Glossary of terms used


Refers to either the Geosynchronous orbit or to the Geostationary Orbit. Geosynchronous Orbit is an orbit around Earth with the same orbital time (24 h) as the rotation of Earth around itself. This occures when the satellite is at 35.786 km above the surface of Earth. Geostationary is a Geosynchronous orbit that is also circular and has zero inclination as regards the orbital plane with respect to the equator of Earth. This is the most important orbit as regards the telecommunication satellites. Other important orbits are LEO and MEO.


Small and Medium Enterprises.



Commercial S/C

Meaning that the scientific and Military spacecrafts are not included in this category.


Latin term, meaning at the same place with respect to another object being mentioned. Very often used in exploration related literature into the term In-Situ Resource Utilizations ISRU.


Technical term, meaning any unforeseen behaviour of the spacecraft which is arrtibuted to some kind of technical failure.

Designed Life Time

The satellites are designed to live for a certain number of years to fullfil certain operational needs and render their investment justifiable. For implementing the designed lifetime, certain components and configurations of a particular, and well predictable lifetime, are selected. If there are not available components for extensively long durations then the solution of redundancy or back-up circuits / units is implemented in the design. Current telecommunication satellites are designed for use of about 12-15 years.

According the designed lifetime the satellites are filled-in with the respectively appropriate amount of fuel.


It describes the set of tests that a spacecraft has to undertake to prove that it is operational on orbit, after the shocking experience of the launch. Sometimes it is performed by the manufacturer. Alternatively, if there is sufficient experience in house, by the operator and owner of the satellite


Is the part of the tests that intend to measure the actual performance of various parameters of the satellite.


Adjustment of operational parameters of an instrument in an optimum way with respect to interpretation of its results.

Alive satellites

Meaning most probably responsive. This term is used to contrast with the “sick/dead” that is used later.


According to ESA terminology this means the process of placing a spacecraft at a higher orbit, in effect releasing its current position, for letting its occupancy slot to a new spacecraft. This new orbit that a satellite obtains after the re-orbiting operation is also called grave-yard orbit.


Client Spacecraft.



Orbit Injection parameters

Or Orbit insertion, in the launcher literature, meaning the orbital parameters at the point where the spacecraft is separated from the upper stage. Usually refers to the parameters of the GTO orbit.


These were satellite engines that were used in the beginning of the life of the satellite in order to place it in the right operational orbit (after separation occurred). They were changing the orbit, from the eliptical one that the launcher was injected them to, into a circular orbit. In order to do so they were ignited at the moment the satellite was passing its apogee point of the elliptical orbit. These motors were separated by the satellite (ejected) after their use in order to let the satellite lighter. This practice has been abandoned to avoid creating additional space debris. It has been replaced by the introduction of the so called UPS Unified Propulsion Subsystem, a propulsion subsystem of the satellite that is being used both for circularizing its orbit in the beginning of its life and also later on for the stationkeeping. We believe the concept of ejectable AKM can come back as preferable solution because the On-Orbit-Services will make available safe and tidily re-orbiting of the AKM.

Upper stages

Meaning the last stages of the launchers. Some launchers PROTON (RU), Delta (US), Long March (CN) have an upper stage that reaches the GEO ring to deliver the satellite it transports. It is usually been re-orbited after this at the cost of using the remaining chemical propellants they carry for this purpose. To achieve the reorbiting manoeuvre the upper stage needs to be restartable


The nose of the launcher that covers and protects the satellites during the ascent phase, when it passes through the atmosphere. It is being jettisoned soon after the launcher exits the atmosphere. Its shape is dictated by aerodynamic considerations.

Utility Base

This NEW TERM, is used to describe the fact that the spacecraft in question will have resources commonly required by all satellites “Utility” and also provide facilities and services. The “Base” is used to denote that it will be more or less stationary. Due to its large size it will not be economical to perform trips to visit the client satellites but vice versa.

In addition, it is foreseen to perform multiple operations, simultaneously in different directions, using multiple instances of Utility Agents. Most optimum therefore is that the UB is stationary, placed in an optimum position, to minimize the overall travel time/cost of visiting a community of client satellites.

Utility Agent

This NEW TERM, is used to denote that this spacecraft will mediate for the availability of “Utilities” between the Utility Base and the place where they might be needed.

Escort Agent

This NEW TERM, is used to denote that this spacecraft will escort a Utility Agent or a client spacecraft. Its main use will be to inspect and to retransmit signals.

Engine Module

This NEW TERM, is used to denote that this spacecraft will be as simple as an engine. It will not have its own navigation capabilities and it will be an add-on module to be attached externally on the main thruster nozzle of a client spacecraft.


The telecommunication channel of a satellite that has the direction up-wards from the earth to the satellite. Otherwise we can call it the reception channel of the satellite.

Station keeping

The set of manoeuvres and their application that are required in order to keepthe satellite within its specified window (station) of operation with respect to the East-West drift or North-South drift.

The satellite normally drifts out of this window in a few weeks due to gravitational forces of the Moon, Venus, Sun and due to solar radiation pressure on its surface and due to gravitational asymmetry of the Earth. (its is being pulled by forces of different size by different points of the Earth "surface".)


The angle of the orbital plane of a satellite with respect to the orbital plane that is defined by the equator. Normally the satellite needs to operate within a fraction of degree of inclination but may drift several degrees by the end of its life.


To-Be-Defined by further analysis.


Term meaning originally the ship that is being used to assist larger ships to manoeuvre in close ports or to be carried in a port if it has become uncontrolled due to a mechanical failure.

Listening of telemetry

Meaning that the telemetry is received and analysed.

ECHO command

Is a special telecommand that its use is just to be relayed back to Earth to the controlling station in order that the control station can be assured that the satellite can receive telecommands and transmit telemetry and all processing in-between is functioning properly.


Opposite of aggregated, meaning that the totality of functionality been required has been divided in multiple segments and distributed to different entities.


Denotes the launcher Ariane-5 in such a configuration that is equipped with an upper stage which uses cryogenic propellants, in contrast to the previous version that were using less efficient storable propellants.

Attitude behaviour

Orientation of the spacecraft with respect to the Earth and the Sun.

Chaser spacecraft

Terminology used in the On-Orbit-Servicing literature meaning a spacecraft that has the capability to chase after another spacecraft that is usually described as the target. The target may be co-operative or non-cooperative.


With respect to the On-Orbit-Servicing literature we describe as target (co-operative or non-cooperative) a spacecraft that a chaser spacecraft has as objective to seek and catch.


GEO Transfer Orbit, It is specific to the launcher literature meaning the intermediate (transfer) orbit before we attempt to transfer to the GEO (Geosynchronous circular) orbit. It is an elliptical orbit and each launcher defines its own geometry of GTO orbit.


Frequently asked questions.


On a satellite we mean electrically powered heating elements that are used to heat the propellants and the boxes with the electronics and other equipment to ambient temperature.


Stands for "pyrotechnically activated devices", which are devices that can be activated only once using electrically ignited explosives, to perform release of clamp bands, antenna booms, solar panels etc.


Electro-Magnetic Compatibility. It is very important from the safety point of view that a spacecraft will not emit to the proximity electromagnetic radiation that would ignite fuel or pyros unintentionally or disrupt communication of the launcher or the data processing of the launcher. (It has same root as to the issue of mobile phones in the aircrafts during taking-off and landing, and the prohibition of mobile phones close to the gas stations).


Meaning in this case the metallic material that is used inside the monopropellant thrusters to ignite the fuel. (In the case high purity Hydrazine).


National Oceanic and Atmospheric Administration (the Meteorological organization of USA).




Engineering term meaning lower than Subsystem and higher that subassembly.

Momentum wheels

Equipment used in configuration of 4 units in a pyramid like redundant scheme, in order to stabilize a three axis satellite. It has a wheel shape and is driven by an electrical motor generating torque each time that is accelerated or decelerated. The torque is transferred to the body of the satellite and so the satellite changes attitude.

Part Level

Engineering term meaning the lowest level of testing of parts (components)


Name of inventor of certain gravitationally balanced points between celestial bodies. Are considered ideal to establish there bases because there is no need for continuous orbit correction and missions originating from these points can serve two distinct points in two different celestial bodies (Earth-Moon, Earth-Sun) in the most optimal way as regards propellant consumption.

Propellant and fuel

In our case we consider only liquid propellants that can be transferred through pipe lines from one spacecraft to another. In this project and at this stage only the “storable” propellants are considered in contrast to the cryogenic which can not be stored for long time due to thermal penetration into the Propellant storage media (tanks), which temperature change alters the properties of the propellants. This term also interchangeable with the term fuel in the case that the fuel can be ignited without the need of an oxidizing propellant. This is the case of decomposable fuels like Hydrazine. They require a catalyst to ignite or sustain fuel decomposition. In the later case we talk about mono-propellant propulsion system in contrast with the case where an oxidizer is present and we talk about bi-propellant propulsion system.

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