smaller, the requirements of the optical system become more exact. Such problems as general system noise and, in particular, optical distortion become acute. There seems to be a lower limit of about 5-10 centimeters set by atmospheric scattering. Since there is a loss of detail as the size of the picture element is increased, the obvious answer is to have two sensors systems, one with a large field of vision and large picture elements for large areas survey and locating areas of interest and a second sensor system for small discrimination within smaller areas. To try to put all of this in perspective - without giving too much detail - let's look at a sketch of a single sensor for an earth satellite. The sensor's lenses continuously scan a strip of the planet at right angles to the motion of the satellite. The motion of the satellite determines the length, direction and speed of the scan. The strip scanned at any one instant will be broken down into picture elements. Thus a strip 185 km (100 miles) long might be broken into picture elements 15 meters long. Further, exposure time would be such that the satellite motion would be 15 meters for each exposure. Thus each exposure or reading of the sensor would sense a strip 185 km by 15 meters at a resolution of 15 meters square. The radiation coming through the lenses of the sensor would be broken by prisms or filters into the various spectral bands required. The spectral bands being chosen to SATELLITES AND COMPUTERS HELP MANAGE EARTH'S RESOURCES GREENBELT, MD - Operations Command and Control Center console for the ERTS-1 spacecraft at NASA's Goddard Spaceflight Center. The ERTS program is a first step in the merger of space and remote sensing technologies into a system devoted to developing the ability for more efficient management of Earth's resources. Design of the observatory based on the highly successful Nimbus Meteorological satellites which have regularly returned pictures of the Earth weather status since 1964. The ERTS observatory will operate in a polar orbit 900 kilometers (about 560 miles) above the Earth and return images from two independently functioning multispectral sensors. A data collection system onboard the observatory will gather environmental information from Earth-based platforms and relay this data to the ground processing facility, at NASA's Goddard Space Flight Center, Greenbelt, Maryland. Federal agencies participating with NASA in the ERTS-1 project are the Department of Agriculture, Commerce, lnterior, Defense and the Environmental Protection Agency. maximize the characterization of surface features of interest, while minimizing the number of such bands required. Each of the 4 or 5 such bands would have its own set of photodectors. Each set of photodectors would have 12333 elements. Each element would record one of 256 (8 bits) levels of radiation intensity. The digital output from each of the sets of dectors would be digitally compressed and encoded and then multiplexed with the signals from the other spectral bands. The digital compression reduces the volume of data by up to 60% but the encoding for error detection and correction adds back about 25% overhead to the reduced data. This multiplexed signal is then transmitted to earth receiving stations to be stored for later analysis. The analysis consists of filtering out signal noises, enhancing the desired images and identifying what has been scanned. These are all done by digital techniques. The area of the surface that was scanned becomes a matrix with each element of the matrix being the intensity output from one photodector. Since the atmospheric absorption is different for different wavelengths, the data from each spectral band is corrected differently for the atmospheric effects to that band. Digital enhancement continues with the elimination of as much blurring as possible and providing as much contrast as possible for objects of interest. The objects of interest vary from analysis to analysis. For example, one researcher may be interested in corn crops in a given area while another is interested in subsurface water and soil composition in the same area. While outlines may be detectable in several of the spectral bands it is the distinctive patterns across the different spectral bands that give the actual identification of objects. For example, it could be seen from any of the matrices that there was a one acre square in the middle of a much larger area. But by considering the different spectral patterns it could be identified as either a square island in a lake or a pond in a pasture or a poppy patch in the middle of a cornfield. This sort of analysis only tells about the physical characteristics within a small area. To reveal cultural levels and patterns, it is necessary to accumulate data into a larger picture. The amounts of artificial illumination and heat given off from an area can be checked to find a city and determine its general energy consumption. The comparison of the number of roads to the number of fields identifies an area as being primarily agricultural or industrial. The comparison of the number of forests to the number of cultivated fields helps to identify the level of agricultural development. These techniques can be done today, although not in real time as the Enterprise's sensors could. The Earth Resources Technology Satellite of 1972 (ERTS-1) had sensors of this general nature to be used for analysis of the earth's resources. The ERTS have the advantage of being able to calibrate their sensors and analysis by scanning known areas. However, when it comes to foreign planets the problems may be more difficult - for as the crew of the Enterprise often discovered, not all planets and civilizations developed in the same manner. THE AUTHOR Tom Kibler, Technology Editor of Creative Computing is Manager of Scientific Programming at the Computer Centen, Georgia State University where he is also a part-time instructor in information systems. Prior to coming to GSU in 1973, Tom was a designer and researcher for IBM and prior to that a consultant and systems programmer at UC, Berkeley and Stanford Univ.