Communications for Human Spaceflight

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Communications for Human Spaceflight

The first serious proposal for space-based human communications was in Arthur C. Clarke's famous article titled "Can Rocket Stations Give Worldwide Radio Coverage?", which appeared in the October 1945 issue of the British magazine Wireless World. In this article, Clarke made the case for manned space stations in geosynchronous orbit . In addition to conducting research, these stations were to be used to relay radio signals back and forth from Earth's surface. Clarke's article is generally recognized as the origin of today's communications satellites.

Early Communications Systems

During the first piloted spaceflight in April 1961, Yuri Gagarin was able to maintain voice communications with Moscow Ground Control throughout his 108-minute trip. As well as being the first human in orbit, he was the first to communicate from space to Earth. On this spaceflight, telemetry, defined as a constant stream of data, was sent back to Earth from his capsule, Vostok 1. Gagarin used radios that transmitted via the very high frequency (VHF), high frequency, and shortwave bandwidths. He tried to maintain communications with a network of six or seven ground stations, all based inside the borders of the Soviet Union.

Later, the Soviets would build and deploy a small fleet of massive radio-relay ships equipped with huge dish antennas. These ships would allow them to maintain constant worldwide communications with their spacecraft without relying on land-based antennas. For the Mercury and later programs, the Americans were able to set up a network of thirteen large antennas in friendly and/or neutral nations, especially Bermuda, Spain's Canary Islands, Nigeria, Zanzibar, Australia, Canton Island, and Mexico; on U.S. territory, in the Hawaiian Islands, California, and Florida; and on ships.

Types of Signals and Antennas

Since Gagarin's flight, piloted spacecraft transmit three main types of signal to ground stations: voice, television, and telemetry, also referred to as data. One of the best-known forms of telemetry is biomedical monitoring where sensors attached to an astronaut's body send an uninterrupted flow of data concerning heartbeat, breathing, and blood pressure to medical personnel on the ground. Other signals used by spacecraft include interferometry for measuring microwaves, radar , and automated beacons that provide mission control with the capsule's precise location in space. Recovery beacons are for use during and after landing back on Earth.

Early Russian piloted spacecraft transmitted mostly in the AM and FMbandwidths, while later ones also used more sophisticated pulse compression techniques. To this day, Russian spacecraft tend to use separate antennas for each communications function. Thus, their vehicles tend to be festooned with whip antennas.

In contrast, the Americans either integrate their antennas into the skin of their spacecraft or use small blade antennas, such as the VHF scimitar ones on the Apollo service module. For Apollo's long-range communications needs, the National Aeronautics and Space Administration (NASA) installed a steerable, S-band, 2-gigahertz high-gain antenna. This assembly, composed of four small (78-centimeter [31-inch]) parabolic dish antennas attached to a boom, was more difficult to design and build than all of Apollo's other communications gear combined. This antenna group was deployed as the Command and Service Module docked with the Lunar Excursion Module.

The TDRS System

For the space shuttle, NASA designed and built a series of tracking and data relay satellites (TDRS; pronounced T-dress). The TDRS series is the backbone of the U.S. space communications system in the early twenty-first century. Ground stations are used almost entirely as backups. There are currently seven TDRS spacecraft in equatorial geosynchronous orbit 35,786 kilometers (22,300 miles) above Earth. The first of the series, TDRS-A (now referred to as F-1), was launched by the space shuttle Challenger in April 1983. TDRS-B was onboard the Challenger when it blew up in January 1986. TDRS-C was launched in September 1988, TDRS-D (F-4) in March 1989, and TDRS-E in August 1991. They have been joined by TDRS-F (F-5), TDRS-G (F-6), and the first of a new generation built by Hughes, TDRS-H, launched in June 2000. TDRS-H provides Ka band service for NASA's international partners.

The TDRS system operates in the C, the S, the high-capacity Ku, and the Ka radio bands. The system is controlled from the White Sands Ground Terminal in New Mexico. A support ground station has been built on Guam. The TDRS does not process any data by itself. It is strictly a relay system. The two principal antennas are 4.9 meters (16 feet) in diameter, parabolic, and dual-feed S band/Ku band, and they are held together by a set of umbrella-like ribs. The S-band multiple access phased-array antennas can simultaneously receive signals from five spacecraft while transmitting to one.

The TDRS system provides service to a wide range of orbiting spacecraft, both those with crews and those without. Spacecraft supported by this system include the space shuttle, the International Space Station (Alpha), and the Hubble Space Telescope; Earth observation satellites, which help monitor and control pollution; and astrophysical satellites, such as the X-Ray Timing Explorer, which are showing scientists some of the wondersof the universe. On March 8, 2002, Atlas IIA launched the TDRS-I. Due to a problem with one of the propellant tanks it failed to achieve the proper orbit. Boeing, the prime contractor, has said that eventually they will succeed in getting it to its proper slot in geosynchronous orbit.

see also Communications, Future Needs in (volume 4); Ground Infrastructure (volume 1); Guidance and Control Systems (volume 3).

Taylor Dinerman