In 1945, an article authored by Sir Arthur C. Clarke was published in Wireless World magazine that stated an extra-terrestrial satellite relay would “appear” in one spot in the sky if the satellite were at approximately 35,787 kilometers (22,237 miles) above the Earth’s equator. As a matter of fact, a satellite does indeed appear to stay in one place in the sky by traveling at the same speed as the Earth’s rotational speed – called a sidereal rotation period.
For operational purposes, Intelsat’s world-class operations teams strive to keep its satellites within an “operational box” of 0.05 degrees – plus or minus – horizontally and vertically from the location we are allowed to operate it (also known as East/West and North/South travel or station-keeping).
For example, if we are allowed to operate a satellite at 123º West Longitude over the equator, the satellite would be considered station-kept while the satellite “drifts” within 122.95º West to 123.05º West for horizontal and plus/minus 0.05 degrees North and South travel vertically from the equator in a 24-hour period.
However, many factors affect a satellite’s speed, such as the solar wind and the gravitational fields of the Earth, moon and sun. Each one of these factors can speed up or slow down a satellite depending on which axis the force and direction is applied to the satellite.
Over time, and left on its own, this speeding up and slowing down will cause a satellite to drift away from its operational box. To counteract these undesired forces, early engineers designed small thruster systems placed onboard the satellite for station-keeping purposes. These thruster systems have built-in redundancies to ensure satellites can be controlled at all times. These systems can be bi-propellant (bi-prop or chemical fuel) or Xenon-Ion Propulsion systems (XIPS – pronounced “zips” or ion propulsion).
To keep a satellite in its proper operational box, Intelsat’s Flight Dynamics Department models the travel pattern for a satellite at its orbital location for several days in the future. This is called ephemeris data, which is then posted to enable customers who use antennas to track our satellites. Flight Dynamics also predicts when and how long a station-keeping maneuver is needed to keep the satellite within the station-kept box.
Intelsat engineers then compare telemetry readings and daily periodic measurements from the ground to the satellite (called ranging) to the theoretical travel to ensure proper station-keeping. Any variance in actual from theoretical travel will be evaluated to determine the proper action necessary to keep the satellite in a station-kept orbit.