High Resolution

A satellite in orbit around the Earth with onboard sensors tirelessly and continuously collecting data. Some satellites take pictures of the Earth at a spectacular level of detail. The high-resolution camera: <i>I know what it is, I know what it does, but how does it work?</i>

This issue of GIM International presents a Product Survey on high-resolution satellite imagery. The first permanent, earth-observation satellite today known as Landsat 1 was launched 34 years ago. With a ground resolution of 80 metres, the Landsat Multispectral Scanner images provided an astonishingly comprehensive and panoramic view of many areas of the Earth never before mapped. The ground resolution of the US Landsat family has subsequently been improved, first to 30 metres with the launch of Landsat 3, and then to 15 metres with Landsat 7 ETM. Nearly twenty years ago, in 1986 to be precise, the French/European SPOT (Satellite Pour l’Observation de la Terre) satellite was put into orbit.

Since 2000 a number of commercial satellites have been in orbit producing Very High-Resolution (VHR) images: these are SPOT 5, Ikonos and Quickbird. VHR means better than 5-metre ground resolution, sometimes also called Ground Sample Distance (GSD). The SPOT 5, Ikonos and Quickbird sensors record panchromatic (black and white) and multispectral images; ground resolution in the panchromatic mode being respectively 5m, 1m and 61cm. The resolution of the panchromatic band is typically four times better than that of the multispectral bands; the ground resolution of the Ikonos multispectral images is, for example, 4 metre, whilst this is 2.44 metre for Quickbird multispectral images. The recently created company GeoEye, a merger of Orbimage and Space Imaging, plans in the first quarter of 2007 to launch the GeoEye-1 satellite, which will have a resolution of 0.41 metre in the panchromatic mode and 1.64 metre in the multispectral mode. The satellite will be able to acquire images of about 700,000km² per day (see interview GeoEye). Typical orbiting alti-tudes are around 700 kilometres above the surface of the Earth. How is it possible from such distances to derive images of the surface of the Earth of such fine resolution that individual houses and even cars in the streets can be identified with the naked eye? The answer lies in the use of very advanced camera systems. How do such camera systems work?

In answer, we can take a closer look at the Ikonos (a word of Greek origin, meaning image) camera, the world’s first commercial, very high-resolution imaging satellite. Launched in September 1999 following launch failure of Ikonos 1, Ikonos 2 started to produce its first images in the year 2000. The Ikonos telescope has the equivalent resolving power of a 10-metre (10,000mm) telephoto lens, as compared to a normal camera lens of 35mm. The telescope features three, concave mirrors each precisely configured to capture and focus the electromagnetic waves reflected by the Earth’s surface onto imaging sensors at the focal plane. Two additional flat mirrors ‘fold’ the imagery across the inside of the tele-scope. This significantly reduces the focal length, saving on size and weight. The total weight of the camera system is 171kg and the assembly size of the optical telescope 1524mm in length and 787mm in width. The three-mirror anastigmat design results in an amazing focal length/focal ratio: 10,000mm/f14.3. The dimensions of the primary mirror are 0.7m diameter x 100mm thick, with weight of 13.4kg. Images are processed by two sensors: (1) a panchromatic sensor consisting of 13,500 pixels, each with a size of 12micron, and (2) a multispectral sensor consisting of 3,375 pixels each of 48micron.