Free Space Optical Communications will enable space missions to return 10 to 100 times more data with 1% of the antenna area of current state-of-the-art communications systems, while utilizing less mass and power.

Optical Communications is being developed at NASA / JPL for future space missions generating high data-volumes. Laser communications is seen as the technology that will meet these needs for future near-Earth, solar system, and interstellar missions.

The inherent advantage of laser communication over traditional deep space communication systems is its narrower transmitted beam-width, which concentrates a larger fraction of the transmit power onto the ground receiver.

Since the optical signal wavelengths are 3 to 5 orders-of-magnitude shorter than typical RF wavelengths, the theoretical advantage of optical communication is 6 to 10 orders-of-magnitude However, the practically realizable advantage is at least an order-of-magnitude in data rate assuming 10 times reduction in aperture size for the space-borne terminal. Because of its high-bandwidth, low mass and low power-consumption, optical communications enables missions to communicate further into deep space.

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Types of Optical-communication Links

• Shortrange terrestrial, Air-to-Ground, Air-to-Air
• Earth orbit (LEO, MEO, or GEO) to ground, aircraft, or to other spacecraft
• Deep-space (other planets) to terrestrial neighborhood

Benefits of Optical Communications Technology

• Four orders of magnitude more bandwidth than RF frequencies
• Smaller equipment for reduced payload weight
• Narrow beams enhance communication security

The advantage of optical communications comes from the narrow transmitted beam .001 degree beam divergence compared with several milliradians for RF systems. By concentrating the energy, the power is delivered more efficiently to the ground receiver. As an example, for comparable RF and optical systems (i.e., equal transmitting and receiving antenna sizes, equivalent power and conversion efficiencies, and similar receiver signal-to-noise characteristics) upon scaling the wavelengths, the physics principles dictate a factor of one billion (90 dB) in favor of optical communications in link margin.

Typical engineering considerations for deep space missions result in:

• Flight terminal telescope diameters on the order of 0.1 to 0.3 m vs.1.5 m to 3 m RF antenna
• 10 m optical antennae on the ground compared with the existing network of (3) 70 m RF dish antenna
• Quantum noise limited optical receivers result in less detection sensitivity
• Limited laser transmitter electrical-to-optical efficiencies

Consideration of these differences results in realizable advantages to communication of a factor of 100, assuming a 30 cm telescope on the spacecraft and a 10 meter telescope on the ground. This advantage may be utilized to reduce precious deep-space mass and power and/or to increase data rate. Utilizing less mass and power, an order of magnitude increase in data rate relative to RF systems can easily be achieved.

Why is Optical Communication Important?
NASA is poised to reap unprecedented volumes of data in support of the desired virtual presence and interplanetary network concepts. The expanding set of missions, combined with the increase in sensor resolution, demands orders of magnitude enhancement in communications capacity. Data from high-resolution hyperspectral imagers (HSI), Synthetic Aperture Radar (SAR), terrain-mapping radar, and life-identification missions are only increasing the demand on our communications capacity. Moreover, optical communications will be necessary for high-definition TV images of future human missions to Mars.

Free-space optical communication addresses both NASA's upcoming data capacity needs and its spacecraft size reduction goals. The NASA Strategic Plan 2000 call for infusion of revolutionary technologies into operational missions by the 2006-2011 time frame. Among the technologies listed is: "Optical Communications systems for ultra-deep space probes". Also, JPL's Implementation Plan and Strategies (FY2000) and NASA's Strategic Road Map call for High Definition TV (HDTV) from Mars using fully operational Optical Communication and Global Operational Optical Communications between the years 2010 and 2023.

In addition, the 1998 National Research Council report identified optical communication as one of six key technology areas: "Wideband high data-rate communication over planetary distances could enable live transmission of high resolution images from robotics rovers, orbiters, and astronauts on missions to other planets. Although several U.S. Department of Defense agencies and some private companies are currently working on wideband high data-rate communications, NASA will need to take the lead in developing technologies - including high precision spatial acquisition and tracking systems and high efficiency lasers to support such communications over planetary distances".

NASA/JPL Optical Communications

Optical Communications is being developed at NASA’s Jet Propulsion Laboratory (JPL) for future space missions generating high data-volumes. Laser communications is seen as the technology that will meet these needs for future near-Earth, solar system, and interstellar missions.

Current plans call for building 10-m-class ground receiving telescopes during the next decade. NASA is currently building a optical R&D optical telescope laboratory at Table Mountain Facility in Southern California to answer key implementation questions of ground receiver technologies. The telescope is equipped with fast tracking capability, and will function as a testbed for development of ground acquisition, tracking, and communications technologies and strategies applicable to future operational stations. The optical telescope laboratory and other programs currently under development are described within the Flight and Ground R & D sections of our website located at http://lasercomm.jpl.nasa.gov.

Optical Communication Challenges

The advantage of optical communication derives from its comparatively narrow beam, which introduces the difficulty of high-precision beam pointing. RF beams require much less pointing precision, the ‘shotgun’ approach to transmitting information. For example, from Mars at closest approach, the beam footprint of an X-band radar using a 3-m antenna is over 10,000 times the projected area of the Earth. At optical frequencies, the energy transmitted from a 30-cm antenna (1% of the area of the radio antenna) can be concentrated to about 1% of the projected area of the Earth. But such great advantage only results if we are able to point the beam accurately enough to reliably hit the part of the Earth with the receiver.

With the aid of Sun-sensors and star-trackers, an interplanetary spacecraft can find the Earth and maintain its attitude relative to the Earth with an accuracy of a few milliradians (.02 degrees). At RF communication frequencies (S-band and X-band) this level of pointing is perfectly adequate. With optical systems, this spacecraft attitude control must be augmented with a fine-pointing mirror which removes the spacecraft vibration. To maintain this fine-pointing direction, it helps to have a reference point, or a beacon. Thebeacon could be a laser emanated from Earth, the Sun-illuminated Earth itself or background stars.

Currently, few lasers are powerful enough to be useful as a beacon, and introduce logistical difficulties since one has to reliably maintain this beacon. Current cost and implementation considerations direct us toward natural sources of light as beacons. Our recent work indicates that combination of star tracking and inertial sensors (accelerometer, gyro, or rate sensors) will be most suitable for acquisition, tracking and pointing (ATP) purposes. To sense the direction of the homing beacon, the remote laser-communication terminal contains a focal plane array (such as a high-rate CCD camera) to track the apparent motion of the beacon and commands the fine pointing mirror to correct for that motion. In this way, the modulated laser beam (carrying data) can be pointed with high accuracy towards the Earth. To account for the relative motion of the Earth and the spacecraft, the fine-pointing system must also calculate and implement a ‘point-ahead’ angle.

Another challenge for optical communications is the difficulty of maintaining a communication link through cloud cover. A good astronomical site is typically sufficiently clear of clouds for about 65% of the time. A cluster of three stations separated by about 500 km would be far enough apart that they would experience statistically independent weather conditions, raising the probability of sufficiently good weather at at least one of the stations to over 95% at any given time. In the long run, the preferred optical communications connection with deep-space spacecraft would be through an optical receiver satellite, high above the clouds.

NASA/JPL Experience

To bring the promise of free-space optical communications to fruition, a long-term strategy of developing the appropriate technology, and demonstrating its capability must be followed. A laser-communication terminal consists of lasers, optics (telescope, lenses, filters…), electronics (drivers, mini-processor…), and the ATP subsystem (focal planes array, fine-pointing mirror…). Each of these components has been independently demonstrated on various space missions. Indeed, some have flown many times. The only remaining critical technology to demonstrate for deep-space optical communications is a demonstration of long-range acquisition, tracking and pointing. Ranges as high as 35,000 km (GEO orbit) were demonstrated successfully with a Japanese optical communications terminal (on the ETS 6 spacecraft) during the Ground-to-Orbit Laser Communication Demonstration (GOLD). GOLD was the first bidirectional laser communications link between the ground and a spacecraft in geostationary orbit.

The Galileo Optical Experiment (GOPEX) demonstrated the ability to point lasers precisely to objects in deep space, and to sense long-distance optical pulses. Laser beams were transmitted from JPL's Table Mountain Facility and the US Air Force's Starfire Optical Range in Albuquerque, New Mexico. Over an 8-day period, the optical beams were successfully detected by the Galileo spacecraft at ranges of up to 6 million kilometers. A downlink demonstration of optical comm ATP has not yet been attempted from deep space.

NASA and JPL have experience developing state-of-the-art optical communications transceivers, suitable for deep-space missions. As a demonstration of our technical capabilities, the 10 cm (4 inch) diameter Optical Communications Demonstrator (OCD) was designed and built by JPL. It efficiently integrates the projection optics, beacon sensing unit, beam steering assembly, laser and receiver into one lightweight, compact unit. With minor modifications, OCD can accommodate optical communications links from airborne platforms to near-earth space-borne satellites to planetary-spacecraft.

Summary:

• NASA missions are generating progressively more data which must be transmitted
  back to Earth reliably, efficiently, and inexpensively
• Optical communications will be the key to realizing the NASA goals of a virtual
  presence in space and development of an interplanetary network
• Optical communications technologies promise an immediate payoff in the ability of
  NASA to accomplish its goals within its schedule and budget, including:
  - Increased data transmission of factors of 10-100 times
  - Reduction of the antenna area to 1% of current antennae sizes
  - Reductions of weight, moment and power
• Optical communications technologies have been independently demonstrated
  - High-data rate laser pulse transmission
  - Space-based optical receiver
  - Space-based optical pointing
• The Optical Communications Group has the complete range of experience,
  capability, and facilities to support development of a deep-space optical comm
  demonstration experiment

The Optical Communications Group (OCG) is part of the Communications Systems and Research Section and the Telecommunications and Science at Jet Propulsion Laboratory (JPL). JPL is managed by the California Institute of Technology; OCG is NASA's premiere Optical Communications research facility.

Points of Contact:

Dr. H. Hemmati, 818-354-4960
hamid.hemmati@jpl.nasa.gov

Dr. S. Townes, 818-354-7525
steve.townes@jpl.nasa.gov

Dr. J. Lesh, 818-354-2766
james.r.lesh@jpl.nasa.gov
Programmatic point-of-contact

Optical Communications Group website:
http://lasercomm.jpl.nasa.gov

The Optical Communications Group (OCG) is part of the Communications Systems and Research Section (Section 331) and the Telecommunications and Science (Division 33) at Jet Propulsion Laboratory (JPL). JPL is managed by the California Institute of Technology, OCG is NASA's premiere Optical Communications research facility.

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