Satellite constellations (from “Satphones making sense” article)

Before we look at Globalstar and Iridium in detail, let�s understand how satellites work in general. For decades satellites have been placed in high-earth orbits, where they remain stationary 22,000 miles above a single point on the globe. Sending a signal 22,000 miles into space requires lots of energy, or a precisely directed beam using a gyro-stabilized dish antenna. That hardware is large, heavy, expensive and not very portable.

Both Globalstar and Iridium opted for a constellation of 45 to 66 satellites about 500 miles above the earth in polar orbits. These satellites orbit the earth every 90 minutes, and their 15,000 mph speed creates sufficient centrifugal force to counteract gravity and keep the satellites in orbit. Using the right frequencies and advanced electronics, communications can be achieved with low-power hand-held phones just slightly larger than cell phones. But Iridium and Globalstar designed their low-earth-orbiting (LEO) satellite networks differently.