Many kinds of observations can be made from satellites, including military reconnaissance, terrain mapping, astronomical photography, international inspection, cloud observation, and photography of the Earth-useful in the Earth sciences.
Appreciation of the value of such satellites must depend upon some understanding of the kinds of obtainable information, the problems in obtaining information, and the ways of getting this information back from the satellite.
As soon as man could get a better view from whatever height he had access to, he climbed and used his eyes. When photography became a practical tool-about 100 years ago-he started using cameras from towers, mountaintops, and balloons, and later from rockets and air-planes.
Nadar 1 (1820-1910), a famous French photographer, was a pioneer in aerial photograph. In 1858 he started the photographic balloon ascents described in his book, Les Memoires du Geánt (1864) . Nadar's views on military applications of balloon reconnaissance changed from a refusal to work for Napoleon III in 1859 to active participation as commander of the balloon corps during the siege of Paris (1870-71).
In 1860 a J. W. Black of Boston joined a "Professor" Sam A. King, a well-known aerialist, to take a balloon photograph of Boston from an altitude of 1,200 feet. This was, for many years, widely regarded as the most successful aerial photograph on record. Oliver Wendell Holmes immortalized this photo with the phrase "Boston as the eagle and the wild goose see it."
Gen. George B. McClellan used balloon photographs in several Civil War battles (1862). He made huge maps, superimposed grids on these maps, and furnished telegraph connection between division head-quarters and the balloon-borne observer, anticipating by about 80 years the role of aerial observers for artillery adjustment.
All of the early balloon photographers had rather small perspectives compared with an American named George Lawrence who started doing aerial photography from balloons in the early 1900's. This remarkable man devised various cameras weighing more than a thousand pounds, taking pictures as large as 4 by 8 feet, and successfully raised them by means of balloons, kites, and associated control apparatus to heights of several thousand feet. One of his earliest cameras was a panoramic camera of the type now proposed for lunar reconnaissance and useful also in observation satellites. 2
1 Pseudonym for Gaspard Felix Tournachon.
2 Davies, M. E., A Photographic System for Close-up Lunar Exploration, The RAND Corp., Research Memorandum RM-2183 (ASTIA No. 156021), May 28,1958.
As soon as airplanes were thought practical and safe, photographs were taken from them. In World War I, and more extensively in World War II, photographs were major tools of reconnaissance and intelligence.
Photography from rockets is not a new phototechnique. At a meeting in Stuttgart in 1906, A. Bujard presented a paper, Rockets in the Service of Photography, describing the work of Alfred Maul, who wanted to use camera-carrying rockets for military reconnaissance. 3 He started with a camera taking pictures 40 millimeters square (the same size as the picture taken by a miniature Rolliflex camera). In spite of many difficulties he devised in 1912 a rocket stabilized by a gyroscope, with a takeoff weight of 92.5 pounds. This rocket carried an 8- by 10-inch camera to about 2,600 feet. By this time, however, the airplane was coming into its own, and photos from airplanes were easily made. The success of airplanes caused loss of interest in rockets as photographic platforms. We are now witnessing, in current interest in observation satellites, a return swing of the pendulum.
In satisfying both national and military intelligence requirements, data are needed on a real or possible enemy's capabilities and intentions. Included in the data is information that will help answer such questions as: What does the enemy have! Where is it? How many does he haves How does he use it? How good is he at using it? Is he about to use it? etc. Further needs are for mapping, charting, and weather reconnaissance.
Data which ultimately can be used to produce intelligence can and do come from many sources of varied character.4 Reconnaissance is one possible source. Spaceborne reconnaissance will be valuable because of the extremely large areas that can be covered rapidly, its objective nature, and the possibility of cyclic operation.
Observation can be carried out in many ways, using sensors operating in different portions of the electromagnetic spectrum. The first sensor used by man was the unaided eye. Next came photography with its ability to record in permanent form large amounts of detailed information. Then followed the development of radar reconnaissance, electronic interception, and infrared reconnaissance. 5
For reconnaissance in peacetime the all-weather requirements normally associated with combat operations can usually be waived. Because ground objects of military interest are, in some important eases, growing smaller (e. g., missile sites compared with airfields), there is a growing requirement for very refined detail in reconnaissance. These considerations point to photography as the most likely tool for satellite operations.
3 Ley W., Rockets, Missiles, and Space Travel, revised edition, Viking, New York, 1957.
4 Katz, A. H., Contributions to the Theory and Mechanics of Photo-Interpretation From Vertical and Oblique Photographs, Photogrammetric Engineering, vol. XVI, No, 3, June 1950, pa. 339-386.
5 See Footnote 4.
Various factors enter into an estimate of the degree of detail that can be detected or identified 6 by a visual sensor system. Among these are distance from the sensor to the object viewed and the focal length of the viewing lens, from which is usually computed a scale number. The scale number, S. is the ratio of altitude to focal length:
For example, consider a camera with a focal length of 6 inches at an altitude of 150 miles. The scale number of the photograph is then:
This means that 1 inch on the photograph corresponds to Sv inches on the ground, or about 25 miles. In general, the larger the scale number, the harder it is to see fine detail. For pictures taken other than straight down, scale numbers vary from point to point, getting larger as the view moves toward the horizon. 7 8
Another useful parameter is resolution, a term originally used by astronomers to specify the ability of a telescope to separate double stars. As applied to photographs, resolution refers to the ability of a film-lens combination to render barely distinguishable a standard pattern consisting of black and white lines. When a lens-film combination is said to yield a resolution of 10 lines per millimeter, it means that it can make distinguishable lines and spaces when there are 10 of them per millimeter. Lines of coarser spacing are seen more clearly. There are limitations on the usefulness of this single parameter for analytical purposes, but it is a convenient basis for gross comparison of sensing systems. 9-18 Ground resolution, R is often used in discussing performance. It is simply the ground-size equivalent to one line, at the limit of resolution.
6 A common view held by experienced workers In aerial photography is that detection is much easier than identification.. For example, one might be able to detect objects on a road, and be unable to decide whether they are trucks, tanks, or smaller vehicles, It usually requires roughly five times better ground resolution to identify objects.
7 See footnote 4, p. 172.8 Katz, A. H., The Calculus of Scale, Photogrammetric Engineering, vol. XVIII, No. 1, March 1952, pp. 63-78.
9 See footnotes 2, p. 171: 3-4, p. 172: 6 and 8.
10 Brook, G. C., Physical Aspects of Air Photography, Longmans, Green & Co., New York, 1952.
11 Schade, Otto, Electro-Optical Characteristics of Television Systems, RCA Review, vol. IX, Nos. 1-4, March-December 1948.
12 Higgins, G. C., and R. N. Wolfe, The Relation of Definition to Sharpness and Resolving Power In a Photographic System, Journal of the Optical Society of America, vol. XLV; No. 2, February 1955.
13 Higgins, G. C., and R. N. Wolfe, The Role of Resolving Power and Acutance in Photographic Definition, Journal of the Society of Motion Picture and Television Engineers, vol. LXV, No. 1, January 1956, pp. 26-30.
14 MacDonald, D., Some Considerations of Resolution, Sharpness, and Picture Quality In Technical Photography, Photographic Society of America Journal, vol. XIX, No. 5, May 1953 pp. 49-55.
15 MacDonald; D., Air Photography, Journal of the Optical Society of America, vol. XLIII, No. 4, April 1953, pp. 290-298.
16 Optical Image Evaluation, proceedings of the NBS Semicentennial Symposium on Optical Image Evaluation held at the NBS In October 1951, National Bureau of Standards Circular 526, April 1954.
17 Rose, A., Television Pickup Tubes and the Problem of Vision, Advances In Electronics, vol. I, 1948, pp. 131-165.
18 Katz, A. H., Air Force Photography, Photogrammetric Engineering, vol. XIV, No. 4 December 1948, pp. 584-690.
Thus, if a film-lens combination yields X lines per millimeter, and the scale number is S, the ground resolution is S/R. To use familiar units, and rounding off a bit-
For the same focal length and altitude used in the example above, if the film resolution is 100 lines per millimeter, the ground resolution is about-
From the formula for ground resolution one would expect to obtain the same ground resolution by trading resolution and scale number. Thus one should expect that 10 lines per millimeter at a scale number of 100,000 should yield the same ground resolution as 100 lines per millimeter at a scale number of 1 million. However, this type of reciprocity is never the case, either in practice or in theory-if one can trade scale for resolution in a design he should trade in the direction of lower resolution and smaller scale number. There are great differences in the graininess characteristics of different aerial photographic emulsions, and these affect interpretability much more than they influence resolution.
It is convenient to define four levels of photographic detail: A, B, C, and D. These levels are, in terms of ground resolution:
Photographic operations at resolution level A would be useful in covering large areas, measured in millions of square miles. From such pictures one should be able to see and identify most lines of communications, railroads, highways, canals, urban centers, industrial areas, airfields, naval facilities, seaport areas, and the like.
Level B would be appropriate to covering areas measured in hundreds of thousands of square miles. With reconnaissance detail, the character of many major installations could be detected and identified; aircraft could be seen on airfields; almost all lines of communication could be found and plotted; and, in general, items merely detected at level A could be seen more satisfactorily. It would be desirable, of course, to eliminate category A and cover millions of square miles at resolution level B.
Level C: is indicated for areas of specific interest measured in terms of hundreds of square miles. At this level, extremely detailed analyses of sites, airfields, industries, and activities could be made. Most World War II photography was accomplished at this level of detail.
Level D is appropriate for rather small areas, say, of the order of 1 square mile. Such operation would provide data in very fine detail about new activities, sites, and installations.
A reasonable concept of satellite operation would cover all areas of interest at level A (or, preferably, level B) at intervals of perhaps 6 months to a year. With such an operation, new major installations could be detected, perhaps patterns of use found, and hints and clues obtained for the direction of other, higher resolution reconnaissance systems. An overall observation capability, whether for reconnaissance or inspection, should be based upon a family of systems able to operate at each of the levels indicated.
It is extremely difficult if not impossible, to take a given number-such as a ground resolution of say, 80 feet-and describe specifically what can be seen. The conditions of observation-the illumination, the contrast, the context, and many other important factors that determine detection and identification, are so variable that specification of ground resolution alone is insufficient. For example, rocket photographs are available that were taken with a 6-inch lens from an altitude of about 150 miles, with resolution estimated to be about 10 lines per millimeter-indicating a ground resolution of about 500 feet. 19 This is much poorer than level A. On one of these photographs (fig. 1), however, major railroads show up clearly (fig. 2) to even casual observation; two major airfields are easily seen, with runways clearly distinguishable; and major streets in a nearby city are fairly easily resolved and can be plotted. (This is an example of the phenomenon of long lines being more easily detected than small square objects.) The only thing that is really clear is that small ground resolution is better than large ground resolution-much more detail can be seen with a system yielding 10-foot ground resolution than one yielding but 50-foot ground resolution.
It is realistic to suppose than one can find large installations in the process of construction by using systems performing at level A or B. At such levels it is certainly possible to get clues of sufficient interest to warrant dispatching a system operating at levels C and D to verify, confirm, or further inspect these operations.
Observation at level A is a fundamental and first-thing-first job. It could supply a matrix in which other data can be imbedded, and it could furnish a planning guide for future reconnaissance.
19 Baumann, R. C., and L. Wunkler, Rocket Research Report XVIIl-Photography From the Viking XI Rocket, NRL Report 4489. Naval Research Laboratory, February 1, 1955.
This photograph, made by the Naval Research Laboratory, was taken from an altitude of 158 miles by a K-25 camera mounted in the Viking II rocket, on May 24, 1954. Film used was infrared, shutter speed was 1/500 sec, at f/8, with a red filter. The Ilex Paragon Anastigmat f/4.5 lens has a 163-mm focal length. Scale around the airports is about 1.5 x 106, with a (calculated) ground resolution of about 500 ft. The fact that airfields and railroad lines are resolved illustrates the caution with which calculations of ground resolution must be viewed. NOTE: The print was made by copying a print made from the original negative. Prints made from the original negative are considerably clearer. ADDENDA: The photo quality presented in this work was further degraded. The original text was printed on velum. Text lines which showed through from the back on white sections were removed by a combination of masks and digital filters. Finally, The photo was stored as a gif with a resolution of 75 dots per inch.
The camera used to make the photograph of figure 1 is now considered obsolete. Lens designs, camera precision, and film characteristics have all improved since the time of its construction. New types of cameras, in particular the shutterless, continuous-strip camera and panoramic cameras have emerged as important and useful tools. 20-22
Modern panoramic cameras are derivatives of a rather old camera design, wherein a lens, which itself covers only a narrow angle, sweeps out a wide-angle photograph by sequentially scanning the scene and depositing the resultant image on a relatively long but narrow strip of film. A basic characteristic of strip or panoramic cameras is the fact that the photograph is taken sequentially, and possible distortions thus produced make precise measurements extremely difficult. Cameras with between-lens shutters avoid this difficulty, and are universally used for mappings. 23
Increased speed of modern aircraft has motivated development of methods for compensating for image speed during exposure24 Image speeds, for cameras mounted vertically in a satellite, may be readily calculated. At an altitude of 300 miles, for example, satellite velocity is about 25,000 feet second. The image speed is simply the vehicle speed divided by the scale number. As an example, consider a 6-inch focal-length camera at 300 miles altitude. The scale number is about 3 million. The image speed is therefore about 0.1 inch per second.
To achieve high resolution-say 100 lines per millimeter-the blur caused by uncompensated image motion must be restricted to less than about one-two-hundredths millimeter.25 This can be accomplished by a combination of image-motion compensation and short exposure time.
If sensitive film emulsion or extremely high-speed optics are used, exposure times in satellite observation of the order of one five-thousandths second are not unreasonable. Clearly, with a very short exposure time, blur is minimized. Even the high satellite speed of 25,000 feet per second will yield a blur corresponding to only about 2½ feet on the ground, if exposure time is one five-thousandths second.
There has also been progress in lens design. Camera lenses with focal lengths of 100 and 240 inches have been developed by the Air Force. 26 Such lenses can be expected to find eventual utility in satellite observation systems to obtain very fine ground resolution.
An idea of what can be seen with comparatively small ground resolution can be gained from figure 3, with a calculated ground resolution of 48 feet. This photograph was taken from a balloon at an altitude of 87,000 feet. The camera used film 70 millimeters wide, and the lens had a focal length of about 1.5 inches. The scale number is 700,000. This photograph covers an area approximately 24.9 miles on a side (620 square miles). Objects such as dam sites, railroads, farm
20 See footnote 18. p. 173.
21 Katz, A. H., Aerial Photographic Equipment and Applications to Reconnaissance, Journal of the Optical Society of America, vol. XXXVIII, No. 1, July 1948, pp. 604-610.
22 Goddard, G. W., New Developments for Aerial Reconnaissance, Photogrammetric Engineering, vol. XV, No. 1, March 1949, pp. 51-72.
23 Katz A. H., Camera Shutters, Journal of the Optical Society of America vol. XXXIV No. 1, January 1949, pp. 1-22.
24 See footnotes 10 and 18, p. 173.
25 See footnote 23.
26 See footnote 22.
ponds, and highways are easily distinguishable, as are some of the larger structures in the town of Truth or Consequences, N. Mex. Agricultural patterns too are readily distinguishable. This experiment in high-altitude photography is described in the reference noted below. 27
The following table of approximate resolutions and focal lengths for a satellite at about 150 miles altitude is of interest, especially when comparing the calculated numbers with those applicable to figure 3 (ground resolution of 48 feet) .
27 DiPentima, A. F., High Altitude Small-Scale Aerial Photography, Rome Air Development Center, Technical Note 58-165, ASTIA Document No. AD-148777, July 1958.
|Focal length (inches):||Scale number||Ground resolution (feet) At 40 lines per millimeter||Ground resolution (feet) At 100 lines per milli meter|
Comparison of this table with the actual photograph of figure 3 is striking evidence of the ability of satellites to secure useful photography. Mapping photography from 30,000 feet during World War II secured a ground resolution of perhaps 15 to 20 feet. Thus, satellite performance possibilities are very attractive in these terms also. Throughout this discussion, reference has been to photographic film systems for consistency of illustration only. Television techniques are also entirely applicable to use in observation satellites, and the concepts, formulas, and calculations presented apply also to television systems.
Photographic film exposed in a camera aboard a satellite, like film exposed in any camera, would have to be processed before the data can be used. Conventional automatic processing techniques could be used, with the film being stored until the satellite is in the vicinity of a ground receiving station. It could then be scanned, using television techniques, and the data transmitted to Earth. Received on Earth, it could be stored as a video tape, or reconstituted into a photographic record for interpretation, study, and analysis at a suitable time and place.
If a television camera system were used, a similar problem would arise. The data would have to be stored-on magnetic type, for example-to be read out and transmitted to a ground station at an appropriate time.
Photographic film could also be returned to Earth directly for processing. The use of television techniques in the case of physical recovery of data is not clear.
There are two distinct ways in which information picked up by a sensor in a satellite can be returned to Earth: it can be transmitted by radio or physically returned to Earth in a data capsule.
Choice of the method of return depends upon such factors as the requirements for timeliness, the rate-of information flow, the total volume of information, and the form in which the information is to be used. There certainly may be military reconnaissance or international inspection operations which require minimum time delay between observation and transmission of data. An obvious example is a satellite system to observe missile firings as part of an international system for warning of surprise attack. The useful life of such information is short—a matter of very few minutes-and direct radio transmission of data is rather clearly indicated.
Inspection of large areas, or detailed examination of smaller areas, may result in the collection of volumes of data so large as to saturate the radio transmission capability of any reasonable satellite. Physical recovery might well be a better way to deliver such data. Mapping, where the principal concern is with geometric fidelity, is another case where physical data recovery may be better than radio transmission.
Preliminary to an attempt to assess the relative merits of either of these recovery methods, it is necessary to reconcile the many ways of estimating the information content of a photograph. From the viewpoint of the communications engineer, a photograph represents a calculable number of "bits" of information; this number permits calculation of the capacity of the communication channel and the time required to transmit the photograph.
The Photointerpreter tends to evaluate the photograph in terms of the useful detail he can find and the extent of the coverage. The aerial photographic scientist will think in terms of scale, resolution, and area covered.28
The relationships between these viewpoints permit some valuable comparisons to be made. It is specifically interesting to consider the way in which one can compare a photograph and television image for the purpose at hand. A high quality live television transmission system will use a communication band width of about 6 megacycles per second. A high-quality photograph will have a resolution of about 100 lines per millimeter. Information theory shows that A communication system with a band width of 6 megacycles per second will have to operate continuously for 22.5 minutes to transmit the quantity of information that can be stored on a single 9- by 9-inch photograph at 100 lines per millimeter.
Satellite systems can readily be postulated that would collect in 1 day many times more data than could be transmitted by video link. For example, consider a system with a 36-inch focal length (this is a median example: for mapping much shorter lenses could be used; for extremely detailed observation, much longer lenses would be needed) which covers (either through single lens panoramic techniques or through a conventional multiple camera installation) an angle of about 90° from about 150 miles altitude. Such a system could cover about 3 million square miles per day, and consume at least 1,500 feet of 9-inch wide film. If a resolution level of 100 lines per millimeter is obtained, this amount of information would require over 1 month to transmit, operating 24 hours per day. Such a photographic system seems to be well within the state of the art. Systems with larger capacity are conceivable and may prove desirable.
Clearly there are important cases where a video link simply cannot transmit information as fast as it can be acquired. Small improvements in transmission systems, increasing the numbers of ground stations, etc., can make only minor improvements in a serious and fundamental limitation. This is one of the main reasons for the importance of physical recovery of a film payload.
28 See footnote 4, p. 172.
The problem of mapping is radically different from most other kinds of observation. Ground resolution, however calculated, is a poor statistic by which to measure the performance of a mapping system. To map a surface with geometric accuracy, "hard photography" free of distortions is needed. Mapmakers have generally required wide-angle photography with a highly corrected, essentially distortionless lens, the entire photograph being taken simultaneously (that is, with a between-lens shutter) instead of sequentially as with focal-plane shutters, strip photography, and panoramic photography. 29 Distortionless wide-angle optics, capable of yielding 50 lines per millimeter on film over the entire field, are now available.
While the requirements of mapping are very stringent with respect to distortion, they are rather modest as regards ground resolution. The distance between 2 points may be measured on a photographic plate to within an error perhaps 10 times less than the resolution figure. For example, a system with a ground resolution of 2,500 feet could yield a mapping precision of about 250 feet. Such resolution can be obtained, for example, from an altitude of 4,000 miles with focal length of 6 inches, using 50-lines-per-millimeter film.
There are mans other problems associated with mapping. In particular, the use of film instead of glass plates (as would be necessary in satellites) imposes the requirement for careful registration marks in the form of a reseau or fine grid on the plate, carefully calculated and fiducial marks along the edges of the format. It is felt that techniques such as these can yield precision measures. Measurements to 2 seconds of arc should be achievable.
These numbers can be used to go up and down the scales of altitudes and focal lengths. For example, coming in to an altitude of 1,000 miles with a 6-inch lens gives numbers 4 times better than those given above. homing in to 1,000 miles with a 12-inch lens (not unreasonable for a mapping satellite) gives numbers 2 times better than these. Thus, in principle, we could measure to perhaps 30 feet. The speed in orbit of a satellite at 300-mile altitude is about 25,000 feet per second. An exposure time of one five-hundredth second implies a maximum blur corresponding to 50 feet on the ground. At much greater altitudes, the orbital speed is even less. This amount of blur could be ignored or, alternatively, a very simple and fixed amount of image motion compensation could be applied to the film during the exposure to eliminate this blur.
The flat-Earth approximation would yield a ground coverage for the conventional mapping angle of 76° fore and aft of 11/2 times the altitude; since the Earth is curved, the coverage is in excess of this. It would not take very many pictures from altitudes of 1,000 miles or more to map the Earth successfully.
It is important to recognize that we can measure distances with accuracy greater, in principle, than that specified by the resolution limit as calculated above. Furthermore, excellent modern wide-angle optics are available.
A mapping satellite would likely require a platform stabilized with respect to the horizons. It would require recovery. It is mandatory
29 See footnotes 4, p. 172; 16, p. 173; 23, p. 178.
to return a precision photograph directly for examination rather than to incur possible geometric degradation via an electronic relay station. Auxiliary ground tracking stations, and perhaps visual and electronic beacons aboard the satellite, might be required and useful to locate the satellite precisely, as might simultaneous star photography for precision determination of the satellite attitude at moments of exposure.
In general, the fewer photographs which have to be tied together to map a given area the better. Thus, with mapping lenses capable of photographing approximately a 1,500- by 1,500-mile square from 1,000 miles altitude, the number of photographs required to map even huge areas is small. If the photographic sequence starts in an area which is mapped, the map can be extended to the new areas.
Progress in satellites for intelligence or inspection purposes will inevitably proceed in the direction of higher resolution systems selectively covering smaller regions on the ground. Weather reconnaissance, on the other hand, can be done at resolutions measured not in feet but perhaps in hundreds of feet. Instead of narrow-angle views of the Earth's surface, it requires extremely wide angle views, covering as much as possible simultaneously. These rather different requirements indicate that the purpose of both fields would be served by separate satellites rather than by trying to develop one all-purpose type.
Proposals for utilizing aerial inspection systems as a part of atomic-energy controls were made as early as 1947 in a United Nations report. 30 Since that time, aerial inspection systems, used alone or in conjunction with ground systems, have been proposed for disarmament inspection and surprise-attack warning systems.
Observation satellites could certainly contribute to, if not, in fact, provide, an inspection system. 31-35 An observation satellite could be used to detect and locate missile firings, for example, serving either to monitor agreements regarding missile launchings or to assist in reducing the measure of surprise from an attack. In conjunction with aircraft, satellites could perform installation inventory, assist ground inspectors in locating previously unknown or new military sites, monitor shipping, and carry out numerous other tasks.
Attempting to list all possible kinds of observation satellites- taking note of varying operational altitudes, sensors, purposes methods of returning data etc., makes for a bewildering array. The following chart (table 2) represents an attempt to sort out and classify some of the possible combinations. It is in no way intended as a complete listing.
30 Second Report to the security council, U. N., AEC September 11 1947 (AEC-36, September 11 1947) pt. II.
31 Leghorn Col. R. S., No Need To Bomb Cities To Win War, U. S. News & World Report January 28, 1955 pp. 78-94.
32 Leghorn Col. R. S., U. S. Can Photograph Russia From the Air Now, U. S. News & World Report, August 5, 1955, pp. 70-75.
33 Final report Subcommittee on Disarmament committee on Foreign Relations, U. S. Senate 85th Cong., September 3, 1958, p. 15.
34 The congressional Record vol. 104 No. 18, February 4, 1958, p. 1389.
35 The Congressional Record vol. 104 No. 18, February 4, 1958, pp. 1401-1402.
| Basic booster
| Typical weight
|Typical ground resolution 1 (feet)|| Useful life
(week or less)
| Useful life
(month or more)
| Data recovery
| Data recovery
|IRBM||300-500||150||Photo||60||X||-||X||-||Coverage of millions of square miles (level A).|
|IRBM||300-500||150||Photo||20||X||-||X||-||Higher resolution search over limited areas (level B).|
|IRBM||300-500||300||Photo-TV||200-500||-||X||-||X||Weather reconnaissance.||ICBM||2,000-10,000||300||Photo-TV||200-500||-||X||-||X||Weather reconnaissance.|
|2,000-10,000||150||Photo||20||X||-||X||-||Higher resolution coverage of millions of miles (level B).|
|2,000-10,000||300||Photo-TV||8-12||-||X||-||X||Cyclic survelliance of selected areas - warning (?) (level C).|
|2,000-10,000||300||Infrared||-||-||X||-||X||Warning (ICBM Firings).|
|2,000-10,000||300||Electronic||-||-||X||-||X||Electronic and communication intelligence, communication relay, etc.|
| Nuclear or
large chemical rocket
|20,000-100,000||500-25,000||all types||(2)||-||X||X||X||All missions listed above plus level D.|
Observation satellites can serve uses other than military reconnaissance, inspection, weather forecasting, and mapping. They can also make observations of other celestial objects far superior to those obtainable within the Earth's atmosphere. 36
The advantage to astronomical photography of reducing atmospheric effects has been shown dramatically by Sun photographs from a balloon-borne telescope. 37 Project Stratoscope employed a special photographic telescope weighing 300 pounds, and photographed the Sun from 80,000 feet, above about 90 percent of the Earth's atmosphere. According to the project scientists, the photographs taken of the Sun are the sharpest ever secured, showing Sun detail never before seen.
The enormous advantages of clear photographs of celestial objects can be appreciated from a comparison of the pictures of figure 4. The photograph of the Moon shown in this figure shows the region of Clavius. Taken by the 200-inch telescope on Mount Palomar, it is considered to be one of the finest photographs ever made. The ground resolution is about 1 mile. In an effort to show how little lunar detail we can readily see in such photographs, despite their high quality, a photograph of Washington, D. C., was systematically degraded to show ground resolutions of 200, 500, and 1,000 feet. Note the low level of detail of Washington at the 1,000-foot resolution level. This is five times better than the "sharp" Moon photograph.
36 Introduction to Outer Space, the White House, March 26, 1958.
37 Project Stratoscope Reveals Sun Data, Aviation Week. October 28. 1957.
Aerial photography of the Earth has been applied to exploration; Earth sciences; land planning; crop, soil, and forest inventories; engineering; ecology; geology; geography; physiography; geomorphology; hydrography; urban-area analysis and planning; and archeology 38-43
38 Gwyer, J. A., and V. G. Waldron, Photointerpretation Techniques-A Bibliography Library of Congress, Technical Information Division, March 1956.
39 Selected Papers on Photography and Photointerpretation, Committee on Geophysics and Geography, Research and Development Board, Washington, D. C., April 1953.
40 Report of and Proceedings Commission VII-Photographic Interpretation to the International Society of Photogrammetry, Washington, D. C., September 1952.
41 Manual of Photogrammetry, 2d ed., the American Society of Photogrammetry, Washington, D. C., 1952.
42 Gutkind. E. A.; Our World From the Air. Readers Union, London, 1953.
43 Le Lause. P. C., La Decourverte Aérienne du Monde, Horizons de France, Paris, 1948.
The application of aerial photography to these varied fields depends first upon the large view afforded and on the recording of fine enough detail to permit accurate identification, measurement, and comparison.
Satellites will yield a grander view, a larger perspective, than has ever been attained before. Photographs from rockets at altitudes of 150 miles have already yielded spectacular views. The possibility of seeing, as a whole, relationships, formations, and terrain features which require the perspective of distance is an exciting prospect. The world today is still poorly mapped, and its resources still not measured.
At least several novel applications of observation satellites can be foreseen. Ice and snow surveys over vast areas, iceberg patrol, and studies of ocean-wave propagation-all require the coverage of large areas hitherto impossible by conventional airborne observation systems.
The emergence and utilization of such a radically new tool as the observation satellite will undoubtedly result in the development of applications and techniques not yet imagined or foreseeable.
Satellite observations can be obtained with sufficient detail for military reconnaissance, inspection operations, mapping, and Earth-science studies.
The satellite whose period is exactly that of the Earth-commonly called the 24-hour satellite-is of special interest. If placed over the Equator, moving eastward in its orbit, it would appear to remain always at the same point in space, as viewed from the Earth. The orbit altitude required for this 24-hour period is about 22,000 miles. Such a satellite would have in view, at usable angles of incidence, about 38.2 percent of the Earth's surface, or approximately 75 million square miles (fig. 5) .
Interest in the 24-hour satellite for observation stems mainly from the somewhat intuitive notion that it would be useful to observe activities on Earth from a fixed position. However, it is not altogether clear that useful observations can be made from such a great distance. A worthwhile preliminary examination of the problems involved can be conducted by assuming a desire to achieve observations with a ground resolution of 100 feet. This is a better order of resolution, considering the viewing distance, than is commonly achieved at astronomical observatories such as Mount Wilson and Mount Palomar.
However, the troubles that limit achievable resolution at Earth-bound observatories are, in large part, chargeable to the atmosphere. The optical system in a 24-hour satellite will have many problems, but not this one. The problem here is simply one of size. Taking into account practical limitations on photographic film, etc., it appears that achievement of the assumed resolution of 100 feet would require a lens diameter approaching 8 feet, with a focal length of about 64 feet. Such optics are huge, even by terrestrial standards. This is the size of the Mount Wilson telescope.
Suppose that such a camera were used to take photographs 4 by 5 inches in size of an area at about 45° latitude. The region photographed would be about 152 by 197 miles, an area of approximately 30,000 square miles. This is only slightly less than the combined area of Connecticut, Massachusetts, New Hampshire, and Vermont.
Clearly, the achievement of ground resolution such as the one calculated above would be an engineering and scientific triumph of the first magnitude. The requirements for stability of the camera system, given any reasonable exposure time (such as one-hundredth second), are severe. Angular motions which tend to blur the photograph must be kept to, say, 0.05 seconds of arc during the exposure time. This is a formidable requirement, indeed.
There is one application of a 24-hour satellite that would not require such huge optical systems: weather observation. Here, relatively poor ground resolution would be adequate; furthermore, an enormous view would be afforded from such a high altitude.
A need for very large satellite payloads-say, of the order of 100,000 pounds-can already be discerned. Current thinking about payloads for observation satellites has been shaped to a considerable extent, not by the inspection of observational needs, not by optics design, but by booster availability and performance. Thus, designers of observation satellites have first looked at rocket vehicles in development, and attempted to use whatever weight such vehicles may allow them to put on orbit.
These weights have, to date, been fairly small. Roughly, they range from something like 20 pounds to a few tons.
Payload limitations constrain the performance of an observation satellite in three respects: quality of data obtainable; quantity of data secured; and timeliness with which this data can be delivered to the user. Current payload limitations preclude observations from satellites of the sort available from aircraft. Ground objects of interest have been observed successfully with reconnaissance equipments operating in aircraft at altitudes measured in miles, not hundreds of miles. The size of the photographic gear useful at high aircraft altitudes (say, of the order of 50,000 feet) are very large, with weights from several hundred pounds to several thousand pounds. As the camera is moved away from the observed objects by a factor of 15 to 100 beyond aircraft altitudes, the size of equipment needed for similar observation quality is going to grow sharply-indicating satellite payloads of many tons. The need for satellite payloads of this order is supportable by the argument that observational capabilities of the kind now available only to aircraft will continue to be needed, perhaps with
growing urgency. Ground objects likely to be of inspection or observational interest in the future will probably not get larger than objects of current interest and may, in fact, get smaller.
Figure 6 shows the relationship among altitude, focal length, and ground resolution on the assumption of a film resolution of 100 lines per millimeter. If poorer resolution is obtained on the film, the required focal lengths increase proportionately.
Observations under twilight, moonlight, and eventually under night-sky illumination only, are goals no longer considered unattainable. There are areas of the world that are continuously dark for considerable periods. Observation, inspection, and surveillance of these areas from satellites might prove impossible unless needed improvements in observation technology are pursued. A capability to make observations in the difficult nighttime hours would be desirable, of
course, for use against attempts to employ darkness as a shield for various activities. Promising developments, such as certain refined television techniques point to one way of achieving such goals. Photography from airplanes by the light of a near-full Moon has been accomplished and described by the Air Force. Either of these kinds of approaches would involve large-diameter, long-focal-length optics to collect as much light as possible and recoup quality loss inherent in low-illumination operation. A speculative example might be a camera system with 100-inch focal length, at a speed of f/2, thus requiring optics at least 50 inches in diameter. In addition to auxiliaries such as power supply, controls, and communication equipment, any large, fast optical systems such as this one would require automatic focusing mechanisms, temperature control, and other refinements not needed for smaller, slower, less sensitive systems -all of which points to payloads of many tons.
Better observational quality -finer detail- will require weight (treater total quantity of information also requires weight. Information quantity is a function of total vehicle life, the total load of film or other storage media carried, and the method of returning data to earth. Vehicle useful life depends, among other things, upon the life of its power supply. For those satellites powered by chemical batteries alone, the upper limit of working life is set by the total energy content of the batteries and the rate at which such energy is used by the apparatus in the satellite. Working life would increase if more batteries (weight) were added. When such power supplies can be supplemented by solar energy or when the power requirements can be cut down, working life win increase for a given total weight. Nuclear powerplants will eventually be available, and will save weight. They may also create added problems, calling for added weight for shielding.
The "quantity" of information deliverable by a satellite which communicates with the ground by video link is a direct function of the band width used and the time available for communication. Generally speaking, greater band width requires more equipment, hence more weight. The available time for communication is determined by the number of ground stations, and the fraction of time the satellite is within view of a given ground station. For 300-mile satellites, this time is about 10 minutes per station. Higher altitude satellites are in view longer, and in addition have less orbital speed. Thus a 1,000-mile-altitude satellite would be in view for more than tWice the time of a 300-mile satellite. Very high satellite altitudes may also permit direct line-of-sight communication from satellite to ground stations at the moment that the satellite is picking up data. This might indeed eliminate the need for data storage aboard the satellite, and would certainly be desirable for warning applications. Higher orbit altitudes require large launching rockets -the same eventual need as that manifested by payload increases.
Environmental factors about which all too little is known at present, include the radiation environment at considerable altitudes. The early data available from Explorer IV are fragmentary, and much more information is needed. It may be suspected that the operation of a film-carrying satellite in the indicated heavy radiation field will
prove to be extremely difficult. The implication of the data at hand is that considerable shielding will be needed. Shielding is directly translatable into pounds of payload. If photographic film proves to be a poor sensor to use under such circumstances, a change to another sensor such as television or electrostatic tape may be required. These would undoubtedly yield poorer resolution than photographic film; the loss could be recouped by using larger, heavier equipment.
If a man is to be put aboard an observation satellite, the total weight required will climb sharply.
Multiple sensors (i. e., combinations of photographic, infrared, radar, and other sensors) in satellites would be of value, and would certainly require larger vehicles than those needed for each of these separately.
One can envision a need for measures to decrease the vulnerability of satellites, changing their orbits, aiming the cameras, and the like. All of these would require weight, hence larger launching rockets.
In brief, large launching rockets are an inevitable requirement if observation satellites are to progress vigorously.
It is apparent that observation satellites can be categorized in several ways -by useful life, purpose, sensor employed, method of returning data, etc.
It would be reasonable to emphasize the potential surveillance capability of the long-lived satellite which transmits its data by video link, and minimize the role of this type of satellite in the collection of huge quantities of data. The latter task is better suited for satellites from which data is recovered physically. Such a family of different types of satellites would constitute a balanced group. The different satellite types are complementary, not competitive.