Satellite constellation
Collaborative configuration of multiple artificial satellites.
Since the launch of Sputnik in 1957, many hundreds of artificial satellites have been put into orbit. Initially they were somewhat crude in both
design and operation but, as the possibilities they offered were explored, many new and potential uses were thought of and implemented. Satellites were built for weather observation, military purposes and to relay telecommunications, and the people and Powers That Be alike thought it was Good. Before too long though, many applications ran into a scalability issue.
Individual satellites have their limitations. As soon as one goes over the horizon in relation to its ground station, we have blackout and therefore interrupted service which is incompatible with continuous use such as would be required for reliable global telecommunications. Geostationary satellites partly solved that problem by remaining in the line of sight of their ground station at all times but their field of vision is clearly defined and limited.
At the same time, some applications required images from more parts of the earth than a single satellite can view or depend on imagery from an angle as close to vertical as possible. One can increase the visible swathe of territory by increasing the satellite's altitude but inevitably a compromise is made as regards image quality and resolution. One can clumsily patch together images from different satellites but the process is practically hand-cranked and cannot compare to a system of satellites meant to work as part of the same system. If you look at a weather images from the 1970s you can see the seams between different snapshots and differences in their resolution and shades.
A constellation is a configuration of satellites, supported by one or more ground stations, designed to increase the range or
coverage of a satellite system or to allow different types of satellites with varying tasks to work together. Typically these are systems used for meteorology or telecommunications. Satellites are carefully positioned in different orbits and orbital planes in order to obtain the desired level
of coverage and redundancy.
In this way, if a geostationary satellite can cover, let's say 30 degrees of longitude, and the goal is coverage of the entire equatorial zone, twelve satellites spaced evenly around the globe will cover the whole earth at the equator but less and less as we move away from it. Sixteen would improve the system by allowing for some overlap and thus improved coverage at the boundaries between the fields of two satellites. Or, one could elevate the constellation's orbit in order for each satellite to "see" a wider area and reduce the number. More territory north and south could be covered by adding satellites in inclined orbits designed to complement each others coverage.
The first constellation to be put into orbit was the Global Positioning System, whose first component, built by Boeing, was launched in 1978, although the system was not declared fully operational until 1993.
A constellation will typically operate by passing data to a ground station which will forward it to another satellite if necessary until it reaches its destination. More high-end constellations work by using the satellites themselves to contact other satellites in the array. When there is a means of sending data to farside satellites by way of relaying traffic through other satellites, a single ground station is sufficient. This system is called crosslinking and was first enabled on the American Milstar II system in 1995.
There are about 25 planned and operational satellite constellations. Some of them are:
- Globalstar is a telecommunications constellation of 48 satellites plus four spares that are spaced around the globe. They use six different orbital planes and their orbit is inclined at 52° in order to obtain coverage up to 70 degrees of latitude on either side of the equator. Anywhere in that zone, you can expect two to four Globalstar satellites to be overhead at any given time. It's also used for IP traffic.
- Iridium, one of the largest constellations, is Motorola's troubled offspring and its 66+6 satellites were originally designed for high-reliability telephone communication anywhere in the world but advances in the reliability, availability and equipment used in other mobile telephony systems left it with only the most demanding customers. It was almost deorbited for financial reasons but may become profitable if it becomes part of a global aircraft monitoring system, a use which was proposed in October 2001.
- Inmarsat is by design a naval communications system and, unlike most others, is primarily geared towards covering oceans rather than land, but does not exclude land areas and will expand to include broadband data traffic. Its eight geostationary satellites and over twenty ground stations are a staple of telecommunications at sea.
- Cospas-Sarsat, operated by several countries, is a constellation itself made up of two small constellations and is designed to aid search and rescue. Its components are a dual-use mixture of geostationary weather satellites and low-orbit communications satellites. This one demonstrates how existing satellites can be used for another purpose in addition to their intended one.
The mathematics of designing and arranging satellite constellations are considerable and all calculations must be made on a case by case basis as
constellations are unlikely to be the same and different applications require wildly varying configurations. Elements such as the locations of ground stations, desired coverage and redundancy, special terrain circumstances, and the amount of available hardware may all have to be taken into consideration. Constellations will also be impacted by the type of satellite used. Communication protocols, security and both ground and orbital software are also important factors as much as they are in standalone satellite systems.
Several software packages have been created to design, increase and maximise the efficiency of a satellite constellation, reduce the number of satellites needed, or allow space for their expansion or interoperability with other constellations. Educated guesses about future changes in available hardware and performance are incorporated but any software of the kind will have to evolve alongside the technlogy it deals with. SATCOS by hot-shot research and technology outfit SAIC promises to deliver the most efficient designs.
There is a free Tcl/Tk based tool for satellite orbit display called SaVi that can simulate satellite constellations like Teledesic and GPS
on your desktop and give a decent approximation of the systems' coverage and movement. It won't build on my system but its screenshots look interesting.
Sources:
http://www.avmdynamics.com/software1.htm
http://savi.sourceforge.net/
http://www.ee.surrey.ac.uk/Personal/L.Wood/constellations/
http://www.cospas-sarsat.org/
http://www.saic.com/cover-archive/space/satcos.html
http://tycho.usno.navy.mil/gps_datafiles.html
http://www.aoc.nrao.edu/vla/interference/survey.shtml
http://www.tbs-satellite.com/tse/online/prog_inmarsat.html