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An Innovative System for Low Cost Airborne Video Imaging
A range of cameras are available for airborne video imaging. They differ in terms of size, quality of image data and cost. The cheapest cameras, and those that offer the poorest quality data, are analogue video cameras. The reason for the poor quality of the data is because moving images in the video data are created by interlacing pairs of sequential image frames. With high frames rates it is not possible for the human eye to see the interlacing, but when the frame is frozen the interlacing effect is clear, especially if features within the images are moving (figure 1).  Fig. 1. Interlacing effects caused by moving features in analogue (interlaced) video imagery (Top: full frame images; bottom: detail from full frame images). Figure 1 shows the effects of interlacing. The images on the left are stationary, and therefore frames extracted from these images appear normal. However, when the camera is moved (the images on the right), features move in the images, and the interlacing becomes evident. It is clear that the frame is actually made up of two sequential frames that are interlaced together. Consequently, when analogue video cameras are used for mapping purposes, the effects of interlacing can often be problematic [7]. In recent years digital video camera technology has superseded its analogue counterpart. Analogue video cameras tend to be older, bulkier, and have less functionality. Consequently there are few compelling reasons to consider analogue video technology as a source of data. However, one strong reason for using analogue video cameras is that some modern cameras are available that are extremely small. These small video cameras, often known as lipstick cameras or bullet cameras, contain the lens and imaging sensor, but are reliant on a camera for recording the data. A typical lipstick video camera is the Sony XC-999 (figure 2). The dimensions of this camera are approximately 120 × 22 × 22 mm.  Fig. 2. Sony XC-999 lipstick video camera. The compact size and light weight of lipstick video cameras means that they can be easily installed on a range of aerial platforms. There is no need to build a robust camera mounting system – these cameras can literally be attached to an aircraft using strong tape and cable ties. Obviously the low cost, ease and flexibility of the installation make cameras of like this extremely useful. For most applications, but not necessarily all, it is necessary to extract individual frames from the video imagery for mapping purposes. To do this with analogue video imagery a video capture device must be used – this is a piece of hardware that links a computer to a video source. Video imagery can then be converted to a common digital file format, such as AVI or MPEG, from which frames can be extracted. The next step up from analogue video is digital video. Digital video is currently the standard in video imaging technology, and offers many advantages over analogue video. In particular, digital video data can be captured in a non-interlaced format known as progressive scan. The problems of moving features showing interlaced artefacts in the imagery (illustrated in figure 1) are removed because each image is captured sequentially, and only one image is used to create a frame of data. Importantly, digital video imagery can easily be converted to a digital file format, such as AVI or MPEG, through the use of a Firewire (IEEE1394) link. This removes the need for a video capture device. Generally the data from digital video cameras is of much greater quality than that from analogue video cameras. Additionally, digital video cameras offer more functionality and flexibility of use compared to their analogue counterparts. Recently the first consumer high definition (HD) video cameras have become available. Since mid-2005, Sony has been marketing a HD Handycam, the HDR-HC1 (figure 3)  Fig. 3. Sony HDR-HC1 consumer high definition Handycam. HD video offers much better image quality through high resolution image capture. Images are recorded over 1080 scanning lines, as opposed to 720 scanning lines for standard definition (SD) PAL video. The aspect ratio of the image is 16:9, compared to 4:3 for SD video. This results in a much wider image, specifically designed for the latest generation of widescreen HD televisions. The implications for airborne video is that data can be collected at a higher resolution than is possible with SD. As can be seen, there are many options for acquiring video imagery for mapping. The choice of imaging system should be determined by the application in question. Once the imaging system has been chosen, the platform on which that system will be mounted can be selected. As with the imaging system, the platform should be chosen according to the specific needs of the application. In many scenarios where airborne video is used, the ideal platform will be a small, low cost system. The next section of this paper describes perhaps one of the smallest airborne platforms, the powered parachutes. 3. The Powered Parachute – An Innovative Aerial Mapping Platform The concept of using of small, lightweight aircraft for remote sensing is not new ([8], [9]). However, in the past the use of small aircraft has not been widespread due to limitations in imaging technology. Nowadays, however, good quality imaging technology has reduced in size and weight, such that it can now be installed into a small aircraft. One of the smallest manned aircraft which can be used for remote sensing data acquisition is the powered parachute (figure 4). A powered parachute (PPC) is a two seat aircraft, powered by a single engine, and slung beneath a parachute canopy. The engine provides the thrust for forward motion, and the parachute canopy provides the lift for vertical motion. A PPC can take off in distances as small as 15m, can fly as high as 5000 feet, and can maintain speeds of up to 40 km/h. Importantly, the lifting capacity of a PPC is 200kg, which is more than sufficient for the pilot, a camera, a computer system etc. Furthermore, PPCs are extremely stable platforms, and can be flown after just a few hours of training.  Fig. 4. Powered parachute, on the ground and in flight. Integrating a camera onto a PPC is a task that can be simply carried out by a suitably qualified engineer. Powering the camera, computer, GPS and any other instrumentation can be done from rechargeable dry cell batteries. In summary, a PPC is the ideal platform for low cost mapping applications that do not cover a wide area. They are especially useful for applications where it is very difficult, or even impossible, to a light aircraft. |