When a satellite activates its thrusters movement takes place — this is known as a maneouvre.
Sometimes we need to correct or change the orbit of a satellite because a number of forces act on a satellite to change its orbit over time.
These include the slight asymmetries in the Earth’s gravitational field due to the fact that the Earth is not completely spherically symmetric; the gravitational pull of the Sun and Moon; the solar radiation pressure; and, for satellites in low earth orbit, the atmospheric drag.
These forces cause two major effects:
The inclination of the satellite orbit is the angle between its orbital plane and the equatorial plane of the Earth. The gravitational pull of Sun and Moon causes a gradual increase in the satellite inclination.
On a Geostationary satellite, whose inclination is very small, this causes an apparent daily north-south motion of the satellite, centred over its nominal longitude location. The maximum excursion north and south of the equator is the same as that of the inclination. While the inclination becomes greater than approximately 1°, action is taken to avoid the inclination becoming too large.
This is performed, at regular intervals (every year), with a so-called 'north-south manoeuvre' which adjusts the orbital plane of the satellite.
North-south station-keeping is expensive in terms of fuel and it is one of the main limiting factors in the lifetime of the satellite.
When the fuel is exhausted, the inclination increases continuously, at about 0.9° each year, and eventually the daily north-south movement may degrade the images taken from the satellite.
On Low Earth Orbit polar satellites it is necessary to maintain the inclination controlled around a specific value. That ensures that the orbital plane rotates at the same rate as the Earth around the Sun, ensuring the Sun is always in the same position, as seen from the satellite. When the inclination diverges from the nominal value, the orbit plane rotation is no longer synchronous with the Sun and, therefore, the operational illumination conditions are not maintained.
On geostationary satellites the uneven shape of the Earth causes a longitude drift, which affects the east-west position. The oceans, in particular, cause the gravitational field of the Earth to depart from a true spherical shape. The effect is as if the satellites were located on hills, which they may slide off, or in valleys, where they may remain stable.
There are two stable locations in geostationary orbit — over the Indian Ocean and over the eastern Pacific Ocean. Meteosat, at 0° longitude, is on the gravitational slope leading to this 'hole' and gradually drifts towards the east.
The satellite is normally maintained within a defined area or 'box' around its nominal longitude location (typically of 0.1° or larger if there are no collision risks with neighbouring satellites). When it reaches the eastern extremity of the permitted box, the satellite is kicked back toward the western extremity of the box, with a so-called 'east-west manoeuvre'.
|Satellite||Defined box area||Nominal location|
This cycle repeats every few months (depending on the current size of the permitted box and of the longitude of the satellite), but is not costly in terms of fuel use.
On Low Earth Orbit polar satellites it is necessary to maintain the control of the longitude of the ascending nodes around specific values (defined as the point at which the satellite's ground track passes in a northerly direction over the equator). That ensures that the orbital evolution over the Earth repeats regularly after a fixed number of days, with large benefits in observation and operations. When the longitude of the ascending nodes diverge from the nominal values — because of the altitude reduction caused by the atmospheric drag force, which means the orbital period is no longer synchronous with the Earth rotation — then a manoeuvre to correct the altitude, and then stop the divergence, is necessary.
The other reasons why satellites manoeuvres take place are:
- Transition from initial to operational orbit.
- Spin rate adjustments for spin stabilised satellites
- Debris collision avoidance.
- De-orbiting at end of life.
On the Meteosat satellites manoeuvres are also used to modify the attitude status, either to spin-up/down the satellite, to keep the rotation rate close to the nominal one of 100 revolution per minutes, or to modify the orientation of the spin axis to keep it normal to the orbital plane, ensuring proper pointing toward the Earth while the inclination increases.
Executing a manoeuvre
A manoeuvre is carried out by firing thrusters (small reaction motors burning hydrazine fuel and ejecting the combustion gases at a very high velocity of around 8000 km/h) to change the magnitude (for longitude control) or direction (for inclination control) of the satellite's velocity. Because the orbital speed of satellites is so large, the velocity changes required for manoeuvring may also be large (above all if the direction is to be changed), requiring the thrusters to use large amounts of propellant (fuel).
How much and how quickly a satellite can manoeuvre depends on the amount of propellant it carries; the propulsion system performances, and the satellite’s mass. On geostationary satellites, such as Meteosat, the majority of propellant is used during the transition from launch orbit to operational orbit. For Low Earth Orbit polar satellites, such as Metop, the largest fraction of the fuel is used for End-of-Life operations, to bring the satellite as close as possible to the Earth surface and reduce, as much as possible, the permanence in orbit of the dead satellite.
Manoeuvres required for attitude control and collision avoidance use up very little propellant.
Two basic manoeuvres are used to change orbits:
- Changing the size (and optionally the shape) of an orbit within the orbital plane.
- Changing the orbital plane by changing the inclination of the orbit and, optionally and only for geostationary satellites, the direction of the orbital plane around the Earth’s axis.
An orbital manoeuvre may involve applying thrust:
- perpendicular to the orbital plane used to change the inclination of the orbit, if such a manoeuvre is not executed at the ascending node, the direction of the orbital plane around the Earth’s axis is also modified at the same time;
- in the orbital plane, normally on the direction of the orbital motion to speed up the satellite and shifts it into a higher orbit, in order to induce a drift in longitude. Two of such manoeuvres (one of the two often against the direction of the orbital motion) allow the shape (eccentricity) of the orbit to be modified at the same time.
Meteosat has a group of thrusters on its side. Because it is a spin-stabilised satellite, an east-west manoeuvre of Meteosat is complicated because the thrusters are spinning with the satellite. The thrusters are designed to allow burns to be controlled to a fraction of a second.
Meteosat spins at 100 revolutions per minute (one revolution takes 0.6 seconds) with its spin axis normal to its orbital plane. One revolution takes 0.6 seconds. The thrusters are fired over an angle of 60° centred on the required direction, i.e. for one-sixth of a revolution.
The result is a staccato firing as the thrusters repeatedly burn for 0.1 seconds and switch off for 0.5 seconds, for the duration of the burn period. So a typical burn for placing a satellite into a graveyard orbit takes place over 20 minutes, but thrust is only applied for about three minutes.
For Metop the challenge is provided by the execution of the inclination manoeuvre, as the satellite has thrusters only on the flight and against the flight direction. It is, therefore, necessary to perform a very large rotation of around 90 degrees to put the thrusters in the right direction before the execution of the thrust, and then rotate back the satellite into its nominal attitude. All of these operations have to take place during an eclipse, to avoid direct sunlight illuminating the instruments. The time available for the thrust is, therefore, not very large, so inclination corrections need to be split in two manoeuvres.
Satellites in orbit can all be affected by debris. It has been assessed that every five years, if no mitigating actions are put in place, a collision involving a LEO satellite can be expected.
As well as weather satellites, there are thousands of other types of satellites in space, including those needed for Defence and telecommunications. Currently the warnings EUMETSAT receives come from the US Air Force, which tracks everything larger than 10 cm⊃2; — anything smaller can't be picked up on radar but can still be damaging, even if normally the energy of these objects may not be sufficient to cause a fatal outcome.
The greatest risk is to satellites in Low Earth Orbit, where there are around 20,000 tracked objects (including 600 operating satellites) &— around almost 2 million kg (4 million lbs) of space debris — everything from small screws and bolts to big chunks of spacecraft. All of this has the potential to hit and severely damage, or even destroy, a satellite.
From 2010–2014 EUMETSAT had around 100 warnings of possible conjunctions (collisions) with debris objects and had to undertake five Collision Avoidance Manoeuvres to move Metop-A (three times) and Metop-B (twice) away from the trajectory of debris.
Analysis of the trajectory of both the satellite and the debris showed that the risk in all cases was very large (more than 1:3000, with a maximum risk of 1:300) — anything higher than 1:10,000 is considered as too high a risk — so the satellites had to be moved.
The actual manoeuvres were in every case quite small — each using only around 70 g of fuel — and for each Metop-A manoeuvre products for the Space Environment Monitor (SEM) and Global Ozone Monitoring Experiment (GOME) instruments were unavailable for a short period.
Impacts of a collision
Although moving a satellite obviously has impacts, they are minimal compared to the costs and impacts of having a satellite badly damaged or destroyed. At a minimum it would take more than a year to replace the satellite in orbit with significant additional risks to the longer term continuity of service. Moreover, a collision on a Metop satellite would create a debris cloud which would significantly increase the risk on the other Metop satellite, flying in the same orbit.
The main impact from the loss of Metop would be on weather forecasting. The Metop satellites make a considerable contribution to Numerical Weather Prediction (NWP) and NWP underpins the forecasts of the National Meteorological Services. Satellite data are particularly valuable in the Southern Hemisphere, where there are few conventional observations.
The loss of Metop would also impact ocean and climate monitoring. Accurate climate monitoring needs a constant stream of historic data — a break of a year could have a significant effect on trend analysis.
Having two Metop satellites mitigates the risk, somewhat. But it also means there is another operational satellite exposed to conjunction risks. A different form of mitigation is removing redundant satellites from Low Earth Orbit by moving them into re-entry orbit. This is planned for Metop-A, once Metop-C has been launched.
Mike Williams, Head of Control Centre Division, sums it up: “We are interested in having as much conjunction data as possible. The more data we have, the greater risk analysis we can do. We have a process in place — MIAMI (Manual In-plane Avoidance Manoeuvre Insertion) — which means the impact of moving a satellite can be minimal, especially versus the nominal risk. The loss of a satellite would be a disaster.”
That is why EUMETSAT is one of a number of European agencies working together to investigate setting up a European Space Situational Awareness system. SSA systems can detect hazards that could threaten critical space and ground infrastructure.
SSA is meant to monitor risks coming from:
- man-made space objects, including launchers and parts of satellites (collision risk for operated satellites);
- in-orbit explosions and collisions (increase of collision risk coming from large increase of debris);
- re-entries (casualty risk posed to the population);
- the effects of space weather (both on space and ground assets).
- Near Earth Objects (assessing and mitigating risk to the Earth from asteroids).