Idea Transcript
News You are here
Directory
Events
Images
FedEO Catalogue
Explore more... Follow
Home › Directory › Satellite Missions
LCRD (Laser Communications Relay Demonstration) LCRD (Laser Communications Relay Demonstration) Mission Overview Launch Payload Ground Segment References 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) 5) 6)
The LCRD project is NASA's pathfinder mission towards an optical relay capability. LCRD is a Space Technology Mission Directorate (STMD) technology demonstration mission that is co-funded by NASA's SCaN (Space Communications and Navigation) program. The LCRD architecture and experiment plan are designed to address the critical questions remaining to move the proven technology to operational readiness. LCRD will address questions beyond whether or not the technology will work; it will address questions about how the technology can be optimally implemented and applied. 7) 8) The NASA TDRS Tracking and Data Relay Satellite) System is comprised of a constellation of spacecraft in GEO (Geostationary Orbit) and associated ground stations and operation centers. There are currently three generations of TDRS on orbit, with the most recent launch of TDRS-L in January 2014. The TDRS are distributed around the equator to provide global coverage to satellites and users in LEO (Low Earth Orbit) or below. The global coverage allows for complete realtime communications with users such as the ISS (International Space Station), as well as communications on demand for users such as the Swift Gamma Ray Burst Mission. NASA has been evaluating the expected life of the current fleet and future mission support requirements and is currently targeting a next generation of relay capability on orbit in the 2025 timeframe. 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.
Directory Directory Home Satellite Missions A B C D E F G H I J K L M N O P Q R S T U V, W, X, Y, Z Airborne Sensors Observation of the Earth
NASA has been developing optical communications for both Near Earth and Deep Space applications. Optical communications (or laser communication or "lasercom") is a revolutionary technology that enables NASA to undertake more complex missions in the future that require transmitting more data and/or decreasing the communication system's mass, size, and power burden on the spacecraft: 9) 10) • For approximately the same mass, power, and volume, an optical communications system will provide significantly higher data rates or data volume than a comparable radio frequency system • For the same data rate (e.g. 1 Gbit/s of output), an optical communications system will require less mass, power, and volume than a comparable radio frequency system. There exist some differences between the technological approaches to optical communications specifically designed for Near Earth missions versus Deep Space missions. Due to the vastly differing ranges and data rates for Near Earth versus Deep Space missions, some of the technologies applicable to each domain differ in profound ways; however, there are also many technologies which are similar to both! Coordination of system development for these two domains maximizes NASA's return on investment. The Laser Communications Relay Demonstration, to launch in 2019, is NASA's flagship optical communications technology demonstration for Near Earth applications. Its purpose is to prove the technology is ready for the Next Generation TDRS (Tracking and Data Relay Satellites) and that it is ready to provide mission critical communications for users.
Figure 1: Artist's rendition of the hosted LCRD payload on the commercial spacecraft in GEO (image credit: NASA) The LCRD demonstration will have two hosted optical communications terminals in space on a single commercial communications satellite in geostationary orbit and two optical communications terminals in Southern California and Hawaii to allow the mission to demonstrate for Near Earth applications. The LCRD 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. • High rate bi-directional communications between Earth and GEO • Real-time optical relay from Ground Station 1 through the GEO flight payload to Ground Station 2 • PPM (Pulse Position Modulations) suitable for power limited users, such as small Near Earth missions • DPSK (Differential Phase Shift Keying) modulations suitable for Near Earth high data rate communications • Various mission scenarios through spacecraft simulations at the Earth ground station • Coding, link layer, and network layer protocols over optical links over an orbiting testbed. At the same time that it is developing LCRD, NASA is also working on a Next Generation LEO (Low Earth Orbit) User Terminal that is compatible with LCRD. Thus, the LCRD flight payload 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 current plan is to launch the Next Generation LEO User Terminal to the ISS (International Space Station) to demonstrate interoperability with LCRD and to demonstrate LEO-to-GEO-to-Earth relay operations. The LCRD project isn't NASA's first foray into laser (i.e., optical) communications. NASA hired the MIT-Lincoln Laboratory to develop a laser communications payload for LADEE (Lunar Atmosphere and Dust Environment Explorer), which was launched on Sept. 7, 2013. The main goal of LADEE is proving fundamental concepts of laser-based communications via the LLCD (Lunar Laser Communication Demonstration) device and transferring data at a rate of 622 Mbit/s, which is about five times the current state-of-the-art from lunar distances. However, the LADEE payload is equipped with only one modem, the lower-speed model is best suited for deepspace communications. In addition, LADEE was a short-duration mission. The LLCD experiment is expected to operate for only 16 days of the LADEE mission, not enough time to demonstrate a fully operational network. However, LLCD will ably demonstrate: • Pulse Position Modulation • Photon counting on the downlink • Inertial stabilization • High-efficiency transmission and reception of PPM (Pulse Position Modulation) • Very low size, weight, and power space terminal • Integrating an optical communications terminal to a spacecraft • Link operation under some conditions (limited due to the very limited operating time) • Scalable array ground receiver. Prior to LADEE, NASA had begun developing the Mars Telecommunications Orbiter, but canceled the project in 2005 because of budget constraints. Had it launched in 2009, it would have exhibited high-speed data rates between Earth and several Mars landers and orbiters for as long as 10 years. Several space agencies are currently working on spaceborne optical communications. The EDRS (European Data Relay Satellite System) will provide intersatellite optical links at data rates up to 1.8 Gbit/s. EDRS is currently planned to consist of two satellites; one is a dedicated ESA (European Space Agency) spacecraft while the other is a commercial satellite carrying a EDRS hosted payload. The standard mode of operation calls for LEO spacecraft to communicate with the relay using optical signals; the relay then transmits the information to Earth via Ka-band RF signals. NASA has successfully completed the LLCD (Lunar Laser Communication Demonstration) from lunar orbit in late 2013 as part of of a demonstration experiment on the LADEE mission (the LADEE mission ended on April 17, 2014). The LLCD system consists of a space terminal, the LLST (Lunar Lasercom Space Terminal) on LADEE, and a primary ground terminal, the LLGT (Lunar Lasercom Ground Terminal), at White Sands, NM for the mission. Two alternate ground terminals were also tested, namely the LLOT (Lunar Lasercom OCTL Terminal) at JPL's Optical Communications Telescope Laboratory in Table Mountain, CA, and the LLOGS (Lunar Lasercom Optical Ground System), residing at ESA's OGS on Tenerife, the Canary Islands. The operation of the space and ground terminals were all coordinated from the LLOC (Lunar Lasercom Operations Center) which resided at the MIT Lincoln Laboratory in Lexington, MA. The entire LLCD program was overseen by NASA/GSFC , and the LADEE spacecraft was designed, built, and operated by the NASA(ARC ( Ames Research Center). LLCD had 15 Lunar Days of operations between mid-October and mid-November 2013. During those days, LLCD had a total of 101 scheduled passes – 56 passes with LLGT, 22 with LLOT, 15 with the LLOGS, and 8 passes lost because no available terminal was cloud-free. The project met or exceeded the mission goals. The LLCD pointing, acquisition, and tracking (PAT) systems and protocols allowed both the uplink and downlink to lock and begin data communications within seconds. After configuration refinements over the first few passes, the links would lock up every pass with error-free performance at uplink rates up to 20 Mbit/s and downlink rates up to 311 Mbit/s with the LLGT ground station. Downlinks at 622 Mbit/s also met performance requirements, but included a codeword error rate floor of about one error per minute due to a LLGT hardware limitation found during ground testing. LLCD and the progress towards EDRS have moved optical communications closer to readiness for an optical relay operational geosynchronous relay system, but challenges remain that are likely beyond what would be acceptable for the commitment to deploy the new capability today. The challenges that remain mainly focus on providing operational services that include an optical space-to-ground link. Though the technology demonstrations and progress on operational intersatellite links greatly increase the confidence in the success of a future TDRS optical capability, many challenges remain for the specification, development, and operations of a future system. The LCRD (Laser Communications Relay Demonstration) mission will build upon the recent successes to serve as a pathfinder for the next generation TDRS network.
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 applications. 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 GS-1 (Ground Station-1) on Earth through the GEO flight payload to GS-2 (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. The LCRD architecture is illustrated in Figure 2. The LCRD flight payload, consisting of two OSTs (Optical Space Terminals) and associated electronics, will be hosted onboard a SSL-built communications satellite. The satellite operator will have a HMOC (Host Mission Operations Center) through which payload commands and telemetry will be routed. LCRD will employ simulators to demonstrate forward and return relay links and direct uplink/downlink. The flexibility and scalability of the LCRD architecture enables support to terrestrial, air-borne, and Low Earth Orbit (LEO) users. LCRD will also demonstrate optical communications networking capabilities including the use of an LMOC (LCRD Mission Operations Center), multiple OGSs (Optical Ground Stations), OGS handovers, degraded operations, user service recovery from link interruption due to clouds, operating through orbital events and spacecraft maneuvers, and coordinated network flight and ground segment operations. LCRD will fly two optical modules, as opposed to the single optical module on LLCD. The CE (Controller Electronics) of each optical space terminal will be a commercially procured update of the LLCD CE. There will be a SSU (Space Switching Unit) to connect the two optical links together for realtime relay. The LLCD optical module design has been modified for GEO applications and to make the system more robust. The optical module components have been transferred to Industry and are being procured and integrated by NASA/GSFC. There will also be two flight modems. The LCRD flight payload system diagram is seen in Figure 3.
Figure 2: LCRD design reference mission (image credit: NASA)
Figure 3: Block diagram of the LCRD flight payload (image credit: NASA) The LCRD flight modems are based on a multi-rate modem designed by MIT/LL. This is a different design than the LLCD modem. The LCRD modems will be capable of the PPM (Pulse Position Modulation) demonstrated on LLCD, as well as DPSK (Differential Phase Shift Keying) modulation. Though PPM allows for communications in photon-starved scenarios, such as deep space links, DPSK will allow for extremely high data rates (10's of Gbit/s). The data rates for all links will be PPM up to 311 Mbit/s and DPSK up to 1.244 Gbit/s. Project status: - February 2016: All elements of the Space and Ground segments are either complete or at the final stages of integration. LCRD ground modems successfully met required bit error rate performance. The LCRD development is on schedule for the planned launch in 2019 (Ref. 10). - On April 1, 2014, NASA awarded a digital processor assembly contract for the LCRD flight payload. 11) - In early Dec. 2013, the LCRD mission passed a PDR (Preliminary Design Review). 12) - In Sept. 2012, the LCRD mission has successfully completed a Mission Concept Review, a major evaluation milestone of the engineering plan to execute the build and launch of a space communications laser system. 13)
Figure 4: Illustration of the LCRD spacecraft (image credit: NASA) Launch: The LCRD hosted payload is scheduled for launch in 2019 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. 14) 15) Orbit: Geostationary orbit, location =
LCRD payload: 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 that can also support low data rate PPM (Pulse Position Modulation) • A space switching unit to interconnect the two optical modules and to interface to the host spacecraft. The LCRD architecture will allow the mission to: 16) • Demonstrate high rate 24/7 optical communications operations over a 2 year period from GEO to Earth (ground station) • Demonstrate real-time optical relay from one Ground Station through the GEO flight payload to the second ground station • Demonstrate both a Near Earth (DPSK) and a Deep Space (PPM) compatible modulation and coding • Demonstrate 1.244 Gbit/s (2.880 Gbit/s uncoded) uplink and downlink using DPSK (Differential Phase Shift Keying) modulation • Demonstrate 311 Mbit/s uplink and downlink using PPM (Pulse Position Modulation) • Demonstrate the Next Generation TDRS compatible optical terminal capable of supporting both direct to Earth and GEO to LEO (ISS Terminal) communications • Demonstrate operational concepts for reliable, high-rate data delivery in face of terrestrial weather variations typically encountered by real NASA missions • Demonstrate control of handover among ground sites • Support performance testing and demonstrations of coding and link layer protocols over optical links via an orbiting testbed. Near-Earth optical communications commercialization efforts: In order to make optical communications more easily available to future NASA science and exploration missions and to reduce costs, NASA would like at least one commercial provider for an entire optical communications terminal. NASA's current vision is to use an LLCD / LCRD type optical module in as many scenarios as possible. Studies show that the terminal can be used from Low Earth Orbit out to the Sun-Earth L1 and L2 Lagrange Points with only a few modifications depending on the mission profile. Each optical module, shown in Figure 5, is a 10 cm reflective telescope that produces a ~15 µrad downlink beam. It also houses a spatial acquisition detector which is a simple quadrant detector, with a field of view of approximately 2 mrad. 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 and stabilized via a MIRU (Magnetohydrodynamic Inertial Reference Unit). Angle-rate sensors in the MIRU detect angular disturbances which are then rejected using voicecoil 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 modem is provided by a controller. Each optical module is held and protected during launch with a cover and one-time launch latch. 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). Wavelength
1550 nm
Terminal mass
69 kg (total mass of ~ 175 kg)
Optical transmit power
130 W DC
Telescope diameter
108 mm
Data rates
2.880 Gbit/s using DPSK, and 622 Mbit/s using PPM
Table 2: Parameters of the LCRD optical terminal 17)
Figure 5: Inertially stabilized LLCD / LCRD type optical module (image credit: NASA) Optical Module Commercialization: Commercialization of a LLCD / LCRD type optical module has already started, and the module has been divided into four subassemblies that are commercially available: • OA (Optical Assembly) • GLA (Gimbal and Latch Assembly) • ISP (Inertially Stable Platform) • SWA (Solar Window Assembly). The OA (Optical Assembly) consists of a beryllium Cassegrain telescope and small optics bench. The small optics bench accommodates three separate wavelengths, each boresight aligned to the telescope. The LCRD OAs will be fabricated and tested by Exelis Geospatial Systems of Rochester, NY.
Figure 6: Schematic of the OA (Optical Assembly), image credit: NASA, Exelis The GLA (Gimbal and Latch Assembly) contains four distinct subassemblies: the two-axis Gimbal Assembly, the Latch Assembly, the Instrument Panel Assembly, and the Bridge Mass Assembly. The Instrument Panel Assembly is used to mount the Gimbal Assembly and Latch Assembly, and serves as the base for the full OM (Optical Module). The Bridge Mass Assembly is a stand-in to represent the mass and inertia of the other OM subassemblies. The LCRD GLAs are being fabricated and tested by the SNC (Sierra Nevada Corporation) facility in Louisville, CO.
Figure 7: Illustration of the GLA (image credit: NASA, SNC) The core of the ISP (Inertially Stable Platform) is the MIRU (Magnetohydrodynamic Inertial Reference Unit) which provides the inertial stabilization system for the OA, once the OM has been assembled. The ISP also contains environmental covers and mass stand-ins for the OA and SWA. The LCRD ISPs are being fabricated and tested by Applied Technology Associates of Albuquerque, NM.
Figure 8: Illustration of the IPS (image credit: NASA, Applied Technology Associates) The SWA (Solar Window Assembly) provides environmental protection for the OA once the OM has been assembled. The main component of the SWA, the solar window, was designed to minimize the amount of solar energy that reaches the OA. The window attenuates optical energy of all wavelengths, except for the band in which the OA operates. The LCRD SWAs are being fabricated and tested by L-3 Integrated Optical Systems of Wilmington, MA.
Figure 9: Illustration of the SWA (image credit: NASA, L-3 Integrated Optical Systems) Controller Electronics Commercialization: The CE (Controller Electronics), used in LLCD and LCRD, is basically a commercial off-the-shelf space qualified computer with just a few modifications; the software is the critical component and it was modified in going from LLCD operations at the Moon to LCRD operations in GEO. The CE interprets commands to configure the OM (Optical Module) in order to provide proper pointing for optical communications operations. The CE contains the processor for the PAT (Pointing, Acquisition and Tracking) algorithm and all the analog interfaces for the OM. The CE is being fabricated and tested by Moog Broad Reach of Golden, Colorado. Modem commercialization: The LCRD will support DPSK (Differential Phase Shift Keying) which has better sensitivity and fading tolerance than simple on-off-keying, although less sensitivity than PPM (Pulse Position Modulation). DPSK can be used at extremely high data rates using commercial components, and because of the use of a single-mode receiver (received light is coupled into a single-mode optical fiber which serves as a spatial filter) and optical bandpass filtering, supports communications when the Sun is in the field of view. LCRD leverages a MIT /LL (Lincoln Laboratory) 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 fixed duration 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 single Mach-Zehnder optical interferometer regardless of data rate, the clock rate remains fixed. The DPSK transmitter utilizes a MOPA (Master Oscillator Power Amplifier) architecture similar to the PPM transmitter used in the LLCD. The DPSK receiver has an optical preamplifier 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 the up- and downlink errors are corrected together in a decoder located at the destination ground station. The LCRD DPSK modem will also support pulse position modulation (PPM). The transmitter utilizes the same 2.88 GHz clock rate, and modulates the signal with a sequence of 16-ary PPM symbols (signal is placed in exactly one of each 16 temporal slots). When operating in PPM mode, the receive modem utilizes the same optical preamplification and optical filter as is used in DPSK. The optical signal is converted to an electrical signal by means of a photodetector. The electrical signal in each slot is compared to a threshold (which can be varied to account for atmospheric turbulence) in a simple, yet sensitive PPM receiver implementation. This method leverages previous work performed by MIT Lincoln Laboratory. This particular modem design has not been commercialized yet, but NASA and MIT/LL have been studying several approaches to do just that. Having a commercial supplier for the modem will make all of the components needed for a space optical communications terminal be readily available for future Near Earth science and exploration missions. SSU (Space Switching Unit) commercialization: The SSU is basically the "glue" on the LCRD flight payload that interconnects the two optical communications terminals. The unit is the central C&DH (Command and Data Handling) unit for the flight payload. The SSU provides the following core functions: - Passes high speed data frames between multiple optical space terminals based on frame addressing - Loads firmware and software into each Integrated Modem at start-up - Receives commands from host spacecraft and optical ground stations - originating from the LMOC (LCRD Mission Operations Center) - Sends health and status telemetry to LMOC via host spacecraft and optical ground stations - Distributes time packets via SpaceWire interfaces. The SSU hardware is being produced by SEAKR Engineering of Denver, Colorado, and the software is designed by MIT Lincoln Laboratory, making the entire unit basically commercially available for future missions.
Figure 10: Block diagram of the LCRD payload (image credit: LCRD partnership, Ref. 17)
Figure 11: Benefits of optical communications (image credit: LCRD partnership)
Ground segment: The LCRD ground segment is comprised of the LMOC (LCRD Mission Operations Center) and two ground stations. The LMOC (at WSC, NM) will perform all scheduling, command, and control of the LCRD payload and the ground stations. In addition, there is a PMOC (partial Mission Operations Center),located at GSFC to support experimenters and the engineering team, and the flight payload in geostationary orbit. The design reference mission for LCRD is illustrated in Figure 2. 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 groundbased beacon. Turbulence spreads the beam, reducing mean irradiance at the terminal in space, and causes fluctuations in the instantaneous received power. JPL will enhance its OCTL (Optical Communications Telescope Laboratory) so that it can be used as Ground Station 1 of the demonstration. OCTL is located in the San Gabriel mountains of southern California and houses a 1 m F/76 Coudé focus telescope. The large aperture readily supports the high data rate DPSK and PPM downlinks from the LCRD space terminal with adequate link margin. The Coudé configuration allows for LCRD operations to continue during the setup, integration, test and operations of other concurrent experiments. The fully enclosed design of ground station 1 systems protects users in the Coudé laboratory, as well as protects the optical systems from accidental jostling and atmospheric disturbances. A fail-safe LASSO (Laser Safety System at the OCTL) will ensure safe laser beam transmission through navigable air and near-Earth space. A state-of-the-art AO (Adaptive Optics) system is being implemented to facilitate coupling the downlinked signal into a single-mode fiber, which conducts it to a high rate detector. The AO system is capable of coupling more than half the received signal into the single mode fiber even under poor seeing conditions and at low elevation angles. A comprehensive monitor and control system will coordinate the receipt of schedules, and direct subsystems to set up and operate according to those schedules. Ground Station 1 also supports extensive simultaneous multi-user, multichannel services such as bitstream, symbolstream, Internet Protocol and AOS (Advanced Orbiting Systems). Resident simulators (User MOC Simulators and User Platform Simulators) have been implemented to exercise the system to its full capacity, allowing users to fully characterize the system capability under a wide variety of nominal configurations. An Atmospheric Channel Monitoring system will measure and record real-time data on weather conditions, photometry conditions, and seeing conditions for real-time and subsequent correlation with link performance. A general block diagram for either ground station is shown in Figure 12. The green blocks in the diagram denote common elements that are being developed for LCRD, while the other boxes are specific to a particular ground station.
Figure 12: Ground station block diagram (image credit: NASA) The telescope assembly subsystems are capable of transmitting and receiving laser light while pointing towards the LCRD flight payload. GS-1 will use a 1 m telescope for both the uplink and downlink, while GS-2 will use a single 15 cm telescope for uplink and 60 cm telescope for downlink. LCRD GS-1 (Ground Station-1): NASA/JPL will enhance its OCTL (Optical Communications Telescope Laboratory) at Table Mountain, CA, so that it can be used as GS-1 of the demonstration. 18)
Figure 13: 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 Figure14. 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 14: Schematic of the integrated optical system to be located at coudé focus in OCTL (image credit: JPL) LCRD GS (Ground Station-2): MIT/LL (Lincoln Laboratory) is developing Ground Station 2 to be deployed somewhere in Hawaii. Ground Station 2 will have a 60 cm receive aperture, a 15 cm transmit aperture, and be located within an approximately 5.5 m diameter dome on one of the islands in Hawaii; ideally it will be located at a summit of one of the volcanoes there to get above the clouds and have excellent atmospheric seeing conditions. NASA has been specifically studying Haleakala, a dormant volcano on the island of Maui, Mauna Kea and Mauna Loa, on the Big Island (Ref. 9). Ground Station 2's laser subsystem consists of a custom photonics assembly that produces a low power (