SP-399 SKYLAB EREP Investigations Summary




[1] REMOTE SENSING of the Earth from orbital altitudes was recognized in the mid-1960's as a potential technique for obtaining information important for the effective use and conservation of natural resources. These studies began when the Tiros satellites (1960) provided man's first synoptic view of the Earth's weather systems. The manned Gemini and Apollo Programs (1965-72) led to further consideration of spaceage remote sensing for study of the Earth. The Earth Resources Technology Satellite, now designated Landsat, provided repetitive multispectral scanner data in the visible and infrared regions of the electromagnetic spectrum for large areas of the Earth. The Landsat series consisted of Landsat-1 launched in 1972 and Landsat-2 in 1975. Skylab, the largest manned space station ever placed in low Earth orbit, was launched in 1973 and carried into space the Earth Resources Experiment Package (EREP), which was designed to view the Earth with sensors that recorded data in the visible, infrared, and microwave spectral regions. Thus, EREP became another step in space exploration by testing the use of high-resolution camera systems with film return capability, narrow frequency bandwidth scanner systems in the visible through thermal-infrared spectral regions, and the initial use of active and passive microwave systems in Earth resource surveys. A significant feature of EREP was the use of man to operate the sensors in a laboratory fashion. Skylab objectives also included scientific observations of the Sun, the stars, and near-Earth space; materials research and manufacturing in a weightless environment; observation of living-organism functions in a near-zero-g environment; and development of techniques for long-duration manned-space-flight operations.

The Skylab spacecraft was launched eastwardly on a 50° azimuth from the NASA John F. Kennedy Space Center, Florida, on May 14, 1973, into a near-circular orbit at an altitude of 435 km above the Earth. It orbited the Earth every 93 minutes and repeated the groundtrack every 5 days. The launch azimuth inclined the orbital plane 50° with respect to the Equator and limited observations of the Earth to latitudes between 50° N and 50° S (fig. 1-1).

Skylab was occupied by three three-man crews during the period May 25, 1973, to February 8, 1974, for a total of 171 days in space (table 1-1). At the end of the Skylab 4 mission, the vehicle was deactivated and remains today in a near-circular orbit about the Earth.

The EREP was designed to explore the use of the widest possible portion of the electromagnetic spectrum for Earth resource investigations. It consisted of two photographic and four electronic sensor systems (table 1-11) that simultaneously permitted remote sensing of the Earth's surface in the visible, infrared, and microwave regions of the spectrum (fig. 1-2). The ground coverage for each sensor is shown in figure 1-3. The EREP sensor systems and their data products are described in appendix A.

The EREP Program began in December 1970 with the announcement by NASA that data collected by the EREP would be made available to qualified investigators for Earth resource investigations. From the response to this announcement, NASA selected 164 tasks to be performed by 148 Principal investigators representing academic, governmental, and industrial firms in the United States and 19 other countries. (A list of the Principal investigators is contained in appendix B.) These investigators analyzed EREP data and ap-...




FIGURE 1-1. Skylab ground coverage. [For a larger picture, click here]


FIGURE 1-2. Wavelength sensitivity of Earth-viewing Skylab sensors. [For a larger picture, click here]


FIGURE 1-3.- The ground area coverage provided by EREP sensors (S-73-005-S). [For a larger picture, click here]


[3] TABLE 1-1.-Skylab Mission Summary.




Launch date

Splashdown date

Duration, days


a Skylab 1


May 14, 1973

Not applicable (NA)


Skylab 2

Charles Conrad, Jr., commander (CDR)

Joseph P. Kerwin, science pilot (SPT)

Paul J. Weitz, pilot (PLT)

May 25,1973

June 22,1973


Skylab 3

Alan L. Bean, CDR

Owen K Garriot, SPT

Jack R. Lousma, PLT

July 28, 1973

Sept. 25, 1973


Skylab 4

Gerald P. Carr, CDR

Edward G. Gibson, SPT

William R. Pogue, PLT

Nov. 16, 1973

Feb. 8, 1974


a Launch of Skylab orbital workshop. The vehicle was operated unmanned between manned missions.



-...plied their results to nine major investigative areas that included agriculture, range, and forestry; geologic applications; continental water resources; oceanographic and atmospheric investigations; coastal zones, shoals, and bays; remote-sensing technique development; regional planning and development; and cartography. Within each of these areas, studies of specific features and phenomena were conducted using EREP, Landsat, aircraft, and ground measurement data.

Each investigation required specific EREP data, which were defined by the Principal investigators and used to preplan the operation of the EREP during the mission. Depending on the scope of the investigations, the data requirements ranged from a series of photographs obtained by the S190A and S190B camera systems to computer-compatible tapes and color-composite images derived from the S192 multispectral scanner. The microwave sensors (S193 and S194) recorded data on magnetic tape for processing and analyzing with computers.

Integration of the data requirements for each investigative area resulted in a mission requirements document that was used by the EREP Team to plan, in the Mission Control Center at the NASA Lyndon B. Johnson Space Center (JSC), each EREP data pass on a daily basis during the three manned missions. Each EREP pass was planned within mission constraints to obtain the specific data needs of as many investigators as possible. Because most investigations required data obtained with minimum cloud cover, the Space Flight Meteorological Group of the National Weather Service at JSC was an essential part of the operation planning team. Before each EREP pass, a detailed time schedule for operation of each sensor was uplinked to the crew.


Table 1-II.- EREP Sensors.



Wavelength range, µm

Frequency range, GHz


Multispectral Photographic Camera system (S190A)

Six 70 mm boresighted cameras; color, black-and-white, color-infrared. and black and white infrared films

0.4 to 0.9


Earth Terrain Camera (S190B)

Single 127-mm camera; color, black-and white and color-infrared films

0.4 to 0.88


Infrared Spectrometer (S191)a

Filter-wheel spectrometer; 1-sec scan rate:16-mm camera that records Earth scenes; crew pointed

0.4 to 2.5; 6.6 to 16.0


Multispectral Scanner (S192)a

13 channel optical-mechanical scanner

0.4 to 2.35; 10.2 to 12.5


Microwave systems a K-band: Microwave Radiometer b/ Scatterometer c and Altimeter c (S193)

3-sensor facility; uses single 1.1-m, pointable parabolic antenna


13.8 to 14.0

L-band: L-Band Radiometer b (S194)

Single sensor: fixed antenna


1.4 to 1.427

a data recorded on magnetic tape.
b Passive sensors.
c Active sensors.



[4] To conduct a conventional EREP pass, the crew oriented Skylab to point the EREP sensors normal (perpendicular) to the Earth's surface. In this position, the Z-axis of the spacecraft was alined with local vertical (Z-LV). With the exception of a few passes, all EREP data passes were accomplished in a Z-LV position.

During the 171 days of manned Skylab operation, 110 EREP passes were completed (table 1-III). These passes resulted in more than 35 0110 frames of photography and 72 725 m (238 600 ft) of magnetic tape.

Skylab orbital groundtracks permitted the EREP sensors to view the United States; the continents of South America, Africa, and Australia; the southern part of Europe and Asia; and large areas of the Atlantic and Pacific Oceans (fig. 1-1). Each manned mission resulted in an abundance of Earth resource data, some of which is unique. Skylab missions were flown during the summer, fall, and winter months in the Northern Hemisphere, and some repetitive data were collected during each season for selected test sites. Mission constraints required that during the Skylab 2 mission, only descending (north to south) passes could be performed; during the Skylab 3 and 4 missions, both descending and ascending (south to north) passes were conducted. In general, the data collection period for each pass ranged from 15 to 25 minutes and covered a groundtrack distance of 6482 to 11112 km (3500 to 6000 n. mi.).

During Skylab 2, EREP data were collected on descending groundtracks over the United States, the Gulf of Mexico, the Caribbean Sea, and northern regions of South America. About midway of this mission, an intense hurricane (Hurricane Ava), located approximately 1000 km (550 n. mi.) southwest of Acapulco, Mexico, was visible to the EREP sensors, and unique....


Table 1-III.- EREP Data Summary.


EREP passes

Photographs (frames)

Magnetic tape, m /(ft)


Skylab 2



13 716


Skylab 3


13 429

28 529

(93 600)

Skylab 4



30 480

(100 000)


....photographs and microwave data were obtained concurrently with U.S. Air Force reconnaissance aircraft flights. The Skylab 3 crewmen returned large quantities of photographic and magnetic tape data obtained on ascending and descending groundtracks over the United States and 28 countries in Central and South America, Europe, western Africa, Asia, and Australia. Single data passes were accomplished over Japan and adjacent ocean. Israel, Ethiopia, Malaysia, Australia, and New Zealand. During the Skylab 3 mission, specific features sensed by the EREP included the active volcano Mount Etna, Sicily; the drought regions of Mali and adjacent countries; and tropical storm Christine in the Atlantic Ocean northeast of the coast of Venezuela.

Skylab 4 was flown during the winter months in the Northern Hemisphere; therefore, lighting conditions were not favorable for EREP data collection in December 1973. Nevertheless, the 84-day mission resulted in successful EREP passes over portions of the world generally covered during the Skylab 3 mission. In addition, some unique data collection passes were completed including predawn and near-local-noon sensing [5] of selected areas in California for testing the thermal mapping capability of the S192, overflight of the largest extratropical cyclone in the North Atlantic in a decade, and a 360° S192 altimeter data pass that began at longitude 39° W and ended at longitude 61° W to measure the configuration of the Earth. Photographic data in both oblique (solar inertial) and Z-LV modes were obtained by the Skylab 3 and 4 crewmen over Paraguay to test the utility of space photography in topographic mapping of remote regions.

The vast quantity of EREP photographic, scanner, spectrometric, and microwave data returned from the 171 days of manned Skylab operations attest to the operational success of the sensor systems. To operate the sensor systems, the astronauts had to perform a series of tasks that included replacing film cassettes for the S19OA and S190B cameras, manually pointing the telescope and operating the S191 spectrometer, and alining the S192 scanner detector system to optimize the signal received during data collection. Late in the Skylab 4 mission, the astronauts replaced the S192 channel 13 thermal detector with a more sensitive detector and thermal data were collected over sites in southern California. The complex S193 microwave system operated satisfactorily during the Skylab 2 mission; however, the scan motion compensator malfunctioned during the Skylab 3 mission. The Skylab 4 crew reestablished the antenna capability in the roll direction (normal to the groundtrack), but the pitch motion was not repaired. The loss of the S193 antenna came in the latter part of the Skylab 4 mission and resulted in degradation of the data. The S194 microwave radiometer operated without malfunctions for the three manned missions. In summary, EREP data collection provided, for each Principal investigator (Pi), a comprehensive set of photographs and magnetic tapes containing data in the visible through microwave spectral regions over a wide variety of scenes, features, and phenomena of the Earth's surface.

Upon completion of each manned mission, the EREP data were returned for processing at JSC according to the requirements of individual investigators. The photographs were distributed soon after completion of each mission, but the complexity of the electronic data resulted in delays in the distribution of such data to the Pl's. Information on geographic areas for which data were obtained by the EREP sensors is contained in the Skylab Earth Resources Data Catalog (ref. 1-1). The PI reports on analysis of the EREP data can be obtained from the National Space Science Data Center, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771.

The purpose of this summary volume is to describe the significant accomplishments of the EREP data analysis program in the areas of agriculture, range, and forestry; geology and hydrology; oceans and atmosphere; land use and cartography; and data analysis techniques. The results presented in this report indicate the manner in which space remote sensing is applied in Earth resource surveys today, the needs of future space remote-sensing systems, and some potential applications of space data in conservation and utilization of natural resources.



1-1. Skylab Earth Resources Data Catalog. NASA Rep. JSC 09016, U.S. Government Printing Office (stock no. 3300-00586) (Washington, D.C.), 1974.