Trajectory corrections
Satellites, probes and crewed spacecraft are tracked continuously to determine their precise trajectory. When they deviate too far from their nominal orbit, commands are sent to correct this trajectory.
Tracking station at Issus-Aussaguel near Toulouse (France). Crédits : CNES/E.GRIMAULT,2000
Teams of expert engineers on the ground communicate with a satellite by radio signals using large antennas at facilities called tracking stations. Their mission begins as soon as the satellite separates from the launcher—during the positioning phase—and continues throughout its service life, during which various stationkeeping operations need to be performed. Their main task is to locate the spacecraft, predict its trajectory, define operations and send manoeuvring commands.
The satellite’s instruments and antennas are pointed precisely by rotating the spacecraft bus. Its attitude cannot be determined from the ground, so it also has sensors that measure its position with respect to the Sun, stars or a point on Earth.
A lot of trajectory corrections are performed using the satellite’s chemical thrusters. The most important manoeuvres like circularizing the orbit require powerful thrusters, whereas finer positioning and stationkeeping manoeuvres rely on smaller motors. Consumption of propellant required for these operations is a key factor determining a satellite’s operational life: once it has spent all its fuel, it can no longer be controlled and therefore becomes inoperable. For this reason, new ion and plasma propulsion technologies are now increasingly finding favour.
A lot of trajectory corrections are performed using the satellite’s chemical thrusters. The most important manoeuvres like circularizing the orbit require powerful thrusters, whereas finer positioning and stationkeeping manoeuvres rely on smaller motors. Consumption of propellant required for these operations is a key factor determining a satellite’s operational life: once it has spent all its fuel, it can no longer be controlled and therefore becomes inoperable. For this reason, new ion and plasma propulsion technologies are now increasingly finding favour.
Not all trajectory corrections require propulsion, however. Interplanetary missions use “swing-by” manoeuvres—also called a gravity assist—that exploit the gravitational attraction of other stars or planets to modify a deep-space probe’s trajectory.
