Satellite constellation.html
 
| ca | de | en | es | fr | it | nl | no | pl | pt | ru | ro | fi | sv | tr | vo |


 

The GPS constellation calls for 24 satellites to be distributed equally among six circular orbital planes

A group of electronic satellites working in concert is known as a satellite constellation. Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, synchronised so that they overlap well in coverage and complement rather than interfere with other satellites' coverage.

Contents

Overview

Low Earth orbiting satellites (LEOs) are often deployed in satellite constellations, because the coverage area provided by a single LEO satellite covers a small area, and the satellite travels at a high angular velocity to maintain its orbit. Many LEO satellites are needed to maintain continuous coverage over an area. This contrasts with geostationary satellites, where a single satellite, moving at the same angular velocity as the rotation of the Earth's surface, provides permanent coverage over a large area.

Examples of satellite constellations include the Global Positioning System (GPS), Galileo and GLONASS constellations for navigation and geodesy, the Iridium and Globalstar satellite telephony services, the Disaster Monitoring Constellation and RapidEye for remote sensing, the Orbcomm messaging service, Russian elliptic orbit Molniya and Tundra constellations, and the large-scale Teledesic and Skybridge broadband constellation proposals of the 1990s.

Broadband applications benefit from low-latency communications, so LEO satellite constellations provide an advantage over a geostationary satellite, where minimum theoretical latency is about 125 milliseconds, compared to 1–4 milliseconds for a LEO satellite. A LEO satellite constellation can also provide more system capacity by frequency reuse across its coverage, with spot beam frequency use being analogous to the frequency reuse of cellular radio towers.

Satellite constellation coverage and geometry – determining the minimum number of satellites needed to provide a service, and their orbits – is a field in itself.

A group of formation-flying satellites very close together and moving in almost identical orbits is known as a satellite cluster or Satellite formation flying.

Walker Constellation

There are a large number of constellations that may satisfy a particular mission. Usually constellations are designed so that the satellites have similar height, eccentricity and inclination so that any perturbations affect each satellite in approximately the same way. In this way the geometry can be preserved without excessive station keeping thereby reducing the fuel usage and hence increasing the life of the satellites. Another consideration is that the phasing of each satellite in an orbital plane maintains sufficient separation to avoid collisions or interference at orbit plane intersections.

A class of circular orbit geometries that has become a standard is the Walker Delta Pattern constellation. This has an associated notation to describe it which was proposed by John Walker in 1984. The notation is:

i: t/p/f

where: i is the inclination, t is the total number of satellites. p is the number of equally spaced planes f is the relative spacing between satellites in adjacent planes. The change in true anomaly (in degrees) for equivalent satellites in neighbouring planes is equal to f*360/t.

For example, the Galileo Navigation system is a Walker 56°:27/3/1 constellation. This means there are 27 satellites in 3 planes inclined at 56 degrees. The "1" defines the phasing or mean anomaly difference between satellites in adjacent planes.

Example satellite constellations

External links

Satellite constellation simulation tools:

More information:

References
All Right Reserved © 2007, Designed by Stylish Blog.