PROJECTILE RANGING WITH DIGITAL MAP

Abstract

A terrain-referenced navigation system for an aircraft comprises: a stored digital terrain map; a position calculation unit arranged to calculate aircraft position relative to the stored digital terrain map to determine a terrain-referenced aircraft position; a fall line calculation unit 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 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 in order to find an expected impact point on the terrain.

Claims

1. A terrain-referenced navigation system for an aircraft, the system comprising: a stored digital terrain map; a position calculation unit arranged to calculate aircraft position relative to the stored digital terrain map to determine a terrain-referenced aircraft position; a fall line calculation unit 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 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 in order to find an expected impact point on the terrain.

2. The system of claim 1, wherein the impact point calculation unit is arranged to incrementally compare a height of the projectile along the fall line with a height of the terrain by searching map data in the stored digital terrain map.

3. The system of claim 2, wherein the impact point calculation unit is arranged to incrementally compare a height of the projectile along the fall line with a height of the terrain according to a coarse search of the digital terrain map and then according to a fine search of the digital terrain map.

4. The system of claim 1, further comprising: a local memory; wherein the stored digital terrain map comprising a plurality of subsets of map data cached in the local memory.

5. The system of claim 4, further comprising: a processor arranged to calculate a maximum terrain height within each subset of map data while caching the plurality of subsets of map data.

6. The system of claim 1, wherein the position calculation unit, fall line calculation unit and impact point calculation unit are arranged in a single processor.

7. The system of claim 1, further comprising: an output unit arranged to output the expected impact point on the terrain.

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

9. The system of claim 1, wherein the position calculation unit includes or receives input from a radar altimeter.

10. The system of claim 9, wherein the position calculation unit is arranged to match radar altimeter measurements of the terrain already traversed with the stored digital terrain map.

11. A terrain-referenced navigation method comprising: calculating aircraft position relative to a stored digital terrain map to determine a terrain-referenced aircraft position; calculating a fall line for a projectile starting from the terrain-referenced aircraft position as a launch point; and directly comparing 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 in order to find the expected impact point on the terrain.

12. The method of claim 11, wherein the step of incrementally comparing a height of the projectile along the fall line with a height of the terrain comprises searching map data in the stored digital terrain map.

13. The method of claim 12, wherein incrementally comparing a height of the projectile along the fall line with a height of the terrain comprises a coarse search of the digital terrain map followed by a fine search of the digital terrain map.

14. The method of claim 11, further comprising: outputting the expected impact point on the terrain on a display.

15. The method of claim 11, further comprising: inputting projectile data.

16. The method of claim 11, wherein calculating aircraft position relative to a stored digital terrain map comprises matching radar altimeter measurements of the terrain already traversed with the stored digital terrain map.

17. A computer-readable medium or computer program product comprising: instructions that are executable by a processor to cause a terrain-referenced navigation system to perform the method of claim 11.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0035] One or more non-limiting examples will now be described, with reference to the accompanying drawings, in which:

[0036] FIG. 1 is a diagram illustrating a prior art iterative process to calculate an estimate of the impact point of a projectile;

[0037] FIG. 2 contains diagrams illustrating examples of stable and unstable prior art iterative loops;

[0038] FIG. 3 is a flow chart illustrating an exemplary method, according to the present disclosure, for calculating a predicted impact point of a projectile;

[0039] FIG. 4a schematically illustrates a plan view of a digital terrain map interacting with a coarse search algorithm, and FIG. 4b schematically illustrates a plan view of a digital terrain map sheet interacting with a fine search algorithm, according to such an exemplary method;

[0040] FIG. 5 schematically illustrates a side view of the fall line of a projectile relative to a side profile of the terrain with an impact point being determined according to such an exemplary method; and

[0041] FIG. 6 is a schematic block diagram representing an exemplary terrain-referenced navigation system according to the present disclosure.

DETAILED DESCRIPTION

[0042] FIG. 1 illustrates a prior art iterative process to estimate the impact point of an unguided projectile to be released from an aircraft 100 travelling at an absolute altitude above ground level (AGL) 102, where 104 illustrates a side profile of the terrain being travelled over by the aircraft 100.

[0043] The AGL 102 of the aircraft 100 may be measured using a radar altimeter or any other height measuring system (such as 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 recent AGL measurements of the aircraft and comparing with stored digital terrain map data, containing elevation information about the terrain. Comparing the AGL measurements with the digital terrain map data can allow for a more accurate estimate of the aircraft's position relative to the terrain than through the use of Global Positioning Systems (GPS) or Inertial Navigation Systems (INS) alone, as these position estimation systems can generate errors and drift, which can be observed and corrected for by the TRN system. In addition to this, the stored digital terrain map data itself may contain map shift errors. To estimate the position of the aircraft 100 relative to the terrain any combination of TRN, INS and/or GPS (or similar) may be used. The TRN system may be the TERPROM® (Terrain Profile Matching) digital terrain system available from Atlantic Inertial Systems.

[0044] In FIG. 1, 106 illustrates a predicted fall line of a projectile; wherein the term ‘fall line’ refers to the path that a projectile would follow, once released, if uninhibited by objects or terrain. Conventionally, a ranging system on the aircraft 100 calculates an estimate of the horizontal range 108 for a projectile; wherein the term ‘forward throw’ refers to the horizontal distance a projectile would travel before falling to a specified altitude, if uninhibited by terrain. In a first iteration, the forward throw of the projectile is calculated using the aircraft's current AGL, giving an initial prediction of the impact point. A marker 116 demonstrates the point in the iteration process where the ranging system has computed a forward throw in this manner. The TRN system then returns terrain elevation data for this point 110. A marker 118 is used wherever the TRN system has returned terrain elevation data in this manner. The ranging system then recalculates the forward throw 112 given this elevation data. These calculations are repeated back and forth in an iterative loop; typically the loop iterates at between 12.5 and 25 Hz. The predicted impact point 114 calculated from this loop, denoted by marker 120, is presented to the pilot in the form of a Constantly Computed Impact Point (CCIP) displayed on the pilot's Heads-Up Display (HUD).

[0045] FIG. 2 illustrates one problem with this prior art iterative loop: that the stability of the loop is dependent on the geometry of the fall line 208 and the gradient of the terrain 210 around the impact point. 200 and 202 are examples of stable iterative loops; wherein it is possible to calculate a reasonable estimate of the impact point within a small number of iterations. 204 and 206 are examples of unstable iterative loops; wherein it is not possible to calculate a reasonable estimate of the impact point within a small number of iterations.

[0046] FIG. 3 is a flow chart illustrating an exemplary terrain-referenced navigation method, according to the present disclosure, that is capable of calculating a predicted impact point of a projectile. Such a method may conveniently be implemented within a TRN system rather than requiring a separate ranging system. The method involves calculating the fall line for the projectile and comparing it directly with the terrain map data to find the impact point. Optionally, this may be implemented through a coarse search algorithm 300 followed by a fine search algorithm 302.

[0047] At steps 304-316, the method starts by implementing a coarse search algorithm 300. At step 304, projectile data (e.g. mass, cross-sectional area, drag coefficient) is taken as an input in order to allow for more accurate prediction of the impact point of the projectile. At step 306, the terrain-referenced aircraft position, which is being continuously calculated, is then used as the starting position for a fall line calculation at step 308. At step 308, the fall line of the projectile may be calculated using standard techniques. At step 310, the coarse search algorithm 300 then increments along the fall line to the next map sheet boundary in a stored digital terrain map. The digital terrain map data may be stored locally in a computer readable storage medium, e.g. Hard Disk Drive (HDD), or stored remotely e.g. in the cloud.

[0048] In at least some examples, the digital terrain map may consist of rows and columns (map posts) of elevation data, where a specific number of these map posts comprises a subset of map data referred to as a “map sheet”. This is visualised in FIG. 4a, which schematically illustrates a plan view of a digital terrain map 400 interacting with the coarse search algorithm. An edge of a map sheet 402 is denoted by a solid line 408, and an edge of a map post is denoted by a dashed line 410. The boundaries of both map sheets and map posts may be a row or a column of the digital terrain map 400.

[0049] At 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 just crossed by the predicted fall line 406 of the projectile. At step 314, the coarse search algorithm 300 determines whether the height of the projectile at the boundary 408 is greater than the maximum height in the map sheet 402. If so, the coarse search reiterates and the coarse search algorithm 300 increments along the fall line 406 to the next map sheet boundary. If the height of the projectile at the boundary 408 is smaller than the maximum height in the map sheet 402, the coarse search algorithm 300 proceeds to return to the previous map sheet boundary at step 316, as it is now known that the fall line 406 of the projectile is likely to intersect with the terrain within the area of this map sheet 402.

[0050] At steps 318-326, the method then proceeds to implement a fine search algorithm 302. FIG. 4b schematically illustrates a plan view of a digital terrain map sheet 402 interacting with the fine search algorithm. At step 318, the fine search algorithm 302 increments along the fall line 406 to the next boundary row or column 410 of the individual map posts. At step 320, the height of the projectile at the boundary 410 is then compared to the terrain height as interpolated from the nearest map posts. According to step 322, if the calculated height of the projectile is greater than height of the terrain as interpolated from the nearest map posts, the fine search algorithm 302 reiterates and increments along the fall line 406 to the next map post boundary. If the height of the projectile at the boundary is smaller than the interpolated terrain height, the fine search algorithm 302 proceeds at step 324 to interpolate between the final two points on the fall line 406 in order to estimate the impact point 416 of the projectile. Optionally, the impact point 416 is then presented to the pilot at step 326 e.g. in the form of a CCIP on a display.

[0051] As described above, FIG. 4a schematically illustrates a plan view of a digital terrain map 400 interacting with the coarse search algorithm, and a map sheet 402 within the digital terrain map 400 interacting with the fine search algorithm, wherein the map sheet 402 comprises a number of map posts 403. In this example, the map sheet 402 covers an area of 3 km by 3 km, and each map post 403 covers an area of 1 m by 1 m. It can be seen in FIG. 4a how the fall line 406 of a projectile to be released from an aircraft 404 intersects with map sheet boundaries 408 at points denoted by markers 412 as calculated by the coarse search algorithm. Then, the fine search algorithm calculates how the fall line 406 intersects with the map post boundaries 410 within a map sheet 402 at points denoted by markers 414 in FIG. 4b. Finally, the predicted impact point 416 of the projectile is calculated by interpolating between the final two markers 414 determined by the fine algorithm.

[0052] FIG. 5 illustrates a side view of the fall line 506 of a projectile to be released from an aircraft 500 relative to a side profile 504 of the terrain. As described above, the coarse search algorithm determines how the fall line 506 of a projectile to be released from the aircraft 500 intersects with vertical projections of map sheet boundaries 508 at points denoted by marker 512. Then, as described above, the fine search algorithm determines how the fall line 506 intersects with vertical projections of the map post boundaries at points denoted by marker 514. The final predicted impact point of the projectile with the terrain side profile 504 is denoted by marker 516.

[0053] There is seen in FIG. 6 a block diagram of a terrain-based navigation system 600. 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; and the term memory refers to any computer storage media including but not limited to volatile and/or non-volatile memory such as read only memory (ROM), and random access memory (RAM). Stored within the memory 604 is a digital terrain map 606. The processor 602 comprises a position calculation unit 608, a fall line calculation unit 612 and an impact point calculation unit 614. In this example, the position calculation unit 608, fall line calculation unit 612 and impact point calculation unit 614 are arranged in a single processor 602.

[0054] During use of the terrain-based navigation system 600, the position calculation unit 608, coupled to the stored digital terrain map 606, calculates an estimate of the terrain referenced aircraft position as described previously using recent altitude measurements, in this example input from a radar altimeter (radalt) unit 607. The position calculation unit 608 is also coupled to the fall line calculation unit 612, wherein the fall line calculation unit 612 uses the terrain-referenced aircraft position as provided by the position calculation unit 608 as the start point for the fall line calculation. The fall line calculation unit 612 is optionally coupled to an input unit 610 which is arranged to provide projectile data in order for a more accurate calculation of the fall line of a projectile. The fall line calculation unit 612 is also coupled to the impact point calculation unit 614, which is arranged to iterate along the fall line as provided by the fall line calculation unit 612 and compare with the stored digital terrain map 606, as described previously. The impact point calculation unit 614 is optionally coupled to an output unit 616 which is arranged 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 previously.

[0055] It will be appreciated by those skilled in the art that the present disclosure has been illustrated by describing one or more specific examples thereof, but is not limited to these aspects; many variations and modifications are possible, within the scope of the accompanying claims.