Better communication from space
The Deep Space Network (DSN) consists of antenna complexes at three locations around the world, and forms the ground segment of the communications system for deep space missions. These facilities, approximately 120 longitude degrees apart on Earth, provide continuous coverage and tracking for deep space missions. Each complex includes one 70-meter antenna and a number of 34-meter antennas. These antennas may be used individually or in combination (antenna arraying) to meet each space mission’s communications requirements.
A large portion of deep space communications research addresses communications system engineering, radios, antennas, transmitters, signal detectors, modulation techniques, channel coding theory, data compression, and simulation. This research also includes optical communications as well as related expertise in optical instruments, optics systems design, optical detectors, lasers, and fine-pointing systems.
Deep space communications research facilities include a 34-meter research and development antenna (at the DSN complex at Goldstone, California), and the Optical Communications Telecommunications Laboratory with a 1-meter telescope (at the Table Mountain Observatory in Wrightwood, California).
Optical Communications (Laser Communications, or Lasercom)
The field of interplanetary telecommunications in the radio-frequency (RF) region has experienced an expansion of eight orders of magnitude in channel capacity since 1960. During the same period, resolution of spacecraft angular tracking, a function performed by the telecom subsystems, has seen improved by a factor of 105, from 0.1-mrad to nearly 1-nrad. Continuous performance enhancements over the past five decades were necessitated by the ever-increasing demand for higher data rates, driven in part by more complex science payloads onboard spacecraft.
Efficiency of the communications link, namely the transmitter and receiver antenna gain, are frequency dependent. JPL engineers have successfully enhanced data-rate delivery from planetary spacecraft by employing higher radio frequencies (X-band and Ka-band). Stronger signal power density can be delivered to the ground receiver using even higher optical frequencies and taking advantage of the lower achievable beam divergence.
The 1/f dependence of transmitted beam-width can be practically extended to near-infrared (laser) frequencies in the 100 to 300 THz range. These frequencies can serve both planetary links over interplanetary distances, as well as shorter-distance links near Earth or near planets.
Spectral-congestion in the RF spectrum and/or performance needs should strongly motivate missions to adopt optical communications in the future; orders-of-magnitude increase in performance for the same power and mass are possible. Areas of emphasis in optical communications research and development at JPL include:
- long-haul optical communications
- optical proximity link system development
- in-situ optical transceivers
These technologies can enable streaming high definition imagery and data communications over interplanetary distances. Similarly, optical proximity link systems with low complexity and burden can boost surface asset-to-orbiter performance by a factor of 100 (20 dB) over the current state of the art. This improvement would benefit planetary and lunar orbiters to communicate with assets such as landers or rovers.
Interplanetary Optical Communications
Interplanetary laser communications concept demonstrating links from a Mars orbiter to Earth, and proximity links from Mars surface assets to orbiters.
Optical communications is being developed at JPL for future space missions generating high volumes of data. Laser Communications (lasercom) could meet these needs for future missions to near-Earth space, the Solar System, and potentially, interstellar missions. The primary motivation for augmenting NASA’s telecommunication data rates is to enhance the science data volume returned with higher-resolution instruments and to prepare for future human deep-space exploration missions.
Optical communication can provide mass, power, and volume allocation benefits over radio frequency (RF) systems, as well as bandwidth allocation restrictions.
Key challenges facing deep space optical communications include maturity of efficient, robust and reliable space laser transmitters, and a lack of data on the operating lifetime of lasers in space. Efficient laserscom links from deep space require the detection of extremely faint signals.
During daylight hours, the presence of additive optical background noise despite the use of narrow band-pass filters poses a challenge to their performance. These challenges can be overcome by use of atmospheric correction techniques, which have been demonstrated successfully on meter-class ground-receiving apertures. However, atmospheric correction techniques are not yet cost effective on the 8-12 meter-diameter aperture ground receivers necessary for deep-space communications.
The operation of lasercom links with sufficient availability in the presence of weather, clouds and atmospheric variability also requires cost-effective networks with site diversity.
Deep Space Optical Communications (DSOC)
DSOC architecture view.
The objective of the Deep Space Optical Communications (DSOC) Project is to develop key technologies for the implementation of a deep-space optical transceiver and ground receiver that will enable data rates greater than 10 times the current state-of-the-art deep space RF system (Ka-band) for a spacecraft with similar mass and power.
Although a deep-space optical transceiver with 10 times the RF capability could be built with existing technology, its mass and power performance would not be competitive against existing RF telecommunications systems. The FY2010 NASA SOMD/SCaN funded Deep-space Optical Terminals (DOT) pre-phase-A project identified four key technologies that need to be advanced from TRL 3 to TRL 6 to meet this performance goal while minimizing the spacecraft’s mass and power burden. The four technologies are:
- a low-mass Isolation Pointing Assembly (IPA)
- a flight-qualified Photon Counting Camera (PCC)
- a high peak-to-average power flight Laser Transmitter Assembly (LTA)
- a high photo-detection efficiency ground Photon Counting Detector array
DSOC’s objective is to integrate a Flight Laser Transceiver (FLT) using key space technologies with an optical transceiver and state of the art electronics, software, and firmware to support a risk-retiring technology demonstration for future NASA missions.
Such a technology demonstration requires ground laser transmitters and single photon-counting sensitivity ground receivers. Lasers and detectors can be integrated with existing ground telescopes for cost-effective ground transmitters and receivers.