Chapter 7 Space Systems
SECTION 1 - Overview 7-1 Space Systems Segments
Introduction Space systems are complex pieces of equipment designed to perform specific functions for a specified design life. Space systems are not merely robots launched into space and then left to perform their mission. To keep a space system functioning over many years requires almost constant attention through a complex network of equipment and the involvement of many people.
Space Systems Space systems have three distinct segments:
  • Space Segment: The satellites placed into orbit or components used to launch the satellites.
  • Control Segment: The personnel, equipment and facilities responsible for the operation and control of the satellite and, in many communications systems, control of users' transmissions through the satellites.
  • User Segment: The personnel, equipment and facilities that use the capabilities provided by the satellite payload. The user segment is covered in detail in subsequent chapters which cover specific space systems.
Space Segment There are numerous types of space systems providing a wide variety of capabilities and services. In spite of this diversity, there are similarities among all satellites because they all must operate in the environment of space.
Satellite subsystems All satellites have two principal subsystems:
  • The platform
  • The payload
The Platform The platform is the basic frame of the satellite and the components which allow it to function in space, regardless of the satellite's mission. The control segment on the ground monitors and controls these components. The platform consists of the following components:
  • Structure of the satellite
  • Power
  • Propulsion
  • Stabilization and Attitude Control
  • Thermal Control
  • Environmental Control
  • Telemetry, Tracking and Command
Structure of the satellite The structure or body of a satellite holds all of the components together as an integral unit and provides the interface with the launch vehicle. The structure must be strong enough to withstand the rigors of launch yet light enough to not unduly restrict payload weight. Many different shapes and materials have been used. Most satellites are built in low quantities. Each is designed to accomplish specific functions using technology and materials available at the time. As technology develops, new or improved materials and components are used to build new replacement satellites. It is not uncommon, therefore, that follow­on systems are significantly different in design and configuration even though they perform functions identical to earlier systems.
Power Satellites require power to operate the electrical equipment that is on board. Satellites which have high power sensors or transmit strong or continuous signals require more power than those which have low power sensors and radios. For example, a satellite with a radar emitter and receiver requires a significant amount of power. Communications satellites which receive, process, amplify and retransmit signals sent from users on the Earth or from other satellites require more power than a scientific satellite with only few sensors and a small radio to transmit the data to researchers on the ground.
7-1 Space Systems Segments, cont'd
Types of energy sources There are three types of energy sources:
  • Solar Energy. The most dependable and constant energy source in space is the Sun. The power from the sun arriving at the Earth is about 130 watts per square foot. Solar cells convert light to electricity but have only about 10% efficiency. To generate high power there must be a large area of solar cells. Satellites in low Earth orbit can spend up to half of their time in Earth's shadow. On satellites which do not rotate and are stabilized so the payload always faces the Earth, the solar cells are usually mounted on panels which extend from the main body. These panels are controlled by motors which keep them oriented to face the sun. Spinning satellites usually have solar cells mounted on the outside of the body of the satellite so that some are always facing the sun.
  • Chemical Energy. Chemical energy sources include batteries and fuel cells. When a battery is used as a continuous source of energy and is not recharged it is called a primary battery. Batteries which are recharged by some other energy source are called secondary batteries. Short lived satellites and sounding rockets use primary batteries. Most spacecraft use secondary batteries to provide power during peak usage periods or when another energy source is not capable of meeting demand. Batteries are required so the satellite can continue to function when it is in the Earth's shadow or when peak power is required for a short time. Fuel cells are another source of power; however they are not used on systems designed to operate for more than three months because they consume so much fuel.
  • Nuclear Energy. Nuclear energy sources, either fission reactors or Radioisotope Thermoelectric Generators (RTG), can generate tremendous amounts of heat, which is then converted into electrical energy. RTG s have been used on some Earth orbiting satellites that required high power levels; however, they are more commonly used to provide power to space exploration systems which must operate for many years at greater distances from the Sun. Radioisotopes generate heat when the unstable nuclei decay into a more stable nuclei.
The most common type of power system in use today is a combination of solar cells and rechargeable batteries. Nuclear energy sources are rarely used to provide power to satellites in Earth orbit.
Propulsion Most satellites have an on board propulsion system which is used to achieve initial orbit and to make major position changes. Shortly after reaching initial orbit the satellite is separated from the final stage of the launcher. The final orbit is achieved by firing a kick motor to move the satellite into the final desired orbit and position. Some satellites are designed so that they can be repositioned. In general, changing the orbital plane requires more force than changes within the orbital plane. The kick motor is used to make major changes in the satellite's orbit. Sufficient propellant must be carried on board to last the lifetime of the satellite system. After a satellite's payload is no longer useable, the kick motor is often used one last time to either increase or decrease its orbital velocity. Increasing the speed raises the altitude of the orbit. Decreasing the speed lowers the altitude, sometimes enough so that the satellite is deliberately destroyed reentering the Earth's atmosphere. Either technique allows a replacement satellite to assume the same orbital position.
Stabilization and attitude control Stabilization and attitude control are necessary to ensure that the satellite maintains the proper attitude. Satellites are subjected to a number of forces in space such as particles streaming from the Sun, meteorites, atmospheric drag, gravity from the Moon, gravity gradients and other perturbations. These forces cause satellites to wobble, spin, drift, or move in other ways not desired. Most satellites which provide visual or electronic images of the Earth or its environment maintain three axis stabilization (roll, pitch and yaw). Many communications satellites are designed to rotate about their longitudinal axis (roll) and thus have only two axis stabilization. Two and three axis stabilization allow sensors and antennas to be pointed in specific directions. Devices such as momentum wheels on the satellite help to stabilize the satellite while in orbit. Position, velocity and attitude data from on­board sun sensors, star trackers, horizon scanners and other devices is transmitted to ground control stations. When momentum wheels and other such passive devices cannot compensate or adjust the orbit, the satellite controllers send signals to the satellite to fire thrusters in short spurts to control roll, pitch, yaw and to make corrects in orbital altitude. To reduce size, mass, complexity and cost some small satellites are designed to tumble freely through space without any stabilization or attitude control.
7-1 Space Systems Segments, cont'd
Thermal control The temperature in a satellite must be controlled so that components do not become too hot or too cold. The temperature in a satellite is affected by both internal and external sources. On board electronic equipment and other devices which consume power generate heat. The sun is a source of a vast amount of radiant energy. Radiant energy absorbed by the satellite heats the satellite surfaces and components unless it is dissipated. The ambient temperature of space is a few degrees above absolute zero (­459 degrees F) however, since there is almost no atmosphere, heat transfer between the satellite and the space around it by convection or conduction is almost nonexistent. In very low Earth orbit, however, there can be significant heat generated from friction as the satellite moves at very high speed through the outer reaches of the atmosphere. The most common heat transfer device is the passive radiator which radiates heat from the satellite into space to maintain temperatures within design parameters. A satellite may also require a liquid or gas filled cooling system to transfer heat from internal components to the passive radiator.
Environmental control Manned spacecraft require precise environmental control to ensure that air quality, humidity, water and temperature are maintained within operating limits. Most unmanned satellites do not require environmental control.
Telemetry, tracking and control The TT&C subsystem monitors and controls all of the other systems on the spacecraft, transmits the status of those systems to the control segment on the ground, and receives and processes instructions from the control segment. Telemetry components include sensors throughout the satellite to determine the status of various components, the transmitters and antennas to provide the data to the control segment and even the data itself. (In some documents TT&C stands for Telemetry, Tracking and Control. The tasks are, however, the same).
The payload The function and capabilities of the payload are the reasons a satellite is placed in orbit. The payload provides space­based capabilities to the users. The payload distinguishes one type of satellite from another. The general types of satellite systems are:
  • Communications
  • Position/Navigation
  • Reconnaissance, Surveillance and Target Acquisition (RSTA)
  • Weather and Environmental Monitoring
  • Scientific/Experimental
  • Manned
7-2 Control Segment
Introduction The control segment is responsible for the operation of the overall system which includes platform control, payload control and network control. The control segment consists of ground satellite control facilities and systems on the satellites.
Platform control Platform control involves satellite station keeping, relocation maneuvers and the proper functioning of onboard systems. Platform control must be accomplished for almost all satellites. The tasks involved are accomplished through telemetry, tracking and commanding (TT&C).
Payload control Payload control involves operation and control of the payload on the satellite. Data is provided by the satellite and commands are sent to the satellite through TT&C.
Network control In general, there are two types of networks involved with a satellite system, both of which must be controlled:
  • The network of satellite control and monitoring stations. Control and monitor stations are strategically located around the world to perform platform and payload control.
  • The network of user terminals. Satellite systems which receive and process transmissions from multiple users (such as DSCS, FLTSATCOM and AFSATCOM) require a controlled earth terminal segment (users) to insure that the system provides service to authorized users according to established priorities and within the operating parameters of the system. These systems can provide service to a limited number of users who at any one time. Satellite systems which only transmit signals to users (such as GPS, DMSP and GOES) may have an unlimited number of users and do not require control of user terminals. More information on network control of user terminals is contained in the sections of this chapter pertaining to specific systems.
Telemetry, tracking and commanding (TT&C) Satellites are controlled through TT&C. Some TT&C is required for all satellites, regardless of the payload they carry.
Telemetry Telemetry consists of measurements of the health of the satellite taken by sensors and transmitted to a ground monitoring station. There are two basic categories of telemetry: Health and Status, and Payload Data. A ground monitoring station receives and relays the telemetry to the satellite control facility where it is used to determine the operational status of the satellite and its subsystems.
Health and status Sensors throughout the satellite monitor the condition and performance of various subsystems such as fuel status, attitude and output of solar panels, bus voltages, the output and orientation of star sensors and many others. The data is encoded and transmitted to a ground monitoring station.
Payload data Telemetry also includes data on the operation and status of the satellites payload. For example, on a communications satellite, telemetry would include data on power output of transponders, pointing direction of antennas, and antenna and transponder switch configurations.
Tracking Tracking involves determining a satellites position, altitude and other orbital parameters. Many satellites carry a beacon which transmits a signal to help ground tracking receivers locate the satellite. On­board sensors, such as star trackers, horizon scanners and inertial navigation sensors provide other tracking data. Tracking information is essential to determine a satellite's orbital parameters so that an accurate predictions can be made of where the satellite will be in the future. In this way, the satellite's orbit can be adjusted so that it will be where it is supposed to be when it is supposed to be. To attain the accuracy that is needed usually requires relatively large antennas, therefore the monitoring stations are normally fixed sites.
Commanding Commanding is the act of controlling a satellite. Commanding of a satellite is accomplished by sending signals to it which initiate an action or change the configuration in some way. Commands may be executed by the satellite immediately upon receipt or stored for later execution. Some commands are part of onboard software that allows the satellite to execute certain functions autonomously when a predefined condition exists. Commands may direct the thrusters to fire to change the orbit or may reconfigure the payload to meet the needs of users.
7-2 Control Segment, cont'd
Reliability of satellite systems Satellites are designed to operate dependably throughout their operational life, usually a number of years. This is achieved through stringent quality control and testing of parts and subsystems before they are used in the construction of the satellite. Redundancy of key components is often built in so that if a particular part or subassembly fails, another can perform its functions. In addition, hardware and software on the satellite are often designed so that ground controllers can reconfigure the satellite to work around a part that has failed.
Failures The majority of failures have occurred during launch or during initial deployment and checkout of the satellite.
U.S. satellite manufacturers U.S. satellite manufacturers have repeatedly demonstrated their ability to build satellites which continue to operate well beyond their design life. The result is that satellite systems have proven themselves to be very reliable.
7-3 Air Force Satellite Control Network
Introduction The Air Force is the designated service responsible for platform control of DoD satellites. The organizations and facilities involved are organized into the Air Force Satellite Control Network (AFSCN). The principal organization in the AFSCN is the 50th Space Wing of the Air Force Space Command with headquarters at Falcon Air Force Base, Colorado.
Mission The Air Force Satellite Control Network (AFSCN) provides support for the operation, control and maintenance of a variety of DoD and some non­DoD satellites. This involves continual execution of the tasks involved in TT&C. In addition, the AFSCN provides prelaunch simulation, launch support and early orbit support while satellites are in initial or transfer orbits and require maneuvering to their final orbit. The AFSCN provides tracking data to help maintain the catalog of space objects and distributes various data such as satellite ephemeris, almanacs, and other information.
AFSCN facilities The AFSCN consists of satellite control centers, tracking stations and test facilities located around the world. Mission Control Centers are located at the Consolidated Space Operations Center (CSOC) at Falcon Air Force Base near Colorado Springs, Colorado and Onizuka Air Force Base, Sunnyvale, California. These centers are manned around the clock and are responsible for the command and control of their assigned satellite systems. The control centers are linked to remote tracking stations (RTS) around the world. Space vehicle checkout facilities are used to test launch vehicles and satellite platforms to insure that the onboard systems operate within specifications. The remote tracking stations provide the link between the satellite being controlled and the control center. A similar relationship exists for dedicated networks. Remote tracking stations around the world are needed to maintain frequent communications with the satellite. Without RTSs, the control centers would only be able to contact a satellite when it came into the control center's view. Some satellites, especially those in geostationary orbit, never come within view of their control center. Each antenna at an RTS is referred to as a "side". Side A is normally a 60­foot diameter antenna which is best for TT&C of a geosynchronous satellite because of the distance to the satellite and the fact that the large antenna does not need to move quickly to maintain contact with the satellite. Side B is either a 44­foot or 33­foot diameter antenna. These are commonly used for TT&C of low earth orbit satellites which can move more quickly across the sky. These smaller antennas are more responsive and require less energy to control.
Remote tracking stations Vandenberg Tracking Station (VTS) The Vandenberg Tracking Station is a dual station which can communicate simultaneously with two satellites. It supports ballistic missile test and space launches from the Western Test Range at Vandenberg Air Force Base.
New Hampshire Tracking Station The New Hampshire Tracking Station is located in New Boston, New Hampshire. It is also a dual antenna site and provides considerable support to the Eastern Test Range at Cape Kennedy, Florida.
Thule Tracking Station The Thule Tracking Station is located in Greenland. This station is unique because of its three antennas. Thule is so far north that it can communicate with polar orbiting satellites on every revolution around the Earth but is not positioned to communicate with satellites in geostationary orbit. For this reason, "Side A" is only a 14­foot diameter antenna.
Indian Tracking Station The Indian Ocean Tracking Station is located on Mahe in the Seychelle Islands. It only has one 60­foot antenna. Its location is ideal for communicating with geosynchronous satellites over the Indian Ocean.
Guam Tracking Station The Guam Tracking Station has two antennas. It can provide TT&C for on­orbit support for low earth orbiting satellites and geosynchronous satellites over the western Pacific Ocean.
Hawaii Tracking Station The Hawaii Tracking Station is located on Oahu. It supports ballistic missile testing conducted from Vandenberg Air Force Base and other areas in the Pacific. It also provides on­orbit support for low earth orbiting satellites and geosynchronous satellite over the eastern and central Pacific.
7-3 Air Force Satellite Control Network, cont'd
Remote tracking stations, cont'd Colorado Tracking Station The Colorado Tracking Station is located at Falcon Air Force Base near Colorado Springs, Colorado.
Oakhanger Telemetry and Command Station Oakhanger Telemetry and Command Station is located at Borden Hauts, England.
Diego Garcia Tracking Station The Diego Garcia Tracking Station provides enhanced tracking of low earth orbiting satellites over the Indian Ocean area since its antenna is smaller than the one at the Indian Ocean Tracking Station.
Automated Remote Tracking Stations (ARTS) The Remote Tracking Stations are being modernized with the addition of the Automated Remote Tracking Station (ARTS). ARTSs provide more responsive support and reduce manpower required at each site because the antennas and supporting systems can be controlled by the Mission Control Centers. There are four ARTS installed at the following locations:
  • Colorado Tracking Station at Falcon Air Force Base, Colorado. It is used for support of various DoD satellites. Additional equipment has been installed to support the Global Positioning System satellites.
  • Thule Tracking Station at Thule Air Force Base, Greenland. The third "side" at Thule is an ARTS.
  • Oakhanger Telemetry and Command Station at Borden Hauts, England. The second "side" is an ARTS.
  • Camp Parks Communications Annex at Pleasanton, California. This ARTS is used to test and analyze signals form communications and navigation satellites.
AFSCN Dedicated Control Networks There are also some dedicated satellite control networks to control specific satellite systems. The Defense Meteorological Satellite Program (DMSP) has a dedicated network operated by the 1000th Satellite Operations Group with the Mission Control Center at Offutt Air Force Base, Nebraska. The Global Positioning System has a Mission Control Center at Falcon Air Force Base operated by the 2d Satellite Control Squadron. There are also dedicated GPS monitoring stations around the world. It is planned that Milstar will have a dedicated Master Control Station at Falcon AFB, Colorado.