Technology

Deep space communications use microwave radio frequencies within a radio-frequency band (30-100,000 megahertz) that is also used for television, FM radio, cellular telephones, data communications networks, and radar. Within that range, a band from 2-32 gigahertz is allocated exclusively for deep space communications. A microwave beam travels in a straight line, it can be reflected from a smooth surface, and it can be focused by a lens or a curved reflector to increase its strength (brightness). A deep space radio link is basically the same as other point-to-point microwave communications systems except for two major differences - the enormous distances involved and the ultra low-level spacecraft signal transmitted to Earth.

A Whisper from Space

To reduce costs and save onboard weight and power, spacecraft communications equipment transmits signals at very low power, usually about 20 watts, approximately the same amount required to light a refrigerator light bulb. The spacecraft antenna focuses the signal into a narrow beam aimed at Earth. As the signal travels, it continues to lose energy as it loses its focus; by the time it reaches Earth, the signal arriving at the antennas can be as weak as a billionth of a billionth of a watt.

As microwave frequencies increase, more radiated power from the spacecraft reaches the reflecting surface of the ground antenna because the transmitted radio beam has a tighter focus, resulting in an improved signal-to-noise ratio -strong signals with weak noise levels. The DSN has instituted communication capabilities at higher frequencies to take advantage of these benefits.

The Problem of Noise

To hear the spacecraft's faint signal, the antennas are equipped with amplifiers, but there are two obstacles. First, the signal becomes degraded by background radio noise (static), which is radiated naturally from nearly all objects in the universe. The background noise is amplified along with the signal. Second, the powerful electronic equipment amplifying the signal adds noise of its own. Because noise is always amplified with the signal, it is the signal-to-noise ratio - an indication of the ground-receiving system's ability to distinguish the signal in the midst of unwanted noise - that makes the critical difference.

The technology elements used to minimize noise are the state-of-the-art sensitivity and efficiency of DSN antennas, very cold low-noise receivers, and noise-combating coding techniques.

Parabolic Reflector Antenna

Extraordinary demands are placed on ground-system antenna. The DSN's parabolic reflector antennas are focusing mechanisms that concentrate power when receiving and transmitting data. The efficiency and high gain of parabolic reflectors are critical to the success of deep space communications. Precision pointing of the antenna is also crucial. Because of its very narrow beam, an antenna can "see" only a small portion of the sky (equivalent to viewing the sky through a soda straw) and must be pointed directly at a spacecraft, whether transmitting or receiving.

As mission support requirements have become more demanding, some DSN antennas have been retired and others upgraded. The 34-meter beam waveguide antennas - the new workhorses of the network - are designed to route microwave energy between the main reflector and a room located in the basement. In this way, many feeds and amplifiers can be placed in a laboratory environment and be illuminated selectively. Moving the electronics from the external environment to the basement protects them and makes it easier to maintain and modify the antenna.