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 70m antenna at Goldstone, California against the background of the Mojave desert. The antenna on the right is a 34m High Efficiency Antenna.
Communication systems are among the most critical functions in space exploration. The communication system provides the link to the spacecraft from Earth and brings scientific data from spacecraft to Earth. It also tracks the spacecraft and provides it with information required to perform its job. Without effective communication systems, a successful mission would be impossible. The demands on deep space communications systems are ever increasing. NASA estimates that the deep space communications capability will need to grow by a factor of 10 during each of the coming decades. In addition, deep space communications is just one of a set of enabling technologies that allow the development of new mission concepts. The principal challenge of deep space communications is the enormous distances that our spacecraft travel: up to tens of billions of miles from Earth. Current and future space missions also demand that increasing information be transmitted. For example, Mars return data rates (the number of bits per second) have increased by a factor of 10 over the last decade and are likely to continue increasing at this rate into the future. Another important challenge is posed by the extreme reliability that space missions require. After launch, spacecraft problems can only be diagnosed, repaired, or mitigated through the communication system. Since planetary missions often last more than a decade, communications reliability must also be maintained throughout the very long system lifetimes. In addition to mission communications, direct science observations are possible using existing communication links. Radio science uses perturbations in the link to deduce either spacecraft motion, e.g., motion induced by unknown gravity fields, or properties of the medium through which the signal has passed, e.g., densities of planetary atmospheres. Additional uses of the radio link include interferometry for an even finer measure of spacecraft motion and radar to measure or even image bodies in space.
The Earth end of the communications system for deep space missions is the Deep Space Network (DSN), comprising antenna complexes at three locations around the world. These facilities, approximately 120 degrees apart on Earth, provide continuous coverage for deep space missions. Each complex includes one 70 m diameter antenna and a number of 34 m 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, coding theory, data compression, and simulation. Deep space communications research includes optical communications as well as related expertise in optical instruments, optics systems design, optical detectors, lasers, and fine-pointing systems. Deep space communications facilities include a 34 m research and development antenna (at the DSN complex at Goldstone, California), and the Optical Communications Telecommunications Laboratory with a 1 m telescope (at the Table Mountain Observatory in Wrightwood, California).
Selected Research Areas
Deep space communications are critical to the success of space missions that require data transmission from spacecraft to Earth, spacecraft tracking, and the ability to instruct the spacecraft to perform necessary actions. To overcome the enormous communication distance and spacecraft mass and power limitations in space, JPL’s deep space communications technologies developed for NASA’s spacecraft and the Deep Space Network have enabled every JPL space mission ever flown and contributed to the development of exciting new mission concepts. To continue meeting the increasing demand on deep space communications systems, the Deep Space Network must increase its capability by a factor of 10 during each of the coming decades.
High communication rates are dictated by future science data requirements.
High-Rate Communication Techniques
High-rate communication techniques are essential if projected future mission requirements are to be met. New methods are being investigated to allow current radio systems to accommodate the ever-increasing need to reliably move more bits between Earth and deep space. Specific areas of investigation include: very low complexity error correction coding to improve Ka-band link availability for gigabit per second (Gbps) links, software configurable radios that adaptively mitigate amplifier distortions throughout the life of long-duration missions, and integrated wideband array combiner and telemetry receivers for bandwidth-efficient signals.
Analysis of the NASA Agency Mission Planning Model and future mission concepts studies suggest that, by 2025, space missions would require a 100 times increase in communications capabilities. This level of capability could be achieved with radio-frequency technologies, including the adoption of Ka-band (26 to 40 GHz) as the deep space workhorse communications frequency.
At some point, spectral or performance needs would force missions to adopt optical communications. Several orders-of-magnitude increases in performance for the same power and mass are possible. Areas of emphasis at JPL in optical communications research and development include: long-haul optical communications, optical proximity link system development, and in-situ optical transceivers. These technologies would be essential to enable streaming video and data communications over the long distances involved in interplanetary distances. Optical proximity link system development would be required to meet projected future mission requirements of enabling high-rate communications translating into a minimum of 20 dB improvement over the state of the art. This improvement would be needed for planetary and lunar orbiters to communicate with landed assets such as landers or rovers and to support optical navigation. The performance goal for optical proximity link systems is a 0.1–2.5 Gbps data rate.
Autonomous and Cognitive Radios
Intelligent systems embedded in DSN communications terminals would allow significant operational cost savings while also offering the ability to adapt to new mission situations as they arise. Autonomous and cognitive radios would simplify operations by autonomously detecting data rates, modulation, and Doppler rates at space and ground receivers. These radios adaptively establish spectrum functionality, such as usage and data rate, based on dynamic probing (cognizance) of spectrum utilization and channel quality. Performance goals include 150 Mbps proximity links at Mars.
Flight Transponder Technology
The communications transponder is the mission’s portal to the interplanetary network. It is also the element that requires the most reliability and longevity in the spacecraft system. Existing flight transponders are approaching their performance limit and are not expected to meet projected requirements of future missions. Improvements to this technology would enable the higher data rates required, support multiple spacecraft communications, and improve the precision of deep space navigation.
JPL uses arrays of DSN antennas to form virtual antennas of a size effectively equal to the sum of its constituents. As we move to a future with orders-of-magnitude greater demand on space communications systems, arraying of DSN antennas would become pervasive, since arraying is more economical and more flexible than building ever-increasingly large monolithic antennas. Arraying would be essential to providing the highest data rates, and eliminating the communications bottleneck to the outer planets. Critical technologies in this area include low-cost electronics, low-cost antennas, signal processing, and remote operations. This array approach would support the future communications need by operating at X-band and Ka-band frequencies at high data rates ( > 100 Mbps). Near-term performance targets are 25 Mbps uplink and 150 Mbps downlink data rates.
Selected Research Projects
Acquisition, Tracking & Pointing (ATP)
The objective of the Acquisition, Tracking, and Pointing (ATP) project is to develop and validate a complete set of ATP systems to enable free-space optical communication for ranges from Near-Earth to Deep Space (beyond moon).
Before data transmission can occur the flight transceiver must be pointed in the direction of the receiver. This is followed by acquisition of the impinging beam from the receiver. The operation that maintains this pointing and acquisition during the duration of the link is tracking.
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
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.
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.
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.
Research 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 a potential optical access (proximity) link for communications between rovers/landers on Moon and Mars to orbiting spacecraft along with efficient pulsed fiber laser amplifiers, and optical pulse-position modulation (PPM) transponder. Activities also involved radiation testing of photo-counting detectors. This transceiver would be for use onboard spacecraft.
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.
Hyper spectral image compression
Projects in this area include currently investigating techniques for compressing hyper spectral 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 spacecraft, 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.
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.
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.
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.
Schematic of the bi-directional link between landers and orbiters
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.