Patent Application
20080002858
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
The present invention is of a device and an associated method which can be used for photogrammetric mapping. Specifically, the present invention can be used to map inaccessible terrain.
The principles and operation of photogrammetric mapping according to the present invention may be better understood with reference to the drawings and the accompanying description.
Referring again to the drawings,
GPS receiver 30 usually is preferred to inertial navigation system 50 because GPS receiver 30 is inherently more accurate than inertial navigation system 50. Inertial navigation system 50 must be initialized with its absolute geographic coordinates before it is deployed. Then, while inertial navigation system 50 is deployed, errors in the measured absolute geographic coordinates accumulate. The advantage of inertial navigation system 50 over GPS receiver 30 is that for GPS receiver 30 to determine its absolute geographic coordinates GPS receiver 30 must receive signals of adequate strength from an adequate number of GPS satellites.
In a variant of device 40 intended for use behind enemy lines, light source 46 emits only infrared light, so as not to be easily visible to enemy soldiers. To this end, if light source 46 is a xenon lamp, light source 46 also includes a filter that blocks visible radiation. Alternatively, dome 44 is transparent to infrared light but not to visible light. In addition, also to reduce the visibility of device 40 to enemy soldiers, device 40 includes opaque shield 58 that, along with housing 42, ensures that, although device 40 emits light in all azimuthal directions, device 40 emits light only upwards and not to the sides.
The first embodiment of the present invention uses two aerial platforms 10 and three or more devices 20 or 60. One of aerial platforms 10 signals devices 20, via RF receivers 26, to turn on GPS receivers 30 so as to measure the absolute geographic coordinates of devices 20 or 60. As is known in the art, such measurements often require several seconds to perform, depending on how many GPS satellites are visible to each device 20 or 60 and on the strength of the signals from those satellites. As soon as each device 20 or 60 has obtained a fix of its absolute position, that device 20 or 60 transmits its absolute geographic coordinates to the interrogating aerial platform 10, using RF transmitter 28.
When all the deployed devices 20 or 60 have finished transmitting their absolute geographic positions, one of aerial platforms 10 issues a command to all the deployed devices 20 to detonate their explosive charges 34, using detonators 36, or to all deployed devices 60 to open their valves 64 to release gas 62. At the same time, both aerial platforms 10 photograph the targeted terrain from their respective vantage points. The two respective photographic images acquired by the two aerial platforms 10 include records of the signature flashes of the explosions of explosive charges 34 of devices 20 or of the signature cold plumes of gas 62 of devices 60. A standard stereo correlation algorithm is used to convert the photographic images to a relative DTM of terrain 12 as described above. Except in degenerate cases (e.g., three devices 20 or 60 deployed at the vertices of an equilateral triangle), there is enough information in the relative DTM of terrain 12 obtained from the two images to associate each signature flash or cold plume with its respective absolute geographic coordinates. The signature flashes or cold plumes then serve as ground control points for transforming the coordinates of the DTM to absolute geographic coordinates, i.e., to provide and absolute DTM of terrain 12.
Alternatively, only one aerial platform 10 is used. Aerial platform 10 signals devices 20 or 60 to measure their absolute geographic coordinates as described above. Devices 20 or 60 transmit their absolute geographic coordinates to aerial platform 10. Aerial platform 10 then issues a command to all the deployed devices 20 or 60 to produce their photographically recordable signatures (the flashes of the explosions of explosive charges 34 or the cold plumes of gas 62). As the deployed devices 20 or 60 produce their photographically recordable signatures, aerial platform 10 photographs the targeted terrain from its vantage point while using its on-board navigation system to measure both its own absolute geographic coordinates and its own absolute orientation. Then, aerial platform 10 flies to a second vantage point that is different from the first vantage point and photographs the targeted terrain again while using its on-board navigation system to measure both its own absolute geographic coordinates and its own absolute orientation. Based on the absolute positions and orientations of aerial platform 10 at both vantage points, the locations, in the terrain image that is acquired at the second vantage point, where the photographically recordable signatures would have been recorded if the deployed devices 20 or 60 had produced their photographically recordable signatures while aerial platform 10 was at the second vantage point, are computed by triangulation. A standard stereo correlation algorithm is used to convert the photographic images to a relative DTM of terrain 12 as described above. As before, the signature flashes or cold plumes serve as ground control points, via their actual records in the first photographic image and their inferred locations in the second photographic image, for transforming the coordinates of the DTM to absolute geographic coordinates.
In variants of the first embodiment that normally are less preferred but that may be necessary under some circumstances, photography of the signatures of deployed devices 20 or 60 is separated from photography of terrain 12. For example, a single aerial platform 10 may be used to photograph the signatures from a first vantage point and then to photograph terrain 12 from second and third vantage points that are different from each other and from the first vantage point. At all three vantage points, aerial platform 10 uses its on-board navigation system to measure is both its own absolute geographic coordinates and its own absolute orientation. The locations, in the two terrain images, where the photographically recordable signatures would have been recorded if the deployed devices 20 or 60 had produced their photographically recordable signatures while aerial platform 10 was at the second and third vantage points, are computed by triangulation. A standard stereo correlation algorithm is used to convert the terrain images to a relative DTM of terrain 12, and the signature flashes or cold plumes serve as ground control points, via their inferred locations in the terrain images, for transforming the coordinates of the DTM to absolute geographic coordinates.
In a second less preferred variant, two aerial platforms 10 are used. The first aerial platform 10 photographs the signatures from a first vantage point. The second aerial platform 10 photographs terrain 12 from second and third vantage points. The second and third vantage point must be different from each other, but one of the second or third vantage points may be the same as the first vantage point. A DTM with absolute geographic coordinates is produced from the two images of terrain 12 as in the first less preferred variant.
The second embodiment uses one aerial platform 10 and three or more devices 40. Aerial platform 10 signals devices 40, via RF receivers 52, to turn on their light sources 46. In each device 40, control unit 54 obtains the absolute geographic coordinates of that device 40 from inertial navigation system 50 and uses modulator 48 to modulate the light emitted by light source 46 in a manner that encodes the absolute geographic coordinates of that device 40 in that emitted light. An optical sensor on board aerial platform 10 receives these optical signals, and a processor on board aerial platform 10 decodes the signals to obtain the absolute geographic coordinates of devices 40.
Now aerial platform 10 flies to a first vantage point above terrain 12 and photographs terrain 12. The resulting photographic image includes records of the light emitted by devices 40, as signatures of devices 40. Then, while devices 40 continue to emit their signature light, aerial platform 10 flies to a second vantage point above terrain 12 and photographs terrain 12. Again, the resulting photographic image includes records of the light emitted by devices 40, as signatures of devices 40. Further processing of the two photographic images to obtain an absolute DTM is as in the first variant of the embodiment.
Alternatively, aerial platform 10 signals devices 40 to turn off their light sources 46 after aerial platform 10 has photographed terrain 12 from the first vantage point. At both the first and second vantage points, aerial platform 10 uses its on-board navigation system to measure both its own absolute geographic coordinates and its own absolute orientation. Only the first photographic image of terrain 12 then includes records of the light emitted by devices 40; but the locations in the second photographic image of terrain 12, where records of the light emitted by devices 40 would have been if devices 40 had continued to emit light, are determined as in the second variant of the first embodiment. Further processing of the two photographic images to obtain an absolute DTM is as in the second variant of the first embodiment.
The third embodiment uses one or two aerial platforms 10, and either devices 20 or devices 40 or devices 60, but devices 20, 40 or 60 are operated sequentially rather than simultaneously. For definiteness, the third embodiment will be described in terms of two aerial platforms 10 and n≧3 devices 20. First, one of aerial platforms 10 signals a device 20, via RF receiver 26, to turn on GPS receiver 30 so as to measure the absolute geographic coordinates of that device 20. When that device 20 has obtained a fix of its absolute position, that device 20 transmits its absolute geographic coordinates to the interrogating aerial platform 10, using RF transmitter 28. Then one of aerial platforms 10 issues a command to that device 20 to detonate its explosive charge 34. At the same time, either both aerial platforms 10 photograph a portion of terrain 12, including the flash from the explosion of explosive charge 34, from their respective vantage points; or one aerial platform 10 photographs the portion of terrain 12, including the flash from the explosion of explosive charge 34, from a first vantage point, followed by photography by the same aerial platform 10 or by a second aerial platform 10 of the same portion of terrain 12 but not including the flash from the explosion of explosive charge 34, from a second vantage point different from the first vantage point. In the latter case of sequential photography of the portion of terrain 12 in which that device 20 is located, while the aerial platform(s) 10 photograph(s) that portion of terrain 12, the aerial platform(s) 10 also use its/their on-board navigation system(s) to measure its/their respective absolute geographic coordinates and its/their absolute orientations. This is repeated for all n devices 20.
2n photographic images thus have been acquired, of respective portions of terrain 12. In each photographic image, either a record of a signature of a device 20 tied to a known absolute geographical location appears, or the absolute location of that device 20 can be inferred as described above. The n portions of terrain 12 are chosen to overlap. Known stereo correlation algorithms are used, in conjunction with known algorithms that cross-correlate photographic images that correspond to overlapping portions of terrain 12 so as to chain the photographic images together, to transform these images to an absolute DTM of terrain 12.
While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.
