Every NASA mission has a communications system to receive commands and other information sent 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. Thus, after launch, a spacecraft’s tracking and communications systems is the only means with which to interact with it. In addition, any issues with the spacecraft can only be diagnosed, repaired, or mitigated via the communications system. Without a consistently effective and 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 March 2016, the Mars Reconnaissance Orbiter (MRO) had returned more than 298 terabits of data – an impressive feat. However, NASA estimates that the deep space communications capability will need to grow by nearly a factor of 10 each of the next three decades. This trend is in step with our increasing knowledge of the cosmos -- as more detailed scientific questions arise, the ability to answer them requires ever more sophisticated instruments that generate even more data. Even at its maximum 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 put further demands on their communications system, requiring even higher data rates.
The principal challenge to deep space communications systems is posed by the enormous distances to which our spacecraft travel. The two Voyager spacecraft, for example, are each more than 15 billion kilometers away, about 100 astronomical units (AU; 1 AU is the average distance between 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. 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 Deep Space Network (DSN): The Earth End of the Communications System
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).
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.
Radio Frequency (RF) Technologies
High-rate RF communications techniques are essential to meeting projected future mission requirements. JPL researchers are investigating new methods that would 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 that increase the data rate that can be reliably transmitted within a given spectral band
- power-efficient technologies that reduce the amount of energy needed to transmit a given number of bits
- propagation effects, to better understand atmospheric modeling and allocate frequency bands
- improved flight and ground transceivers that enable future radio systems
- antennas, both flight and ground, that 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)
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.
Selected Research Projects
Interplanetary Optical Communications
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)
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.
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 improved sufficiently to achieve data rates close to a provable theoretical maximum known as the Shannon limit.
For NASA's RF missions, JPL has developed two families of capacity-approaching codes. For the higher data rate missions, a family of low-density parity-check (LDPC) codes, now an international standard, delivers the maximum data volume within a constrained spectral band. Data rates in excess of 1 Gbps are feasible with existing commercial FPGA technology. For the lower data rates of extremely distant missions, such as to the outer planets or beyond, JPL has designed turbo codes, which can operate effectively on channels in which the noise power is more than five times the signal power.
For NASA’s optical communications missions, a fundamentally new channel coding approach is necessary to overcome the fading and phase-corrupting characteristics of a turbulent atmosphere. To meet this challenge, JPL has developed channel interleavers and photon-efficient channel codes for use with direct detection systems. The channel code, called Serially-concatenated Convolutionally-coded Pulse Position Modulation (SCPPM), provides a capacity-approaching method. SCPPM has been used on the Lunar Laser Communication Demonstration (LLCD) on the Lunar Atmosphere and Dust Environment Explorer (LADEE), which demonstrated up to 622 Mbps from lunar orbit using a 0.5 W 15 μrad beam at 1550 nm.
Along with research in increasing the data rate, JPL is deeply involved finding ways to compress the data as much as possible prior to transmission. Common compression techniques used 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 their imagery. Both lossless and lossy compression were used, and for the imagers used by Spirit and Opportunity, excellent image quality was typically obtained with approximately a 10:1 compression ratio.
Hyperspectral imagers represent an emerging data demand for deep-space missions. These instruments take an image at hundreds or thousands of wavelengths simultaneously, revealing the mineral content or other scientific treasures that cannot be revealed in a single visible-wavelength image. This makes the image hundreds or thousands of times larger. New image compression technology has been developed to utilize both the spatial and spectral correlations present, and to operate within the limited constraints of spacecraft processors or FPGA resources. Both lossless and lossy versions of the hyperspectral image compressors have been developed.
Reconfigurable Wideband Ground Receiver
Efforts at JPL have recently focused on 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 Block V, the standard DSN receiver. This work will address the challenge of processing 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. Unlike other radios that can only process signals that are integrally related to their internal clocks, the RWGR uses specially developed noncommensurate sampling to accommodate data rates that are arbitrary with respect to its internal clock. 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 spacecraft. Unlike the Internet, however, whose protocols require latencies in the milliseconds, deep-space communications requires new protocols that can tolerant latencies or disruptions of up to several hours. For example, the roundtrip 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. In response to this need, NASA has established a long term, readily accessible communications test-bed onboard the International Space Station (ISS), with an arbitrary simulator of multiple network nodes in the Protocol Technology Laboratory at JPL.
Flight Transponder Technology
A spacecraft’s communications transponder is the mission’s portal to the interplanetary network. It is also the element of the spacecraft that requires the most reliability and longevity. 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 precision in deep space navigation.