lower level to the caution envelope; any extensions of the BETA sink rate boundary 308 of figure 17 need to be made with this in mind. For exemplary puφoses only, a LEADTIME of 6 seconds is reasonable for small civil aircraft. Figure 51 shows a plan view of the pre-caution envelope 610. Figure 50 shows a side elevation view of the pre-caution envelope 610. Referring to figures 50 and 51, the pre-caution look ahead boundary 614 is shown. The precaution look ahead boundary will normally be used under all flight conditions. The look down or offset pre-caution boundary 613, shown in figure 50, will normally be used only during descent. The pre-caution envelope can further include lateral eΛensions 611 and 612 of figure 51 to account for turning. The lateral extensions are set at a preselected lateral extension angle DALPHA. For exemplary puφoses, DALPHA may be selected as 3°. Selecting DALPHA as 3° makes the distance from the center look-ahead vector 617 to either outboard vector 618 or 619 less than the width of a 15" (arc seconds) cell. The lateral extension angles begin a preselected distance D off from the instantaneous aircraft position 616, which is indicated by coordinates X 0 and Y 0 . Selecting D off as 0J25 naut M is preferred since this distance is one-half of the width of a 15" tier terrain cell. FIG. 51 further shows the terrain blocks 620 (bordered by the heavier line) which are affected by (identified with terrain features by) the pre-caution algorithm. Specifically, the look down or offset expansion, the look ahead expansion, and the lateral extensions. The pre-caution envelope may further include a look-up expansion boundary 615. The look-up pre-caution boundary 615 is preferably established as being offset from the look-up boundary 366, 386 the offset distance of equation 37. When any terrain violates this pre-caution envelope, the affected background terrain cell of the display is oveφainted with the lowest density caution color, for example, the lowest density yellow dot pattern, but preferably only if the normal background is green at that location. Figure 26A shows the terrain caution display 402 of figure 26, but further comprising the pre-cautions 601. As can be seen, the pre-cautions 601 are the lowest density feature shown on the display. Where cautions 436 are yellow of a first density, pre-cautions 601 are yellow of a lower density than cautions 436. A flowchart showing the pre-caution algorithm is shown in Fig. 52. Referring to Fig. 52, the flowchart 630 includes an initial step 632 which initializes the pre-caution algorithm. In the nej t step, 634, the pre-caution envelope offset 613 of Fig. 50 is calculated using equation 37. In the nejct step of the pre-caution algorithm, the pre-caution envelope for the look-ahead distance is calculated in set 636 using equation 36. In the next step of the pre-caution algorithm, cells on the display which indicate a pre-caution condition are marked (in this case with a low-density yellow color) to indicate a pre-caution condition. The algorithm terminates at step 640 and can resume again at step 632. Preferably, the algorithm continually cycles between step 632 and step 640 to continually refresh the pre-caution warnings of the display. The pre-caution envelope preferably applies to the 'Look Down" portion of the caution envelope. The 250 foot/500 foot bias which can be added to the background terrain yellow/green boundary can also be modified to allow for precautions in the manner described. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above. The invention has been described in compliance with the applicable statutes. Variations and modifications will be readily apparent to those of skill in the art. It is therefore to be understood that the invention is not limited to the specific features shown and described, since the disclosure comprises preferred forms of putting the invention into effect. The invention is, therefore, to be inteφreted in light of the appended claims appropriately inteφreted in accordance with the doctrine of equivalents.

Claims: CLAIMS 1. An aircraft terrain awareness system comprising: a processor configured to evaluate terrain over which an aircraft can fly and generate cautions for terrain features which will come within a first predetermined range of the aircraft, based on a current position and groundtrack of the aircraft; and wherein the processor is further configured to generate pre-cautions a predetermined precaution distance beyond the first predetermined range, the precautions for a given terrain feature being generated at an earlier point in time than cautions for the given terrain feature. 2. The aircraft terrain awareness system of claim 1 wherein the processor is further configured to generate warnings for terrain features which will come within a second predetermined range of the aircraft based on a current position and course of the aircraft, said second predetermined range being less than the first predetermined range. 3. The aircraft terrain awareness system of claim 2 wherein the cautions and warnings are displayed on a visual display representative of a plan view of the terrain over which the aircraft can fly, wherein: the cautions are displayed in a first color having a first color density; the warnings are displayed in a second color; and the pre-cautions are displayed in the first color having a second color density, the second color density being less than the first color density. 4. The aircraft terrain awareness system of claim 1 wherein at least a portion of the predetermined precaution distance is generated based on a descent rate of the aircraft. 5. The aircraft terrain awareness system of claim 4 wherein the first predetermined range comprises a forward-looking component and a downward- looking component, and wherein the predetermined precaution distance for the downward looking component is generated based on a descent rate of the aircraft, and the predetermined precaution distance for the forward-looking component is generated based on a preselected percentage of the forward-looking component. 6. The aircraft terrain awareness system of claim 5 wherein the first predetermined range further comprises a sideways-looking component a preselected lateral distance either side of the aircraft, and wherein the predetermined precaution distance for the sideways-looking component is generated based on a pre-selected offset and angle. 7. An aircraft terrain awareness system comprising: a navigational system configured to determine latitude, longitude, altitude, groundtrack, and ground speed of an aircraft employing the system and to generate signals in response thereto; a terrain database comprising altitude data for terrain over which the aircraft can fly; a processor configured to identify from the terrain database altitude data within a range of the aircraft; a look-ahead warning generator configured to receive the altitude identified by the processor and generate an altitude data subset of terrain altitudes within a first determined zone around the aircraft, the warning generator further configured to receive the signals generated by the navigational system, wherein the warning generator processes the altitude data subset, as well as the signals generated by the navigational system, to identify terrain which can intrude the first predetermined zone, and generate a warning signal in response to thereto; a warning device configured to receive the warning signal and generate a primary warning in response thereto; wherein the warning generator is further configured to generate an incremental data subset of terrain altitudes a determined incremental range beyond the first determined zone, and to process the incremental data subset to identify terrain which will intrude the incremental range, and generate a caution signal in response to thereto; and wherein the warning device is configured to receive the caution signal and generate a secondary warning in response thereto. 8. The aircraft terrain awareness system of claim 7 wherein the first determined zone around the aircraft comprises a look-ahead distance in front of the aircraft and a look-down distance below the aircraft, and wherein the determined incremental range comprises an incremental look-ahead distance beyond the look-ahead distance, and an offset distance beyond the look-down distance. 9. The aircraft terrain awareness system of claim 8 wherein the offset distance is a function of the descent rate of the aircraft and a preselected lead time. 10. The aircraft terrain awareness system of claim 8 wherein the incremental look-ahead distance is a preselected percent of the look-ahead distance. 11. The aircraft terrain awareness system of claim 8 wherein the look-down distance comprises a terrain floor distance, the terrain floor distance being proportional to a distance from the aircraft to a preselected runway, and wherein the terrain floor distance is limited to a preselected terrain floor maximum. 12. The aircraft terrain awareness system of claim 11 wherein the look-down distance further comprises a cut-off boundary configured to limit the terrain floor distance within a cut-off zone proximate the aircraft, the cut-off boundary beginning at a predetermined cut-off offset distance immediately below the aircraft and extending at a cut-off angle therefrom to the terrain floor distance, the cutoff angle being a function of an instantaneous climb angle of the aircraft. 13. The aircraft terrain awareness system of claim 12 wherein the look-down distance further comprises a beta sink rate enhancement boundary configured to extend the terrain floor distance beyond the cut-off boundary, the beta sink rate enhancement boundary beginning the terrain floor distance immediately below the aircraft and ejctending therefrom at a sink-rate enhancement angle to a preselected fraction of the look ahead distance, and thence horizontally forward to the look-ahead distance. 14. The aircraft terrain awareness system of claim 13 wherein: the sink rate enhancement angle is a function of the climb rate of the aircraft, wherein a negative climb rate indicates descent of the aircraft; and the preselected fraction of the look-ahead distance is approximately one- half. 15. The aircraft terrain awareness system of claim 14 wherein the offset extends the look-down distance by a distance D 0 = ALTO + I R s I X ( T i ), wherein ALTO is a preselected offset distance, R s is a descent rate of the aircraft, and T j is a preselected lead time. 16. The aircraft terrain awareness system of claim 8 wherein the look-ahead distance is a function of a speed of the aircraft times a look-ahead time, the look-ahead time being determined as a function of aircraft maneuvering time required to avoid terrain contact with the aircraft. 17. The aircraft terrain awareness system of claim 16 wherein the look ahead time comprises a time to turn the aircraft plus a time to prevent inadvertent terrain contact as a result of the turn plus a reaction time. 18. The aircraft terrain awareness system of claim 8 wherein the incremental look-ahead distance is a preselected percent of the look-ahead distance. 19. The aircraft terrain awareness system of claim 7 wherein the warning generator is further configured to generate a caution signal when the aircraft is descending. 20. The aircraft terrain awareness system of claim 8 wherein the first determined zone around the aircraft further comprises a look-up terrain advisory envelope, the look-up terrain advisory envelope ejctending a second predetermined distance beyond the look-ahead distance, the upper limit of the look-up terrain advisory limit being defined by a line extending at an angle from a third predetermined distance immediately below the aircraft upward at a look-up angle, the look-up angle being a function of the absolute value of a flight angle of the aircraft. 21. The aircraft terrain awareness system of claim 20 wherein the determined incremental range further comprises a look-up offset, the look-up offset being a function of a rate of descent of the aircraft. 22. A method for generating terrain feature caution notifications for an aircraft which can fly over a terrain, comprising: defining a terrain advisory envelope in a region prorimate the aircraft, such that when terrain features intrude within the terrain advisory envelope a caution signal detectable by a pilot of the aircraft is generated; defining a pre-caution envelope ejctending beyond at least a portion of the terrain advisory envelope, such that when terrain features intnide within the terrain advisory envelope a pre-caution signal detectable by the pilot is generated; and wherein the caution signal is configured to be more easily detectable by the pilot than the pre-caution signal. 23. The method of claim 22 further comprising: providing altitude data for terrain over which the aircraft can fly; comparing the altitude data immediately below the terrain advisory envelope and the pre-caution envelope against an altitude of the aircraft; identifying terrain features which intrude the terrain advisory envelope and the pre-caution envelope; generating the caution signals when terrain features intrude the terrain advisory envelope; and generating the pre-caution signals when terrain features intrude the precaution envelope.

Inventors: CONNER KEVIN J, MULLER HANS RUDOLF DI, CONNER, KEVIN, J., MULLER, HANS, RUDOLF DI
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1998
Main classification: G01C5
IPC codes: G01C5, G01C23