Satellite launch Launching a communications satellite into orbit is a complex and expensive process. This first stage in a satellite's airborne life may cost several million dollars. The cost for launching is primarily a function of the satellite's weight and size. Traditional geosynchronous communications satellites tend to be large and more costly to launch, although the more compact digital communications payloads and longer satellite life will reduce life cycle costs. Low earth orbit communications satellites tend to be smaller and more economical to launch, but will have shorter in orbit life shortage of launch vehicles influenced the economics of the launch industry following the 1986 U.S. Space Shuttle Challenger disaster. The shortage has now given way to other launch alternatives. The dominant player in the satellite launch business is the French company Arianespace. Major U.S. players in the satellite launch usiness are Lockheed Martin, McDonnell Douglas, and Orbital Sciences Corporation. China and Russia have also begun providing launch services The launch of a satellite payload into the geosynchronous orbit involves many complex steps. Using the launch vehicle, the payload is first placed in a parking orbit. This is a nearly circular orbit which places the satellite approximately 300 km above the earths surface. After reaching this orbit, the next step is to fire a motor known as the payload assist module(Pam)to place the payload in a transfer orbit. The PAM motor is discarded afterwards. The transfer orbit is an elliptical orbit whose perigee matches the parking orbit and whose apogee matches the geostationary orbit. Perigee is defined as the point in the orbit closest to the earth, while apoge is the point in the orbit furthest from the earth. The payload itself consists of the satellite with an apogee kick motor(AKM). Once in a transfer orbit, the AKm is fired at the point when the satellite has reached apogee. This firing will place the satellite in a nearly circular orbit. Final positioning of the satellite in geosynchronous orbit can then take place [ Pritchard and Sciulli, 1986] Spacecraft and Systems A satellite spacecraft employs several major subsystems. These are propulsion, electrical, tracking, telemetry command and control, and the communications subsystem Figure 102. 4 is a diagram of a typical commercial satellite. The propulsion subsystem consists of thrusters oriented in north-south and east-west directions and is used to maintain the spacecraft in the proper orbit and orientation. An electrical subsystem is used to generate lectricity in the spacecraft by means of solar cells. Backup batteries are used during periods of equinoxes. The used to chan commands from the controlling ground station, as well as to allow the ground station to monitor on-board systems The spacecraft requires some form of stabilization to prevent it from tumbling in space. There are two types of stabilization techniques: spin stabilization and three-axis stabilization. Spin stabilization uses an outside rinder to spin, creating the effect of a gyroscope providing spacecraft stabilization. An internal platform is decoupled from the cylinder, whose orientation is fixed towards the earth. Three-axis stabilization uses internal gyros which sense movement of the spacecraft. Any movement in the axes is detected and can be compensated by firing thruster jets The communications subsystem consists of receiver and transmitter sections. The receiver system consists of wideband redundant units. The transmitter subsystem consists of separate amplifiers(transponders)for each channel utilized. Satellite systems make use of orthogonal polarized signals in order to transmit two signals Itaneously on the same frequency, a technique known as"frequency reuse. Two different polarization thods for signals are used: horizontal and vertical linear polarization, or clockwise and counterclockwise circular polarization Figure 102.5 shows a simplified block diagram of a typical satellite. A matrix-type switching arrangement provided on the input and output of the transmitter subsystem for switching to backup transponders. This satellite is three-axis stabilized and operates at Ku-band. There are 16 operational transponders with a bandwidth of 54 MHz each. The employment of frequency reuse provides nearly 1000 MHz of usable bandwidth. Fourteen of the 16 operational transponders use 20-w traveling wave tube amplifiers(TWTA)to provide ground commandable east or west regional coverage, for 48-state( CONUS)coverage. The remaining two transponders provide 50-state coverage using 27-W TWTAs. For the 50-state channels, one spare 27-W TWTA provides e 2000 by CRC Press LLC© 2000 by CRC Press LLC Satellite Launch Launching a communications satellite into orbit is a complex and expensive process. This first stage in a satellite’s airborne life may cost several million dollars. The cost for launching is primarily a function of the satellite’s weight and size.Traditional geosynchronous communications satellites tend to be large and more costly to launch, although the more compact digital communications payloads and longer satellite life will reduce life cycle costs. Low earth orbit communications satellites tend to be smaller and more economical to launch, but will have shorter in orbit life. A shortage of launch vehicles influenced the economics of the launch industry following the 1986 U.S. Space Shuttle Challenger disaster. The shortage has now given way to other launch alternatives. The dominant player in the satellite launch business is the French company Arianespace. Major U.S. players in the satellite launch business are Lockheed Martin, McDonnell Douglas, and Orbital Sciences Corporation. China and Russia have also begun providing launch services. The launch of a satellite payload into the geosynchronous orbit involves many complex steps. Using the launch vehicle, the payload is first placed in a parking orbit. This is a nearly circular orbit which places the satellite approximately 300 km above the earth’s surface. After reaching this orbit, the next step is to fire a motor known as the payload assist module (PAM) to place the payload in a transfer orbit. The PAM motor is discarded afterwards. The transfer orbit is an elliptical orbit whose perigee matches the parking orbit and whose apogee matches the geostationary orbit. Perigee is defined as the point in the orbit closest to the earth, while apogee is the point in the orbit furthest from the earth. The payload itself consists of the satellite with an apogee kick motor (AKM). Once in a transfer orbit, the AKM is fired at the point when the satellite has reached apogee. This firing will place the satellite in a nearly circular orbit. Final positioning of the satellite in geosynchronous orbit can then take place [Pritchard and Sciulli, 1986]. Spacecraft and Systems A satellite spacecraft employs several major subsystems. These are propulsion, electrical, tracking, telemetry command and control, and the communications subsystem. Figure 102.4 is a diagram of a typical commercial satellite. The propulsion subsystem consists of thrusters oriented in north-south and east-west directions and is used to maintain the spacecraft in the proper orbit and orientation. An electrical subsystem is used to generate electricity in the spacecraft by means of solar cells. Backup batteries are used during periods of equinoxes. The solar cells are also used to charge the batteries. The tracking, telemetry, and command subsystem is used to receive commands from the controlling ground station, as well as to allow the ground station to monitor on-board systems. The spacecraft requires some form of stabilization to prevent it from tumbling in space. There are two types of stabilization techniques: spin stabilization and three-axis stabilization. Spin stabilization uses an outside cylinder to spin, creating the effect of a gyroscope providing spacecraft stabilization. An internal platform is decoupled from the cylinder, whose orientation is fixed towards the earth. Three-axis stabilization uses internal gyros which sense movement of the spacecraft. Any movement in the axes is detected and can be compensated by firing thruster jets. The communications subsystem consists of receiver and transmitter sections. The receiver system consists of wideband redundant units. The transmitter subsystem consists of separate amplifiers (transponders) for each channel utilized. Satellite systems make use of orthogonal polarized signals in order to transmit two signals simultaneously on the same frequency, a technique known as “frequency reuse.” Two different polarization methods for signals are used: horizontal and vertical linear polarization, or clockwise and counterclockwise circular polarization. Figure 102.5 shows a simplified block diagram of a typical satellite. A matrix-type switching arrangement is provided on the input and output of the transmitter subsystem for switching to backup transponders. This satellite is three-axis stabilized and operates at Ku-band. There are 16 operational transponders with a bandwidth of 54 MHz each. The employment of frequency reuse provides nearly 1000 MHz of usable bandwidth. Fourteen of the 16 operational transponders use 20-W traveling wave tube amplifiers (TWTA) to provide ground￾commandable east or west regional coverage, for 48-state (CONUS) coverage. The remaining two transponders provide 50-state coverage using 27-W TWTAs. For the 50-state channels, one spare 27-W TWTA provides