Air University Review, March-April 1978

The Meteorological Satellite

As far as I am concerned, this weather picture is probably the greatest innovation of the war. I depend on it in conjunction with the traditional forecast as a basic means of making my decisions as to whether to launch or not launch the strike. And it gives me a little bit better feel for what the actual weather conditions are. The satellite is something no commander has ever had before in a war.

So stated General William Momyer while Seventh Air Force Commander during a nationally televised interview on CBS in May 1967.¹ General Momyer was referring to the value of the Defense Meteorological Satellite Program (DMSP) in providing pictorial weather coverage over the data-void regions of North Vietnam, Laos, and the South China Sea. The use of meteorological satellite photography was conceived during the technology explosion of the 1940s and 1950s. During the 1940s, cloud photography was studied from high-altitude platforms such as rockets or manned and unmanned free-floating balloons.² Also, from 1945 to 1950, the military services were involved in several independent missile and preliminary space projects. In 1946, the Army Air Forces started specific satellite studies through Project Rand, a consultant group of scientists and technicians.³

By the end of the l940s, however, very little money had been committed to any of the space programs. The United States was not really interested in satellites or missiles. We were the most powerful nation in the world, and we were demobilizing. In addition, although the Rand study contended that a 500-pound satellite could be put into a 300-mile orbit by 1951, it would be impossible to lift a heavy atomic bomb to orbit altitude. Therefore, no active military purpose could be projected for satellites: only passive missions such as communications or weather seemed feasible.⁴

However, the initial 1946 Rand study did make the following interesting observation:

A satellite vehicle with appropriate instruments can be expected to be one of the most potent scientific tools of the Twentieth Century. The achievement of a satellite craft by the United States would influence the imagination of mankind, and would probably produce repercussions in the world comparable to the explosion of the atomic bomb. To visualize the impact on the world, one can imagine the consternation and admiration that would be felt here if the United States were to discover suddenly that some other nation had already put up a successful satellite.⁵

In 1949, the Soviets detonated their first nuclear weapon. and we immediately reestablished our missile programs. Then on 4 October 1957, the Soviets launched Sputnik I, and it caused all the psychological and political impacts that had been predicted by the 1946 Rand study. The Soviet threat of the 1950s, resulted in top priority and funding for our space efforts, including meteorological satellites.⁶ By the end of the 1950s, experimental meteorological packages were actually in orbit, and the stage was set for the launch on 1 April 1960 of the first meteorological satellite, Tiros I (Television and Infrared Observation Satellite). Tiros I was a forerunner to the civilian polar-orbiting meteorological satellites that provide the APT (Automatic Picture Transmission) data to several hundred civilian (both U.S. and foreign) and military installations throughout the world.

Not long after this the Department of Defense realized that the civilian system would not be sufficiently responsive to constantly changing military requirements. Thus, the DMSP was subsequently established and has been providing military commanders with meteorological satellite imagery.

Since the early 1960s, meteorological satellite technology has continued to evolve and advance. The early satellites used television cameras and took photographs of the Earth's cloud cover only during the daylight hours. The photographs were transmitted to ground receiving stations where the individual frames were assembled into mosaics to provide a total picture covering the area of interest. Today, meteorological satellites use a multitude of sophisticated sensing instruments covering a wide portion of the electromagnetic spectrum. They provide a variety of data, including images of the Earth, and operate day and night in both polar and geostationary orbits.

Of the several types of orbits that can be used by meteorological satellites, experience has shown that two specific kinds are preferred for the meteorological satellite role: the earth-synchronous, geostationary orbit and the sun-synchronous, near polar orbit.

The geostationary orbit is defined by a spacecraft flying in the equatorial plane at sufficient altitude to require 24 hours to complete one orbit. This means that the spacecraft is traveling at the rotational speed of the Earth, and, therefore, the satellite remains essentially stationary over a fixed point on the Earth's equator. The altitude required for geostationary satellites is 35,786km (l9,323nm). The fixed position combined with the high altitude allows the geostationary satellite to view a large portion of the Earth on a nearly continuous basis. The current civilian system was developed by NASA and is operated by the National Environmental Satellite Service (NESS) of the National Oceanic and Atmospheric Administration (NOAA); it is called the Geostationary Operational Environmental Satellite (GOES). The GOES spacecraft views an area illustrated by Figure 1 routinely every 30 minutes or, when desired, subsets of that area as frequently as every minute, depending on the size of the area viewed. It should be noted, however, that geostationary satellites cannot view areas north or south of approximately 60º latitude.

The sun-synchronous polar orbit is one in which the orbital plane is inclined nearly 90º to the Earth's equatorial plane. The altitude and inclination (the angle specifying the departure from the equatorial plane) of the orbit are adjusted so that the orbit plane precesses or shifts exactly 360/365th of a degree per day. This precision shifts the orbit plane so that it makes one complete revolution as the Earth makes one revolution around the Sun and thereby maintains a constant orientation of the orbit plane to the Sun. This sun-synchronous orbit means that the satellite passes over a given latitude at the same local sun time, an important characteristic for meteorological satellites.

It is possible to select the proper altitude and inclination to match desired sensor coverage requirements so as to obtain full global data coverage. NOAA polar-orbiting meteorological satellites, for example, fly at a 1300km (890nm) nominal altitude circular orbit inclined at 102º to the equator with an orbital period of 115 minutes. The DMSP spacecraft are at an 833km (450nm) nominal altitude circular orbit inclined at 98.7º to the equator (8.7º from true polar). This results in an orbital period of about 101 minutes. The Earth rotates just over 25º during each DMSP orbit.

While aerial coverage obtained from geostationary orbit is relatively fixed (the satellite can be relocated at different longitudes by proper thrusting), aerial coverage from polar orbit is much more complex. The coverage obtainable from real-time readout of a polar-orbiting satellite at a single ground station depends on line-of-sight communication. Because of the much lower altitude, coverage is much smaller than from geostationary altitudes. (See Figure 2.)

To receive data on a global basis, polar-orbiting satellites must carry on-board recording equipment. Recorders on the DMSP satellite collect and store as much as four orbits of data. These data are subsequently transmitted to command readout stations located at Loring AFB, Maine, and Fairchild AFB, Washington, for relay via a communications satellite to the Air Force Global Weather Central (AFGWC) located at Offutt AFB, Nebraska. Through this system, global imagery data are received at the AFGWC with minimum delay.

The Soviets have flown decidedly different orbits for their meteorological satellites. The geostationary orbit is of little use to them because of their extensive area in high northern latitudes. The Soviets thus have typically flown a combination of non-sun-synchronous polar orbits and the Molniya orbit, which is highly elliptical. Tile latter has the advantage that when apogee is in the Northern Hemisphere the spacecraft can view most of the Soviet Union continuously for up to 12 hours.

The fundamental imaging system used on both geostationary and polar orbiting satellites is the scanning radiometer. Figure 3 illustrates the scanning concept for the current DMSP polar-orbiting system. The radiometer consists of a telescope-detector combination that sweeps across the Earth's surface. In polar orbiters the scanner sweeps perpendicular to the orbit plane. For geostationary orbiters the entire spacecraft rotates in one direction, and the radiometer is mechanically stepped in the other direction.

The DMSP primary imager covers a swath width of about 2960km (1600nm) which equates to about 26º at the equator. (See Figure 3.) Therefore, the DMSP satellite will image every point on the Earth at least twice each 24-hour day, once ascending (traversing from south to north) and once descending (traversing from north to south).

The Block 5C DMSP imager senses in the visible (0.4-1.1µm) as well as in the window region (8-l3µm) of the infrared portion of the electromagnetic spectrum and obtains images at two different surface resolutions, 0.6km (one-third nm) and 3.7km (2nm). Some improvement in resolution is being achieved with the Block 5D satellite now in operation.

The resolution of the imagery defines the smallest detectable element that can be displayed in the data directly below the satellite. Because of the geometry of the Block 5C scanning radiometer systems the resolution of the imagery degrades by a factor of about six from the center toward the edges. The resolution of the DMSP VHR and WHR data, for example, degrades from one-third NM at picture center to about two NM at the edges.⁷ The resolution of the data from the Block 5D satellite is more constant, resulting in a degradation of less than two to one.

In addition to the imaging systems, most meteorological satellites carry a complement of instruments that measure a number of atmospheric, exoatmospheric, and solar parameters. One of the most important of these instruments is the infrared profiling radiometer, which measures upwelling energy from narrow spectral intervals in a region of strong atmospheric absorption.. The energy data are then inverted to temperature or absorber concentration. By careful selection of the proper spectral intervals, a mean vertical profile of temperature and water vapor concentration (humidity) can be obtained. This information is input into global numerical weather analysis models. Both NOAA and DMSP polar orbiters have carried a vertical temperature profile radiometer since 1972, and the retrieved temperature information is used operationally to prepare forecasts at the AFGWC.

Instruments that measure upper atmospheric, exoatmospheric. or solar parameters are still in their infancy. They cover the electromagnetic spectrum, ranging from measurements of cosmic and x-rays to the monitoring of high frequency radio waves to determine the critical frequency for over-the horizon communication and navigation systems.

Meteorological satellites provide weathermen and military decision-makers with large area observations or depictions of the existing weather. This imagery is especially valuable over the vast data-sparse regions of the globe and is indispensable over unfriendly, data-denied areas during times of conflict.

Geostationary satellites provide wide area coverage that is almost continuously available on demand. This can be a significant advantage for example, for battlefield support. However, the resolution and usefulness of the data degrades north-south from the equator as well as east-west from the longitude of the satellite subpoint; useful coverage does not extend much beyond 55º north or south. Because of their much higher altitude, geostationary satellites provide data that are much more difficult to locate geographically with certainty. Geostationary satellites also cost roughly an order of magnitude more than corresponding polar orbiters.

Since the military needs worldwide high-resolution satellite weather imagery that can be precisely gridded, the DMSP system has relied on sun-synchronous polar-orbiting satellites. Two satellites are routinely kept in operational orbits to provide coverage four times per day over all areas of the globe. One satellite is in an early morning/evening orbit, and the other is in a near noon/midnight orbit.

The DMSP system has designed the ground-processing and display equipment to be responsive to military needs. The display equipment produces a high-quality positive film transparency that is available within five minutes after receipt of the last line of data from the satellite. The display system also has many enhancement and processing options. For example, the brightness of the visible imagery can be enhanced to accentuate the clouds, or the ground, or water; infrared imagery is presented as Kelvin temperature, and selected temperature levels can be displayed separately. The foreshortening of the imagery at the edges caused by the Earth's curvature is reduced in the display system by using a sinusoidal sweep rate on each scan line. While this produces an equal-area-rectified image, it does not compensate for the loss of resolution at the edges.⁸

The DMSP direct readout equipment has been installed in trailers that are transportable by C-5 aircraft. With this equipment, known as Transportable Terminal Systems (TTS), direct readout of DMSP imagery can be made available to military commanders anywhere on the globe within a matter of hours after arrival on site.

As mentioned previously, the AFGWC receives the stored global imagery and other data from the DMSP satellites. These data are reproduced on positive transparency film for immediate use. Simultaneously the data are input into electronic data processing equipment and used in developing the many AFGWC computer-assisted analyses and forecasts. Examples of the support that is enhanced by these computer-processed meteorological satellite data include detailed cloud information for computerized flight plans, cloud cover forecasts for aerial refueling operations, point analysis information for the environmental impact determination for new weapon system testing, and a comprehensive cloud climatological data base for the development of algorithms for computing probabilities of cloud-free line-of-sight (CFLOS) for electro-optical guidance systems.

The role of the meteorological satellite and the DMSP program will continue to grow. In 1976, the first of the newest generation of DMSP satellites, referred to as Block 5D, was launched. These satellites are designed for longer on-orbit operational lifetime, improved gridding and data location accuracies through improved satellite positioning techniques, and increased data resolution by making the resolution of the imagery nearly constant across the photograph. The constant resolution is accomplished by varying the detector site and orientation (smallest at data edge, largest at data center) while scanning at a sinusoidal raw (slowest at data edge, fastest at data center). In addition, a feasibility model of a smaller direct readout system has been tested. The smaller Transportable Terminal Systems will be transportable by C-l30 or C-141 aircraft and suited for tactical bare base deployments.

Military and civilian scientists are also testing and evaluating new satellite-borne sensors that promise to overcome some of the limitations of today's systems. Microwave sensing instruments, depending on frequency, are not sensitive to higher, drier cloud formations; they can (in a sense) see through many cloud types and depict the areas of concentration of rainfall or clouds with larger water droplets. Microwave sounders and imagers have been flown on NASA experimental satellites, and a sounder will be flown on future DMSP and NOAA satellites.

The Space and Missile Systems Organization of Air Force Systems Command is sponsoring a mission analysis and follow-on studies that include analyses of meteorological satellite support to strategic and tactical forces. These studies will review such topics as the adaptation of geostationary satellite Systems to meet military needs as well as the development of computer forecasting models based entirely on data inputs from satellite-borne sensors.

In addition, the World Meteorological Organization is conducting Global Atmospheric Research Projects (GARP) in an attempt to improve man's understanding of the basic atmospheric circulation patterns and associated weather phenomena. The First GARP Global Experiment (FGGE) will be conducted during 1978. Nations around the world will join in taking detailed simultaneous observations of the Earth's atmosphere in the mid-latitudes and the tropics. Geostationary meteorological satellites are scheduled to be placed around the globe to ensure total coverage of the earth's surface between 60ºN and 60ºS. FGGE should provide some new insights to observing and forecasting techniques using geostationary platforms.

Much has been accomplished since the launch of Tiros I in 1960, yet military meteorologists and space engineers are continually working to improve the weather support provided to the military decision-maker. Satellite meteorology, as supported by the Defense Meteorological Satellite Program, is an invaluable aid. In unveiling some of the DMSP photos before a Pentagon press conference in March 1973, *Dr. John L. McLucas (then Secretary of the Air Force) said that DMSP "furnishes the best data possible to decision-makers anywhere in the world whose operations are affected by weather."

Notes

1. John F. Fuller, Weather and War, Scott AFB, Illinois: Military Airlift Command, December 1974, p. 16.

2. Charles W. Dickens and MSgt Charles A. Raverstein edited by John F. Fuller. Air Weather Service and Meteorological Satellites 1950-1960, Air Weather Service Historical Study No. 5, Scott AFB, Illinois: Military Airlift Command, December 1973, p. 1.

3. Eugene M. Emme, The History of Rocket Technology (Detroit: Wayne State University Press, 1964), pp. 46-47.

4. Ibid., p. 75.

5. Ibid., p. 74.

6. Ibid., pp. 108-9.

7. James R. Blackenship and Richard C. Savage, "Electro-Optical Processing of DAPP Meteorological Satellite Data," American Meteorological Society Bulletin, January 1974, p.9.

8. Ibid., p. 5.

Major Ernie R. Dash (M.S., University of Southern California) is Commander of Detachment 75, 3d Weather Squadron (MAC), Hurlburt Field, Florida. He has served as a Defense Meteorological Satellite Program tactical site commander and recently was assigned as the DMSP Staff Officer in the DCS/Operations of Headquarters Air Weather Service. Major Dash is a graduate of Air Commander and Staff College and Industrial College of the Armed Forces.

Major Walter D. Meyer (Ph.D., University of Washington) is Special Assistant for Air Force/Army to the Defense Meteorological Satellite Systems Program Office, Headquarters, Space and Missiles Systems Organization, Los Angeles. He has served as operations officer of the meteorological satellite data processing section at the Air Force Global Weather Central, Offutt AFB, Nebraska, and recently was assigned as DMSP Staff Officer in the DCS/Aerospace Sciences of Headquarters Air Weather Service. Major Meyer is a graduate of Air Command and Staff College and of the Industrial College of the Armed Forces.

Disclaimer

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.

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