Issues Magazine

The Deep Space Network: Connecting the Solar System and Beyond

By Glen Nagle

It’s quite incredible to see groundbreaking space events unfold live on television and on the internet, but who or what connects us to them? Around the world, a unique communication system and a dedicated team of people bring the universe down to Earth.

On 24 December 2013, as we all prepared to celebrate another Christmas on Earth, a special milestone was reached in the history of space exploration. On that day, 50 years ago, NASA’s Deep Space Network (DSN) was established.

The DSN is a worldwide system of large antenna dish facilities capable of two-way radio communication with multiple robotic spacecraft. The network provides 24/7 coverage to every part of the solar system and beyond.

Without the DSN, which can be thought of as the air traffic control for space, no spacecraft bound for another celestial body would be able to go anywhere, arrive anywhere, or get information there or back again. It is the unseen and often unheard-of connection between interplanetary spacecraft and their Earth-bound scientists and engineers.

What is now known as the Deep Space Network first existed as just a few small antennae called the Deep Space Instrumentation Facility (DSIF). The facility was originally operated by the US Army and used to track sub-orbital and Earth orbiting scientific satellites. In 1958, the DSIF was moved over to the jurisdiction of a newly created civilian agency: the National Aeronautics and Space Administration (NASA).

On 24 December 1963, as the space program continued to expand and more ambitious missions to the Moon and beyond were being planned, the DSIF was formalised as the Deep Space Network.

During the past 50 years, antennae of the Deep Space Network have communicated with most of the missions that have gone to the Moon and those that have ventured further into deep space. The highlights include relaying through their “manned spaceflight” wing the moment that astronaut Neil Armstrong stepped onto the surface of the Moon in a “giant leap for mankind”; transmitting data from numerous encounters with the outer planets, comets and asteroids of our solar system; communicating images taken by rovers exploring the surface of Mars; and relaying the historic data confirming that NASA’s Voyager 1 spacecraft had finally entered interstellar space.

The DSN doesn’t support only NASA missions. Space agencies in Europe, Japan and Russia have also relied on it when planning and communicating with their own missions over the decades. The DSN has been used most recently by India’s first interplanetary probe, the Mars Orbiter Mission, which is due to arrive at the red planet in September 2014. The DSN has been so critical to so many missions over the decades, the network’s team members like to use the phrase “Don’t leave Earth without us”.

The DSN’s Earth-based communications network is an essential component for controlling a spacecraft’s operating modes, loading and reprogramming the onboard computers, navigating the spacecraft to its destination, and sending scientific data back to Earth.

During its first year of operation, the network communicated with just three spacecraft. Today, it maintains around-the-clock coverage with nearly 50 deep space robotic probes via three large antenna complexes located in Goldstone in California, near Madrid in Spain, and near Canberra.

Each of the antenna complexes consists of several deep space stations equipped with large, steerable parabolic reflector antennae, powerful radio transmitters and ultrasensitive receiving systems.

The flagship antennae of the Deep Space Network are massive 70-metre diameter dishes. These towering 22-storey, 7 million kilogram structures can be moved with better than hairwidth accuracy to any point in their local horizon to communicate with spacecraft hundreds of millions to billions of kilometres from Earth and receive signals that can be a billion times weaker than the power of a watch battery.

Each site also hosts several 34-metre diameter high-efficiency and beam waveguide antennae that can be used individually or in arrays to increase overall sensitivity.

All of the complexes are operated by teams of radio communication experts from a centralised signal processing centre at each facility. The centres house the electronic subsystems that point and control the antennae, receive and process the telemetry data, transmit commands and generate the spacecraft navigation data.

Once the data is processed at the complexes, it is relayed via a dedicated optical ground communications network to the Jet Propulsion Laboratory (JPL) in Pasadena, California, for further processing and final distribution to mission science teams.

Missions literally make an “online” booking to the network for the time their particular spacecraft requires. A schedule is then worked out by the Jet Propulsion Laboratory, which provides sufficient resources for transmitting commands to spacecraft and receiving data. The tracking schedule must take into account special activities such as maintenance and upgrades at the facilities, mission training simulations, and the occasional unexpected issue such as a spacecraft emergency.

To maintain and operate all of the systems at a DSN complex, each complex is staffed by teams of mechanical and structural engineers, electronics and computer technicians, specialists in transmitter, receiver and cryogenic systems, plus logistics, general maintenance and administration personnel.

The DSN team’s dedication to “mission” and “data capture” is evident at all times. Spacecraft never sleep. If an antenna stops operating at 2 am they are on duty to ensure it gets fixed. If a spacecraft is due to land on Mars on Christmas day, then they are at their consoles and equipment to make sure it happens. DSN personnel are known in science circles as the unsung heroes of space exploration.

Even though each DSN station is operated independently by personnel from its host nation, the three stations work as one team with a single purpose to keep the lines of communication open between distant space probes and scientists on Earth.

The two-way communications link between Earth and the spacecraft consists of two types of data: uplink and downlink (telemetry). Uplink includes commands, which are coded instructions sent from Earth to control a spacecraft’s operating modes and install new mission software. Downlink consists of science information and “housekeeping” status data sent from the spacecraft back to Earth and forwarded to mission controllers.

The radio link itself is used to generate information about the position of the spacecraft (ranging). By accurately calculating the time a radio signal takes to reach the spacecraft and return (similar to the radar “ping” performed by submarines to detect other vessels), plus knowing the precise pointing coordinates on the antennae, the mission teams can calculate, knowing the speed of their spacecraft, the location of the robotic craft to within a few metres or better.

In addition to allowing missions to upload and download data, the network helps navigators pinpoint spots for landings and conduct engine burns that place spacecraft into orbit around other planets or fine-tune their trajectory.

While some scientists would probably love to have 24-hour coverage for their particular mission, the relatively limited number of antennae compared with spacecraft usually means that each mission can only get a few hours per day of DSN time. However, some places in the solar system have so many active spacecraft operating or have missions returning such huge volumes of data that they do receive virtually 24/7 attention from the network.

To provide for continuous communications, the complexes are located to compensate for the Earth’s daily rotation by being situated approximately 120° apart in longitude. This arrangement provides an 8–14-hour view of the spacecraft at each complex plus suitable overlap to transfer contact with the spacecraft from one complex to the next.

Right now, Mars is considered the “traffic jam” of the solar system, with three missions in orbit and two rovers exploring the surface. They will be joined later this year by two more orbiting craft. By providing around-the-clock coverage, the DSN can handle the huge volume of data that needs to be relayed between them.

Communications for deep space missions are complicated by the extreme distances between the spacecraft and Earth, with lengthy intervals required for transmission and return of signals; a wide range of environments in which spacecraft must operate; and unusual navigation scenarios such as gravity-assist trajectories and aerobraking. Locating the spacecraft’s signal over vast distances, commanding the spacecraft, verifying that the transmission has been correctly understood, and receiving and decoding the faint transmitted signal are fundamental challenges for the DSN.

To reduce costs and save onboard weight and power, spacecraft transmit signals at very low power, usually about 20 Watts, approximately the same amount required to light a refrigerator light bulb. The spacecraft’s antenna focuses the signal into a narrow beam aimed at Earth. As the signal travels, it continues to lose energy and loses its focus, spreading out; by the time it reaches Earth, the signal arriving at the antennae 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.

To detect the spacecraft’s faint signal, the antennae are equipped with amplifiers, but these face several obstacles. First, the signal becomes degraded by background radio noise (static) radiated naturally from nearly all objects in the universe. So 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 biggest issue is radio noise generated on Earth. Deep space communications use microwave radio frequencies within a radio-frequency band of 30–100,000 MHz. This is in the same range used for television, FM radio, mobile telephones, data communications networks and radar. 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. Hearing those signals through all the ground-based radio frequency interference is becoming more difficult as the electronic traffic increases.

With 50 years of history behind it, the future of the Deep Space Network looks bright indeed. As we continue to explore the solar system with both robotic and even human-piloted spacecraft, the need for expanded, efficient deep space communications will intensify.

The future DSN will have optical communications terminals on Earth or in orbit, enabling additional improvements in data collection. Early tests of an optical laser communication system were conducted on NASA’s LADEE lunar mission launched late last year. It beamed data back to Earth ground stations hundreds of times faster than currently allowed at radio bandwidths.

New antennae are currently under construction at each complex to deal with the ever-increasing spacecraft traffic across the solar system. Optical communication is on the horizon to augment the traditional radiofrequency technology, providing a dramatic increase in data return from science missions.

The DSN team envision a day not so far off when, in addition to returning still photos of robotic wheel tracks on the surface of Mars, they will be streaming high-definition video to a wide-eyed public as the first humans leave their footprints on its dusty surface.