This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2014

Lifetime of satellite

1. The useful lifetime of satellite is determined by the maximum acceptable deviation in inclination of the orbit and orbital location of the satellite with the reliability of the satellite systems.
2. A satellite is maintained in its orbital location by firing thrusters regularly using stored fuel. Hence lifetime of satellite depend upon fuel storage capacity of the satellite.
3. But there is a practical limit to the fuel storage capacity. Hence alternative method used is to increase fuel efficiency used for propulsion. The life of satellite also depends on quality of batteries used in the satellite and amount of power consumed.

Reliability

1. Reliability is defined as the probability that the system will not fail to work before the given time. Reliability theory is nothing but the mathematical attempt to predict the future of the satellite.
2. Reliability is done for following reason: (1)To know what is the probability that the system will work after a given period. (2)To provide redundant subsystems where probability of failure is not acceptable
3. Reliability of component is given as $R=e^-∫^t_0λdt$ Where λ=failure rate of the component and is specified in FIT i.e.number of failures in 109 or 106 hours.

Therefore once the failure rate of the component is known then the reliability can be calculated. Space particles and their impact

1. There are several types of particles existing in the space. These are cosmic rays, protons, electrons, meteoroids and man made space debris.

2. The intensity of particles emitted from the sun mainly depends on the solar activity.

3.The bombardment of the particles on a satellite causes degradation in solar cells and certain solid state components present in the satellite.

1. Besides planets, natural and artificial satellites, many other particles like cosmic rays, protons, electrons, meteoroids and manmade space debris exists in space. These particles collide with the

2. Space debris, also known as orbital debris, space junk and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to explosion and collision fragments.

3. The debris can include slag and dust from solid rocket motors, surface degradation products such as paint flakes, clusters of small needles, and objects released due to the impact of micrometeoroids or fairly small debris onto spacecraft. As the orbits of these objects often overlap the trajectories of spacecraft, debris is a potential collision risk.

4. The vast majority of the estimated tens of millions of pieces of space debris are small particles, like paint flakes and solid rocket fuel slag. Impacts of these particles cause erosive damage, similar to sandblasting. The majority of this damage can be mitigated through the use of a technique originally developed to protect spacecraft from micrometeorites, by adding a thin layer of metal foil outside of the main spacecraft body.

5. Impacts take place at such high velocities that the debris is vaporized when it collides with the foil, and the resulting plasma spreads out quickly enough that it does not cause serious damage to the inner wall. However, not all parts of a spacecraft may be protected in this manner, i.e. solar panels and optical devices (such as telescopes, or star trackers), and these components are subject to constant wear by debris and micrometeorites.

6. The present means for spacecraft shielding, such as those used for the manned modules of the International Space Station, are only capable of protecting against debris with diameters below about 1 centimeter. The only remaining means of protection would be to maneuver the spacecraft in order to avoid a collision. This, however, requires that the orbit of the respective object be precisely known.

7. If a collision with larger debris does occur, many of the resulting fragments from the damaged spacecraft will also be in the 1 kilogram mass range, and these objects become an additional collision risk.

8. As the chance of collision is a function of the number of objects in space, there is a critical density where the creation of new debris occurs faster than the various natural forces that remove these objects from orbit. Beyond this point a runaway chain reaction can occur that quickly reduces all objects in orbit to debris in a period of years or months.