LCRD (Laser Communications Relay Demonstration)
LCRD is a NASA/GSFC-led technology demonstration mission of a spaceborne optical communications system. The project promises to dramatically increase data rates, but achieving these speeds will be technically challenging — particularly when transmitting and collecting these tight, data-packed laser beams and then compensating for distortions that occur when the light travels through a turbulent or cloudy atmosphere. 1) 2) 3) 4)
LCRD is a joint project between NASA/GSFC, NASA/JPL (Jet Propulsion Laboratory) and MIT/LL (Massachusetts Institute of Technology / Lincoln Laboratory. The mission goal is to provide two years of continuous high data rate optical communications in an operational environment from GEO, demonstrating how optical communications can meet NASA’s growing need for higher data rates, or for the same data rate provided by a comparable RF system, how it enables lower power, lower mass communications systems on user spacecraft. In addition, LCRD’s architecture will allow it to serve as a testbed in space for the development of additional symbol coding, link and network layer protocols, etc.
Figure 1: Artist's rendition of the hosted LCRD payload on the commercial spacecraft in GEO (image credit: NASA)
The demonstration involves a hosted payload on a commercial communications satellite developed by SSL (Space Systems/Loral), of Palo Alto, CA, and two specially equipped ground stations in California and Hawaii. The demonstration is expected to launch in late 2016 and operate two to three years.
The LCRD mission requirements call for:
• Enable reliable, capable, and cost effective optical communications technologies for near Earth applications and provide the next steps required toward optical communications for deep space missions
• Demonstrate high data rate optical communications technology necessary for:
- Near-Earth spacecraft (bi-directional links supporting hundreds of Mbit/s to Gbit/s)
- Deep Space missions (tens to hundreds of Mbit/s from distances such as Mars and Jupiter)
• Develop, validate and characterize operational models for practical optical communications
• Identify and develop requirements and standards for future operational optical communication systems
• Establish a strong partnership with multiple government agencies to facilitate crosscutting infusion of optical communications technologies
• Develop the industrial base and transfer technology for future space optical communications systems.
Table 1: Some background on NASA involvement in optical communications
Unfortunately, LLCD does not go far enough. To make optical communications useful to future projects, long mission life space terminals must be developed and proven. Operational concepts for reliable, high-rate data delivery in the face of terrestrial weather variations and real NASA mission constraints needs to be developed and demonstrated. To increase the availability of an optical communications link and to handle cloud covering a ground terminal, there needs to be a demonstration of handovers among multiple ground sites. For Near Earth applications, a demonstration needs to show the relaying of an optical communications signal in space. There also needs to be a demonstration of the modulation and coding suitable for very high rate links.
NASA’s new LCRD optical communications project will answer the remaining questions for Near Earth applicaitons. LCRD’s flight payload will have two optical communications terminals in space and two optical communications terminals on Earth to allow the mission to demonstrate:
• High rate bi-directional communications between Earth and GEO (Geostationary Earth Orbit)
• Real-time optical relay from Ground Station 1 on Earth through the GEO flight payload to ground station 2 on Earth
• Pulse Position Modulations suitable for deep space communications or other power limited users, such as small Near Earth missions
• DPSK (Differential Phase Shift Keying) modulations suitable for Near Earth high data rate communications
• Demonstration of various mission scenarios through spacecraft simulations at the Earth ground station
• Performance testing and demonstrations of coding, link layer, and network layer protocols over optical links over an orbiting testbed.
Launch: The LCRD hosted payload is scheduled for launch in late 2016 on a commercial communications satellite developed by SS/L (Space Systems/Loral). A contract was signed in April 2012. The agreement marks the first time NASA has contracted to fly a payload on an American-manufactured commercial communications satellite. 5) 6)
The LCRD Project Office is also working closely with NASA HQ to possibly demonstrate LCRD with an optical communications terminal flying on a LEO (Low Earth Orbit) spacecraft, such as the ISS (International Space Station). Thus, the flight payload on the GEO spacecraft has a requirement to be able to support high rate bi-directional communications between LEO and GEO as well as between Earth and GEO.
The LCRD flight payload will be flown on a GEO spacecraft and consists of:
• Two optical communications modules (heads)
• Two optical module controllers
• Two DPSK (Differential Phase Shift Keying) modems
• Two PPM (Pulse Position Modulation) modems
• High Speed Electronics to interconnect the two optical modules, perform network and data processing, and to interface to the host spacecraft.
An optical communications terminal on LCRD consists of an optical module, a DPSK modem, a PPM modem, and an optical module controller.
Each of the two optical communications terminals to be flown on the GEO spacecraft will transmit and receive optical signals. When transmitting, the primary functions of the GEO optical communications terminal are to efficiently generate optical power that can have data modulated onto it; transmit this optical power through efficient optics; and aim the very narrow beam at the ground station on earth, despite platform vibrations, motions, and distortions. When receiving, the GEO optical communications terminal must provide a collector large enough to capture adequate power to support the data rate; couple this light onto low noise, efficient detectors while minimizing the coupled background light; and perform synchronization, demodulation, and decoding of the received waveform (Ref. 3).
Table 2: Parameters of the LCRD optical terminal 7)
Figure 2: Photo of the inertially stabilized optical module (image credit: LCRD partnership)
Each optical module (terminal) features a 10 cm aperture reflective telescope that produces a ~15 microradian (µrad) downlink beam. It also houses a spatial acquisition detector which is a simple quadrant detector, with a FOV (Field of View) of approximately 2 µrad. It is used both for detection of a scanned uplink signal, and as a tracking sensor for initial pull-in of the signal. The telescope is mounted to a two-axis gimbal via a MIRU (Magnetohydrodynamic Inertial Reference Unit). Angle-rate sensors in the MIRU detect angular disturbances which are then rejected using voice-coil actuators for inertial stabilization of the telescope. Optical fibers couple the optical module to the modems where transmitted optical waveforms are processed. Control for each optical module and its corresponding modems are provided by a controller. Each optical module is held and protected during launch with a cover and one-time launch latch (Ref. 3).
There exist some differences between the technological approaches to optical communications specifically designed for Near-Earth missions versus deep space missions. This is mostly due to the vastly differing ranges and data rates for Near-Earth versus deep space missions. One area that has been looked at for some time within NASA is the appropriate modulation, coding, and detection scheme for the two different classes of missions. Photon counting and PPM (Pulse Position Modulation) has been identified as the technique of choice for deep space missions, while DPSK (Differential Phase Shift Keying) is the current preferred choice for Near-Earth missions. LCRD will demonstrate both techniques.
Photon counting PPM is highly photon efficient although the ultimate data rate is limited due to detector
The PPM flight transmitter encodes data with a rate-½ serially concatenated PPM (SC-PPM) turbo code. The encoded data stream is convolutionally interleaved (to mitigate the effects of atmospheric fading) and modulated with a 16-ary PPM modulation scheme (signal is placed in exactly one of each 16 temporal slots). The maximum data rate is achieved using a 5 GHz slot clock rate; lower data rates are accomplished by combining consecutive slots, effectively lowering the clock rate, with a minimum slot rate of 311 MHz. The optical modulation is accomplished with a MOPA (Master-Oscillator Power Amplifier ) architecture. A CW (Continuous Wave) laser (at ~1550 nm) is modulated with a Mach-Zehnder modulator, and amplified with a two stage EDFA (Erbium-Doped Fiber Amplifier) to a 0.5 W average power level.
The PPM flight receiver is an optically pre-amplified direct detection receiver. After amplification and filtering, the signal is optically split to perform spatial tracking, clock recovery, and communications. The uplink communications signaling is 4-ary PPM, with a simple two comparator demodulator performing binary hard decisions. The received uplink data stream are de-interleaved and decoded (rate-½ SC-PPM coding is applied on the uplink).
LCRD will also support DPSK (Differential Phase Shift Keying) which has superior noise tolerance, can be used at extremely high data rates, and supports communications when the Sun is in the field of view. LCRD leverages a MIT/LL previously designed DPSK modem as a cost effective approach to providing a DPSK signal. It can both transmit and receive data at an (uncoded) rate from 72 Mbit/s to 2.88 Gbit/s. In future relay scenarios, it could be replaced by a higher rate DPSK modem that would support data rates beyond 10 Gbit/s.
The DPSK modem employs identical signaling for both the uplink and downlink directions. The DPSK transmitter generates a sequence of pulses at a 2.88 GHz clock rate. A bit is encoded in the phase difference between consecutive pulses. As demodulation is accomplished with a Mach-Zehnder optical interferometer, the clock rate remains fixed. The DPSK transmitter utilizes a MOPA architecture similar to the PPM transmitter. The EDFA amplifies the optical signal to a 0.5 W average power level. Data rates below the maximum are accomplished via “burst-mode” operation, where the transmitter sends pulses only a fraction of the time, sending no optical power the remainder of the time. Since the EDFA is average power limited, the peak power during the bursts is increased; thus the rate reduction is accomplished in a power efficient manner.
The DPSK receiver has an optical pre-amplifier stage and an optical filter, at which point the light is split between a clock recovery unit and the communications receiver. The receiver uses a delay-line interferometer followed by balanced photo-detectors to compare the phases of consecutive pulses, making a hard decision on each channel bit. While coding and interleaving will be applied in the ground terminal to mitigate noise and atmospheric fading, the DPSK flight receiver does not decode nor de-interleave. The modems instead support a relay architecture where up-and downlink errors are corrected together in a decoder located at the destination ground station.
HSE (High Speed Electronics):
To be an optical relay demonstration, LCRD will create a relay connection between two ground stations. A significant objective of LCRD is to demonstrate advanced relay operations on the GEO spacecraft. LCRD will enable a wide variety of relay operations through the HSE that connect the two optical terminals. In addition to real-time relay operations, the electronics will allow scenarios where one link uses DPSK signaling and the other PPM.
A known challenge with optical communication through the atmosphere is the susceptibility to cloud cover. The HSE will include a significant amount of data storage in order to demonstrate store-and-forward relay services for when the uplink is available but the downlink is unavailable. The HSE will support DTN (Delay Tolerant Network) protocols. To support DTN over the DPSK optical links, the HSE will implement any required decoding and de-interleaving so the payload can process and route the data (at a rate less than the maximum DPSK throughput). The link operations will be configurable to allow support for a variety of scenarios (Ref. 3).
Figure 3: Block diagram of the LCRD payload (image credit: LCRD partnership, Ref. 7)
Figure 4: Benefits of optical communications (image credit: LCRD partnership)
The LCRD ground segment is comprised of the LMOC (LCRD Mission Operations Center) and two ground stations. The LMOC will perform all scheduling, command, and control of the LCRD payload and the ground stations (Ref. 3).
Each Earth ground station must provide three functions when communicating with one of the two optical communications terminals on the GEO spacecraft:
- receive the communications signal from the GEO space terminal
- transmit a signal to the GEO space terminal, and
- transmit an uplink beacon beam so that the GEO space terminal points to the correct location on the Earth.
The receiver on Earth must provide a collector large enough to capture adequate power to support the data rate; couple this light onto low noise, efficient detectors while trying to minimize the coupled background light; and perform synchronization, demodulation, and decoding of the received waveform.
The uplink beacon, transmitted from each Earth ground station, must provide a pointing reference to establish the GEO space terminal beam pointing direction. Turbulence effects dominate the laser power required for a ground-based beacon. Turbulence spreads the beam, reducing mean irradiance at the terminal in space, and causes fluctuations in the instantaneous received power.
LCRD ground station 1:
JPL will enhance its OCTL (Optical Communications Telescope Laboratory) so that it can be used as ground station1 of the demonstration.
Figure 5: Current view of the OCTL telescope at JPL (image credit: JPL)
OCTL telescope will be modified with an optical flat to support links in the presence of more windy conditions.
The integrated optical system at the telescope coudé focus is shown in Figure6. A shutter controlled by a sun sensor protects the adaptive optics system should the telescope inadvertently point closer to the sun than specified. The downlink is collimated by an off axis parabolic mirror is incident on a fast tip/tilt mirror and dichroic beam splitter before reflecting off a DM (Deformable Mirror). A fraction of the beam is coupled to the wavefront sensor to measure the aberrations in the downlink beam. A scoring camera monitors the quality of the corrected beam that is focused into a fiber coupled to the DPSK/PPM receiver. A waveplate adjusts the polarization into the fiber to the DPSK Mach-Zehnder interferometer and a slow tip/tilt mirror ensures maximum signal input to the fiber. In the uplink system the beacon and communications beams are first reflected from slow tip/tilt mirror to track out satellite motions and is then coupled to the telescope through a dichroic mirror.
Figure 6: Schematic of the integrated optical system to be located at coudé focus in OCTL (image credit: JPL)
LCRD ground station 2:
MIT/LL designed and is building the LLGT (Lunar Lasercom Ground Terminal for NASA’s LLCD (Lunar Laser Communications Demonstration). The LLGT (Figure 7), will be refurbished and enhanced to serve as ground station 2 for LCRD.
Figure 7: Illustration of the LLGT (image credit: MIT/LL)
For LCRD, the DPSK modem requires the received light to be coupled into single-mode fiber. For this reason, at least one of the receive apertures will utilize an adaptive optics system to support the DPSK receiver. The current LLGT design will continue to support the PPM functionality for LCRD.
The LLGT is an array of four 40 cm receive reflective telescopes and four 15 cm transmit refractive telescopes. For the uplink, the optical signal (PPM for LLCD, to include DPSK for LCRD) is modulated onto four separate carrier wavelengths, each very slightly detuned. Each modulated signal is amplified to a 10 W average power, and coupled to a transmit aperture via single-mode fiber. For the downlink, the receive apertures couple into few-mode multi-mode fibers connected to an array of SNSPDs (Superconducting Nanowire Single Photon Detectors. The SNSPDs must be cryogenically cooled to ~3 K, and it is impractical to locate them in the focal planes of the receive apertures. The multi-mode fiber was designed to efficiently couple the received light from the aperture to the detector over a distance of 22 m. By using multi-mode fiber, efficient coupling is achieved without an adaptive optics system.
1) “Goddard Wins ‘Paradigm-Shifting’ LaserComm Mission,” Cutting Edge - Goddard's Emerging Technologies, Vol. 8, Issue 1, pp. 3 and 9, Fall 2011, URL: http://gsfctechnology.gsfc.nasa.gov/newsletter/Current.pdf
2) “NASA Announces Technology Demonstration Missions,” August 22, 2011, URL: http://www.nasa.gov/offices/oct/crosscutting_capability/tech_demo_missions.html
3) Bernard L. Edwards, Dave Israel, Keith Wilson, John Moores, Andrew Fletcher, “Overview of the Laser Communications Relay Demonstration Project,” Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL: http://www.spaceops2012.org/proceedings/documents/id1261897-Paper-001.pdf
4) “NASA to Demonstrate Communications Via Laser Beam,” NASA, Sept. 22, 2011, URL: http://www.nasa.gov/topics/technology/features/laser-comm.html
6) NASA Space Technology Program selects Space Systems/Loral platform to help enable next era of space communications,” SS/L, April 12, 2012, URL: http://www.ssloral.com/html/pressreleases/pr20120412.html
7) Bernard Edwards, “Overview of NASA’s Laser Communications Relay Demonstration,” April 2012, URL: http://cwe.ccsds.org/sls/docs/SLS-OCM/Meeting%20Materials/2012_04_Darmstadt/Presentations/LCRD%20Overview%20for%20IOAG%20-%20Germany%20-%20April%202012.pdf
8) B. S. Robinson, D. M. Boroson, D. A. Burianek, D. V. Murphy, “The Lunar Laser Communications Demonstration”, ICSOS (International Conference on Space Optical Systems and Applications), Santa Monica, CA, USA, May 11-13, 2011
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.