GPS Satellite Constellations
The Global Positioning System consists of a constellation of 24 satellites orbiting the earth at an altitude of 10,900 nautical miles. By definition, a satellite constellation is a set of satellites distributed over space intended to work together to achieve common objectives. Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, and synchronized so that they overlap well in coverage and complement rather than interfere with each other’s coverage or functioning.
Low Earth orbiting satellites (LEOs) are often deployed in satellite constellations, because the coverage area provided by a single LEO satellite only covers a small area that moves as the satellite travels at the high angular velocity needed 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.
There are a large number of constellations that may satisfy a particular purpose. Usually constellations are designed so that the satellites have similar orbits, 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. Circular orbits are popular, because then the satellite is at a constant altitude requiring a constant strength signal to communicate.
The Walker Constellation:
A class of circular orbit geometries that has become popular is the Walker-Delta Pattern constellation. This has an associated notation to describe it which was proposed by John Walker (Walker J.G. 1984. Satellite Constellations. Journal of the British Interplanetary Society, Vol 37, pp 559-571). His notation is:
where: i is the inclination; t is the total number of satellites; p is the number of equally spaced planes; and 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 Delta 56°:27/3/1 constellation. This means there are 27 satellites in 3 planes inclined at 56 degrees, spanning the 360 degrees around the equator. The "1" defines the phasing between the planes, and how they are spaced. The Walker-Delta is also known as the Ballard rosette, after A. H. Ballard's similar earlier work. Ballard's notation is (t,p,m) where m is a multiple of the fractional offset between planes.
Another popular constellation type is the near-polar Walker-Star, which is used by Iridium (a communication constellation). Here, the satellites are in near-polar circular orbits across approximately 180 degrees, travelling north on one side of the Earth, and south on the other. The active satellites in the full Iridium constellation form a Walker Star of 86.4°:66/6/2, i.e. the phasing repeats every two planes. Walker uses similar notation for stars and deltas, which can be confusing.
The NAVSTAR Constellation
NAVSTAR satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,187 km (10,900 nautical miles). Each satellite circles the Earth twice a day. The satellites in the NAVSTAR constellation are arranged into six equally-spaced orbital planes surrounding the Earth. Each plane contains four ‘slots’ occupied by baseline satellites. This 24-slot arrangement ensures users can view at least four satellites from virtually any point on the planet.
The US Air Force normally flies more than 24 GPS satellites to maintain coverage whenever the baseline satellites are serviced or decommissioned. The extra satellites may increase GPS performance but are not considered part of the core constellation.
In June 2011, the Air Force successfully completed a GPS constellation expansion known as the ‘Expandable 24’ configuration. Three of the 24 slots were expanded, and six satellites were repositioned, so that three of the extra satellites became part of the constellation baseline. As a result, GPS now effectively operates as a 27-slot constellation with improved coverage in most parts of the world.
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