How Do I Rate This?
The blue stars show the average user rating for this item. To add your own rating, move your cursor over the stars to highlight them in gold, and click to show your rating. One star highlighted is the lowest rating, all five is the highest. Once you have rated an item, your rating is added to the average.
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.
The 70m antenna at Goldstone, California against the background of the Mojave desert. The antenna on the right is a 34m High Efficiency Antenna.
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.
Selected Current Efforts
Free Space Optical Communications
Optical communications is being developed at NASA / JPL for future space missions generating high data-volumes. Lasercom is seen as the technology that would meet these needs for future near-Earth, solar system, and interstellar missions.
Illustration of a high-level downlink block diagram of the optical link and the different blocks (subsystems) that are involved at different stages of the link.
Higher frequency of the optical band compared with radio-frequency (RF) band results in significantly narrower transmitted beam-width. The narrower beam concentrates a larger fraction of the transmit power onto the ground receiver,resulting in higher link power efficiency. Free space optical communications (laser communications or lasercom) would enable space missions to return 10 to 100 times more data with 1% of the antenna area of RF antennas, while utilizing less mass and power. Other advantages include secure and difficult –to-jam link, virtually unlimited bandwidth, and no regulation (other than eye safety) on use of the band.
Primary lasercom challenges include:
Precision laser beam pointing from distant planets
Efficient lasers for spacecraft
Lightweight opto-mechanically stabile optics and structures
Signal attenuation due to clouds, and signal disturbances by atmospheric index-of-reflection turbulence
Low-cost multi-meter diameter ground apertures for photon-collection
Illustration of a high-level downlink block diagram of the optical link and the different blocks (subsystems) that are involved at different stages of the link.
There are several projects in development that address both flight and ground component, subsystem, and system level technologies, as well as the optical atmospheric channel measurements, modeling, and effects mitigation.
Information Processing
For five decades, JPL has provided leadership and expertise in information theory, coding theory, and communications systems, in line with the missions of JPL and NASA. Research ranges from fundamental research on performance limits to practical details of infusing and supporting our technology on flight missions.
Researchers conduct studies and development activities in the following broad areas:
Channel coding: In this area, the goal is to design low complexity forward error correcting codes that have performance near the Shannon limit. Researchers have provided coding solutions to NASA space missions since nearly the beginning of the space age.
Data compression: Recently, researchers have developed a progressive wavelet image compression algorithm, ICER, which has been implemented as the compression technology for Mars Exploration Rover.
Systems analysis: Researchers provide expertise in the end-to-end performance of communications systems, including analysis of statistical channel models, link continuity, radio losses, and weather prediction.
Many researchers also consult with current or planned NASA missions, and sometimes non-NASA projects, on these topics when critical issues arise. They sometimes play a role in the hardware development and implementation of channel decoders and data compressors.
Selected Research and Development Projects
There are several technologies that JPL is developing in the areas of free optical communications and information processing. Below is a quick description of efforts being undertaken to advance deep space communications.
Flight Transceiver
Researcher in this area is being conducted to develop a lasercom transceiver with 15-cm of aperture diameter based on a new architecture that incorporates innovative technologies resulting in superior performance to state-of-the-art. This research also focuses on an optical access (proximity) link for communications between rovers/landers on Moon and Mars to orbiting spacecrafts along with efficient pulsed fiber laser amplifiers, and optical pulse-position modulation (PPM) transponder. Activities also involved radiation testing of photo-counting detectors. This tranceiver will be for use onboard spacecrafts.
Hyperspectral image compression
Projects in this area include currently investigating techniques for compressing hyperspectral image data, using extensions of the wavelet-based ICER image compression system that is being used by the Mars Exploration Rovers. A “3-D ICER” (two dimensions of the image, and a third for the frequency) has already been developed.
Ground (Earth based) Transceiver
Work here is dedicated to the development of a 1-meter diameter telescope capable of tracking planes, and spacecrafts, dedicated to laser communications with the capability to look at small sun angles. It is being designed with novel technologies that are single-photon sensitive (photon counting detectors) along with high-rate receivers (scalable to Giga-bits/s) for PPM modulated signals and universal decoders for PPM modulated signals.
Part of a Mars brassboard being developed, this model of a 15 cm transceiver with low frequency isolation platform is designed to retire lasercom risks on a Mars pathfinder mission. Its aperture can be scaled to larger sizes for improved performance.
Low-Density Parity-Check (LDPC) Codes
Design and analysis of protograph-based LDPC codes are near-capacity approaching codes that have an embedded sub-structure that makes them practical to encode and decode. The LDPC codes JPL has developed have been selected to fly on NASAs Constellation Program series of human missions to low earth orbit, the Moon, and eventually Mars. In preparation, 100 Mbps encoders and decoders have been developed, to support the demanding needs of human flight. Codes are being considered for an international space-coding standard.
Optical Channel
A suite of instruments is being created to characterize the atmospheric attenuation and turbulence effects. Modeling tools characterize the link, the atmospheric channel, and laser beam acquisition, tracking and pointing. Laser safety strategies, hardware and software implementation for safe laser beam propagation to spacecraft.
Schematic of the bi-directional link between landers and orbiters
Codes and Signal Structures for Ka-Band and Antenna Arrays
The move to Ka-band brings with it a promise of increased spectrum to support dramatically higher data rates, but it also has a greater sensitivity to weather. Antenna arrays have a similar trade-off in increased capability and risks associated with combining and synchronization. New codes, modulations, and signal structures to combat weather outages and array-specific impairments are being developed. For example, very long erasure correcting codes may be needed to combat weather outages, and new or modified modulations may be needed to get the best performance out of an antenna array.
Autonomous Software-Defined Radios
Developing first-of-a-kind technology to enable a receiver to autonomously determine the data rate, coding scheme, Doppler profile, modulation type, etc., of an incoming signal, and configure a radio to receive it.
Codes and Modulations for Optical Channels
To develop coded modulations for the optical channel. Development includes a thorough study of the capacity of optical channels, with respect to the available average power, peak power and bandwidth of lasers and optical detectors.
RSS
Send
Bookmark
