On the wall of the palace of the President of Peru a few years ago, there hung a photo-map of the Andean and coastal regions of the country, the first small-scale photo-map Peru had ever had. President Belaunde explained that the map was “an indispensable aid for relating Peru’s complex physiography to the many problems associated with development projects.”

“This overview of my country,” he added, “clearly illustrates the very limited coastal plain, the dominant Andean mountain barrier, and the vast Amazon forest. These features show up on ordinary maps, but the satellite mosaic also shows the oil slicks and industrial pollution originating along the coastal strip and being spread by the wind and the offshore currents of the sea. You can see old avalanche scars on some of the Andean mountainsides, almost signaling a warning to the settlements in the valleys below. The windblown smoke of brush fires in the jungle shows which areas are being cleared for colonization. You just don’t see these things on ordinary maps, or even on existing black and white aerial photos which cover relatively limited areas.”

The map on the wall was a mosaic of 11 photographs taken from manned satellites orbiting the earth. The same map by aerial photography would have required extensive processing of several thousand photographs from about 50 flights made over a period of a year or more. Even then, it would have had serious defects.

Satellite photography may turn out to be a major technological breakthrough in resources surveys, an advance comparable to the introduction of aerial surveys before World War II. A typical use of satellite photo-maps is to guide more detailed measurements by aerial photography and ground survey. Timber producers in the southeastern United States, for example, stand to save millions of dollars as a result of an estimate by “multistage sampling” of the volume of timber in forests slated for cutting. Satellite photographs are used to select typical areas with different densities of timber for aerial and ground surveys. Measurements provided by these surveys make possible a much better estimate of the amount of timber to be cut, an improvement that will be reflected in more efficient dispatch of men and equipment to the cutting site.

Earth survey module envisaged by the National Aeronautics Space Administration of the United States

Satellite photography offers great advantages to less developed countries, which are in general poorly mapped and greatly in need of resources surveys. In many cases, satellite photographs provide small-scale maps where none existed before. On the other hand, much of the value of satellite photography is realized only after careful analysis by trained experts, often in conjunction with traditional methods of aerial and ground survey.

To take advantage of this new technology, developing countries must train and equip experts. Nations planning extensive surveys may want to build or share in national or regional stations to receive satellite data and convert it into useful photographs.

The United Nations Working Group on Remote Sensing of the Earth by Satellites will consider some of the technical and institutional problems raised by the earth resources satellites, once information has accumulated from the first experimental satellites. The Working Group was formed by the Scientific and Technical Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space.

A New Satellite Program for Earth Surveys

The first opportunity for developing countries to employ this technology in a systematic way will come from earth resources satellites about to be launched by the United States. Proposals for experiments have already been submitted by 19 developing countries. These range from isolated projects, like the location of locust breeding sites in the Middle East, to countrywide resources surveys in Brazil and Mexico, surveys that will involve both aircraft and satellites.

Some of these proposals were submitted by universities or aid-giving institutions in the developed countries; a few were submitted with the help of the World Bank, the UN Development Program, and the Food and Agriculture Organization. The World Bank and other international organizations are following these experiments with interest. A seminar held at the Bank last January provided much of the information for this article.¹

The first of these satellites, called ERTS-A (for Earth Resources Technology Satellite), is scheduled for launching in May or June 1972, with plans to conduct experiments with at least 22 other countries.² The second satellite, ERTS-B, is scheduled for launching November 1973. Satellites and instruments are being developed by the National Aeronautics and Space Agency (NASA), while applications of the data are the responsibility of the U. S. Department of the Interior and other user agencies. NASA is also planning an Earth Resources Experimental Package (EREP) as part of its manned Skylab program.

An Orbital Platform for Remote Sensing

“Remote sensor” is a generic term for a camera or other device that permits an object to be studied at a distance. Physical objects can be sensed through the electromagnetic radiation which they either emit or reflect. Visible light is one form of this electromagnetic radiation. Another form is infrared radiation, invisible to the human eye, but detectable by artificial equipment.

Two kinds of infrared radiation are important in remote sensing: near infrared, which originates from the sun (as does visible light) and is reflected toward the satellite by objects on earth; and thermal infrared, “heat rays” emitted to varying degrees by objects at different temperatures. Earth features—vegetation, water, soil, geothermal areas, etc.—can be distinguished by an analysis of the radiation coming from them. Each kind of surface has, so to speak, its own color fingerprint, or “spectral signature,” by which it can be recognized.

To record these spectral signatures, there will be three television-type cameras of a kind called return beam vidicon (RBV), on ERTS-A. They operate simultaneously and can record an image covering 10,000 square miles every 25 seconds. Each camera responds to light in a particular region of the spectrum, thus providing “multispectral” data. The first camera, operating in the blue-green spectral band, can penetrate into water. The red spectrum covered by the second camera can give useful information on land features. The third camera covers the near-infrared band, which can provide information on vegetation, particularly on plant vigor. The three cameras will be complemented by an experimental sensor called a multispectral scanner, which operates across four spectral bands. ERTS-B will add a band in order to be able to operate in the thermal infrared.

A satellite merely provides a platform from which remote sensing devices view the earth’s surface. The ERTS-A and ERTS-B satellites will employ a platform design based on that of the Nimbus weather satellites, which have operated successfully since 1964. The satellites will be launched into a near-polar orbit at an approximate altitude of 912 kilometers. This orbit will be sun-synchronous; that is, it will always obtain data over a given location on the earth at the same time of day, under constant conditions of light and shadow. This is a very important aid to data interpretation. The satellite is expected to have an operating life of about one year. It will make 14 revolutions a day and will complete its coverage of the globe every 17 days.

Three ground stations will receive the image data from the satellite and will pass it initially to NASA’s data processing facility at Goddard Space Flight Center near Washington, D. C. This center will process, store, and distribute the data in the form of images or coded magnetic tape to the international team of scientists participating in the program.

The data will also be available to anyone for purchase at cost from the U. S. Geological Survey’s EROS data center, which is being established in Sioux Falls, South Dakota. The EROS center will provide training programs on techniques for processing and analyzing the space-derived data.

Data collected on the far side of the globe will be stored on a tape recorder until it can be read out to a ground station. Until additional reception and processing facilities are constructed (Canada has one near completion), the capacity of the tape recorder limits the amount of data which the satellite can gather from these areas on any given orbit. Priority areas for coverage are the United States and international test sites proposed for coverage by the scientists participating in the program.

Color infrared photograph of the Imperial Valley-Salton Sea, California, U.S.A., taken by the Apollo 9 spacecraft on its 121st revolution of Earth. The layers of the film are sensitized so that areas of water and vegetation are easier to discern than in normal color photographs.

Color infrared photographs taken by satellite at high altitudes are used to obtain information on boundaries and ecological changes, etc. Their usefulness depends on expert interpretation.

“Ground Truth”

A satellite offers many advantages over ground and aerial surveys, possibly the most important being speed, the ability to obtain an overall view of a region, and repetitive coverage. The use of satellites should not, however, be thought of as replacing more traditional surveys; the various methods are complementary. Data from a satellite survey can be used to indicate where a more intensive survey on the ground or by aircraft might be useful. In addition, ground and aerial surveys are needed to help identify and calibrate satellite data. Supplementary information from such surveys aids in the interpretation of high-altitude images, and is often referred to as “ground truth.” A major part of this work is the identification of surface and subsurface features from their typical characteristics on satellite photographs. Greater knowledge of spectral signatures will increase the effectiveness and reduce the cost of using remote-sensing.

Satellite photographs are generally only an intermediate product, and their usefulness depends on expert interpretation. In general, four kinds of skills are needed:

• Data selection and evaluation. The pictures best suited for interpretation must be selected from the large amount of data available and must be checked for technical errors in transmission.

• Photo-interpretation. Modern techniques of multi-spectral photo-interpretation are used by trained geologists, hydrologists, and agricultural experts.

• Image enhancement. Inexpensive optical exercises, including color composites, bring out fine details of photographs.

• Ground truth data collection. This is a different skill from the others and requires separate training.

What Can We Expect?

What can we expect of remote sensing by satellites? While the ERTS program is described as experimental and some uncertainty exists as to how it will work in particular areas, quite a lot is already known about the capabilities of the satellite technique. Some of the projects proposed by the developed countries are research oriented, but most of the projects in developing countries have almost immediate applications.

The satellite is an excellent mapping tool, and photo-maps are richer in detail and more accurate and up to date than conventional maps. The satellite’s great speed permits it to photograph a strip 3,000 miles long and 100 miles wide in about ten minutes. These photographs can be assembled in minutes into a map which would have taken years to complete by earlier methods that would have required hundreds of aerial photographs, each taken under varying light conditions and cloud cover. Distortions, such as perspective displacement, are more severe with aerial than with satellite photographs.

ERTS-A photographs will be used to correct or update regional maps and will provide the first opportunity for detailed mapping of many remote or inaccessible regions. Such maps will be very useful, for example. In planning long highways and railroads. Projects to be conducted by scientists in France, the Federal Republic of Germany, and the United Kingdom aim at improving the techniques for mapping from satellite photographs.

Closely allied to its cartographic applications is the satellite’s capability in the mapping of land use and in land classification. Spacecraft imagery can assist in taking inventories of a country’s crops, areas with potential mineral deposits, water, snow cover, and other resources. Furthermore, the repetitive coverage makes it possible to watch changes over time. A uniform global resources inventory is now feasible, and is suggested by environmentalists as a move toward improved world-wide resources management.

There are numerous applications of spacecraft imagery to agriculture and forestry. Various crop inventories are to be conducted in several South American countries. Once the spectral signature of a normal crop has been established, variations due to changes in plant vigor show up in the spectrum of the reflected light.

In pictures taken from aircraft with infrared sensitive film, insect-infested trees appear blue, while healthy trees are red or pink. Often plant diseases can be identified through these methods even before they become visible from ground observations. An aerial remote sensor project to study coconut palm blight in India has indicated, for example, that the blight can be detected on infrared pictures as much as two years before it becomes evident to the naked eye.

Satellite imagery can also make a valuable contribution to geology. Volcanic activity and geothermal areas show up in the thermal infrared. Furthermore, the imagery shows up large-scale features, such as geological faults, that are not recognizable in either ground or aerial surveys, but are related to mineralization and major ore deposits. Such views of the overall geology have proved useful in selecting the best location for irrigation projects—from large dams to small wells.

The science of oceanography has always been plagued by the practical impossibility of obtaining, by conventional means, overall observations of the oceans, which cover about 70 per cent of the earth’s surface. Thermal infrared sensors, which will be available on ERTS-B and Skylab, will be capable of identifying ocean currents, discontinuities in temperature and salinity, the extent of pollution, and many other factors. Identification of more-or-less homogeneous areas will aid in the efficiency of ground sampling techniques, such as in the placement of pollution meters. Forms of pollution difficult to detect directly often give rise to smoke or algal blooms which can be seen with visible light on small-scale photographs.

Satellite imagery is particularly valuable in the study of shorelines and river deltas. Satellite-borne sensors registering blue-green light can penetrate water at least to some depth and show the configuration of the sea floor adjacent to the coast. The dynamics of estuaries can be nicely depicted; currents, the location of fresh and salt water, and the presence of oil and other pollutants can all be readily identified. Future development of the oceans as a source of food and minerals may be advanced by information acquired by satellite.

In addition, natural disasters may be averted or mitigated. Knowledge of preflood conditions, for example, may enable preventive actions to be taken in time. There can be earlier warnings of the need for evacuation because of approaching cyclones, typhoons, etc. Japan will use ERTS-A to study monsoon clouds and snow cover, and Norway will study ice movements to help predict the timing, magnitude, and direction of spring floods.

Changes in land use can be followed as they develop or can be planned with the aid of satellite information. Urban growth and demographic changes between censuses can be inferred, and suburban and industrial sites might be chosen where they minimize the loss of good agricultural land. Venezuela has an ERTS-A project focusing on urban and regional planning, and both Japan and Norway are planning studies on environmental quality.

The Next Step

Even though the ERTS program is officially labeled an experiment, countries in all stages of development should consider the possibility of using satellite data in their mapping and resources survey programs. Those wishing to participate in the ERTS-B and Skylab programs should file an application with NASA’s Office of International Affairs through their respective national space or scientific research organizations. The application, should state the proposed use of the data and the location to be covered.

Proposals are evaluated on the basis of scientific interest, the degree to which the proposed project will help to document the usefulness of the new techniques, the feasibility of the project, and the capability of the proposers to fund the data analysis. In order to take advantage of the data made available by NASA, facilities must be established and technical personnel trained for photo-interpretation, obtaining ground truth, and other processes needed to turn photographs and other sensor data into usable resource information.

It is not too soon to think about the long-term institutional and political problems that are certain to arise when the value of resources satellites has become better known around the world. Somewhat oversimplified, the questions boil down to:

• Who will receive the data and information from earth resources satellites?

• Who will pay for it?

• Who will manage the technical aspects of the operation: launch, data reception, photo-interpretation?

• What should be the role of the “user” countries, particularly the developing countries, in the formation of policies concerning earth resources satellites?

The initiative in developing the potential of earth resources satellites inevitably lies with the spacefaring powers, the United States and the U.S.S.R. (The U.S.S.R. has carried out earth resources experiments from manned and unmanned spacecraft and may conduct a program on the ERTS model.)

The other nations of the world, whose resources are by and large less adequately surveyed than are those of the two major space powers, have a strong stake in the institutional arrangements that are ultimately devised for planning and management of operational resources satellites. It may turn out that a system of multinational control is the best way to ensure that the interests of all nations are adequately represented.

The recent proposal for a UN data bank to receive and distribute satellite data is one partial approach to this question. Experience with international technical cooperation should be carefully evaluated. The question of cost must be fully considered (some tentative estimates are presented below). The relationship between participating governments and the technical management of the satellite system must be so arranged that the organization can respond to the needs of its less developed members and at the same time keep up with a rapidly changing technology.