Exploring the Unknown - NASA's History Office

192
OBSERVING THE EARTH FROM SPACE
around the 56th parallel. In any event, the coverage, as mentioned here, if achieved with
any measure of success, should produce good weather reconnaissance results.
RESOLUTION AND LIMITING CONTRASTS
Since it is now obvious that clouds will be the chief meteorological element directly
observable from high altitude photographs, it must be ascertained how closely these clouds
may be identified and what may be determined from them, either directly or indirectly.
As can be seen from Tables 1, 2, and 3, when a set of conditions such as aperture, illumination,
exposure time, and focal length-aperture diameter ratio of a given camera have
been established, the remaining factor for determination of the minimum resolution
attainable is the contrast value. In cloud photography of the type to be attempted from
the satellite, one is unable to choose the surrounding photographic conditions. Features
such as background, lighting at time of observation, etc., are examples of the uncontrolled
variables, and, as a consequence, any system of data gathering by photographic
means must be flexible enough to give adequate results over a wide range of limiting factors.
The question is: If the camera and optical system are chosen, 10 and if the various conditions
of lighting, background, etc., are assumed to remain within the limits providing
[9] usable resolution, will the resulting limiting contrast values still enable one to observe
the weather under a wide-enough range of actual conditions?
Before endeavoring to answer this question it is desirable to define the term “usable
resolution.” It was thought that details of cloud structure as small as several hundred feet
in diameter might possess significance when an attempt was made to form a synoptic picture
by means of cloud analysis. This was borne out when high-altitude rocket photographs
were examined. Further reasons for asserting this to be the appropriate
minimum size to be resolved were found when a simple test was conducted on these photographs.
(The heights at which these pictures were taken varied between 50 and 70 mi.)
Using an adjustable viewer, the photograph was taken slowly out of focus until it was
impossible to identify definitely the forms of clouds other than by saying that they were
widespread or were in small clusters. For example, beyond this point it was impossible to
distinguish between closely packed cumulus and a deck of altocumulus, and also between
a dense layer of stratus or altostratus and the fibrous texture of cirrostratus. A study of
other parts of the photograph, where recognizable or measurable objects were located at
ranges about equal to those of the clouds, showed that the limiting resolution at which the
clouds lost their distinguishability was from 500 to 1000 ft. This is what is meant by “usable
resolution.” As may be imagined, this is at best only a rough approximation, but because
of its apparent agreement with previously estimated values it should serve very well as a
working basis.
It was mentioned above that in order to obtain a known, usable resolution, once the
camera and lighting conditions are chosen, the limiting contrast value must also be spec-
10. Previous studies at RAND have shown that one of the best available television cameras for use in the
satellite would be one employing an image orthicon pick-up tube. The characteristics of this tube approach those of
the human eye over part of its operating range, it has a greater sensitivity than earlier types, and it is capable of
stable operation without adjustment over a wide range of illumination intensity. Since it is not the purpose of
this report to delve too deeply into the technical aspects of the the problems of a television viewing system, only
results of resolution computation of the image orthicon tube are presented here. These are summed up in
Tables 1, 2, and 3. For technical information and equations involved, see R. B. James, R. E. Johnson, and R. S.
Moore, RCA Review, Vol. 10 July 1949, pp. 191–223; and A. Rose, “Television Pickup Tubes and the Problem of
Vision,” Advances in Electronics, Vol. 1, Academic Press, New York, 1948.