Autopiloted Model Airplane

Pilot-operated model airplanes, as shown by the previous example, are not the easiest platform for SFAP. They require considerable flying experience of the pilot to start with, and even more experience and technical skills for pin-pointing exact locations with vertical photographs or covering large areas in systematic flightline arrangements. Recent developments in global positioning system (GPS) and inertial

FIGURE 8-38 Close-up view of the Multiplex Easy Star model airplane in the field with small digital camera mounted for oblique views to the forward port (left) side of the plane. The camera and shutter microservo are secured quite simply with rubber bands. Green tape on the nose was used to repair damage from a previous crash. Photo by JSA.

FIGURE 8-38 Close-up view of the Multiplex Easy Star model airplane in the field with small digital camera mounted for oblique views to the forward port (left) side of the plane. The camera and shutter microservo are secured quite simply with rubber bands. Green tape on the nose was used to repair damage from a previous crash. Photo by JSA.

navigation system (INS) technologies are now leading to autopiloted model airplanes which can autonomously follow prescribed flightlines.

In his Master's thesis, C. Claussen, assisted by M. Niesen, developed an automated navigation system for model airplanes which fully controls flightpaths, flying height, velocity, and airplane landing. Both are among the founders of the German MAVinci company (Claussen et al., 2008), which has subsequently expanded and refined the system for controlling unmanned aerial photographic surveys. The autopilot software now integrates the NASA WorldWind viewer; the information about the survey area is fed into the software via this viewer or alternatively via GPS coordinates or placemarks digitized in Google Earth. The direction and parallel distance of the flightlines, flying height, and flying velocity can be set by the operator or automatically computed by the software for an optimal flight plan.

The target flightpath is transferred to the on-board computer in the plane via radio communication. During the flight, this continuous two-way wireless communication reports the plane's location and orientation, so the actual flightpaths can be visualized in the NASA WorldWind viewer or Google Earth in real time. The wireless communication also enables to correct deviations from the flightpath or even force the plane to repeat flightlines which have not been met with sufficient precision due to wind drift, etc. The control can be toggled between the autopilot software and a handheld radio controller so that the operator can take charge of the launching and landing phase, but even this is not necessary when the ground conditions allow fully autonomous takeoff and landing. The maximum flying height in Germany, as demanded by national aviation laws, is restricted by the visibility of the plane and amounts to about 500 m.

Claussen and Niesen's autopilot system makes the MAVinci planes the most technically advanced and most automated SFAP platform employed by the authors so far.

FIGURE 8-39 B. Graves prepares to launch the Easy Star model airplane from his right hand while he holds the radio controller in his left hand. Photo by JSA.

It considerably reduces the role of the operator compared to hand-controlled model airplanes, but some basic flying skills, which could be gained with a similar off-the-shelf model and with a flying simulator, are still indispensable. The plane is launched by hand as shown in Figure 8-41A, and the autopilot takes over immediately, directing the plane in tight circles for gaining height (see Fig. 3-9). However, most ground conditions require the plane to be landed by hand-control to avoid a rough end to the survey (Fig. 8-41B).

So far, the MAVinci team has developed two SFAP airplanes meeting different demands, a smaller one with a digital compact camera and a larger one with a calibrated DSLR camera, as prototypes for photogrammetric survey platforms (Fig. 8-42). For the smaller system, a Multiplex Twinstar II made from Elapor with a wing length of 1.4 m and a weight of 1.7 kg was adapted to accommodate the GPS/INS control system and a Nikon Coolpix camera (ca. 200 g). The camera is completely concealed in the body of the plane and points vertically to the ground through an opening in the plane bottom. A single lithium-polymer (LiPo) rechargeable battery (11.1V, 5000-7000 mAh) provides power for the two propeller motors, the GPS/INS, and camera, allowing a mission time of approximately 40 minutes or mission distance of 35 km. The battery as well as the camera memory card can be reached and removed when the cockpit cover is taken off.

Following the requirements given by two of the authors (IM, JBR) this first system design was modified for a prototype of a larger system capable of conducting stereoscopic surveys with a calibrated DSLR camera suited for photo-grammetric analysis. An existing Canon EOS 300D camera with 28 mm lens, weighing 1.1 kg in total, was to be installed in the plane. This required a larger model that would be able to carry such high payloads without exceeding the 5 kg limit for authorization-free model aircraft in Germany (see Chapter 9.8.). A Multiplex Mentor with a wing length of 1.6 m and a body length of 1.2 m was modified by mounting two propellers to the wings and additional Elapor moldings to the bottom of the plane, where the camera lens needed protection (see Fig. 8-41A). The resulting somewhat bulky appearance earned it the nickname ''der Bulle'' (the bull), and its heavy weight of 3.3 kg reduces the mission time to

FIGURE 8-40 Panorama of the Blue Lake hot spring and marsh complex in the desert basin of western Utah, United States. The boardwalk and dock in lower center provide access for scuba divers to enter the deep pool. View toward southeast, October 2006. Photo courtesy of B. Graves.

about 15 minutes, but these limitations will be improved in a future system based on this first prototype. In cooperation with another company, MAVinci is currently developing a third plane that will carry a 2.5-kg payload and have a mission time of 50 minutes. This plane will be capable of integrating different types of cameras depending on the application.

The velocity of the plane is an important factor in survey planning (see Chapters 3.3 and 9.6). For the two Multiplex models, the nominal airspeed of 45-70 km/h results in a wider range of ground speeds depending on the wind conditions. Usually, the flightlines are arranged parallel to the wind direction, so the airplane is considerably slower out than back, a fact which needs to be taken into account when calculating exposure intervals and image coverage. For stereoscopic coverage from low-flying heights, the slow interval between shots (low frame rate) for continuous exposure series is currently still a problem with many digital cameras. For example, the fastest interval with the Canon EOS 300D is 3 seconds for JPG images and 4 seconds for RAW images, which is (assuming a ground speed of 70 km/ h and 28 mm lens) too slow for achieving 60% overlaps at flying heights below 240 m. The Nikon Coolpix compact camera is not much faster. Recent DSLR camera models and industrial cameras often offer shorter exposure intervals, and this requirement may be satisfied in a future version of the plane. For now, stereoscopic coverage at lower flying heights is simply achieved by repeated overflights on the same flightpaths.

The camera exposure is triggered by the on-board computer in regular pre-set intervals so that a continuous image series is taken from the moment of launching till landing of the plane. In the refined autopilot version of der

FIGURE 8-42 The two autopiloted model airplanes developed by the MAVinci team. Background: Multiplex Twinstar II, carrying a digital compact camera. Foreground: modified Multiplex Mentor (dubbed der Bulle; see also Fig. 8-41) carrying a DSLR camera. The cockpit cover of both planes is removed, giving access to the batteries. The cameras are mounted beneath the detachable wings. Photo by M. Niesen.

FIGURE 8-42 The two autopiloted model airplanes developed by the MAVinci team. Background: Multiplex Twinstar II, carrying a digital compact camera. Foreground: modified Multiplex Mentor (dubbed der Bulle; see also Fig. 8-41) carrying a DSLR camera. The cockpit cover of both planes is removed, giving access to the batteries. The cameras are mounted beneath the detachable wings. Photo by M. Niesen.

Bulle plane, the triggering of the camera can be constricted to a limited nadir angle of the optical axis. If the plane and thus the axis exceed a tilt of, e.g., 5°, no image is taken; this eliminates the oblique situations at the turning point of each flightline and also temporary tilts by wind pressure. GPS position and tilting angles for each image are logged into a file so that this information can later be added to the EXIF data of the image header. This feature is especially helpful for selecting suitable images for stereoscopic viewing and photogrammetric analysis. For use as exterior orientation parameters in photogrammetric triangulation without additional ground control (see Chapter 3), the recorded data are

FIGURE 8-41 (A) MAVinci plane being hand-launched by C. Claussen. The additional Elapor moldings mounted to the bottom of the plane serve as a protection for the camera lens and as a skid for softer landings. (B) M. Niesen landing the plane on a plowed field. In spite of the dirt plume the camera lens was barely dusted. Photos by G. Rock, February 2009.

FIGURE 8-41 (A) MAVinci plane being hand-launched by C. Claussen. The additional Elapor moldings mounted to the bottom of the plane serve as a protection for the camera lens and as a skid for softer landings. (B) M. Niesen landing the plane on a plowed field. In spite of the dirt plume the camera lens was barely dusted. Photos by G. Rock, February 2009.

not precise enough, especially as exact synchronization with the camera trigger is difficult owing to the shutter time lag. However, they are sufficient to enable tagging the images with the georeferencing information used by NASA WorldWind or Google Earth, so the flightpath and whole image block can be visualized in these viewers and checked for completeness and gaps (Fig. 8-43).

The MAVinci airplane is currently the only platform in use by the authors which allows the automatic visualization of flightpaths, image exposure stations, and the images themselves by using the exterior orientation parameters in viewers such as Google Earth. However, these exciting and useful features, which are enabled by recent GPS/INS technology, also could be incorporated in other platforms. This is rather a question of costs than carrying capacity. In terms of transportability, the autopi-loted airplane is quite mobile and can be transported easily by car. The modular assembly design allows uncomplicated replacement of broken or defective components by spare parts.

Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

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