The Iridium Satellite Constellation

Satellite paths
Iridium Satellite LLC is a company that operates a global network of over 70 low earth orbit communications satellites. The Iridium Home Page provides information on their products and services. The satellites are arranged into 6 polar orbit planes with about eleven satellites in each plane plus some spares. The Iridium satellites are in low-Earth orbit (LEO), at an altitude around 800 km.

Each satellite must be oriented in space so that the solar panels can provide power and the communications antennae point toward the specified cell on Earth. The orientation of a satellite in space involves the Attitude Control System (ACS). As part of the ACS, satellites employ an instrument that detects magnetic fields known as a magnetometer. The source of the geomagnetic field may be traced back to electric currents in the Earth.

Magnetometers also respond to electric currents external to the Earth, those that are generated in near-Earth space. A well-known current system in near-Earth space is the Birkeland current, named after the famous Norwegian scientist who studied them using ground based instrumentation around 1908.

At right is the basic look of an Iridium satellite. Each satellite is around 13 m in length, 4 m wide and weighs 700 kg on Earth. They complete one revolution around the Earth in about 100 minutes. This means that an Iridium satellite is in view for a person observing from the Earth surface for about 9 minutes. But dont worry, there are plenty of them.

A person making a voice/data connection through Iridium gets routed from satellite to satellite as the antennae come into and disappear from view. Communication across the constellation is required to route the data to ground stations (gateways). In principle only one gateway is required. However, a number of them are used as described on the Iridium home page.