Overhead surveillance has come a long way since Tournachon (“Nadar”) collected the first aerial photographs from a hot air balloon in 1858. The invention of the plane made overhead surveillance more practical. Images from space began with Explorer IV in 1959.
Today an increasing amount of overhead surveillance imagery is collected by systems that do not require pilots. Satellites are used to collect data high above the earth. (UAVs) (unmanned aerial vehicles) are being used to collect information at lower altitudes. Overhead surveillance systems continue to provide environmental and meteorological data to the scientific community as well as important intelligence data from areas that are difficult to access.
Linking together many steps in an image chain creates the final image product produced by an overhead surveillance system. Each link plays a vital role in the final quality of the image. This article discusses the various steps.
Surveillance systems are generally categorized by the wavelength region of the electromagnetic spectrum that they image. The earliest cameras used black and white film that captured panchromatic images, i.e., film that is sensitive to the visible portion of the electromagnetic spectrum. Even with CCD (charged coupled devices) detectors today, black and white images are the most common because the light is captured over a broad spectrum of wavelengths, thus allowing a high enough signal to collect high‐resolution images. Visible imaging systems generally acquire images using the sun as the illumination source.
After the image data has been downlinked to the groundstation and decompressed, it may be hard to interpret the image due to low contrast, blurred edges, streaks from sensitivity differences between detectors, failed detectors, and discontinuities at the segment boundaries. Image processing techniques can be used to optimize the visual interpretability of the image data. Calibration is performed to remove the streaks, and the data from each detector segment are processed to synthesize an image collected with a single continuous detector array. The image is also processed to remove any image motion effects, such as vehicle oscillation, and geometric distortions that might occur. The image data can also be processed to a specific mapping spatial resolution. The utility of an image should not be equated with quality of the image. For example, geographic surveys can be performed better with overhead images that trade‐off lower resolution for a larger area of coverage. The image acquired at the lower altitude has more detail that can be resolved on the ground, but much of the surrounding information is lost, which may be very important to understand the context of the objects on the ground.