Satellites & Mobile Systems

HW 1.1 – How do inclination and elevation determine the use of a satellite?

The inclination angle is defined as the angle between the equatorial plane and the place described by the satellite orbit. An inclination angle of 0 degrees means that the satellite is exactly above the equator.

The elevation angle is defined as the angle between the center of the satellite beam and the plane tangential to the earth’s surface.  

The inclination of the orbit determines what parts of the Earth the satellite travels over. A satellite with inclination near zero may not be able to observe or communicate with parts of the Earth near the poles. When the inclination angle is 90 degrees, the orbital plane contains the Earth’s axis and the orbit passes over the Earth’s poles. Therefore, inclination generally determines what regions of the Earth that are covered by the satellite.

Elevation determines the quality of the communication link. Depending on the elevation, the signal has to penetrate a smaller or larger percentage of the atmosphere. Additionally, interference from tall buildings and other tall objects tends to be greater at smaller elevation angles. Generally, quality is best when the elevation is 90 degrees (i.e. directly overhead the observer), and an elevation less than 10 degrees is considered useless for communication (reference section 5.3).

In summary, both inclination and elevation are important parameters that need to be considered and constrained to ensure that a satellite will meet the objectives of its given mission.

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HW 1.2 – What characteristics do the different orbits have? What are their pros and cons?

GEO (Geostationary or Geosynchronous earth orbit) satellites require a period of 24 hours to appear fixed in the sky. This results in a distance of 35,786 km and an orbit with an inclination of 0 degrees. Examples are almost all TV and radio broadcast satellites, many weather satellites and satellites operating as backbones for the telephone network.

Advantages:

• Three GEO satellites are enough for a complete coverage of almost and spot on earth
• Senders and receivers can use fixed antenna positions (no adjusting is necessary)
• GEOs are ideal for TV and radio broadcasting
• GEOs do not exhibit and Doppler shift (as the relative movement to earth is zero)
• Lifetime expectations for GEOs are high (~15 years)
• GEOs typically do not need a handover due to large footprint

Disadvantages:

• Northern and southern regions (i.e. near poles) have more problems receiving these satellites

• Shading of the signals in cities due to high buildings and low elevation further away from the equator limits transmission quality
• Transmission power needed is relatively high (up to 10 Watts) which causes problems for battery powered devices and cannot be used for small mobile phones
• High latency (> 0.25 seconds one way)
• Due to large footprint, either frequencies cannot be reused or else satellites needs special antennas focusing on a smaller footprint
• Transferring a GEO into orbit is very expensive

MEO (Medium earth orbit) satellites operate at a distance of about 5,000-12,000 km. Up to now there have not been many satellites in this class, but some upcoming systems (e.g., ICO) use this class for various reasons.