c. easy access to remote areas; and,
d. repeat coverage.
Of these characteristics of satellite remote sensing, perhaps the most
interesting and the one sometimes overlooked is repeat coverage. Scien-
tists are finding that the repeat coverage capability of Landsat is very
significant for improving the identification of wheat and other crops.
Change detection is a technique also made practical by the repeated obser-
vation under similar lighting conditions and viewing angle.
Although there are many types of motion- and energy-measuring concepts
that may be useful in detecting and monitoring Earth resources, the NASA
program has devoted its major effort toward exploitation of the capabili-
ties within the optical and microwave electromagnetic systems. On the
other hand, little effort has been expended to date to determine the tech-
nical value of observing changes in the Earth's gravity and the magnetic
field. Figure 1 shows the types of sensors of primary interest to NASA
in exploiting the optical and microwave portions of the electromagnetic
spectrum for Earth resources activities.
Earth resources activities within NASA began in the early 1960's as an
outgrowth of space photography from the Mercury, Gemini, and Apollo
programs and from aircraft tests of the lunar landing radar. Remote
sensing from airborne platforms initially utilized military design cameras
and electromechanical systems modified for resources observations. By
the early 1970's, however, the airborne systems being tested were speci-
fically designed to provide the technical parameters needed to define future
space systems. As a result, the selection of spectral bands for Landsat
scanners has evolved to provide the data most appropriate to the specific
natural phenomena under observation.
This paper describes the evolution of sensor systems technology and
applications from these early beginnings within NASA and emphasizes future
trends and plans. Also included are brief discussions of plans for
satellite (Landsat) data receiving stations in foreign countries and for
an "operational system." In addition, a bibliography of pertinent
documents containing more detailed information is appended.
Sensor Systems Technology
Optical Sensors
Spaceborne scanning systems during the 1970's have experienced a moderate
evolution. Multispectral line scanners having four-channel and 80m resolu-
tion field of view were flown on Landsats-l, -2, and -3 (with a fifth band
in the thermal region added for Landsat-3), and a conical scanner was flown
on Skylab. The focal plane scanning system developed for Skylab was
partially the result of minimal power availability. Some of the informa-
tion content in the data was lost during rectilinearizing prior to
NASA-S-78-2542 B
