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Geoinformatics_06-2009
article [a] [b] fig. 3 (a) this eurocopter as350 helicopter is equipped with a dart heli-utility-pod which is designed specifically to accommodate a complete airborne laser scanning system. (b) this igi litemapper 5600 airborne laser scanner and its accompanying digicam camera are shown mounted in the heli-utility-pod with its covering lid removed. (c) this complete igi litemapper system has been placed on a purpose-built anti-vibration mount that has been constructed in-house by igi for use in fixed-wing aircraft. at the front, is the aerocontrol imu unit which is housed in the red box placed at the right-hand end of the mount; at the front centre is the digicam camera that is mounted on two silver pillars; while at the left-hand end is a digitherm thermal-ir camera. the litemapper scanner unit is contained in the silver box with the red plate on top that has been placed at the rear of the mount. [c] sm-2000 or somag gsm-3000 units. indeed igi can supply adapter pods to fit digicam cameras into all of these mounts. however, till now, many individual digicam camera units have been supplied as an integral part of igi s litemapper airborne laser systems. for this specific application, igi offers the complete system (laser scanner + digicam + ccns4 flight navigation and guidance system + aerocontrol gps/imu unit) mounted in a dart heli-utility-pod which is certified for operations with eurocopter as350/355 helicopters [fig. 3(a)]. the dart pod is a very lightweight streamlined kevlar box that is attached to the side of the helicopter and has been modified to accommodate a complete litemapper system [fig. 3(b)]. for use with certain other types of helicopters - e.g. the schweizer/ sikorsky 333, bell jetranger, hughes/md 500, eurocopter ec-120 and mil mi-8, as well as in fixedwing aircraft - igi offers an alternative purposebuilt mount that has been constructed in-house and can accommodate all the sensor units of a complete litemapper system, including the digicam [fig. 3(c)]. it features spring elements at each corner that are designed to dampen or remove the effects of aircraft vibration. called the double-digicam system - involves placing the pair of cameras in a suitable mount with both optical axes set parallel to one another, pointing in the vertical direction [fig. 4]. this allows two possible operational modes - (i) the first producing two virtually identical images of the terrain in colour (rgb) and falsecolour (cir) simultaneously; and (ii) the second using the cameras to take alternating images of the terrain when the required overlap, flying height or flight speed values require an image repetition time of less than one second. the two camera systems with regard to the use of twin digicam airborne cameras, a number of alternative configurations are being offered by igi. the first of these latest news? visit www.geoinformatics.com fig. 4 an igi double-digicam system with its twin nadir-pointing cameras mounted inside the cylindrical adapter box that fits into the somag gsm3000 gyro-stabilized mount. an igi aerocontrol imu unit is mounted on the shelf above the two cameras. exposures of the two cameras are synchronised within 1/100ms. a second quite different configuration called the dual-digicam system involves the two digicam cameras being placed in a suitable mount with a relatively small tilted (low oblique) pointing of their optical axes [fig. 5(a)]. again this allows two possible operational modes to be implemented. (i) in the first of these, the two cameras are pointed in opposite directions cross-track and their shutters are fired simultaneously, again with a synchronisation of better than 1/100ms. this produces two oblique photographs with a narrow overlap between them [fig. 5(b)]. the two photos can then be rectified and stitched together using tie points in the common overlap to provide the wide crosstrack coverage of the ground that is required. (ii) the alternative mode of operation is to operate the same system with its two oblique pointing cameras set in the along-track direction. this can be used to acquire twin convergent oblique photographs with 100% overlap if desired, allowing stereo-models to be formed with a favourable base:height ratio. the format size of the merged images that are acquired by the current dual-digicam using its cross-track configuration using the 39 megapixel digital backs is 70 megapixels. when the new 60 megapixel backs become available by the end of this year, the size will increase to 110 megapixels. september 2009 61
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