Projectile Ranging Using Digital Maps

Abstract

To provide an improved terrain-referenced navigation system for an aircraft.SOLUTION: A terrain-referenced navigation system (600) for an aircraft comprises: a stored digital terrain map (606); a position calculation unit (608) arranged to calculate aircraft position relative to the stored digital terrain map (606) to determine a terrain-referenced aircraft position; a fall line calculation unit (612) arranged to calculate a fall line for a projectile starting from the terrain-referenced aircraft position as a launch point; and an impact point calculation unit (614) arranged to directly compare the fall line with the digital terrain map, by incrementally comparing a height of the projectile along the fall line with a height of the terrain according to the stored digital terrain map (606) in order to find an expected impact point on the terrain.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present disclosure relates to a terrain referenced navigation system, a terrain referenced navigation method, and a computer readable medium for an aircraft. In particular, the disclosed terrain referenced navigation system and method enable projectile fall line calculations to be integrated with digital terrain map data to estimate the point of impact of the projectile with the terrain. [Background technology]

[0002] Terrain Referenced Navigation (TRN) systems, which integrate aircraft navigation data, radar altimeter data, and digital terrain elevation data to generate a navigation solution, are used on many aircraft platforms. Such systems provide enhanced navigation, situational awareness, and air-to-ground ranging capabilities.

[0003] Aircraft and other vehicles may be designed to launch projectiles for a variety of reasons, including scientific research, meteorological or geological surveys, civilian support, and military purposes. Assuming a uniform gravitational field and known atmospheric conditions, a projectile launched with known initial conditions (e.g., initial height, launch angle, and velocity) will result in a predictable flight path.

[0004] In a typical operational aircraft mechanization, a standalone ranging system is used to estimate the line of fall of the projectile and calculate the three-dimensional range to the projectile's impact point with the terrain, and then the TRN system uses the horizontal range to determine the terrain altitude at the estimated impact point. The terrain altitude is then fed back to the ranging system, which then recalculates the projectile's impact point based on the updated terrain altitude information. The two functions implemented in separate avionics systems work in an iterative loop to provide the pilot with a real-time visualization of where the projectile will land if released. However, the stability of the iterative loop depends on the shape of the line of fall and the terrain gradient around the impact point. It has been found that the iterative loop can become unstable if the intersection angle between the terrain and the line of fall is unfavorable.

[0005] The present disclosure seeks to provide an improved terrain referenced navigation system for aircraft. Summary of the Invention

[0006] According to a first aspect, the present disclosure provides a terrain referenced navigation system for an aircraft, the system comprising: A stored digital topographical map; a position calculation unit configured to calculate a position of the aircraft relative to a stored digital terrain map to determine a terrain referenced aircraft position; a fall line calculation unit configured to calculate a fall line of the projectile starting from the terrain referenced aircraft position as the launch point; and an impact point calculation unit configured to directly compare the fall line to the digital terrain map by incrementally comparing the height of the projectile along the fall line to the height of the terrain according to the stored digital terrain map to find an expected impact point on the terrain.

[0007] It will be appreciated that unlike previous Terrain Reference Navigation (TRN) systems, the disclosed TRN system performs the fall line calculation itself by making a direct comparison of the fall line to the terrain around the aircraft to find the impact point. This effectively eliminates the iterative loop previously required between the TRN system and a standalone ranging system, thus eliminating the risk of instability resulting from, for example, complex terrain profiles. Iterative loops between the TRN system and a standalone projectile launch system can be avoided entirely.

[0008] In accordance with the present disclosure, the height of the projectile is incrementally compared to the terrain height along the fall line, rather than the terrain height along a straight line. It is convenient to be able to directly compare the height of the projectile to the terrain height according to a digital terrain map. Furthermore, the intersection of the fall line with the terrain can be calculated more accurately because the calculation of the fall line benefits from the terrain referenced aircraft position as the launch point, rather than the absolute altitude of the aircraft, e.g., as measured by a Global Navigation Satellite System (GNSS), Global Positioning System (GPS), or similar navigation technology, where the aircraft position cannot be directly and accurately registered to a terrain map database.

[0009] In various embodiments of the present disclosure, the impact point calculation unit is configured to incrementally compare the height of the projectile along the fall line to the height of the terrain by searching map data of the stored digital terrain map. Taking into account factors such as the available resolution of the map data of the stored digital terrain map and / or the processing speed of the impact point calculation unit, the step of searching the map data may include a single search of the digital terrain map.

[0010] In one or more embodiments, the impact point calculation unit is configured to incrementally compare the height of the projectile along the fall line to the height of the terrain according to a coarse search of the map data of the stored digital terrain map and then according to a fine search of the map data of the stored digital terrain map. In such embodiments, the coarse search may have a resolution on the order of 1 km to 3 km, e.g., about 1 km, 1.5 km, 2 km, 2.5 km, or 3 km. In such embodiments, the fine search may additionally or alternatively have a resolution in the range of 1 m to 100 m, e.g., about 1 m, 5 m, 10 m, 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, 90 m, or 100 m. Of course, the resolution used for the coarse search and / or the fine search may depend on the resolution of the stored digital terrain map (i.e., the digital terrain map database) being used.

[0011] In various embodiments, the stored digital terrain maps may be obtained from satellite and / or aerial reconnaissance. In various embodiments, the stored digital terrain maps may include three-dimensional terrain models. As known in the art, the stored digital terrain maps may include digital terrain map data with a resolution of 1 m to 100 m (e.g., Digital Terrain Elevation Data (DTED) Level 1 and Level 2 as defined in MIL-PRF-89020B).

[0012] In some embodiments, the system may include a communications link configured to access stored digital topographical maps in the cloud.

[0013] In other embodiments, the system does not require such a communications link. In some embodiments, the system includes a local memory that includes a stored digital terrain map. To conserve local memory space, the stored digital terrain map may not be a complete map. Only a subset of map data related to the terrain expected to be traversed on a given flight may be stored. Thus, in at least some embodiments, the stored digital terrain map includes multiple subsets of map data cached in the local memory.

[0014] In at least some embodiments, the system includes a processor configured to cache multiple subsets of the map data while calculating a maximum terrain elevation within each subset of the map data, which is a convenient way of storing the maximum terrain elevation that can be readily compared to the height of the projectile along the fall line.

[0015] The system may include one or more processors to provide a position calculation unit, a fall line calculation unit, and an impact point calculation unit. The one or more processors may form an integrated TRN system. However, it is an advantage of the present disclosure that a single processor may provide the functionality of determining a terrain referenced aircraft position, calculating a fall line, and calculating an impact point. Thus, in at least some embodiments, the position calculation unit, the fall line calculation unit, and the impact point calculation unit are located in a single processor.

[0016] In at least some embodiments, the system includes an output unit configured to output the predicted impact point on the terrain, e.g., configured to display the predicted impact point on the terrain. For example, the output unit may be connected to or may include an aircraft display, such as a pilot's head-up display (HUD). For example, the output unit may be configured to output the predicted impact point in the form of a Constant Calculated Impact Point (CCIP).

[0017] The fall line calculation unit may be configured to calculate the fall line of the projectile using any known technique. The equations of motion for a projectile launched at a given angle at a given speed are widely known, with standard modifications available to optionally take into account factors such as air resistance and drag.

[0018] In at least some embodiments, the system may include stored projectile data. For example, the launch velocity of the projectile may be preset and stored in a local memory. In at least some other embodiments, the system may include an input unit for projectile data, for example, connected to the fall line calculation unit to aid in the calculation of the fall line of the projectile, allowing an operator to input variables such as one or more of the launch velocity, mass, cross-sectional area, drag coefficient, etc. of a given projectile. Such an input unit is unique compared to previous TRN systems.

[0019] In at least some embodiments, the position calculation unit includes or receives input from a radar altimeter. Measurements from a radar altimeter provide an indication of height above the ground, rather than absolute altitude. In comparison to GPS or other satellite-based navigation systems, a radar altimeter is not easily jammed.

[0020] In at least some embodiments, the position calculation unit is configured to match radar altimeter measurements of terrain already traversed with a stored digital terrain map, potentially allowing the position calculation unit to correct for map shift errors and provide a more accurate terrain referenced aircraft position. In at least some embodiments, the position calculation unit additionally or alternatively includes an inertial measurement unit and uses the inertial measurements to refine the terrain referenced aircraft position, which is another way of providing a more accurate terrain referenced aircraft position. In any of these embodiments, a more accurate terrain referenced aircraft position may be provided that is used as a launch point by the fall line calculation unit.

[0021] In at least some embodiments, the position calculation unit could additionally or alternatively include a Global Navigation Satellite System (GNSS) unit, a Global Positioning System (GPS) unit, or the like. However, this may make the TRN system more susceptible to jamming or other interruptions. Thus, in at least some embodiments, the position calculation unit is configured to calculate the position of the aircraft relative to a digital terrain map without using a Global Navigation Satellite System (GNSS), a Global Positioning System (GPS), or similar navigation technology that requires satellite communication.

[0022] According to a second aspect, the present disclosure further provides a method of terrain referenced navigation, the method comprising: calculating a position of the aircraft relative to a stored digital terrain map to determine a terrain referenced aircraft position; calculating a line of fall of the projectile beginning at the terrain referenced aircraft position as the launch point; and directly comparing the fall line to the digital terrain map by incrementally comparing the height of the projectile along the fall line to the height of the terrain according to the stored digital terrain map to find the expected point of impact on the terrain.

[0023] Various exemplary features described above in relation to the present system apply equally to the disclosed methods, which may be computer-implemented methods. As disclosed above, in at least some embodiments, a single processor is configured to perform all steps of the disclosed methods.

[0024] In various embodiments of the present disclosure, the step of incrementally comparing the height of the projectile along the fall line to the height of the terrain may include searching map data of a stored digital terrain map. Taking into account factors such as the available resolution of the map data of the stored digital terrain map and / or the processing speed of the impact point calculation unit, the step of searching map data may include a single search of the digital terrain map.

[0025] In at least some embodiments, incrementally comparing the height of the projectile along the fall line to the height of the terrain includes a coarse search of the digital terrain map followed by a fine search of the digital terrain map.

[0026] In at least some embodiments, the method includes outputting, for example on a display, the predicted impact point on the terrain.

[0027] In at least some embodiments, the method includes inputting projectile data, for example to aid in calculating the fall line of the projectile.

[0028] In at least some embodiments, calculating the position of the aircraft relative to the stored digital terrain map includes matching radar altimeter measurements of terrain already traversed with the stored digital terrain map.

[0029] In at least some embodiments, additionally or alternatively, calculating the position of the aircraft relative to the stored digital terrain map includes refining the terrain referenced aircraft position using inertial measurements.

[0030] The present disclosure also extends to a computer readable medium comprising instructions executable by a processor to cause the terrain referenced navigation system to perform any of the methods described above.

[0031] The present disclosure also extends to an apparatus including a processor and a memory, the memory storing instructions executable by the processor to cause the terrain referenced navigation system to perform any of the methods described above. A single processor may be configured to perform each of the steps in a given embodiment of the disclosed method.

[0032] The methods disclosed herein may be implemented by one or more computer program products or computer readable media provided on one or more devices or on a single device. The computer program product or computer readable medium may include computer code configured to instruct a computer or multiple computers to perform the functions of one or more of the various methods described herein. The computer program and / or code for performing such methods in a computer readable medium or computer program product may be provided to an apparatus such as a computer. The computer readable medium may be transitory or non-transitory. The computer readable medium may be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, or a propagation medium for data transmission, for example, for downloading code via the Internet. Alternatively, the computer readable medium may take the form of a physical computer readable medium, such as a semiconductor or solid state memory, a magnetic tape, a removable computer disk, a random access memory (RAM), a read only memory (ROM), a rigid magnetic disk, and an optical disk such as a CD-ROM, a CD-R / W, or a DVD. An apparatus, such as a computer, may be configured according to such code to perform one or more processes according to various methods described herein. Such an apparatus may take the form of a data processing system. Such a data processing system may be a distributed system. For example, such a data processing system may be distributed across a network. In some examples, some of the processes may be performed by software on a user device, while other processes may be performed by software on a server, or by a combination thereof. In some other examples, all of the processes may be performed by software on a single user device. The user device may be part of an aircraft control system of a manned or autonomous aircraft.

[0033] It is understood that the projectiles disclosed herein can be any airborne object launched from an aircraft. In various embodiments of the present disclosure, the projectile is not a guided projectile that is powered in flight after launch, but rather an airborne object that results in a line of fall determined by the laws of classical mechanics (e.g., taking into account gravity, air resistance, drag, etc.). Thus, in at least some embodiments, the projectile is an unguided projectile.

[0034] One or more non-limiting embodiments will now be described with reference to the accompanying drawings. [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 illustrates a prior art iterative process for calculating an estimate of a projectile's point of impact. [Diagram 2] 1 includes diagrams showing examples of stable and unstable prior art iterative loops. [Diagram 3] 1 is a flowchart illustrating an exemplary method according to the present disclosure for calculating a predicted point of impact of a projectile. [Figure 4] 1A shows a schematic diagram of a plan view of a digital topographical map sheet interacting with a coarse search algorithm, and FIG. 1B shows a schematic diagram of a plan view of a digital topographical map sheet interacting with a fine search algorithm according to an exemplary method. [Diagram 5] 1 illustrates a schematic diagram of a side view of a line of fall of a projectile relative to a side profile of a terrain with an impact point determined according to such an exemplary method. [Figure 6] 1 is a schematic block diagram illustrating an exemplary terrain referenced navigation system according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] FIG. 1 illustrates a prior art iterative process for estimating the point of impact of an unguided projectile released from an aircraft 100 traveling at an absolute altitude above ground level (AGL) 102, where 104 illustrates the side profile of the terrain being traveled by the aircraft 100.

[0037] The AGL 102 of the aircraft 100 may be measured using a radar altimeter or any other height measurement system (sonar, LIDAR, etc.). The position of the aircraft relative to the terrain may be estimated using a Terrain Referenced Navigation (TRN) system by considering the aircraft's most recent AGL measurements and comparing them to stored digital terrain map data that includes elevation information about the terrain. Comparing the AGL measurements to the digital terrain map data may allow for a more accurate estimation of the aircraft's position relative to the terrain than using a Global Positioning System (GPS) or an Inertial Navigation System (INS) alone. This is because these position estimation systems may introduce errors and drifts that can be observed and corrected by the TRN system. In addition, the stored digital terrain map data itself may include map shift errors. Any combination of TRN, INS, and / or GPS (or the like) may be used to estimate the position of the aircraft 100 relative to the terrain. The TRN system may be the TERPROM® (Terrain Profile Matching) Digital Terrain System available from Atlantic Inertial Systems.

[0038] In FIG. 1, 106 indicates the projected line of fall of the projectile, where the term "line of fall" refers to the path that the projectile would follow once released if unobstructed by objects or terrain. Conventionally, the ranging system of the aircraft 100 calculates an estimate of the horizontal range 108 of the projectile, where the term "forward throw" refers to the horizontal distance that the projectile would travel before falling to a specified altitude if unobstructed by terrain. In a first iteration, the forward throw of the projectile is calculated using the current AGL of the aircraft, giving an initial prediction of the point of impact. Marker 116 indicates a point in the iterative process at which the ranging system is thus calculating the forward throw. The TRN system then returns terrain elevation data to this point 110. Marker 118 is used wherever the TRN system thus returns terrain elevation data. The ranging system then recalculates the forward throw 112, taking into account this elevation data. These calculations are repeated back and forth in an iterative loop. Typically, the loop repeats at 12.5 to 25 Hz. The predicted impact point 114 calculated from this loop, indicated by a marker 120, is presented to the pilot in the form of a Constantly Calculated Impact Point (CCIP) that is displayed on the pilot's Head-Up Display (HUD).

[0039] Figure 2 illustrates one problem with this prior art iterative loop. The problem is that the stability of the loop depends on the shape of the fall line 208 and the slope of the terrain 210 around the point of impact. 200 and 202 are examples of stable iterative loops, where it is possible to calculate a reasonable estimate of the point of impact within a small number of iterations. 204 and 206 are examples of unstable iterative loops, where it is not possible to calculate a reasonable estimate of the point of impact within a small number of iterations.

[0040] 3 is a flow chart illustrating an exemplary terrain referenced navigation method according to the present disclosure that is capable of calculating the predicted impact point of a projectile. Such a method can be conveniently implemented within a TRN system without the need for a separate ranging system. The method includes calculating the fall line of the projectile and directly comparing the fall line with terrain map data to find the impact point. Optionally, this can be implemented by a coarse search algorithm 300 followed by a fine search algorithm 302.

[0041] In steps 304-316, the method begins by implementing the coarse search algorithm 300. In step 304, projectile data (e.g., mass, cross-sectional area, drag coefficient) may be obtained as input to enable more accurate prediction of the projectile's impact point. Next, in step 306, the continuously calculated terrain referenced aircraft position is used as the starting position for the fall line calculation in step 308. In step 308, the fall line of the projectile may be calculated using standard techniques. Next, in step 310, the coarse search algorithm 300 increments along the fall line to the next map sheet boundary of the stored digital topographic map. The digital topographic map data may be stored locally on a computer readable storage medium (e.g., a hard disk drive (HDD)) or may be stored remotely, for example, in the cloud.

[0042] In at least some embodiments, a digital topographical map consists of rows and columns (map posts) of elevation data, and a certain number of these map posts comprise a subset of the map data referred to as a "map sheet." This is visualized in FIG. 4a, which shows a schematic plan view of a digital topographical map 400 interacting with a coarse search algorithm. The edges of the map sheet 402 are indicated by solid lines 408, and the edges of the map posts are indicated by dashed lines 410. The boundaries of both the map sheets and the map posts may be rows or columns of the digital topographical map 400.

[0043] In step 312, the coarse search algorithm 300 calculates the height of the projectile at the map sheet boundary 408 and compares it to the maximum height contained within the map sheet 402 that the prediction of the projectile's fall line 406 just intersects. In step 314, the coarse search algorithm 300 determines whether the projectile's height at the boundary 408 is greater than the maximum height of the map sheet 402. If so, the coarse search is repeated and the coarse search algorithm 300 increments along the fall line 406 to the next map sheet boundary. If the projectile's height at the boundary 408 is less than the maximum height of the map sheet 402, the coarse search algorithm 300 proceeds to return to the previous map sheet boundary in step 316. This is because it is now known that the projectile's fall line 406 has a high probability of intersecting the terrain in this area of ​​the map sheet 402.

[0044] Next, in steps 318-326, the method proceeds to implement the fine search algorithm 302. FIG. 4b shows a schematic plan view of the digital topographical map sheet 402 interacting with the fine search algorithm. In step 318, the fine search algorithm 302 increments along the fall line 406 to the row or column 410 of the next boundary of the individual map post. Next, in step 320, the height of the projectile at the boundary 410 is compared to the terrain elevation interpolated from the nearest map post. If, according to step 322, the calculated height of the projectile is greater than the terrain elevation interpolated from the nearest map post, the fine search algorithm 302 repeats and increments along the fall line 406 to the next map post boundary. If the projectile height at the boundary is less than the interpolated terrain elevation, the fine search algorithm 302 proceeds to step 324 and interpolates between the last two points on the fall line 406 to estimate the impact point 416 of the projectile. Optionally, the impact point 416 is then presented to the pilot in step 326, for example in the form of a CCIP on a display.

[0045] As explained above, FIG. 4a shows a schematic representation of a plan view of a digital terrain map 400 interacting with a coarse search algorithm and a map sheet 402 in the digital terrain map 400 interacting with a fine search algorithm, the map sheet 402 including several map posts 403. In this example, the map sheet 402 covers an area of ​​3 km x 3 km, and each of the map posts 403 covers an area of ​​1 m x 1 m. In FIG. 4a, it can be seen how the fall line 406 of a projectile released from an aircraft 404 intersects with the map sheet boundary 408 at a point indicated by a marker 412 calculated by the coarse search algorithm. The fine search algorithm then calculates how the fall line 406 intersects with the map post boundary 410 in the map sheet 402 at a point indicated by a marker 414 in FIG. 4b. Finally, the predicted impact point 416 of the projectile is calculated by interpolating between the last two markers 414 determined by the fine algorithm.

[0046] 5 shows a side view of a line of fall 506 of a projectile released from an aircraft 500 against a side profile 504 of the terrain. As explained above, a coarse search algorithm determines how the line of fall 506 of the projectile released from the aircraft 500 intersects with a vertical projection of a map sheet boundary 508 at a point indicated by marker 512. Then, as explained above, a fine search algorithm determines how the line of fall 506 intersects with a vertical projection of a map post boundary at a point indicated by marker 514. A final prediction of the point of impact of the projectile with the terrain side profile 504 is indicated by marker 516.

[0047] In FIG. 6, a block diagram of a terrain-based navigation system 600 is seen. The system 600 comprises a processor 602 coupled to a memory 604. As used herein, the term "processor" refers to one or more devices, circuits, and / or processing cores configured to process data, such as computer programming instructions. The term "memory" refers to any computer storage medium, including, but not limited to, volatile and / or non-volatile memory, such as read-only memory (ROM) and random access memory (RAM). A digital topographical map 606 is stored in the memory 604. The processor 602 comprises a position calculation unit 608, a fall line calculation unit 612, and an impact point calculation unit 614. In this embodiment, the position calculation unit 608, the fall line calculation unit 612, and the impact point calculation unit 614 are located in a single processor 602.

[0048] During use of the terrain-based navigation system 600, a position calculation unit 608 coupled to the stored digital terrain map 606 uses the most recent altitude measurements (in this example, input from a radar altimeter (radalt) unit 607) to calculate an estimate of the terrain-referenced aircraft position as described above. The position calculation unit 608 is also coupled to a fall line calculation unit 612, which uses the terrain-referenced aircraft position provided by the position calculation unit 608 as a starting point for fall line calculation. The fall line calculation unit 612 is optionally coupled to an input unit 610, which is configured to provide projectile data for more accurate calculation of the fall line of the projectile. The fall line calculation unit 612 is also coupled to an impact point calculation unit 614, which is configured to iterate along the fall line provided by the fall line calculation unit 612 and compare it to the stored digital terrain map 606 as described above. The impact point calculation unit 614 is optionally coupled to an output unit 616 configured to present the predicted impact point to the pilot. For example, the predicted impact point may be presented to the pilot by the output unit 616 in the form of a CCIP on a display such as the pilot's HUD, as described above.

[0049] Although the present disclosure has been illustrated by describing one or more specific embodiments thereof, it will be appreciated by those skilled in the art that the present disclosure is not limited to these aspects, and many variations and modifications are possible within the scope of the appended claims.

Claims

1. 1. A terrain referenced navigation system for an aircraft, comprising: a memory containing a stored digital topographical map having a plurality of map posts of terrain elevations; receiving input from a radar altimeter; a position calculation processing unit for calculating the position of the aircraft relative to the stored digital terrain map and matching the radar altimeter measurements of previously traversed terrain with the stored digital terrain map to determine a terrain referenced aircraft position; receiving said topographically referenced aircraft position; a fall line calculation processing unit for calculating a fall line of a projectile starting from said terrain referenced aircraft position as a launch point; an impact point calculation processing unit that directly compares the fall line to the stored digital terrain map by incrementally comparing the height of the projectile along the fall line to the height of the terrain by searching map data of the stored digital terrain map according to a coarse search of the stored digital terrain map to find an expected impact point on the terrain, and then according to a fine search of the stored digital terrain map; an output processing unit having an aircraft display for displaying to a pilot the predicted point of impact on the terrain; Equipped with During the rough search of the stored digital topographical map, The impact point calculation processing unit includes: incrementing along said fall line to a boundary of a map sheet, said map sheet having a subset of said map posts of said stored digital topographical map, said subset having a plurality of map posts; Calculating the maximum height of the topography of said map sheet; comparing the height of the projectile at the boundary of the map sheet to the maximum height of the terrain on the map sheet; A terrain reference navigation system for aircraft.

2. Further comprising a local memory; 2. The terrain referenced navigation system for an aircraft of claim 1, wherein said stored digital terrain map comprises a plurality of subsets of said map data cached in said local memory.

3. 3. The terrain referenced navigation system for an aircraft of claim 2, further comprising a processor configured to cache a plurality of subsets of the map data while calculating a maximum terrain elevation within each subset of the map data.

4. The system of claim 1 , wherein the position calculation processing unit, the fall line calculation processing unit, and the impact point calculation processing unit are located on a single processor.

5. The system of claim 1 , further comprising an input unit for projectile data.

6. 1. A method of terrain referenced navigation, comprising: receiving an input from a radar altimeter; matching said radar altimeter measurements of previously traversed terrain with said stored digital terrain map to calculate an aircraft position relative to a stored digital terrain map to determine a terrain referenced aircraft position, said stored digital terrain map having a plurality of map posts of terrain elevations; calculating a line of fall of the projectile beginning at said terrain referenced aircraft position as a launch point; directly comparing the fall line to the stored digital topographical map by incrementally comparing the height of the projectile along the fall line to the height of the terrain by searching map data of the stored digital topographical map according to a coarse search of the stored digital topographical map to find an expected point of impact on the terrain and then according to a fine search of the stored digital topographical map; displaying to the pilot the predicted point of impact on the terrain; and Including, The rough search of the stored digital topographical map includes: incrementing along the fall line to a boundary of a map sheet, the map sheet having a subset of the map posts of the stored digital topographical map, the subset having a plurality of map posts; calculating a maximum height of the topography of said map sheet; comparing the height of the projectile at the boundary of the map sheet to a maximum height of the terrain on the map sheet; A topographically referenced navigation method, including:

7. The method of claim 6 further comprising inputting projectile data.

8. The method comprises the steps of: receiving input from a radar altimeter; calculating a position of the aircraft relative to a stored digital terrain map to determine a terrain referenced aircraft position by matching said radar altimeter measurements of previously traversed terrain with said stored digital terrain map, said stored digital terrain map having a plurality of map posts of terrain elevation data; calculating a line of fall of the projectile beginning at said terrain referenced aircraft position as the launch point; directly comparing the fall line to the stored digital terrain map by incrementally comparing the height of the projectile along the fall line to the height of the terrain according to a coarse search of the stored digital terrain map to locate an expected point of impact on the terrain, and then according to a fine search of the stored digital terrain map; causing the terrain referenced navigation system to display to the pilot on an aircraft display the predicted impact point on the terrain; During the rough search of the stored digital topographical map, The instructions are provided to the topographical reference navigation system. incrementing along said fall line to a boundary of a map sheet, said map sheet having a subset of said map posts of said stored digital topographical map, said subset having a plurality of map posts; Calculating the maximum height of the topography of said map sheet; comparing the height of the projectile at the boundary of the map sheet to the maximum height of the terrain on the map sheet; A non-transitory computer readable medium or computer program product.

9. During the rough search of the stored digital topographical map, when the height of the projectile at the boundary of the map sheet is determined to be greater than the maximum height of the terrain on the map sheet, the impact point calculation processing unit increments along the fall line to the next map boundary; 2. The terrain referenced navigation system for an aircraft of claim 1, wherein when the height of the projectile at the boundary of the map sheet is determined to be less than the maximum height of the terrain on the map sheet, the impact point calculation processing unit returns along the fall line to the previous map sheet and initiates a fine search of the stored digital terrain map.

10. During the fine search of the stored digital topographical map, the impact point calculation processing unit: incrementing along said fall line to a boundary of a map post of said stored digital topographical map; Interpolating the terrain height at the boundary of the map post from the map posts closest to the boundary of the map post; 2. The terrain referenced navigation system for an aircraft of claim 1, wherein the height of the projectile at the boundary of the map post is compared to an interpolated height of the terrain at the boundary of the map post.

11. During the fine search of the stored digital topographical map, when the height of the projectile at the boundary of the map sheet is determined to be greater than the interpolated height of the terrain at the boundary of the map post, the impact point calculation processing unit increments along the fall line to the boundary of the next map post; 11. The terrain referenced navigation system for an aircraft of claim 10, wherein when the height of the projectile at the boundary of the map sheet is determined to be less than the interpolated height of the terrain at the boundary of the map post, the impact point calculation processing unit interpolates between the boundary of the map post and the boundary of a previous map post along the fall line to estimate an expected point of impact on the terrain.

12. The rough search of the stored digital topographical map comprises: incrementing along the fall line to a next map sheet boundary when the height of the projectile at the map sheet boundary is determined to be greater than the maximum height of the terrain on the map sheet; 7. The method of claim 6, further comprising: returning along the fall line to a previous map sheet and initiating a fine search of the stored digital topographical map when the height of the projectile at the boundary of the map sheet is determined to be less than the maximum height of the terrain on the map sheet.

13. The fine search of the stored digital topographical map comprises: incrementing along said fall line to a boundary of a map post of said stored digital topographical map; Interpolating the terrain height at the boundary of the map post from the map posts closest to the boundary of the map post; 7. A method of terrain referenced navigation as claimed in claim 6, including comparing the height of the projectile at the boundary of the map post to an interpolated height of the terrain at the boundary of the map post.

14. The fine search of the stored digital topographical map includes: incrementing along the fall line to the next map post boundary when the height of the projectile at the map sheet boundary is determined to be greater than the interpolated height of the terrain at the map post boundary; 14. A method of terrain referenced navigation as claimed in claim 13, comprising, when a height of the projectile at a boundary of the map sheet is determined to be less than an interpolated height of the terrain at a boundary of the map post, interpolating between the boundary of the map post and a boundary of a previous map post along the fall line to estimate an expected point of impact on the terrain.

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