Basic Satellite Communication (3) Components of Communications Satellite Dr. Joseph N. Pelton

Components of a Satellite Payload: Satellite Main Mission Antennas & Communications System Satellite Bus Components Satellite Tracking, Telemetry, Command & Monitoring (TTC&M) Methods to Cope with Rain Attenuation Satellite Power System Satellite “Bus”, Orientation and Stabilization System

Components of a Satellite Main Mission Antennas & Communications System Antennas are constantly pointed to the earth with larger antenna server the lower frequency and smaller antenna service the higher frequency The transponder in the communications system is responsible for receiving the signal, amplifying it and re-transmitting it in the lower frequency back to earth. Most transponders in the C- and Ku- bands now use solid state power amplifiers (SSPAs) to amplify the signal because of their weight, compactness and reliability. Satellites can have 12 to 96 transponders plus spares, depending on the size of the satellite.

Components of a Satellite Satellite Transponders & SSPA The transponder is the “active” part of the satellite communications system that provides the connection between the satellite’s receive and transmit antennas. A transponder bandwidth can frequently be 36 MHz, 54 MHz, or 72 MHz or it can be even wider. A transponders function is to receive the signal filter out noise, shift the frequency to a downlink frequency and then amplify it for retransmission to the ground. The main amplifier may be a Travelling Wave Tube (TWT) or Klystron tube (now usually used for higher frequencies above 20 GHz and at very high power levels (i.e. 100 to 200 watts) or it may be a Solid State Power Amplifier (SSPA) that would be used at lower L, C or Ku bands frequencies. If the transponder is a regenerative transponder then the signal will be converted to base band frequencies and processed there rather than handled at RF bands. The opening price for 36 MHz service on the transponders is around $195000/month, Optus and Pan Am Sat can split their 54 MHz Full transponders into two 27 MHz half transponders. The existing STM-1 in Asia-Pacific was using a 72MHz transponder of the outbound carrier 155Mbps and another 72MHz for the inbound. Presently, 72MHz transponders are mandatorily divided into two transponders. This limitation dramatically reduces the efficiency of 72MHz transponders.

Components of a Satellite Antenna Gain & Path Loss Gain measures the performance of an antenna. The larger the size (or aperture) of an antenna the better it can point to or concentrate a signal towards the desired receiver location on earth. Thus the more the antenna acts like a spot light to illuminate the earth and not send signal uselessly out into space, the higher the gain of the antenna. An Omni antenna that sends signals in all directions equally has a gain of 1 and represents the lowest gain. Path loss comes from the spreading out of a signal from the antenna as it travels from the satellite to user antenna and vice versa. Since the signal spreads in a circle effective power (or flux density) is reduced by the square of the distance traveled.

Components of a Satellite Satellite Transmit Power (EIRP) EIRP = Effective Isotropic Radiate Power. The product of final amplifier power, filtering and coupling losses and antenna gain. Specified in watts or dB (relative to 1 watt) = dBw EIRP density, typically specified in terms of dBw/Hz is a key parameter in determining both uplink and downlink performance.

Components of a Satellite Satellite Transponder & SSPAs Transponder Traffic Loading is a Function of: Transponder Bandwidth Uplink and Downlink satellite and earth station parameters Number of Carriers per transponder. This determines “back-off” which is the reduction in amplifier power from the saturated or maximum output power. Modulation efficiency (i.e. bits/Hz)

Methods to Cope with Rain Attenuation Use lower frequencies where possible Provide link margin + additional link margin in beams where there is heavy rains On-board processing Site diversity Ability to increase dwell time and/or reduce information rate Dwell Time: The amount of time a scanner has to collect photons from a ground resolution cell: (scan time per line)/(#cells per line) It depends on: - satellite speed - width of scan line - time per scan line - time per pixel

Components of a Satellite: Satellite Tracking, Telemetry, Command & Monitoring (TTC&M) Tracking is necessary to know exactly where a satellite is and thus be able to send it commands, receive telemetry and communication with it. Omni antenna is used to be able to track and command satellite even if orbit is disturbed. The TTC&M system is highly automated with computer alarms to sound in case of anomalies are detected. Analogy: School bus represents: spacecraft bus, and passengers represents payload: telecommunications and TV traffic. TTC&M is needed to keep “bus” running and check on status of passengers.

Components of a Satellite: Satellite Power System Main source of power is solar cell panels that are constantly oriented towards the sun. New solar cells are increasingly efficient. The solar cell system is backed up by battery system that provides energy during solar eclipses and other periods of outages. Typical power levels of 2 to 5 KWs for Fixed Satellite Systems and 10 to 12 KWs for Mobile and Broadcast Satellite Systems in GEO. Levels are lower for LEO & MEO Systems.

Components of a Satellite: Satellite “Bus” & Stabilization System The “bus” is the key parts of the satellite that allows its operation in space. The early generation of satellites use gravity-gradient or spin stabilization but now 3-axis body stabilization using momentum wheels is most common because of lifetime & efficiency. The “bus” includes the antenna mast, the thermal control, the sun & earth sensors, the batteries, momentum wheels, solar array controls & on-board computers. Carbon-epoxy structures are used because of strength and light weight.

Components of a Satellite: Satellite “Bus” & Stabilization System Multi-frequency Scanning Microwave Radiometer (MSMR): The MSMR measures Sea Surface Temperature, wind magnitude, water vapor and liquid water content in the atmosphere which are used to compute geophysical parameters of the ocean and weather forecasting. Ocean Color Monitor (OCM): OCM detects the chlorophyll content, suspended sediment and yellow substance (degraded biomass). It is being used in ‘true’ operational sense for identification of Potential Fishery Zones (PFZ) in the sea. OCM is an 8-band multi spectral camera operating in the Visible-Near IR Spectral range. This camera provides an Instantaneous Geometric Field of View of 360 m covering a swath width of 1420 km. Earth Sensor: Earth sensors operate in the infrared domain. They scan a large field of view and then detect signals at the Earth/Space transitions. An optical encoder provides the four angular Earth/Space and Space/Earth transition values, which are then fed to the onboard computer to facilitate roll and pitch angle calculations through a combination of these four transitions. The Dual Cone Scanner (DCS) a single-head earth horizon sensor which provides altitude and two-axis attitude information with minimal sun or moon error.

Assignment Assignment 4: Write short notes on the terms highlighted in yellow color (Slides 4 and 9 of lecture 4) mailto: adeel.akram@uettaxila.edu.pk