Tuesday, April 04, 2006

All of those satellites up there

Ever wonder what all those satellites flying above us are for? I was looking for a list of the satellites in orbit around the Earth as part of understanding the build up of debris in orbit and I found one at the Union of Concerned Scientists website. The Union of Concerned Scientists satellite database is a free list of all of the satellites in orbit around the earth in a very useful excel spreadsheet.

To try to understand where all of the more than 800 satellites are, I put the data into a 2-dimensional histogram (chart below, click it for a larger view) to see the perigee (closest Earth approach) and apogee (farthest distance) at the same time. Satellites with circular orbits fall on the diagonal and stretch from a height of about 300 kilometers to geosynchronous orbits at about 35,700km. These 348 satellites tend to be for communications and have orbital period of 24 hours so that they stay over the same spot on the Earth, geo(earth) synchronous.

The satellite with the most eccentric, non-circular orbit is the Wind satellite, its orbit takes it from 186km above the surface of the earth out to 470,000km with a period of 13 days. Even the moon only orbits at an average distance of 384,403km. Other scientific satellites are the Cluster satellites, which were launched by the European Space Agency and artfully named Salsa, Samba, Rumba, and Tango. These four satellites orbit in a line so that they can used to study the interaction of the Solar wind with Earth's radiation belts. The International Space Station orbits at around 400 miles.

Besides the many geosynchronous communication satellites, there are several groups of communication satellites for use with satphones in circular orbits. The ORBCOMM satellites provide narrow band two-way digital messaging, data communications, and geo-positioning services on a global basis. The Iridium satellites at orbits of 775km are used for a satellite phone system that has certainly seen its ups and down over the years. Globalstar at 1400km is another set of satellites for a satellite voice and data system. Sometimes the communications are mainly one way. For instance, the XM radio satellites are called Rock and Roll (and XM3, no imagination there) and are in geosynchronous orbits at 85W and 115W.

Probably the most famous satellites are the NAVSTAR satellites that are part of the Global Positioning System (GPS). They orbit at about 20,000km. The Russian version is called GLONASS and orbits around 18,000km. Less famous and more hidden are the Keyhole satellites used by the National Reconnaissance Office (NRO) to take pictures of us.

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