Continuation of satellite subsystems

If you liked the previous articles you will also like this one as it complements what we have already learnt.

 

A very decisive parameter for the life of batteries is the temperature in which it can operate, if the battery overheats of freezes the continuation of the mission could be condemned to fail. In this table you can appreciate different temperatures

Table regarding the temperature range of different elements of a spacecraft.

That thermal balance is achieved with the thermal subsystem, which has the mission of maintaining the thermal balance of the spacecraft at all times and positions.

Thermal control subsystem.

It includes all the sensors, radiators and isolating materials used. To maintain a certain temperature there are both active and passive ways of doing so. Active methods: radiators, orientation change or cryogenic systems; passive methods: materials or surface coatings. The thermal capacity of a spacecraft may vary due to the effects of surface degradation, solar radiation (radiation emitted by the Sun), albedo radiation (radiation reflected by Earth) and the pressure difference between the inside of the satellite and the near vacuum of space.  All these factors must be considered so that the spacecraft can be kept functioning and operative to accomplish the mission.

Radiators of the ISS used to regulate the temperature.

After that is the attitude and control subsystem, which allows the spacecraft to achieve and maintain a known and preferred attitude and orbit.

Cubesat rotating looking for the right attitude for its orbit.

This subsystem can be divided in two independent subsystems, the Attitude Determination And Control System  (ADACS), which is in charge of the determination  of attitude and it includes the attitude control (actuators) for achieving that. Secondly the Orbit Control and Maintenance, which has two different elements, such as the orbit determination, using GPS for that, and orbit acquisition and control, which includes propulsion to accomplish it.

To manage to get the right orientation there are both active and passive actuators. Some of the passive methods are the use of gravity to orientate the satellite, or the use of Earth’s magnetic field to orientate the satellite.

Passive orientation techniques.

 

Regarding the active procedures for orientation and attitude control, we can find the reaction wheels, where the spin-up or slow-down of a wheel with an electric motor produces a torque to rotate the satellite.

Cubesat with reaction wheels in action.

For achieving the right orbit and attitude it is possible to also use the propulsion subsystem, which burns fuel in order to arrive to or maintain an orbit. The propulsion subsystem includes all the piping, the engine, the fuel tanks and valves.

Propulsion subsystem.

Then is the communications subsystem, which has the objective to allow telecommands and telemetry to be sent between the spacecraft and the ground stations. For such actions one or more ground stations could be used and when transferring the data, the atmospheric and ionospheric interferences should be considered. The frequency in which the communications are usually made with is S-band, around 2 GHz.

Communications subsystem.

To maintain communications with the spacecraft, sending commands and receiving telemetry data is necessary the ground segment subsystem, some of its activities include mission planning, coordination activities, analysis and archiving of spacecraft data. These centres have antennas to point at the satellite and rather emit or receive the information.

Antennas at a ground control centre.

The more ground stations available the faster and easier it is to download the data.

European Space Agency Estrack ground station network.

Closely related to the communications subsystem is the command and data handling subsystem, which receives, validates, decodes, stores and distributes commands to the spacecraft subsystems and payloads. It also collects, stores, processes and formats spacecraft data for transmission to the ground.

Commanding and data handling subsystem.

The final element we will analyse is not a subsystem, but it is present in all the different elements of the mission, software. It is present from the early beginning until the very end of the lifetime of the spacecraft and it is key for its functioning.

Software is very is essential and if it fails the mission fails, we will now talk about some software incidents that caused missions to fail.

The Mars Climate Orbiter crashed in September 1999 against Mars’ surface because the control team on Earth made use of the Anglo-Saxon System of Units to calculate the insertion parameters and sending the data to the spacecraft, which performed the calculations with the decimal metric system. This mistake improved the systems engineering processes to detect errors at NASA.

Art image of Mars Climate Orbiter.

That same year the Soviet Phobos I Mars probe was lost, due to a faulty software update, at a cost of 300 million roubles. Its disorientation broke the radio link and the solar batteries discharged before reacquisition.

Phobos 2 art image.

Finally, in 1996 the Ariane 5 satellite launcher malfunction was caused by a faulty software exception routine resulting from a bad 64-bit floating point to 16-bit integer conversion. The results of that error can be found in the following link:

https://www.youtube.com/watch?v=PK_yguLapgA&ab_channel=AmazingInfoTV

We hope that you liked this article and that you learnt something today, if you still have any doubts or questions don’t doubt to contact us and we will happily answer.

 

Jesús Lucero Ezquerro

Jesús Lucero Ezquerro

Collaborator in Earth Observation - B. Eng. Aerospace Engineering at T.U. Madrid - MSc. in Astronautics & Space Engineering at Cranfield University.
Share on facebook
Share on twitter
Share on linkedin

Deja un comentario