Introduction
Did you know that most of the communication satellites that are used for transmitting TV signals and weather information are 1/6 of the distance to the moon? Yes, they are 36 000 km away and that is almost 6 times the radius of the Earth. At this orbit the satellite, seen from the Earth, stays at the same position. This orbit is called a geostationary orbit. Satellites in other orbits will move relative to the Earth. For instance, The International Space Station (ISS) with an altitude of 320 km can be seen moving across the sky during night.
Because the satellites are so far away, you will also get a delay when your phone uses a satellite to link your phone call. The radio signals travel at the speed of light (300 000 km/s).
The satellite is placed in a slot in the geostationary arc. The position is named after its longitude: 1° West, 13° East etc. On this arc reside about 300 satellites, both civilian and military from many countries. The Earth is not perfectly spherical and therefore forces act on the satellite. In addition the Sun and the Moon exert their gravitation on the satellite. These forces will make the satellite move sideway from its original position and travel more than 72 000 km, that is about 0.25 seconds delay just because the satellite is so far away.
It is important to limit this movement to secure that the signals arrive to a certain area on the Earth. This is done by executing maneuverss on the satellites, i.e. fire small rocket engines called thrusters.
Payload
The communication satellites orbiting the Earth do perform different tasks. Examples include, broadcasting TV signals to home users and delivering telephony and fax services to ships at sea. When the signals reach the satellite they are received in an antenna onboard the satellite and fed into the payload part of the satellite. The payload makes sure that the received signals are given another frequency and amplified before transmitting back towards the Earth. This is called a transparent payload, meaning that the signals are only relayed onboard the satellite. In the future there will be an increasing amount of payloads containing computers, meaning that the satellite payload can perform some processing of the signals before they are sent back to the Earth.
Platform
The platform is the structure where the payload is mounted and makes sure the platform itself and the payload survive the harsh environment with respect to temperature, pressure and radiation. Usually it is convenient to split the platform into various subsystems in order carry out these tasks. These subsystems must work closely together. The structure is the physical infrastructure of the satellite. The thermal subsystem makes sure the various equipment on the satellite stay within certain temperature limits in order to work. The attitude control subsystem enables the satellite to point in a certain direction. This is important if you need to receive your television signal. The propulsion subsystem keeps the satellite within a defined box on the geostationary arc. Small rocket engines carry out this task. The telemetry and command subsystem sends health and status information about the satellite to the ground and executes certain commands, e.g. to change the status of equipment or fire the small rocket engines. Finally, the power subsystem provides the platform and the payload with electric power. Both solar panels and batteries are necessary for this purpose.
One tries to make the platform as light as possible because the cost per kilo launched into space is very high. Making the platform light and still able to carry out the tasks we just mentioned is the challenging job of many space engineers.
Launch
The launch of a satellite is a critical part of the whole satellite mission. It is also very expensive, typically 100 million Euros for a large satellite. This means that the cost of a kilo in space is more than the cost of a kilo of gold on ground. No wonder that one of the major challenges in the space industry is to reduce mass. There are many providers of launch capacity today. Among them are the Ariane 4 and Ariane 5 from Europe, Atlas, Delta and the Space Shuttle from the US, Sea Launch from an international consortium of Boeing (US), RSC Energia (Russia), SDO Yuzhnoye / PO Yuzhmash (Ukraine), Kvaerner (Norway), Long March from China, H-2 from Japan and Proton from Russia. Unfortunately launch vehicles sometimes fail, but one should remember that the failure rate is usually less than 1 in every 10 that launch. However, this can be disastrous for an investment, so usually a company that launches a satellite will insure it. The cost of this must obviously be more than the failure rate of the launcher, thus many millions dollars. All these launch vehicles, except for the Space Shuttle, are only used once and are therefor expendable. A challenge for the future will be to make launch cheaper, and therefore make access to space more affordable.
Satellite Operation
A satellite orbiting the earth is an autonomous system which does not require continuous commanding from the ground. To ensure that the satellite is in its correct orbital position and in good health, a Satellite Control Centre (SCC) manned with special trained operators monitoring different satellite status parameters in real-time 24 hours a day. The SCC's computer systems communicate with the satellite using a large antenna located at a Telemetry, Tracking & Command (TT&C) site located somewhere at the earth.
A satellite launched into geostationary orbit, would ideally remain at exactly that position forever. Unfortunately the earth is not a perfect sphere and together with the gravitational forces acting from the moon and the sun, this makes the satellite move north-south and east-west in an oscillating motion. To avoid that the antennas at earth pointing toward the satellite loosing their signal and prevent the satellite to collide with other satellites, the SCC have to keep the satellite into a predefined box in space. This is done by SCC when commanding the satellite to fire some small rocket engines onboard, correcting the orbital movements.
