Monday, January 18, 2010

About Deep Space Communications

One of the most important and predominant functions involved in the exploration of space is its communication system. This system is responsible for sending scientific data from spacecraft back to Earth. It also provides the capability of tracking the spacecraft and commanding it to take certain actions. Without an effective communications system a successful mission would not be possible.

The challenge of deep space communication has been the enormous range of distances that spacecraft have traveled in the past 50 years. Planetary spacecraft have reached distant planets tens of billions of miles from Earth, and have successfully performed their functions. The necessity of minimizing spacecraft mass presents a major challenge to communications system engineers, as engineers must consider the issues of providing power supply, antennae, and many other necessary devices and supporting elements for a communications system. Another important challenge is the extreme reliability required of the communications systems on the spacecraft. Once the spacecraft is launched, on-board failures can be repaired only by relying upon redundant and adaptive systems. Communication engineers must take into consideration such factors as system degradation, aging, and imperfect antenna positioning, as well as operations and data procedures.

In the past, spacecraft data return rates have been tens to hundreds of kilobits per second (kbps) and uplink command data rates have been limited to a few kbps. Recent missions such as MRO can transmit data to Earth at rates as high as 6 megabits per second. For more demanding missions in the near future, much higher data capabilities will be required.
DSN image

The Deep Space Network (DSN) operated for NASA by the Jet Propulsion Laboratory (JPL), provides deep space communications, tracking of spacecraft, and performs many scientific experiments. Because future space missions promise to explore the far reaches of the solar system and beyond, the DSN would need to expand its technological and communications capabilities to meet greater science data return rates and the requirements of advanced spacecraft. For example, by one estimate, the DSN might have to support over twice the missions in 2020 as it supported in 2005, and the data rate from each mission could average at least a factor of 10 higher.

The DSN consists of antenna arrays in 3 locations around the world; near Madrid, Spain; near Canberra, Australia; the Goldstone facility in California’s Mojave Desert, and the command center at JPL in California. These facilities, approximately 120 degrees apart on Earth, provide constant coverage for a mission at critical times. Each facility has a number of antennae some of which can be operated as an array, including at least two 34-meter arrays, and a giant 70-meter array in each location. Use of the arrays is scheduled well in advance for all interplanetary missions as their use is in high demand.

To enable future critical space exploration missions, new technology investments are needed so that future programs will continue to be successful and affordable (i.e. no specific program can afford to bear the burden of the technology development by itself). JPL sponsors internal development of several deep space communications efforts.

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