Glossary of terms used

GEO

Geosynchronous Orbit. Orbit with the same orbital time (24 h) as the rotation of earth around itself.

SMEs

Small and Medium Enterprises.

S/C

Spacecraft.

Commercial S/C

Meaning that the scientific and Millitary spacecrafts are not included in the analysis.

In-situ

Lattin term meaning at the same place with respect to another object being mentioned.

Anomalies

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

Designed Life Time

The satellites are desighed to live for certain number of years to fullfil certain operational needs. 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. Current satellites are designed for 12-15 years.

According the designed lifetme the satellites are filled-in with the respectively apropriate amount of fuel.

Commissioning

Is described sellectively the set of tests a spacecraft has to undertake to prove that is operational on orbit after the schocking experience of the launch. Sometimes performed by the manufacturer.

Characterization

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

Calibration

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

Alive satellites

Meaning most propably healthy. This term is used to contrast with the “sick/dead” that is used later. However the term “healthy” is avoided because at the early life stages of a satellite on orbit its status is undefined and every parammeter has to be carefully and accurately analysed.

Re-orbit

According to ESA terminology this means the process of placing a spacecraft at a higher orbit to release its current position from occupation. This new orbit that a satellite obtains after the re-orbiting operation is also called grave-yard orbit.

CS

Client Spacecraft.

EOL

End-Of-Life.

Orbit Injection

Or Orbit insertion, in the launcher literature meaning the point where the spacecraft is separated by the upper stage and it is left at an orbit (intermediate or final). Diagrammatically this orbit is naturally being intersected with the orbit of the composit body orbit (upper stage + spacecraft) and thus is described as being injected into the required orbit due to the zero angular difference (tangential) of th two orbits at intersection.

Apogee-Kick-motors

These are satellite engines that are used in the beginning of the life of the satellite in order to place it in the right operational orbit. They change the orbit, from the eliptical one that the launcher has injected them to, into a circular orbit. In order to do so they are ignited at the moment the satellite passes its apogee point of the eliptical orbit. These motors are separated by the satellite (ejected) after use in order to let the satellite lighter. This practice has been abandoned to avoid creating aditional 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, Delta have an upper stage that reach 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.

Fairing

The hollow part of the nose of the launcher that covers and protects the satellites during the asccent phase, when it passes through the atmosphere. It is being jettisonned 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 ecconomical to perform trips to visit the client satellites but vice versa.

In addition it is foreseen to perform multiple operations, simultaneusly in different directions, using multiple instances of Utility Agents. Most optimum therfore is that the UB is stationaty, placed in an optimum position, to minimize the overal 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 simpe as an engine. It will not have its own navigation capabilities and it will be an add-on module to be attached externaly on the main thruster nozle of a client spacecraft.

Up-link

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

The satellite normaly drifts out of this window in a few weeks due to gravitational forces of the moon, venus, sun and due to solar ratiation pressure on its surface and due to gravirational assymetry of the earth. (its is being pulled by forces of different size by different points of the earth "surface".)

Inclination

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

TBD

To-Be-Defined by furher analysis.

Tug

Term meaning originaly the ship that is being used to assist larger ships to maneuver in close ports or to be carried in a port if 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.

Segregated

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

A5-ECA

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.

ECA

Etage Cryogenique Apogee xx

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-coperative.

Target

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

GTO

GEO Transfer Orbit, It is specific to the launcher literature meaning the intermediate (transfer) orbit before we atempt to transfer to the GEO (Geosynchronous circular) orbit. It is an elliptical orbit and each launcher defines its own geometry of GTO orbit. The GTO of Ariane has a Perigee of xx and Apogee of and inclination of xx.

FAQ

Frequently asked questions.

Heaters

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

Pyros

Stands for "pyrotechnicaly activated devices", which are devices that can be activated only once using electrically ignited explosives, to perform release of booms, solar pannels.

EMC

ElectroMagnetic Compatibility. It is very important from the safety point of view that a spacecraft will not emmit 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 is similar to the issue of mobile phones in the aircrafts during flight, and the prohibition of mobiles close to the gas stations).

Catalyst

Meaning in this case the metallic matterial that is used inside the thusters to ignite the fuel. In the case we use Hydrazine as fuel.

NOAA

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

OOS

On-Orbit-Servicing.

Unit-Level

Engineering term meening lower than Subsystem and higher that subassembly. Xx

Momentum wheels

Equipment used in configuration of 4 in a pyramid like redundant scheme in order to stabilize a satellte usually in a three axis. It has a wheel shape and is driven by an electrical motor generating momentum each time that is accelerated or deccelerated. The momentum is transferred to the body of the satellite and moves the satellite in a different stable attitude.

Part Level

xx

Lanrgange

Name of inventor of certain gravitationaly ballanced points between celestial bodies. Are considered ideal to establish there bases because there is no need for continouous orbit corection and missions originating from these poinnts can serve two dinstict points in two different celestial bodies (Earht-Moon, Eart-Sun) in the optimal propellant consumption way.

Propellant and fuel

In our case we consider only liquid propellants that can be transferred through pipes from one spacecraft to another. In this project and at this stage only the “storable” propellands are considered in contrast to the cryogenic which can not strored 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 oxidizer propellant. This is the case of decomposable fuels like Hydrazine. They require a catalyst to ignite or sustain fuel decomposition. In the late case we talk about monopropelant propultion system in contrast with the case where an oxitizer is presentt and we talk about bi-propellant propultion system.

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