can view virtually every part of the earth many times during the course of a year, and the record of those observations can, in most cases, be made quickly available.

The prospect of achieving a comprehensive view of terrain features, formations, geographic relationships, and surface patterns, which require the perspective of distance, is certainly an exciting one. For the first time, natural and cultural phenomena will be studied in their full regional context. Furthermore, a satellite will provide easy access to areas which are inaccessible to men on the surface, and not conveniently accessible using aircraft.

There are still other advantages. Observations can be made repetitively over long periods of time to record, for example, seasonal changes. Once aloft, unlike an aircraft, the satellite does not require power for propulsion in order to remain there. Thus, repeated surveys from space can be made at the relatively small incremental cost of ground station operation. Again, compared to an aircraft, a satellite provides an inherently more stabilized and vibration-free platform. The earth-orbiting satellite constitutes a most modern instrument available to mankind, a product of the electronics revolution and the space age. It has already proven to be both flexible and versatile in the collection of data. For certain purposes, it is the only effective way to gather information. For other purposes, it is the best, most economical way. In some respects observations of the earth from satellites will constitute an extension of the type of data now produced using aircraft, while in other respects, satellites will provide unique observation capabilities.

Just as observations from aircraft have not replaced men on the surface, so an earth resources satellite system will not do away with the need for remote sensing from aircraft, or personal investigation by men on the ground. Rather, a satellite system will be a new and powerful tool to supplement existing techniques.

Each method of data collection has its peculiar advantages and limitations. All three can make distinctive contributions, and together will constitute a complementary approach to a better understanding of the natural and cultural resources of the world.

Applicable Prior Developments

The use of spaceborne instruments to detect and measure the characteristics of natural and cultural phenomena on or near the surface of the earth would seem to be a logical development in the advancing state of the art of space technology. The color photographs of the earth produced during the Gemini program, for example, established the usefulness of visual observations from orbital altitudes. Those photographs, taken with a hand-held camera, contain a startling clarity of imagery and remarkable detail, in addition to showing large patterns and formations on the surface impossible to observe from a closer vantage point, such as aircraft altitudes.

Development and perfection of an impressive array of sophisticated remote sensing devices with capabilities quite distinct from photographic cameras has been underway for several years. For example, high-resolution television cameras and other sensors developed in the course of space programs such as Ranger, Surveyor, Lunar Orbiter, Tiros, and Nimbus have contributed significantly to the technical base upon which a research and development version of an earth

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resources satellite system can now be built. Moreover, a number of sophisticated new sensing instruments in various stages of evolution hold forth great promise for incorporation in later versions of ERS spacecraft.

Data-handling techniques developed in the many successful U.S. space programs to date provide high confidence that an earth resources satellite system is both feasible and practicable, despite the fact that collection, transmission, analysis, and dissemination of the vast quantities of information expected from an ultimate operational system will certainly present special problems, and will quite likely require the development of special techniques and equipment. In any case, high-speed computers are now available, and data processing techniques and equipment have experienced rapid development in recent years; and the trend can be expected to continue. In fact, the major difficulties may turn out not to be technical, but in achieving needed cooperation and coordination among interested parties. Remote Sensing

Remote sensing is the acquisition of information about objects or phenomena which are not in intimate contact with the data-gathering device. The technique is a modern development, as noted above, introduced in recent decades with the advent of aerial photography.

Photographs and television pictures taken from aircraft and satellites now play an important role in surveying the earth and its environment, and are destined to be increasingly useful in the future, even though they are limited to the visible light portion of the electromagnetic spectrum-a rather narrow band of wavelengths reflected by objects on the ground.

The electromagnetic spectrum has been conveniently divided into several bands. In addition to the visual wavelengths, techniques now available make it possible to produce imagery over a wide spectral range including ultraviolet, infrared, and microwave frequencies; wavelengths both shorter and longer than those which can be seen by the human eye.

Remote sensing of the earth's resources is made possible by virtue. of the fact that every object on the surface, every feature of the terrain, absorbs, reflects, and emits electromagnetic energy at specific distinctive wavelengths, most of which are not in the visible range. These spectral characteristics, when collected, compared, and analyzed, make it possible to distinguish objects, one from another, and furnish information relating to size, shape, density, and other physical and chemical properties.

The use of photographs or video pictures for such purposes is well established, and visual images have proven to be powerful tools in the hands of scientists concerned with the natural environment of the earth. Now, it has become feasible to undertake development of devices which, when placed in earth orbit, will detect radiated energy at frequencies outside the visual wavelengths.

There are a number of advantages to be gained by resorting to various portions of the spectrum in addition to the visual. To begin with, some objects will appear clearly at one frequency, but not be discernible at another. Simultaneous images in various spectral bands may be compared, making it possible to discriminate between objects or phenomena which exhibit no apparent differences in the visual range alone. Images from one spectral region may also be combined

with those from another spectral region, and a single composite image can thus be obtained representing an optimum "picture" for certain analytical purposes.

Second, new types of information not obtainable from visual data can be provided; for example, infrared sensors are capable of producing thermal maps. Since all materials emit heat radiation at various intensities, temperature gradients can be very important parameters for certain studies; for example, monitoring volcanic activity, water pollution, and ocean currents.

Finally, longer wavelengths such as microwaves are not obscured by clouds, as are visual images; and like infrared, microwave sensors can be used at night as well as during the day, thereby producing continuous information around the clock. By contrast, visual images are entirely dependent upon reflected sunlight, or a substitute for sunlight, and are at the mercy of the weather.

It is recognized, of course, that much remains to be learned regarding the "spectral signatures" of various objects on the earth's surface. Energy absorption, emissivity, and reflectivity may vary widely with climate and weather, as well as the seasons of the year. A given object, therefore, may have a variety of "spectral signatures."

Additional complexities are introduced by the fact that the earth's atmosphere, itself a variable absorber, reflector, and scatterer of energy at different wavelengths, will impose severe limitations on certain spaceborne sensors' capabilities to observe objects on the ground. Only those instruments will be useful which are designed to sense in the particular spectral regions where the atmospheric medium permits transmission of energy from the surface to the satellite at least some of the time.

Satellite Systems

There are three basic approaches to the problem of deriving earth resources data from an orbiting satellite. The first would require the placing of automatic sensors at selected places on the earth's surface, on buoys at sea, or on balloons aloft in the atmosphere. As the satellite passes overhead it queries these remote sensors, stores the information in a magnetic tape recorder, and then transmits it to a central receiving station on the ground. In essence, such a satellite would constitute a data collection and relay device and should, therefore, be viewed as a kind of specialized communications satellite.

A second technique involves a satellite equipped with a camera and photographic film. Once exposed, the film must be returned to the ground physically by means of a reentry capsule. Recent advances in both film and camera systems have made possible extremely high resolution data from such a system. Replenishment of film in orbit, of course, requires redundant launches making this technique very expensive.

The third approach to remote sensing from space involves a satellite equipped with television cameras and other sensors, the data from which are transmitted electronically back to a receiving station on earth. This system avoids the expense and inconvenience of physical recovery of film packs as well as replenishment of film in orbit. While television pictures cannot achieve as high resolution as photographs, recent developments make possible sufficiently high resolution for most purposes.

All three systems may some day prove to be useful in an operational earth resources survey program, and each may play a role economically justified by its peculiar capabilities. It is even conceivable that a single satellite system might some day combine the best features of all three.

The initial program proposed by NASA, however, and recommended by representatives of industry and user agencies would undertake development of a satellite of the third category-a spacecraft equipped with electronic sensors which would remain in orbit.

EARTH RESOURCES DATA USERS

Scientists working in six key areas are generally considered to be the most important potential users of earth resources data obtained from space. At this early stage, there is uncertainty as to just where the greatest potential lies. Yet, enough is known based upon experiments performed in aircraft and from data already produced by successful space programs such as Tiros, Nimbus, and Gemini to justify a sense of high confidence in the efficacy of observing the surface of the earth from spacecraft in orbit, and producing data of genuine utility.

As the 1967 interim report of the National Academy of Sciences summer study concluded:

As urban areas of regional extent expand and multiply in the United States, the pressure upon the resources of field, forest, and waters will greatly increase. Effective management of all types of land will become imperative. We believe that the satellite as an instrument of direct observation and data collection could have a revolutionary impact on land-use management and resource utilization. Cartography

The requirement for current, accurate maps is not open to dispute. Geographers are concerned with the location, arrangement, and association of earth features such as rivers and mountain ranges, cultural patterns such as population concentrations; in short, a wide range of natural phenomena and human activities for which maps are indispensable. Foresters and agriculturists need maps for regional land use, inventory, and planning. Many other examples might be cited. Suffice to say that good maps have proven fundamental to all modern resource investigations and assessments.

Mapmaking is an ancient art, and cartographers are constantly searching for better, quicker, and more accurate ways to ply their trade. Of all the techniques at their disposal, aerial photography presently provides the best means of obtaining small-scale maps of relatively large areas. Nevertheless, the U.S. Geological Survey reports that the compilation of small-scale maps by current practices is a slow, laborious process of assembling thousands of observations and subjecting them to photogrammetric processes. As a result, smallscale maps available today are neither uniform nor timely. In fact, experts have stated that 70 percent of the world's small-scale maps are considered inadequate, while the remaining 30 percent are deemed obsolete. Even in this country where aerial photography is done on a more systematic and regular basis than by most other nations of the world, some areas of the United States have not been photographed in 20 years, and the most modern of our national land maps are reportedly at least 10 years out of date. While these may be excellent historical documents, the needs of current planning remain unfulfilled. In terms of regional and national surveys, it must be concluded that our present techniques are simply inadequate.

Fortunately, America's space program has provided the means whereby cartography can become both efficient and expeditious. At present, it takes many years to assemble the thousands of aerial photographs into a mosaic of a large region. Approximately 1 million such photographs would be required to make a photomosaic of an area the size of the United States, and it would cost $60 million to rectify these photographs orthographically. From satellite altitudes, such a photomosaic of the United States would require only 400 pictures and could be assembled in a few weeks.

Even more important is the fact that pictures taken from orbital altitudes with relatively narrow-angle viewing systems will be nearly orthographic; that is, the geometrical distortions will be minimal. Therefore, these images will not require the complex, time-consuming, and expensive photogrammetric processing of the type needed to process aerial photographs whose distortions, due to the use of wideangle lenses, must be corrected.

Placed in an appropriate sun-synchronous orbit, a satellite is capable of producing pictures of the earth under virtually constant lighting conditions, whereas aerial photographs typically are not uniform in shadow patterns due to changes in the angle of illumination as the earth rotates beneath the aircraft. In addition, repetitive pictures obtained from an orbiting spacecraft will record changes in the dynamic features and processes of the surface-a capability that is entirely impracticable and uneconomical using aircraft.

Not only will satellite pictures be geometrically superior to aircraft photographs, they will be more uniform as well, thereby opening the door to automatic processing and interpretation techniques that are difficult or impossible to utilize working with aerial observations. Moreover, the Gemini photographs demonstrate that images recorded at orbital altitudes frequently contain greater detail than aerial photographs, much to the surprise of many interpreters.

All in all, small-scale mapmaking can be done much more accurately and economically in the future using satellites. In point of fact, such a space system apparently provides the only practicable way to produce up-to-date, small-scale maps of large regions; and its capabilities are global.

Agriculture and Forestry

Conventional aerial photography has been used extensively by the Department of Agriculture in surveys of land use and land capability. Officials of the Department believe that remote sensing from orbiting satellites is now technically feasible and is certain to yield considerable quantities of valuable agricultural data.

Visual photographic interpretation techniques are well established. Different crops and different species of trees frequently cannot be distinguished, however, when viewed remotely in the visual portion of the spectrum. Photographs taken from aircraft reveal that the various species tend to blend in together. Tone and texture differences are revealed, on the other hand, when visual images are examined in combination with images produced by sensors using other frequencies, and various species and varieties of plantlife can thus be identified and distinguished. Healthy crops and trees can also be distinguished from diseased or infected ones using multispectral scanning techniques because they reflect or emit radiation differently. Devices sensitive to