Understanding space fundamentals will aid the user in selecting the best space support. Orbital characteristics and space system limitations are two of the fundamentals.
Generally, the orbital characteristics of a space system are related to the function of the satellite. Satellites may be in circular or elliptical orbits which vary in altitude from 200 miles to over 22,500 miles from the earth's surface. Low orbits, being closer to the earth, best support sensing requirements. The disadvantages of a low orbit are a limited field of view of the earth and a short station time over any given earth area. As altitude increases, so does the field of view and station time; but the ability to resolve small objects decreases at higher altitudes.
The time it takes to complete one complete revolution of the earth is known as the orbital period. The period relates directly to the orbit's average distance from earth and is a function of the satellite's velocity. The greater the velocity imparted to the satellite during orbital insertion, the greater the orbital period and average distance from earth. Periods range from 90 minutes for the lowest orbits to 24 hours or more for deep space orbits.
Another element of a satellite's orbit is its inclination, which is the angle at which the satellite's orbital plane crosses the equator. A higher inclination generally means that more of the earth's surface is covered. A polar orbit, with an inclination of 90 degrees, crosses all latitudes, while lesser or greater inclinations only provide coverage for increasingly higher latitudes, particularly for satellites in low earth orbits.
The length of time between satellite coverage of a particular earth location, that is, the satellite's revisit time, depends upon a number of factors. For any given satellite, revisit time depends upon both orbital period and inclination. For earth coverage by like systems, revisit time also depends upon the number of satellites in the constellation, the capabilities of the satellites' payloads, and the footprints of the various onboard sensors.
One type orbit which is particularly useful for wide area-continuous observation has a period of 24 hours and an inclination of 0 degrees. In this orbit, which is referred to as geosynchronous, a satellite orbits the earth around the equator at the same rate that the earth rotates below the satellite. While the satellite orbits at very high speed at an altitude of 22,500 miles, it appears to remain stationary over the same point on the earth's surface. Satellites in geosynchronous orbit provide continuous observation of most of a global hemisphere.
While satellites can provide the Army many valuable capabilities, planners and users must understand some of their general limitations. Though not all-inclusive, the following limitations represent areas that must be considered when planning and requesting space support.
Satellites, and the launch operations which support them, are extremely expensive and manpower intensive. For these reasons, military satellites are national resources, supporting the NCA, CINCs, other services and government agencies, and other tactical users. As a result, requirements can frequently exceed capacity and the system's capabilities. A validation process to determine what requirements will be satisfied is based upon priority and system availability. The NCA, through the JCS, allocates satellite resources to the joint force commander who allocates those resources to users within the theater according to the JFC's priorities.
Satellite systems are vulnerable to environmental conditions in space such as temperature extremes, radiation, meteoroids, and space debris. Satellite systems also can be affected by atmospheric disturbances and solar activity such as solar flares. Clouds, fog, and smoke affect the ability of imaging systems to see, and rain may degrade some radio signal frequencies. Solar flare and glare can blank out areas of the earth's surface from infrared observation for hours each day during the vernal and autumnal equinox period. Since blanked out areas differ for satellites in different orbits, redundant coverage of crisis areas by two or more satellites mitigates the effects of solar flare and glare and ensures continuous coverage of the entire theater.
Operationally, the Army is dependent upon systems currently on orbit, although these systems may or may not be suited to a particular Army mission. Satellites do not provide continual coverage; for example, LANDSAT sensors revisit a point on the earth approximately every 16 to 18 days. Moving a satellite to a more advantageous orbit or position takes time and is limited to the amount of fuel on board since satellites cannot be refueled. As satellites age, components become degraded or fail altogether, decreasing the satellite's utility and reliability. Satellites in geosynchronous orbit have poor viewing geometry towards the edge of their coverage along the limb of the earth. Though overlapping coverage mitigates the effect in the higher latitudes, coverage of the polar regions is poor.
The space systems discussed are divided into five categories. A brief narrative is provided at the beginning of each category. Specific systems are described in terms of their space, control, and user segments.
Communications satellites receive signals from user terminals and retransmit them to other ground, shipboard, or airborne stations. They provide direct line-of-sight communications and eliminate the need for miles of cables or numerous ground relay stations. The Army uses military and commercial communications satellites to provide a significant capability. Satellite communications carry a large portion of intercontinental, intertheater, and a significant portion of intratheater traffic at division level and above. Some tactical intratheater users are also supported. Deploying forces can quickly establish communications within the theater of operations and back to their deployment base, even in areas where there is no established communications infrastructure. During a crisis, demand for satellite communications does, however, exceed current capabilities. Communications satellites operate in a variety of radio frequency bands. The most common are UHF, SHF, EHF, and commercial C-band and Ku-bands.
The Fleet Satellite Communications (FLTSATCOM) system provides worldwide communications for DOD mobile forces, including fleet broadcast services and command and control to surface ships, aircraft, and submarines. FLTSATCOM operates in the UHF band. The FLTSATCOM system consists of a mix of FLTSATs and dedicated, leased satellites. The FLTSATs will be replaced by UHF follow-on satellites in the future. All are positioned in geosynchronous orbits over the equator, spaced around the world at an altitude of 35,800 kilometers (22,250 miles). Each satellite can be repositioned to support specific mission requirements. Each FLTSATCOM satellite can relay 11 separate channels. FLTSATCOM satellites are controlled by the Naval Space Command. Channel capacity and access are allocated to the unified and specified CINCs by the JCS. The Navy has dedicated use of the ten 25-kilohertz channels on FLTSATs. Army users may request access; however, it is usually difficult to obtain. The CINCs apportion their assigned channels among their assigned forces.
The Defense Satellite Communications System (DSCS) provides high-capacity, wideband, jam-resistant super-high frequency for worldwide long-haul communications between fixed stations and critical mobile users. DSCS is essential for the transmission of the large volume of information required to operate and support deployed units. The current space segment consists of a mix of DSCS II and DSCS III satellites in geosynchronous orbit. DSCS satellites are a critical part of the Defense Communications System. Some of the satellites are on-orbit spares which can be activated to provide additional capabilities when needed.
Each DSCS II satellite has two transponders which provide four channels through two earth coverage antennas and two narrow spot beam antennas. Each DSCS III satellite has six transponders providing six channels through earth coverage antennas, narrow beam antennas, and multibeam antennas. The DSCS satellites provide worldwide coverage from 70 degrees north to 70 degrees south latitude. Additional DSCS III satellites are available for launch.
Operational command of DSCS is provided by the US Space Command. Management of user traffic and network configuration is a function of the Defense Information Systems Command. Payload control on DSCS III satellites is accomplished through DSCS operation centers (DSCSOCs). The Army Space Command operates all five of the DSCSOCs. Platform control of DSCS III satellites and both payload and platform control of DSCS II satellites are provided by the Air Force Space Command. Users include the Defense Information Systems Agency which supports many government agencies, US Air Force, US Navy, US Marine Corps, and US Army. DSCS supports the Worldwide Military Command and Control System, the Defense Data Network, and the Defense Switched Network. Channel capacity is allocated to the unified and specified CINCs by the JCS through coordination with Regional Space Support Centers. The CINCs further allocate their channels within their command.
DSCS supports tactical communications through the Ground Mobile Forces Satellite Communications (GMFSC) program. The GMFSC program provides critical communications support for critical command, control, communications, computer, and intelligence requirements. The Army has about 200 GMFSC terminals. These terminals connect other Army communications systems, such as mobile subscriber equipment, to provide connectivity between the dispersed units and to deployment and support bases in CONUS and other theaters.
Satellites have unique capabilities that can be used for RISTA. Space-based sensors have the advantage of unrestricted access over battlefields and denied areas. They can be used to observe enemy weapons development, verify compliance to treaties, determine the deployment of land, sea, and air forces, and provide weather data. If hostilities begin, space systems can provide targeting information, attack warning, battle damage assessment, and technical intelligence on enemy capabilities. When the information provided through space systems is integrated with that gathered by other systems, a more complete IPB is attained.
The Army Tactical Exploitation of National Capabilities Program (TENCAP) provides Army commands with equipment that can receive and process data provided by the national space systems. Initially, ground terminals were developed for use at corps and higher headquarters. The evolution of the TENCAP systems has made it feasible to deploy certain systems to echelons below corps also. The request and dissemination process, system capabilities, and specific applications can be found in the Joint-Tactical Exploitation of National Capabilities (J-TENS) Manual.
Environmental monitoring consists of gathering weather and terrain data. Three satellite systems are the primary contributors.
Weather has a significant impact on the conduct of tactical operations in terms of visibility, temperature, and trafficability. Electro-optically guided weapons and other weapons are particularly sensitive to weather conditions along their flight path. Space systems provide detailed information on the current atmospheric conditions over a wide area, to include areas where there are few weather sensors or where political and military considerations restrict the gathering of weather data. Satellite imagery and data also support the preparation of accurate weather forecasts. The Army requires both imagery and vertical profile data; for example, atmospheric conditions from both military and civil weather satellites.
The Defense Meteorological Satellite Program (DMSP) provides worldwide visible and infrared cloud imagery and other meteorological, oceanographic, and space environmental data for the Department of Defense. Normally, two DMSP satellites are maintained in sun-synchronous, near polar, 833 kilometers (518 miles) high circular orbit. Each satellite provides coverage of the entire earth every 12 hours. With two satellites, a specific area is observed once every four to eight hours. DMSP satellites carry a variety of sensors which collect data of an area up to 2,960 kilometers (1,839 miles) wide. The primary sensor is the Operational Linescan System (OLS) which provides cloud imagery in visible and infrared bands. In the fine mode, resolution of images of areas designated by DMSP control can be 0.6 kilometers. All other areas are imaged in the smooth mode which provides a resolution of 2.8 kilometers in daylight or 3.5 kilometers at night. In addition to the OLS, DMSP satellites have a microwave sensor which provides a vertical temperature profile of the atmosphere. Each satellite also has sensors to measure the space environment and the upper reaches of the ionosphere. Data transmitted by the DMSP satellites is encrypted. DMSP is controlled by the Air Force Space Command.
The human eye sees light across a region of the electromagnetic spectrum known as the "visible" region. Human vision presents an image as a color rendition of the world, visible under typical conditions. A spectral imaging system can be designed to receive in narrow bands, for example, only green. A multispectral imager is one that "sees" in several specific bands (wavelengths) simultaneously and stores the information as separate images. If this is done in the "visible" spectral region, the result is not a color image, but may be a band of green, a band of blue, and a band of red information. If used in other regions of the spectrum, the resultant increase in detail can be dramatic.
Multispectral imagery (MSI) from space has proven invaluable for mapping, geology, agriculture, earth resources, oceanography, and environmental monitoring. The information available through multispectral imagery can be obtained by a satellite in about 25 seconds or through a ground survey taking months, many people, at great expense, and often without the same amount of detail. Many areas cannot be easily surveyed from the ground or by aircraft because they are too heavily congested, too remote, or access is restricted for military or political reasons. There are seven satellite systems collecting MSI. Each provides data with a unique combination of bands, resolution, and times of coverage.
Land satellite (LANDSAT) is a US civil satellite system used to provide worldwide land surface data and some ocean data. LANDSATs are launched into a 705-kilometer high, sun-synchronous, near polar, (98.2-degree inclination) orbit that repeats its ground trace every 16 to 18 days. The primary multispectral sensor is the Thematic Mapper which has 7 different bands. The maximum multispectral resolution is 30 meters. With processing at a ground station, the effective resolution can be enhanced to about 15 meters. The Earth Observation Satellite (EOSAT) Company is under contract to operate the LANDSAT spacecraft. Data from LANDSAT is received at ground-processing stations that have been licensed to process the data. There are more than 30 LANDSAT groundprocessing centers around the world. In the US, the EROS Data Center in Sioux Falls, South Dakota, processes and stores LANDSAT data.
Systeme Probatoire d'Observation de la Terre (SPOT) satellites have the same mission as LANDSAT with slightly different capabilities. They are launched into a sun-synchronous, 832-kilometer high orbit with an inclination of 98.7 degrees and a period of 100 minutes. This orbit repeats its ground trace every 26 days. The two high-resolution visible (HRV) range instruments have one panchromatic band with a resolution of 10 meters and three multispectral (visible wavelength) bands with a resolution of 20 meters. The HRVs operate independently from one another and can image at an angle within 27 degrees either side of vertical. This off-nadir viewing allows the satellite to see a 950 kilometer wide corridor. It also permits the development of stereo views by imaging the same area on different passes at different angles. If only vertical viewing is used, the swath is 60 kilometers wide. SPOT is controlled by the European Space Agency with France as the executive agent. Customers submit coverage requests and payment to SPOT marketing offices located around the world.
The Global Positioning System (GPS) is a satellite-based system developed by DOD to provide continuous, all weather, global position, navigation, velocity, and precision time information. GPS consists of 24 operational satellites in 20,260 kilometers (10,950 nautical miles) altitude, semi-synchronous, circular orbits, evenly spaced in 6 orbital planes. A minimum of 5 satellites is always within view of any ground user, thus providing continuous three-dimensional capability. Satellite life expectancy is 6 to 7 years each. Replacement satellites will be launched to maintain the constellation of 24 GPS satellites.
Satellite control is performed by the US Air Force Space Command assisted by Army personnel assigned to US Army Space Command. All US military services, some allied military services, and certain other designated users have access to Precision Positioning Service (PPS), if equipped with terminals capable of loading a COMSEC code. GPS receivers provide a degree of accuracy for position determination, navigation, velocity, and time synchronization never before possible. GPS receivers are passive, therefore, they cannot be detected by electronic means and there can be an unlimited number of users. Signals from three GPS satellites are required for two-dimensional position determination (user provides altitude). Signals from 4 satellites are required for three-dimensional position determination (no user input required). PPS provides three-dimensional positioning accuracy of 16 meters spherical error probable (SEP) and two-dimensional position accuracy of 10 meters circular error probable (CEP). Depending on the type of receiver used, the time accuracy can be as good as 48 nanoseconds. Velocity accuracy is 0.2 meters per second.
The Defense Support Program (DSP) was initiated as a space-based strategic surveillance system to detect the launch of ICBMs and SLBMs. Its utility has expanded to support theater missile defense operations. DSP satellites are located in geosynchronous orbits to provide continuous coverage over the eastern and western hemispheres. DSP satellites can detect the launch of ICBMs, SLBMs, tactical ballistic missiles, satellite booster rockets, and certain other rockets. USSPACECOM exercises authority over DSP through the Air Force Space Command which operates and controls the satellites. Data is centrally processed and transmitted to users. The principal users are the National Command Authority, US Space Command, Strategic Command, North American Aerospace Defense Command (NORAD), and unified and specified CINCs.
The Tactical Event Reporting System (TERS) is a worldwide distribution system currently made up of the Tactical Receive Equipment and Related Applications (TRAP) system, the Tactical Information Broadcast System (TIBS), and the Joint Operation Tactical System (JOTS). Within two to four minutes of launch, the theater commander is provided tactical missile launch warning data, including the place of launch, time of launch, type missile, and a course azimuth. This warning information can be used to alert friendly forces (defensive weapon systems such as air defense systems) and attack systems.
TERS will transition into the Tactical Event System (TES) in the near future. TES will use the same communications architecture currently used by TERS, but will provide more timely and more reliable missile launch warning. TES warning messages will originate at different ground segments which each contribute different but complementary detection and processing capabilities. The components of TES are the Attack and Launch Early Reporting to Theater (ALERT) system, the Tactical Detection and Reporting (TACDAR) system, and the Joint Tactical Ground Station (JTAGS).
JTAGS is a new satellite receiver that allows direct downlink of missile and other warning data into the theater. Data is processed and disseminated in theater via the TRAP and TIBS networks. JTAGS will also disseminate voice warning to forces in the theater. Within 2 minutes of missile launch, tactical parameters, to include estimated impact area, are available to support tactical missile defense operations. USSPACECOM executes its control of JTAGS through the Army Space Command (ARSPACE).
The Army has been actively involved in space operations since Dr. Wernher von Braun and other German scientists joined the Army's program to develop militarily useful missiles and rockets in 1945. The efforts of Army scientists working at White Sands Missile Range in New Mexico and at Redstone Arsenal and the Marshall Space Flight Center in Alabama resulted in the birth of the United States space program. In 1958, the Army space program became the nucleus of the newly created National Aeronautics and Space Administration (NASA), which assumed responsibility for all civilian space operations. That same year, NASA launched the nation's first satellite into orbit using an Army Redstone rocket. The Redstone was also used to launch a Mercury capsule carrying Commander Alan B. Shepard into space in 1961, marking the beginning of the nation's manned space flight program.
As NASA and the newly created United States Air Force assumed greater responsibility for civilian and military space operations, the Army's role in space declined proportionately. The Army's space interests were limited to development of air and strategic ballistic missile defense capabilities, and to exploitation of national space capabilities for support of tactical operations.
In 1973, the Army Space Program Office (ASPO) was established to improve support by national capabilities to tactical commanders. The Joint Tactical Exploitation of National Capabilities Program has fielded a number of systems for use by Army commanders. The Strategic Defense Command (SDC) has led Army efforts to develop defenses against both strategic and tactical ballistic missiles.
In 1985, DOD established the United States Space Command (USSPACECOM) to exercise combatant command over the service's space operations. The Commander in Chief, USSPACECOM, provides space support to other theater commanders as a supporting CINC. The Army's Space and Strategic Defense Command (SSDC), which consists of ARSPACE and SDC, is the Army component of USSPACECOM. ARSPACE is the Army operational component of USSPACECOM, and SDC continues as the lead agency for missile defense systems development.
The development of the Army's AirLand Battle Doctrine in the 1980s focused on a battlefield that was expanding in time, depth, and lethality. Space offers the Army unique and enhanced capabilities to achieve land dominance. Space systems provide communications, positioning and navigation data, early warning, weather, environmental, and RISTA capabilities which are essential for the successful prosecution of land warfare. FM 100-5 incorporates space support to Army operations. Under this evolving doctrine, space systems are fully exploited to enhance execution of the Army's mission during all phases of force-projection operations.
In the past, space assets were used for support of echelons above corps only. Today, as a result of programs initiated by USSPACECOM, SSDC, and the ASPO, space capabilities are exploited by every soldier on the battlefield. Units involved in counterair operations benefit from the entire suite of national space systems. In addition to intelligence, weather, and terrain support, Army ADA units use space-based communications and early warning of missile attack to significantly enhance air and missile defense operations.