The recent launch of the first Inmarsat I-4 communications satellite marks a significant advancement in global connectivity. This satellite, positioned in geosynchronous orbit, will cater to the eastern hemisphere, with its counterpart scheduled for launch later this year to cover the west. This two-satellite system is designed to provide comprehensive broadband voice and data services across the globe, with a third satellite on standby as a backup.
This achievement builds upon a legacy of space-based relay stations that has spanned almost half a century. The world’s first genuine communications satellite was a component of a missile test. In the early days of space exploration, the ATLAS ICBM was being developed into a space launch vehicle. While it would later make history by carrying John Glenn into Earth orbit, on December 18, 1958, its purpose was to deliver the Atlas B missile into low Earth orbit, conveying a message to the world. The Army was tasked with creating a communications satellite that could relay messages for a few weeks, known as Project SCORE (Signal Communication by Orbiting Relay Equipment).
Project Score’s communication system operated in two modes: an electronic repeater mode, which amplified and retransmitted Earth-based transmissions on different frequencies, and a “store and forward” mode, utilizing tape recorders. This second method involved recording a message from the ground while over the transmitting station and playing it back once over the receiving station, potentially on the opposite side of the globe. Voicemail serves as a contemporary illustration of store-and-forward communication.
The inaugural voicemail message was a Christmas greeting from President Eisenhower, a last-minute decision that added a poignant touch to Project SCORE, exceeding the impact of the initially planned test messages. Recordings of the President’s message were swiftly uploaded to the fueled missile and stored on the primary and backup tape recorders. On December 19, 1958, President Eisenhower’s voice was broadcast 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 descending into the atmosphere a month later, the 9,000-pound rocket-satellite hybrid successfully transmitted 78 voice and teletype messages through direct relay and store-and-forward methods.
Notably, the Inmarsat I-4 satellite alone weighs half as much as the entire Project SCORE rocket and payload combined. Interestingly, it was launched using a modern iteration of the same Atlas rocket. The present-day Atlas V, considerably larger and more potent than the original ATLAS B ICBM, incorporates strap-on solid booster rockets designed to lift massive payloads.
Another difference between Project SCORE and Inmarsat I-4 lies in their orbits. Project SCORE was a low Earth orbiting (LEO) satellite, with an orbit ranging from 114 miles at its lowest point (perigee) to 920 miles at its highest (apogee). While a single LEO satellite can relay messages globally, it relies on the store-and-forward mode. Alternatively, a network of multiple satellites is needed to ensure constant availability. Iridium, a satellite phone provider utilizing 66 low-orbiting satellites, exemplifies modern LEO satellite applications.
In contrast, the Inmarsat I-4, similar to most television and broadband data satellites, occupies a geosynchronous orbit at an altitude of 22,223 miles, also known as a Clarke Orbit, named after science fiction author Arthur C. Clarke, who proposed the concept in the 1940s. The advantage of this specific altitude is that satellites orbit at the same speed as Earth’s rotation, maintaining a fixed position over a specific point on the ground. Consequently, only two or three satellites in geosynchronous orbit are required to provide global coverage, allowing ground-based dish antennas to remain permanently directed at them.
To delve deeper into the latest developments in communications satellites, refer to my article, “Broadband In Space.”
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