Orbital stationkeeping
Encyclopedia
In astrodynamics
Astrodynamics
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and Newton's law of universal gravitation. It...

orbital station-keeping is a term used to describe the orbital maneuver
Orbital maneuver
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...

Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

that are needed to keep a spacecraft in a particular assigned orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

.
For many Earth satellites the effects of the non-Keplerian
Orbital perturbation analysis (spacecraft)
Isaac Newton in his Philosophiæ Naturalis Principia Mathematica demonstrated that the gravitational force between two mass points is inversely proportional to the square of the distance between the points and fully solved corresponding "two-body problem" demonstrating that radius vector between the...

forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere
Shell theorem
In classical mechanics, the shell theorem gives gravitational simplifications that can be applied to objects inside or outside a spherically symmetrical body...

, the gravitational forces from Sun/Moon, the solar radiation pressure and the air-drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

, must be counter-acted.

The deviations of the gravitational force of the Earth from that of a homogeneous sphere
Shell theorem
In classical mechanics, the shell theorem gives gravitational simplifications that can be applied to objects inside or outside a spherically symmetrical body...

and the gravitational forces from Sun/Moon will in general perturb the orbital plane. For sun-synchronous orbit
Sun-synchronous orbit
A Sun-synchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time. The surface illumination angle will be nearly the same every time...

the precession of the orbital plane caused by the oblateness of the Earth is a desirable feature that is part of the mission design but the inclination change caused by the gravitational forces of Sun/Moon is undesirable. For geostationary spacecraft
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

the inclination change caused by the gravitational forces of Sun/Moon must be counter-acted to a rather large expense of fuel as the inclination should be kept sufficiently small for the spacecraft to be tracked by a non-steerable antenna.

For spacecraft in low orbits the effects of air-drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

must often be compensated for. For some missions this is needed simply to avoid re-entry. For other missions, typically missions for which the orbit should be accurately synchronized with the Earth rotation, this is necessary to avoid that the orbital period gets shorter.

The solar radiation pressure will in general perturb the eccentricity (i.e. the eccentricity vector), see Orbital perturbation analysis (spacecraft)
Orbital perturbation analysis (spacecraft)
Isaac Newton in his Philosophiæ Naturalis Principia Mathematica demonstrated that the gravitational force between two mass points is inversely proportional to the square of the distance between the points and fully solved corresponding "two-body problem" demonstrating that radius vector between the...

. For some missions this must be actively counter-acted with maneuvers. For geostationary spacecraft
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

the eccentricity must be kept sufficiently small for a spacecraft to be tracked with a non-steerable antenna. Also for Earth observation spacecraft
Earth observation satellite
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc....

for which a very repetitive orbit with a fixed ground track is desirable the eccentricity vector should be kept as fixed as possible. A large part of this compensation can be done by using a frozen orbit
Frozen orbit
For most spacecraft missions the "perturbing forces" caused by the oblateness of the Earth, the gravitational attraction from Sun/Moon, the solar radiation pressure and the air drag must be counteracted by orbit maneuvers to keep the spacecraft in the desired orbit...

design but for the fine control maneuvers with thrusters are needed.

For spacecraft in a halo orbit
Halo orbit
A halo orbit is a periodic, three-dimensional orbit near the , , or Lagrange points in the three-body problem of orbital mechanics. A spacecraft in a halo orbit does not technically orbit the Lagrange point itself , but travels in a closed, repeating path near the Lagrange point...

around a Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

the station keeping is even more fundamental as such an orbit is instable, without an active control with thruster burns the smallest deviation in position/velocity would result in that the spacecraft would leave the orbit completely.

## Station-keeping in low-earth orbit

For a spacecraft in a very low orbit the air-drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

is sufficiently strong to cause a re-entry before the intended end of mission if orbit raising maneuvers are not executed from time to time. A typical example of this is the International Space Station which has an operational altitude above Earth surface of between 330 and 410 km. Due to atmospheric drag the space station is constantly losing orbital energy. In order to compensate for this loss, which would eventually lead to a reentry of the station, it is from time to time being re-boosted to a higher orbit. The chosen orbital altitude is a trade-off between the delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

needed to counter-act the air drag and the delta-v needed to send payloads and people to the station. The upper limitation of orbit altitude is due to the constraints imposed by the Soyuz spacecraft
Soyuz spacecraft
Soyuz , Union) is a series of spacecraft initially designed for the Soviet space programme by the Korolyov Design Bureau in the 1960s, and still in service today...

. On 25 April 2008, the Automated Transfer Vehicle
Automated Transfer Vehicle
The Automated Transfer Vehicle or ATV is an expendable, unmanned resupply spacecraft developed by the European Space Agency . ATVs are designed to supply the International Space Station with propellant, water, air, payload and experiments...

"Jules Verne
Jules Verne ATV
The Jules Verne ATV, or Automated Transfer Vehicle 001 , was an unmanned cargo resupply spacecraft launched by the European Space Agency . The ATV was named after the French science-fiction author Jules Verne...

" raised the orbit of the ISS for the first time, thereby proving its ability to replace (and outperform) the Soyuz at this task.

## Station-keeping for Earth observation spacecraft

For Earth observation spacecraft
Earth observation satellite
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc....

typically operated in an altitude above the Earth surface of about 700 - 800 km the air-drag is very faint and a re-entry due to air-drag is not a concern. But if the orbital period should be synchronous with the Earth rotation to maintain a fixed ground track
Ground track
A ground track or ground trace is the path on the surface of the Earth directly below an aircraft or satellite. In the case of a satellite, it is the projection of the satellite's orbit onto the surface of the Earth .A satellite ground track may be thought of as a path along the Earth's surface...

also the faint air-drag at this high altitude must be counter-acted by orbit raising maneuvers in the form of thruster burns tangential to the orbit. These maneuvers will be very small, typically in the order of a few mm/s of delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

. If a frozen orbit
Frozen orbit
For most spacecraft missions the "perturbing forces" caused by the oblateness of the Earth, the gravitational attraction from Sun/Moon, the solar radiation pressure and the air drag must be counteracted by orbit maneuvers to keep the spacecraft in the desired orbit...

design is used these very small orbit raising maneuvers are sufficient to also control the eccentricity vector.

To maintain a fixed ground track it is also necessary to make out-of-plane maneuvers to compensate for the inclination change caused by Sun/Moon gravitation. These are executed as thruster burns orthogonal to the orbital plane. For Sun-synchronous spacecraft having a constant geometry relative to the Sun the inclination change due to the solar gravitation is particularly large, a delta-v in the order of 1-2 m/s per year can be needed to to keep the inclination constant.

## Station-keeping in geostationary orbit

For geostationary spacecraft thruster burns orthogonal to the orbital plane must be executed to compensate for the effect of the luni/solar gravitation that perturbs the orbit pole with typically 0.85 degrees per year. The delta-v needed to compensate for this perturbation keeping the inclination to the equatorial plane small amounts to in the order 45 m/s per year. This part of the GEO station-keeping is called North-South control.

The East-West control is the control of the orbital period and the eccentricity vector performed by making thruster burns tangential to the orbit. These burns are then designed to keep the orbital period perfectly synchronous with the Earth rotation and to keep the eccentricity sufficiently small. Perturbation of the orbital period results from the un-perfect rotational symmetry of the Earth relative the North/South axis, sometimes called the ellipticity of the Earth equator. The eccentricity (i.e. the eccentricity vector) is perturbed by the solar radiation pressure.

The fuel needed for this East-West control is much less then what is needed for the North-South control. To extend the life-time of aging geostationary spacecraft with little fuel left one sometimes discontinues the North-South control only continuing with the East-West control. As seen from an observer on the rotating Earth the spacecraft will then move North-South with a period of 24 hours. When this North-South movement gets too large a steerable antenna is needed to track the spacecraft. An example of this is Artemis
Artemis (satellite)
Artemis is a geostationary earth orbit satellite for telecommunications, built for and owned by ESA. The Artemis satellite operates at the 21.5E orbital position....

To save weight, it is crucial for GEO satellites to have the most fuel-efficient propulsion
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

system. Some modern satellites are therefore employing a high specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

system like plasma or ion thruster
Ion thruster
An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

s.

• Orbital maneuver
Orbital maneuver
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...

• Delta-v budget
Delta-v budget
In the astrodynamics and aerospace industry, a delta-v budget is the estimated delta-v requirements for the various propulsive tasks and orbital maneuvers over one or more phases of a space mission.Sample delta-v budget will enumerate various classes of maneuvers, delta-v per maneuver, number of...

• Orbital perturbation analysis (spacecraft)
Orbital perturbation analysis (spacecraft)
Isaac Newton in his Philosophiæ Naturalis Principia Mathematica demonstrated that the gravitational force between two mass points is inversely proportional to the square of the distance between the points and fully solved corresponding "two-body problem" demonstrating that radius vector between the...

• Orbital decay
Orbital decay
Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit.This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar...

• Nautical stationkeeping
Nautical stationkeeping
Station keeping in a nautical situation is when a vessel is to:* maintain a position in relation to another moving vessel or vessels, such as when** conducting underway replenishment or** part of a task force or convoy...