Saturday, March 12, 2005

The Biggest Communications Bird Is Up

The first in the constellation of Inmarsat I-4 communications satellites has been successfully launched into its geosynchronous orbit to serve the eastern hemisphere. A second I-4 will be launched later this year to provide coverage for the western hemisphere. Only 2 satellites will provide broadband voice and data coverage for the entire Earth. The third member of the constellation is a spare which is only needed for backup.

Thus continues an almost half-century of relay stations in space. The very first true communications satellite was actually a passenger on a missile test. In the dawn of the space age, the ATLAS ICBM was being readied as a space launch vehicle. In a few years it would take John Glenn into history as the first American to orbit the Earth. But on December 18, 1958 the mission was simply to place the Atlas B missile into low Earth orbit and make a statement to the world. The Army was asked to design a communications satellite that would be part of the missile and could relay messages for a couple of weeks. It was called Project SCORE for Signal Communication by Orbiting Relay Equipment.

The communications package for Project Score had two modes. The first is the familiar electronic repeater mode where transmissions from the Earth are amplified and retransmitted on another frequency. The second is a "store and forward" mode using tape recorders. The message from the ground is recorded while it is over the transmitting station. It is then played back over the receiving station, which might be on the other side of the Earth. A modern example of store and forward communications is voice mail.

The very first voice mail was a Christmas greeting by President Eisenhower. It was a last minute idea that made Project SCORE far more dramatic than the simple test messages that were originally planned. Tapes of the President's message were hurriedly uploaded to the fueled missile and recorded on the main and backup tape recorders. On December 19, 1958, President Eisenhower broadcast this message from space:

"This is the President of the United States speaking. Through the marvels of scientific advance, my voice is coming to you from a satellite traveling in outer space. My message is a simple one: Through this unique means I convey to you and all mankind America's wish for peace on Earth and goodwill toward men everywhere."

Before burning up in the atmosphere a month later, the 9,000 lb. rocket/satellite combination transmitted 78 voice and teletype messages by direct relay and store and forward.

The Inmarsat I-4 satellite alone weighs half again as much as the entire rocket and payload for Project Score. Ironically, it was launched by the latest version of the same Atlas rocket. Today's Atlas V is far larger and more powerful than the original ATLAS B ICBM, and includes strap-on solid booster rockets to aid in lifting massive payloads.

Besides size and sophistication, another difference between Project SCORE and Inmarsat I-4 is their orbits. Project SCORE was a low Earth orbiting or LEO satellite. Its orbit varied from 114 miles at the low point or perigee and 920 miles at the high point or apogee. One LEO satellite can relay messages around the world, but it needs that store and forward mode to do so. Otherwise you need a constellation of many satellites so that at least one is available to you at all times. An example of LEO satellites today is Iridium, which provides telephone service that is available anywhere through 66 low orbiting satellites.

The Inmarsat I-4, like most television and broadband data satellites, is parked in a geosynchronous orbit at an altitude of 22,223 miles. That distance is also called a Clarke Orbit after the author, Arthur C. Clarke, who proposed it in the 1940s before any satellites flew. The magic of that exact distance is that satellites orbit at the same rate as the Earth rotates so they stay in one point over the ground. Only 2 or 3 satellites will provide worldwide coverage in geosynchronous orbit. Dish antennas on the ground can be permanently pointed at them without any need for tracking.

Read more about the latest communications satellites in my article, "Broadband In Space."

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