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Deep Space Communications
Every NASA mission delving into deep space has a communications system to carry commands and other information from Earth to the spacecraft, and to return scientific data from the spacecraft to Earth. The vast majority of deep space missions never return to Earth, which means that after the launch of these spacecraft, the tracking and communications systems are the only ways in which we can interact with them. After launch, spacecraft problems can only be diagnosed, repaired, or mitigated through the communications system. Without a consistently efficient communications system, a successful mission would be impossible.
Communications: Increasing Demands and Extreme Challenges
The demands placed on deep space communications systems are continuously increasing. For example, as of 2013, the Mars Reconnaissance Orbiter (MRO) has sent us nearly 25 terabytes (TB) of data– already an impressive feat – but NASA estimates that the deep space communications capability will need to grow by nearly a factor of 10 during each of the coming three decades. This trend is occurring because as our knowledge of the cosmos increases and more pressing scientific questions arise, the ability to answer them requires more sophisticated instruments, and these instruments generate more data. Even at its top data rate of 5.2 megabits per second (Mbps), MRO requires 7.5 hours to empty its onboard recorder and 1.5 hours to send a single HiRISE image to be processed back on Earth. New high-resolution hyperspectral imagers produce further demands on the communications system. This means that much higher data rates will be needed. The figure below illustrates increased communication rate demands as dictated by future science data requirements.
High communication rates are dictated by future science data requirements.
The principal challenge of deep space communications systems is the enormous distances over which our spacecraft travel. The two Voyager spacecraft, for example, are each more than 15 billion kilometers away, about 100 astronomical units (AU), the distance between the Earth and the Sun. Another important challenge for deep space communications systems is to maintain their extreme reliability and versatility, in order to accommodate the long system lifetimes of most planetary missions, which often last more than a decade. Though the twin spacecraft launched in 1977, Voyager is still communicating with Earth. These challenges must be met with a communications system that uses no more than a few kilograms of mass, and often uses only about enough power to illuminate a refrigerator light bulb.
The 70m antenna at Goldstone, California against the background of the Mojave desert. The antenna on the right is a 34 m High Efficiency Antenna.
The Deep Space Network (DSN): the Earth End of the Communications System
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 and tracking 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, channel 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
To overcome the enormous distance and spacecraft mass and power limitations in space, JPL develops deep space communications technologies for NASA’s spacecraft and the Deep Space Network (DSN). These technologies 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 more than a factor of 10 during each of the coming decades, with a goal of 200 Mbps from 1 AU by 2022 and 20 Gbps from 1 AU by 2030.
Radio Frequency (RF) Technologies
High-rate RF communications 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 deep space and Earth. Areas of investigation include: spectral efficient technologies, to increase the data rate that can be reliably transmitted within a given spectral band; power efficient technologies, to reduce the amount of energy needed to transmit a given number of bits; propagation, to better understand atmospheric modeling and allocate frequency bands; flight and ground transceivers, to enable future radio systems; and antennas, both flight and ground, to enable NASA’s move to higher radio frequencies such as Ka-band (26 to 40 GHz), and deployable and arraying antenna technology.
Optical Communications (Laser Communications, or Lasercom)
Interplanetary laser communications concept illustrating links form, for example, a planetary orbiter to Earth (or Earth orbit), and access (or proximity) links involving surface-to-orbiter links. Links directly from surface-to-Earth are feasible as well (not shown).
Interplanetary telecommunications in the radio-frequency (RF) region has experienced eight orders of magnitude expansion in channel capacity since 1960. During the same period, resolution of spacecraft angular tracking, a function performed by the telecom subsystems, has seen a factor of 105 improvement from 0.1-mrad to nearly 1-nrad. Continual performance enhancements over the past five decades were necessitated by the ever-increasing demand for higher data rates, driven in part by inclusion of more complex science payloads onboard spacecraft probes. Efficiency of the communications link, namely the transmitter and receiver antenna (telescope) gain, and the magnitude of the space loss both have frequency-dependent components. By taking advantage of the lower beam divergence, and therefore stronger signal power delivered to the ground receiver, system implementers have successfully enhanced data-rate delivery from planetary spacecraft by employing higher frequencies (X-band and Ka-band). Similarly, the 1/f dependence of transmitted beam-width substantially favors optical and near-infrared (laser) frequencies that are in the 100 to 300 THz regime. Deep-space communications include links directly from spacecraft to ground, and those relayed from planetary surfaces via orbiting spacecraft.
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 space. 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 current state of the art. This improvement would be needed for planetary and lunar orbiters to communicate with 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.
Selected Research Projects
Interplanetary Optical Communications Optical communications is being developed at NASA/JPL for future space missions generating high data-volumes. Optical Communications (lasercom) is seen as the technology that would meet these needs for future near-Earth, solar system, and interstellar missions. The primary motivation for augmenting NASA’s telecommunication data rates is to en¬hance the science data volume returned with higher-resolution instruments and prepare for future human deep-space exploration missions. Radio frequency (RF) telecommunications constraints on mass, power, and volume on the spacecraft, as well as bandwidth allocation restrictions, limit increases in data rate. Optical communication can potentially overcome all these limitations while providing increased data rates. By using the narrow beam of an optical carrier frequency near 200 THz (1550 nm) for transmit, Optical communications has the potential to increase the achievable data rate from spacecraft at planetary distances by orders of magnitude with a spacecraft transceiver that is of similar mass and power consumption to a wide beam-width 32-GHz (Ka band) spacecraft transceiver. In a stark difference with RF communications, whose beam footprint, for example transmitted from a Mars spacecraft, is 100-200x the Earth diameter, lasercom beam’s foot print is 1/10th to 1/20th of the Earth diameter. This necessitates precision laser beam pointing, a feet that is accomplished via the lasercom flight terminal itself, and not the spacecraft.
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. 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.
Some of the key challenges facing optical communications from deep space ranges are: availability of laser transmitters that are efficient, suitable for space use, and adequately long-lived; detection of extremely faint signals during daytime and night-time; precision laser-beam pointing in the presence of host platform vibrations; and uninterrupted operation in the presence of adverse atmospheric and weather conditions. Some of the major Lasercom research activities at JPL are described below.
Deep-space Optical Terminals (DOT)
The Deep-space Optical Terminals (DOT) project involves development of flight and ground terminals for interplanetary laser communications. The DOT architecture is meant to provide functionalities of (a) high-rate downlink data (e.g. 270 Mb/s from short distance to Mars) telecommunications, (b) forward data telecommunications, and (c) precision spacecraft ranging. By using an optical carrier frequency near 200 THz (1550 nm) for transmit, the flight terminal can achieve up to a 10× improvement in data rates with a spacecraft transceiver that is of similar mass and power consumption to a 32-GHz (Ka band) spacecraft transceiver. The DOT system is composed of four major subsystems. The Flight Laser Transceiver (FLT) is the DOT subsystem mounted on the spacecraft. It receives the uplink beam and transmits a downlink beam. The Ground Laser Transmitter (GLT) sends an uplink beam to the spacecraft. This beam is used as a pointing reference (i.e., beacon) at the spacecraft, as well as carry¬ing uplink communication data. The Ground Laser Receiver (GLR) receives the downlink light and recovers the communica¬tion data. The DOT Mission Operations Center (MOC) controls DOT operations and performs data analysis and archiving. DOT also supports ranging by measuring the time of flight on both the uplink and downlink beam. Optical beacon-assisted pointing of the downlink beam is as¬sumed. A modulated optical beacon provides forward command and data links. Paired one-way ranging on the bidirectional optical links provides spacecraft range and veloc¬ity. The intent of the DOT flight demonstration is to: (a) retire the major risks perceived for operational deep-space optical telecommunications; and (b) demonstrate a flight terminal concept that is easily scalable from data rates of hundreds of Mb/s to a few Gb/s at spacecraft ranges out to about 10 AU (Saturn). At greater ranges, a switch to a beaconless tracking scheme is antici¬pated, but with today’s technology that is expected require greater system complexity and mass.
(A) Flight concept design, and (B) Flight telescope assembly.
The architecture of the ground laser receiver (GLR) electronics. The subassemblies in the dashed box have been built and are shown on the right. Two elements of a GLR array are shown.
Illustration of some of the components constituting the ground terminal.
Key new technologies that require maturation to TRL 6 in order to develop a low mass and power flight terminal, and to establish a highly efficient link with sub-Gb/s data-rate from interplanetary distances include:
1) An isolation platform for the telescope assembly to effectively mitigate much of the spacecraft vibration before it reaches the flight lasercom assembly
A low mass and power consumption assembly, made from a hybrid of passive and active actuators, has been demonstrated and validated in the laboratory. The next step is maturation to TRL 6.
2) Single photon sensitive (photon-counting) photo-detector array for use with the flight terminal, for uplink signal detection, and for beacon laser acquisition and tracking
Near-room-temperature semiconductor type devices are being matured to high TRLs
3) Single photon sensitive detector arrays for the ground terminal
Detector arrays based on superconducting nanowire devices, operating at below 1° K temperature, have been demonstrated in the laboratory, and are being matured to high TRLs
A channel code enables reliable communications over unreliable channels. By adding specific types of redundancy, the transmitted message can be recovered perfectly with high probability, even in the face of enormous channel noise and data corruption.
For five decades, JPL has used its expertise in information theory and channel coding theory to develop practical, power-efficient channel codes that achieve reliable transmission from deep space to Earth. In the last 15 years, the codes have become so good that they are achieving data rates close to a provable theoretical maximum known as the Shannon limit. For NASA's highest data rate missions, JPL has developed a family of low-density parity-check (LDPC) codes, now an international standard, to deliver the maximum data volume while living within a constrained spectral band. Data rates in excess of 1 Gbps are feasible with existing commercial FPGA technology. For extremely distant missions, such as to the outer planets or outside of the Solar System, JPL has designed turbo codes, which can operate effectively on channels in which the noise power is more than five times higher than the signal power.
Together, the turbo and LDPC codes will meet any channel coding need for deep space missions. To take advantage of dynamic link conditions, JPL is developing protocols to utilize variable coded modulation (VCM), which allows the channel codes and the modulation to vary from code block to code block. This technology alone may double the data return of a mission, without any physical changes to communications hardware.
JPL is also in the process of developing channel codes for the uplink, the communications channel that sends data from the ground to the spacecraft. Historically, power on the ground has not been an issue, but with more distant destinations, and with more missions vying for limited time on the large 70 m antennas of the DSN, it will be important to make the uplink transmissions power efficient as well.
Along with increasing the data rate itself, JPL is deeply involved in research that compresses the data as much as possible prior to transmission. Common compression techniques we use on Earth, such as JPEG, are often too complex for a spacecraft’s limited computing power. JPL developed the ICER image compression technique as a replacement – it achieves the same result with substantially less complexity. ICER was used by the Spirit and Opportunity Mars Rovers to return the vast majority of imagery. Both lossless and lossy compression was used, and for the imagers used by Spirit and Opportunity, excellent image quality was typically obtained with approximately a 10:1 compression ratio.
An emerging data hog for deep space missions is hyperspectral imagery. These instruments take an image at hundreds or thousands of wavelengths simultaneously, revealing the mineral content or other scientific treasures not knowable from a single visible-wavelength image. The image is hundreds or thousands of times larger, however. New image compression technology is being developed to utilize both the spatial and spectral correlations present, and again, to operate within the limited constraints of spacecraft processors or FPGA resources.
Reconfigurable Wideband Ground Receiver
Recently, much effort has been placed toward the development of the Reconfigurable Wideband Ground Receiver (RWGR): a variable-rate, reprogrammable software-defined radio intended to supplement and augment the capabilities of the standard DSN receiver, the Block V Receiver. This effort will meet the challenge to process high data rate telemetry (1 Gbps or higher), and is well-suited to firmware and software reconfigurability. The RWGR is an intermediate frequency (IF) sampling receiver that operates at a fixed input sampling rate, and unlike other radios that can only process signals that are integrally related to their internal clocks, the RWGR uses a specially developed noncommensurate sampling to accommodate arbitrary data rates with respect to its internal clocking. This technology provides the flexibility and reconfigurability needed for the long-term support of NASA’s signals and protocols in future deep space missions.
Disruption Tolerant Networking (DTN)
JPL is developing technology to enable Internet-like communications with space vehicles. Unlike the Internet, however, whose protocols require latencies in the milliseconds, deep space communications requires new protocols that can tolerant latencies or disruptions of several hours. For example, the round trip light time to the Voyager spacecraft and back is more than 24 hours. The disruption tolerant network makes use of store-and-forward techniques within the network in order to compensate for intermittent link connectivity and delay. Many applications can benefit from the reliable delivery of messages in a disconnected network. NASA has established a long term, readily accessible communications test-bed onboard the International Space Station (ISS), with arbitrary simulator of multiple network nodes in the Protocol Technology Laboratory at JPL.
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.