Deep Space Communication

“Astronomy compels the soul to look upwards and leads us from this world to another” ~Plato

What is meant by the term “Deep Space”?

The term “Deep Space” is generally used when referring to large distances from earth. The National Aeronautics and Space Administration (NASA) considers deep space to be any distance further than the moon which is roughly 384,000km (Sharp, 2013) (NASA, 2014). However, the European Space Agency (ESA) use distances of greater than 2,000,000 km from earth (ESA, 2012).

During each deep space mission, reliable communication with spacecraft, to send commands or software updates, track location and receive telemetry, images and scientific data is paramount to the success of the mission. This need for reliable deep space communication fuels a constant source of global interest for research and development in the area.

The term “Deep Space Communication”

“Communication with satellites and spacecrafts active in outer space region as well as collecting radio and radar astronomy observations.”

Need For Deep Space Communication

• Acquire telemetry data (Spacecraft Data)
• Transmit commands to spacecrafts
• Gather scientific data
• Monitor and control performance of a network (Satelite/Probe)

Challenges Faced in Extraterrestrial Communication

• Distance

Distance is the main problem in space communications, since the intensity of electromagnetic radiation decreases according to 1/r², that is why signals from deep space probes are usually very weak when they reach the earth.

Cosmological Scale in AU

• Time Delay

Electromagnetic radiation can’t move faster than the speed of light there are considerable time lag introduced in the communications making real time communications impossible. It takes over 5 hours for a signal from earth to reach the orbit of Pluto in the outer part of the solar system.

• Line Of Sight

In order to communicate with the Earth, the spacecraft must have a free line of sight to the Earth, since radio waves cannot pass through large solid objects such as planets and moons.

• Antennae Alignment

Even if the probe has a free line of sight to the Earth the receiving antenna could be on the wrong side of the Earth.

• Earth’s atmosphere

Since the signal has to pass through the Earth’ s atmosphere some limitations are placed on which frequencies that could be used. The ionosphere is almost opaque to some of the lower frequency bands so space communication mainly uses high frequency bands.

• Shannon Limit

Given a channel with particular bandwidth and noise characteristics, Shannon showed how to calculate the maximum rate at which data can be sent over it with zero error. He called that rate the channel capacity, but today, it’s just as often called the Shannon limit.

Formula to calculate Shannon Limit

The above-mentioned challenges are solved by NASA using Deep Space Networks (DSN’s).

Deep Space Network

• The NASA Deep Space Network (DSN) is a worldwide network of U.S. spacecraft communication facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA’s interplanetary spacecraft missions.

Field of View of DSN from North Pole

• DSN is part of the NASA Jet Propulsion Laboratory (JPL) at Caltech. The DSN is operated by NASA’s Jet Propulsion Laboratory (JPL), which also operates many of the agency’s interplanetary robotic space missions. Other space agencies, such as Europe’s European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), also use the DSN through cooperative agreements.

DSN Control center at Pasadena, JPL @Caltech

• The strategic placement of these sites permits constant communication with spacecraft as our planet rotates; before a distant spacecraft sinks below the horizon at one DSN site, another site can pick up the signal and continue communicating. The Apollo program needed full-time communications support and JPL was busy with its own missions, so DSN engineers helped design and operate a “parallel network.”
• DSN currently consists of three deep-space communications facilities placed approximately 120 degrees apart around the Earth:

1. Goldstone Deep Space Communications Complex outside Barstow, California.
2. Madrid Deep Space Communications Complex, 60 kilometres (37 mi) west of Madrid, Spain.
3. Canberra Deep Space Communication Complex (CDSCC) in the Australian Capital Territory, 40 kilometres (25 mi) southwest of Canberra.

Goldstone, Canberra, Spain DSN Complexes respectively

• Other countries and organizations also run deep space networks. These include the Soviet Deep Space Network, the Chinese Deep Space Network, the Indian Deep Space Network, the Japanese Deep Space Network and ESTRACK of the European Space Agency. These agencies often cooperate for better mission coverage.

Uplink and Downlink

• The radio signal transmitted from Earth to a spacecraft is known as uplink. The transmission from spacecraft to Earth is downlink. Uplink or downlink may consist of a pure RF tone, called a carrier. Such a pure carrier is useful in many ways, including radio science experiments. On the other hand, carriers may be modulated to carry information in each direction.

Beacons

• As part of its new technology, the Deep Space 1 spacecraft has demonstrated beacon monitor operations, a mode which may become more widespread as spacecraft intelligence and capabilities increase.
• In beacon monitor operations, an on-board data summarization system determines the overall spacecraft health. Then it selects one of 4 subcarrier tones to place on its downlink carrier to indicate whether, or how urgently, it needs contact using the Deep Space Network’s larger antennas.

Beacon Mode

Symbols and Bits and Coding

• Generally, modern spacecraft don’t place data bits, as such, onto the carrier or subcarrier. They put symbols there instead.
• A symbol is a wiggle (or a non-wiggle) in the phase of the carrier or subcarrier. A number of symbols make up a bit.
• Two schemes are commonly in use for putting them on the downlink or uplink:

1. NRZ
2. Bi-Phase(Manchester)

Telemetry Lock

• Once a DSN receiver has locked onto a downlink, symbols are decoded into bits, and the bits go to a telemetry system.
• Once the telemetry system recognizes frames reliably, it is said to be in “lock” on the data.

DSN Transmitters

• All the antennas used in the DSN are quasi-parabolic reflectors. This achieves high directivity and gain with electromagnetic waves in the microwave range, with frequencies greater than 1 GHz.
• The shape of the antenna dish also has the advantage of forcing the entire incident wave front that bounces on the different points to travel the same distance until reaching this focus.
• The emission and reception signal is similar to a common microwave radio link. The microwave signals are transmitted through space, by an unguided means. The vacuum waves propagate in a straight line at the speed of light.

Transmission Antenna

Ranging System

• The DSN ranging system measures the round-trip phase delay of a ranging signal sent from an uplink DSS to a spacecraft and back to a downlink DSS.
• In the most common configuration, known as two-way ranging, the uplink and downlink stations are the same, and the measured two-way phase delay permits the determination of the round-trip light time (RTLT) between the DSS and spacecraft.

DSN ranging system architecture

Some of the Successful Missions Conducted with the help of DSN at JPL Caltech:

1. Hubble Space Telescope

The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It is one of the largest and most versatile, renowned both as a vital research tool and as a public relations boon for astronomy.

Hubble

Hubble is the only telescope designed to be maintained in space by astronauts. Five Space Shuttle missions have repaired, upgraded, and replaced systems on the telescope, including all five of the main instruments.

2. Pioneer 10

• Pioneer 10 (originally designated Pioneer F) is an American space probe, that completed the first mission to the planet Jupiter. Pioneer 10 became the first of five artificial objects to achieve the escape velocity needed to leave the Solar System. On November 6, 1973, the Pioneer 10 spacecraft was at a distance of 25 million kilometers (16×106 mi) from Jupiter. Testing of the imaging system began, and the data were successfully received back at the Deep Space Network.

Location of Pioneer 10 as of FEB 2012

3. Voyager Program

• The twin Voyager 1 and 2 spacecraft are exploring where nothing from Earth has flown before. Continuing on their more-than-40-year journey since their 1977 launches, they each are much farther away from Earth and the sun than Pluto. In August 2012, Voyager 1 made the historic entry into interstellar space, the region between stars, filled with material ejected by the death of nearby stars millions of years ago. Voyager 2 entered interstellar space on November 5, 2018 and scientists hope to learn more about this region. Both spacecraft are still sending scientific information about their surroundings through the Deep Space Network, or DSN.

Voyager 1 and 2 have crossed heliosphere and entered Intersteller space

4. New Horizons

• New Horizons is an interplanetary space probe that was launched as a part of NASA’s New Frontiers program. the spacecraft was launched in 2006 with the primary mission to perform a flyby study of the Pluto system in 2015.
• The craft had a communication rate of 38 kbit/s at Jupiter, at Pluto’s distance, a rate of approximately 1 kbit/s. The 70 m (230 ft) NASA Deep Space Network (DSN) dishes are used to relay commands once it is beyond Jupiter.

Deep Space Network (DSN) Now

• At three sites around the globe NASA’s Jet Propulsion Laboratory operates a network of large radio antennas called the DSN. The DSN is used to keep in contact with the spacecraft exploring our solar system and the universe beyond. This online tool will let you see what the DSN is up to right now.
• The grid of antennas show you the current state of the network. ‘Radio Waves’ will show you if data is being sent to, or received from a spacecraft. Often both will be happening at the same time sometimes to more than one spacecraft at a time.

DSN Now

Indian Deep Space Network (IDSN)

• The Indian Deep Space Network (IDSN), commissioned during the year 2008, at Byalalu village near Bengaluru, forms the Ground segment for providing deep space support for India’s Space Science Missions like Lunar mission-Chandrayaan-1, Mars Orbiter Mission (MOM) etc., Indian Space Science Data Centre (ISSDC), located at the IDSN campus, is the primary data centre for data archives of Indian Space Science Missions.

IDSN Campus

• IDSN complex comprises of Deep Space Antennas of 18 m and 32 m capable of supporting interplanetary missions. It also houses 11 m antenna facility to support earth bound scientific missions. The 32 m fully steerable antenna (DSN32) with beam wave-guide, operating in S and X-band and 18 m fully steerable antenna offer excellent facilities for supporting International Deep Space Missions.

Authors:

• Neeraja Khire
• Mahesh Kinge
• Chinmay Kshirsagar
• Parth Kudal
• Isha Lad