Each of the experimental programs on Skylab represents an extension of studies that have been under way on Earth for some time. It is natural, therefore, that experimenters have expressed the desire to compare and correlate the results of Skylab experiments very carefully with data gained from ground-based observations. In fact, several ground-based study programs have been planned and prepared specifically to provide data in support of Skylab experiments. Ground-based supporting projects which will supply back-up services to Skylab will include the Ground-Based Astronomy Program, the National Oceanic and Atmospheric Administration's support to NASA, earth resources ground-truth activities, and ground-based medical studies.
These ground-based supporting projects will supplement Skylab studies in several ways. First, by comparing ground observations and Skylab observations of the same object, an experimenter can determine the kind and the degree of superiority which space observations may have over ground observations. Second, by observing a given object, such as a group of sunspots, over an extended period of time before and after the Skylab mission, insight into evolutionary processes will be gained which may greatly enhance the value of Skylab observations. Third, by observing at close range certain features on the surface of the Earth, such as tectonic formations, ocean currents, or plant growth patterns, a "calibration table" for the interpretation of Skylab pictures of the same features can be established. This calibration can then be used for the interpretation of other Skylab pictures for which no ground pictures exist. Fourth, in the case of biological and medical experiments, the ground-based observations will enable the investigator to isolate the effects of weightlessness upon living organisms by comparing Skylab data with the results of observations taken under similar conditions on the ground.
Details of some of the ground-based supporting projects will be described in the following sections.
1. GROUND-BASED ASTRONOMY PROGRAM
The Skylab Ground-Based Astronomy Program is designed to obtain solar data from observatories around the world at the same time the ATM instruments are observing the Sun from orbit. Data gathered on the ground will support and add to the space-gathered data and thus help gain maximum benefit from the ATM.
 Early in 1971, NASA began to solicit proposals from solar astronomers for ground-based observations that would support and extend solar observations on Skylab. Many organizations submitted proposals; nine of them were selected for implementation.
The participating astronomical observatories and their proposed projects are as follows:
a. The University of Hawaii's Institute of Astronomy at Haleakala (Fig. 215) is constructing a photometer for observations of active regions in the corona. This instrument will measure simultaneously the intensities of several visible coronal lines in a study to determine the rates of energy loss and gain from the active regions and the effects of flare events on the corona.
b. The Lockheed Missiles and Space Company at Palo Alto, California, will operate at Kitt Peak's McMath solar observatory (Fig. 216) a spectroheliograph capable of mapping physical parameters of the solar atmosphere. In conjunction with the Kitt Peak solar telescope and vacuum spectograph, a wide-exit aperture and a specially constructed movie camera with rapid film advance will be used to obtain spectral maps of active regions of the solar disc with high spatial resolution (to one-half arc second). A high speed microdensitometer-computer system will be set up to allow rapid analysis of the spectral data to obtain solar plasma velocity and magnetic field maps of regions of interest.
c. The National Bureau of Standards is upgrading calibration capabilities in support of ATM-related measurements in the following areas:
(1) A hydrogen arc source of known radiant flux for the calibration of spectrometric detector systems over the region from 50 to 370 nm (500 to 3700 Å) is being developed.
(2) A study is being conducted to determine the effects on photocathodes caused by removal or addition of mono-layers of contaminants in vacuum; wavelength region of interest is 50 to 150 nm (500 to 1500 Å).
(3) A capability is being established for radiometric calibration down to 20 nm (200 Å) by utilizing the National Bureau of Standards' synchrotron facility. Windowless diodes are being developed as transfer standards in this spectral range.
d. California Institute of Technology is installing a 65 cm (25.6 in) photoheliograph at its observatory at Big Bear Lake in California (Fig. 217) where observing conditions are exceptionally good. The new unit will be used for extremely high resolution studies of active regions in conjunction with ATM. Filtergrams will be made in lines extending from the 393.3 nm (3933 Å) Calcium K-line to the 1083 nm. (10,830 Å) Helium line.
e. Lockheed Solar Observatory at Rye Canyon, California (Fig. 218) is preparing both of its spar telescope systems for observations during the ATM mission. On one spar, studies in the D3 line of Helium I will be directed at solar flares and transient events during periods of high disc activity and to prominence observations during periods of low disc activity. On the other spar, a telescope with a filter will make high resolution photographic studies in the calcium II line at 854.2 nm (8542 Å). This line is believed to originate at a higher level in the solar atmosphere than the H-Alpha line; it should be very valuable in relating ATM X-ray and extreme W data to filtergrams and spectroheliograms taken at wave lengths originating at lower levels in the chromosphere.
f. The University of California at San Diego is building a photometer system for infrared observations of solar flares at the Mt. Lemmon Observatory near Tucson, Arizona (Fig. 219). Observations with the 1.52 m (60-in) Cassegrainian telescope will extend from the 700 micrometer (700,000 nm) region down to possibly one micrometer. These infrared continuum observations will be compared with observations made on ATM.
 g. The Lockheed Missile and Space Company at Palo Alto, California will perform a theoretical study of Helium emissions in the visible and ultraviolet from solar active regions. Calculations of the statistical equilibrium populations for a 19 level Helium I and a 10 level Helium II ion are being performed. Results of these studies should allow the interpretation of line observations by ATM in terms of electron density and temperature profiles for the active regions on the Sun .
h. The Uttar Pradesh State Observatory in Naini Tal, India, is performing a study of dissociation and excitation equilibria of various molecules in the photosphere, in Sun spots, and in faculae. Detection equipment has been loaned to India for an observational program with its existing horizontal solar telescope and associated spectrograph. The program aims at an improvement of existing models of the solar atmosphere.
i. The Applied Physics Laboratory of Johns Hopkins University in Baltimore, Maryland (Fig. 220), will perform an observational program during the ATM mission of solar radio bursts in the 500 to 1000 MHz range. With the .01 second time resolution of the spectrograph and the 18.3 m (60-ft.) diameter antenna, it should be possible to determine whether a finite frequency drift rate is present in the radio bursts. This in turn will have implications on existing models of emission mechanism and electron densities in the source region. Results of these studies will be correlated with ATM data.
2. NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION SUPPORT TO ATM
The National Oceanic and Atmospheric Administration (NOAA) of the Federal Government has been coordinating a solar data collection network among observatories in the U.S. and in foreign countries. These observatories will furnish data to NOAA for use in its continuing study of solar activities.
By agreement with NASA, the Space Environment Laboratory of NOAA established the Space Environment Support Program. This program is designed to support the unique requirements of the ATM Skylab mission for solar data. It will expand and improve the Space Environment Laboratory's current data gathering and distribution facilities in order to provide continuous solar activity forecasts to the ATM Principal Investigators at the Lyndon B. Johnson Space Center during the Skylab missions.
NOAA will use information from numerous observatories to provide this support to NASA before, during, and after Skylab flights. On the basis of solar data from the ground-based network of observatories, NOAA will forecast flares and other solar activities, inform NASA of important solar events in progress, and, after the Skylab mission, prepare a book of all data collected during the mission for use in data analysis.
During the mission, NOAA representatives will be stationed at the Lyndon B. Johnson Space Center to provide NASA with real-time space environment data, analyses, and forecasts. These representatives will coordinate the operation of the solar data network. Working closely with the Principal Investigators and providing them with current information on the present state of the Sun, they will interpret data for the Principal Investigators and make suggestions on possible observing programs based on solar activity. After the mission, NOAA will continue collection of data from the network for a month.
 3. EARTH RESOURCES GROUND TRUTH ACTIVITIES
"Ground truth" data will be obtained by direct observations on the ground of those areas, objects, and phenomena which will also be observed by Sky lab instruments from orbit. Nearly simultaneous observations will be made of weather, lighting conditions, and other environmental factors which may influence the data gathered from Skylab. By comparing ground truth observations with orbital observations of a test site, calibration factors will be established which will allow the proper interpretation of orbital data from many sites.
The concept to be followed in obtaining ground truth data to correlate with Skylab observational data from the Earth Resources Experiment Package is to employ each individual Principal Investigator's own ground truth capability. Principal Investigators will furnish their ground truth data to the Skylab Ground Truth Office at JSC where they will be included in the archives together with the space-acquired data.
The Lyndon B. Johnson Space Center will establish a communications system so that Principal Investigators will be notified of the times Skylab will overfly their areas. Thus, they can acquire ground truth measurements synchronized with EREP overflight data.
Ground truth data to be acquired in support of EREP observations will include photographs, temperature measurements, terrain data, weather observations, and descriptive material identifying plant growth, soil conditions, snow depth, status of crops, geological features, and other specific details.
4. AIRCRAFT UNDERFLIGHT SUPPORT
NASA-operated and private aircraft will be used to obtain data over the sites to be observed by Skylab. These aircraft will carry a variety of cameras and imaging devices which generally approximate the capabilities available on board Skylab. Like the ground truth data, data acquired by high altitude aircraft underflights will be used to analyze and understand the space-acquired data.