M5-S12: Low Earth and Geostationary Satellites

  • Predict quantitatively the orbital properties of planets and satellites in a variety of situations, including near the Earth and geostationary orbits, and relate these to their uses

Orbital motion depends on the nature of orbit

An object’s orbital motion depends on its velocity, mass of object it is orbiting around and the radius of orbit. 

  • Orbital velocity

Greater orbital velocity is required for smaller orbits and when orbiting planets of heavier mass. This implicates on greater energy need and consumption as greater velocity can only be achieved and maintained by greater fuel consumption. Ultimately, this means satellites in bigger orbit paths need to carry more fuel otherwise they are destined to have short orbit time.

Satellites often encounter orbital decay whereby they are unable to maintain the stable orbit and as such fall back down onto Earth’s surface. There are couple of reasons for this:

  • Radius of orbit

As indicated by Newton’s law of gravitation, smaller orbital radius also causes the distance between two masses (e.g. Earth and satellite) to become smaller. This causes the gravitational force and acceleration experienced by the satellite to increase. This is a problem for satellites with low altitude orbits as it contributes to orbital decay 

  • Air resistance

In the real world, orbiting in space is heavily associated with air resistance due to the presence of atmospheric particles. As a result, during a satellite’s motion, friction significantly contributes to orbital decay. This is more of a concern for low-Earth orbits as the atmosphere is typically denser at low altitudes

  • Depletion of fuel leads to loss of velocity which eventually becomes lower than the required orbital velocity à loses stable orbit.

 

Types of Satellites

There are two main types of satellites: low-Earth orbit (LEO) and geostationary orbit (GEO) satellites. Geosynchronous orbit satellites are similar to geostationary with one key difference: Geostationary satellites orbit the equator whereas geosynchronous orbit at a different latitude. Geosynchronous orbit satellites are not examinable in the HSC Physics syllabus.

 

Comparison between types of satellites

 

Low-Earth (LEO)

Geostationary (GEO)

Orbital Radius

Satellites with low attitude (<2000 km) and short orbital period.

Satellites with much higher altitude of ~35000 – 36000 km. ‘Orbits’ at the same velocity as Earth’s rotation.

 

Thus, it remains at the exactly same spot above Earth’s surface throughout its orbit.

Total energy

Lower (gravitational potential energy is more negative)

Higher

Orbital period

Shorter

Longer. Orbital period is approximately 24 hours.

Orbital velocity

Faster

Slower

Advantage

·     Closer to surface of Earth which enables higher resolution photographs and videos

·     Cheaper to establish due to low attitude and lower fuel requirement to reach there

·     Accurate signal in relation to communication

·    Orbital period is in sync with that of Earth which enables several essential applications

·    Easier than low-Earth to maintain the orbit due to greater orbital radius (thus low orbital velocity)

·    Wide coverage

Disadvantage

·     Harder to maintain a stable orbit due to low attitude (greater orbital velocity)

·     Limited coverage in relation to communication

·     More expensive to establish due to high attitude, more fuel required to reach the orbit

·     Requires strong signal for communication

·     Vulnerable to sun outages (elevation in radiation, occurs twice a year)

Application & uses

·    Cellular communication that only require small coverage e.g. Iridium phone systems

·    Spy satellites

·    International Space Station

·    Television

·    Radio

·    Weather forecast

·    Cell phones

 

HSC Physics: LEO vs GEO satellites

 

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