LCRD (Laser Communications Relay Demonstration) Mission
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.
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) 11)
• 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.
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.
• December 15, 2017: The LCRD mission has begun integration and testing at NASA/GSFC. "There are three phases to integration and testing leading up to launch," said Glenn Jackson, LCRD payload project manager. "We're on track to finish the first phase, payload integration, by the end of December. The next phase is to test the entire payload in a flight environment including electromagnetic, acoustic and thermal vacuum testing." 12)
• February 13, 2017: Orbital ATK announced it has been awarded a contract by the U.S. Air Force Space and Missiles Systems Center to provide payload integration and support services for Space Test Program Satellite 6 (STPSat-6). The multipurpose satellite will operationally demonstrate advanced communication capabilities, collect space weather data and support nuclear detonation detection in the Earth s atmosphere or in near space. STPSat-6 is the primary payload on the STP-3 mission which is set to launch no earlier than June 2019. 13)
- The multiple payloads on board STPSat-6 include the SABRS-3 (Space and Atmospheric Burst Reporting System-3), NASA s LCRD (Laser Communication Relay Demonstration), and seven experiments from the DOD Space Experiments Review Board. Orbital ATK' s heritage bus and avionics product line is designed to support multiple payloads and can be adapted to support the customer s desired mission life.
• 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. 14)
• In early Dec. 2013, the LCRD mission passed a PDR (Preliminary Design Review). 15)
• 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. >16)
The LCRD flight payload will fly on the STPSat-6 (Space Test Program Satellite-6). The STP satellites are a series of spacecraft developed under a Department of Defense (DoD) program to field space capabilities quickly in response to emerging national needs. STPSat-6, which is managed by the Space Test Program office at Kirkland Air Force Base in Albuquerque, New Mexico, is scheduled to launch in summer 2019. The Space Test Program office is in the Advanced Systems and Development Directorate within the Space and Missile Systems Center of the U.S. Air Force. While the satellite is named STPSat-6, the entire mission is referred to as SPT-3 (Space Test Program 3). 17)
The STPSat-6 spacecraft bus is being provided by Orbital ATK in Dulles, Virginia, and is based on a partially assembled satellite in storage; it was originally based on Orbital ATK's high-end modular A-500 bus. The satellite will be directly inserted into an orbit slightly above geostationary. STPSat-6 will carry several experiments into orbit in addition to the LCRD (Laser Communications Relay Demonstration) experiment. The primary payload is the SABRS-3 (Space and Atmospheric Burst Reporting System- 3) from the NNSA (National Nuclear Security Administration), which provides nuclear detonation detection and space environment data, and is designed to complement nuclear detonation detectors currently in orbit. The satellite will also host seven Department of Defense Space Experiments Review Board payloads from the Space Test Program office.
High Data Rate RF: When LCRD was originally conceived and approved, the project contained only two optical space terminals and two optical ground stations. This was considered to be the minimum configuration that would allow NASA to experiment and learn about a geosynchronous-Earth orbit (GEO)-based optical communications relay service. With two ground stations, NASA could simulate a handover from one ground station to another; in a different configuration, one ground station could act as the user and the other could act as the "receiving" ground station (i.e. as the optical trunkline from the relay satellite, (Ref. 9).
This minimum configuration was never considered ideal, and it was recognized that a real operational demonstration required a user terminal in space communicating through LCRD to the ground. Furthermore, the minimum architecture would suffer from cloud coverage at either optical ground station. A benefit of an optical-to-optical relay is that it allows the trunkline to carry the same bandwidth as the link between the relay and the user. An actual optical relay would need to contend with inevitable cloud cover over one or both ground stations. The susceptibility to cloud cover, scintillation, scattering and other atmospheric effects are known challenges associated with optical communications. If a cloud were present, the optical trunkline could either wait for clouds to pass or switch to a different ground station that has a cloud-free line of sight; however, both of these options create a link outage.
Figure 4: The LCRD mission architecture (image credit: NASA)
A high data rate RF trunkline has been added to the LCRD architecture to provide more capability and alleviate concerns regarding lack of optical ground station availability due to cloud cover. NASA expects that future operational GEO relay satellites will have both optical and RF services. Optical communications would be used for extremely high data transfer where some latency might be acceptable; RF would provide high availability of services, but lower data transfer. For example, science instrument files, housekeeping files, software uploads and more can often be delayed as long as they are completely delivered. But most missions also have requirements for real-time or very low-latency delivery, such as commanding, telemetry, science alerts, voice, video, etc. In this way, the capabilities complement each other, enabling both high-volume data delivery and reliable coverage.
The STPSat-6 spacecraft is being augmented with a Ka-band system that consists of two SDRs (Software Defined Radios). The system supports:
- Transmit up to 622 Mbit/s
- Receive up to 64 Mbit/s
- SpaceWire command, control, and telemetry to the spacecraft
- SpaceWire command, control, and telemetry to the spacecraft
- Ability to be reconfigured on orbit.
Each SDR on the spacecraft operates independently; one supports RHCP (Right Hand Circular Polarization) while the other supports LHCP (Left Hand Circular Polarization). The system is equipped to receive and transmit in single or dual polarization modes at any time during the mission. The addition of high data rate RF makes the LCRD demonstration much more valuable to NASA in developing and experimenting with future concepts of operations.
Launch: The LCRD hosted payload is scheduled for launch in the summer of 2019 onboard the STPSat-6 spacecraft. The Air Force selected the Atlas-5-551 launch vehicle of ULA. The mission is part of the Space Test Program (STP-3). In addition, a propulsive EELVSPA (Evolved Expendable Launch Vehicle Secondary Payload Adapter) is used to carry as many as six small payloads. 18)
Orbit: Geostationary orbit (slightly higher than GEO) with direct insertion into GEO.
LCRD (Laser Communications Relay Demonstration) 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: 19)
• Demonstrate high rate 24/7 optical communications operations over a 2 year period from GEO to Earth (ground stations)
• 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.
Each optical module, shown in Figure 2, is a 10 cm reflective telescope that produces a ~15 microradian downlink beam. It also houses a spatial acquisition detector which is a simple quadrant detector, with a field of view of approximately 2 microradians. 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.
Figure 5: Illustration of the optical module 1 (image credit: NASA)
Flight modem: LCRD's primary modulation format is DPSK (Differential Phase Shift Keying) which has superior noise tolerance, it 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. It can both transmit and receive data at an (uncoded) rate from 72 Mbit/s to 2.88 Gbit/s. The 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. 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 photodetectors 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 uplink and downlink errors are corrected together in a decoder located at the destination ground station.
In addition to DPSK, each flight modem will also support PPM (Pulse Position Modulation). The transmitter modulates the signal with a sequence of 16-ary PPM symbols (a signal pulse is placed in exactly one of each 16 temporal slots). The maximum PPM data rate is 311 Mbit/s. The transmitter utilizes the same 2.88 GHz clock rate. When operating in PPM mode, the receive modem utilizes the same optical pre-amplification 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.
Figure 6: Photo of the flight modem 1 (image credit: NASA)
Space Switching Unit: To be an optical relay demonstration, LCRD will create a relay connection between two optical ground stations or between a satellite in LEO and the ground. The Space Switching Unit interconnects the two flight optical terminals and the spacecraft's high rate RF system. In addition to realtime 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. Thus the Space Switching Unit can use the high rate RF system in addition to an optical downlink when necessary. To support DTN (Delay Tolerant Networking) over the DPSK optical links, the Space Switching Unit 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.
SHIM (Secure Host Interface Module): The SHIM isolates the LCRD flight payload, in an Information Assurance (IA) point of view, from the spacecraft. The addition of IA technology was a recent enhancement to LCRD and its purpose is to prevent unauthorized access, use, disclosure, disruption, modification, inspection, recording, or the destruction of information within the LCRD flight payload. IA gets the right information to the right people at the right time while protecting that information from eavesdropping or corruption. NASA's space communication links must be available when needed (e.g. be protected from a denial of service attack) and reliably transmit data (e.g. be protected from information manipulation). Some of NASA's communications, such as medical information and private astronaut communications, also need to be protected from eavesdropping. Thus IA is critical in preserving the integrity and confidentiality of future operational relay satellite systems; this is particularly needed for NASA's human exploration missions, such as the future Orion Crew Exploration Vehicle. With the SHIM, the LCRD flight payload could be hosted on an untrusted non-US government spacecraft and maintain complete information assurance.
Payload Integration: Integration and test of the LCRD flight payload has been occurring at NASA's Goddard Space Flight Center, and is near completion. Each optical terminal has been installed on the LSA (LCRD Support Assembly) provided by the spacecraft provider. Table 2 and Figure 7 identifies and shows the major components of the flight payload as installed on the LSA.
Table 2: LCRD flight payload components
Figure 7: LCRD flight payload configuration (outboard side is on the left), image credit: NASA
Figure 8: Block diagram of the LCRD payload (image credit: LCRD partnership)
The LCRD ground segment is comprised of the LMOC (LCRD Mission Operations Center) and two ground stations. The LMOC at WSC (White Sands Complex) 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 ground-based 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 10. 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 10: 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 Optical 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. 20)
Figure 11: 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 Figure12. 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 12: Schematic of the integrated optical system to be located at coudé focus in OCTL (image credit: JPL)
LCRD Optical Ground Station-2:
MIT/LL (Lincoln Laboratory) is developing Ground Station 2 to be deployed at the Air Force's Maui Optical and Supercomputing Space Surveillance Complex on top of Haleakala in Maui, Hawaii (Figure 13).
Figure 13: OGS-2 (Optical Ground Station 2) location at the Maui Optical and Supercomputing Space Surveillance Complex (image credit: NASA)
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 Maui, 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 (<10 mW) fiber-coupled optical signal, followed by a commercially obtained high-power optical amplifier. The fiber amplifier can produce up to 10 W of optical power but will be limited by software to a maximum power of 7.3 W during operation. After accounting for transmission loss of the transmit telescope and the window in the dome, the maximum power emitted into free space outside the dome is 5.4 W.
Figure 14: Illustration of the LLGT (image credit: MIT/LL)
The LCRD USG (User Services Gateway) is the interface between the LCRD Network and users for data transfer services. The USG supports realtime frame and symbol stream services, as well as interfaces for higher layer services, such as IP packets and DTN (Delay/Disruption Tolerant Networking) bundles. The USG can (de)multiplex different users and data streams within any of the potential trunkline channels. The USG also provides the interface to allow the inline command and telemetry flows between the LMOC and Flight Payload over the optical links.
In order for LCRD to perform demonstrations of complex scenarios and exercise all of the possible data paths and modes, a UPS (User Platform Simulator) and User Mission Operations Center Simulator (UMS) will be located at each ground station. The simulators will allow high rate data streams to flow over the optical links without requiring high data rate connections out from the ground stations. The inclusion of the service interfaces and simulators enables LCRD to be ready for whenever a real user may come online.
Other ground station subsystems which are not part of the user data flow, include ground station monitor and control, atmospheric monitoring instruments, and laser safety systems. The monitor and control system provides the interface for the LMOC to monitor and control the ground stations. The atmospheric monitoring instruments will provide the necessary measurement to improve the models and operational strategies needed for a future operation optical relay network. The laser safety systems will provide the necessary safeguards to shut off laser transmissions in the event that an aircraft is detected or a satellite has been predicted to be in the path by the U.S. Strategic Command's Laser Clearinghouse.
The LCRD mission is planned for two years of operations with a goal for at least five years. This experiment duration, much greater than LLCD's month of operations, will allow extended characterization of hardware performance, link performance, atmospheric effects, and operational approaches. There will be enough data and time to refine and validate models and to repeat scenarios with different operations schemes. The extended time with two ground stations will allow for optimization of handover approaches and provide the key data needed to determine the optimum number of ground stations for a future operational network and other approaches to minimize user data loss and increase system availability. The system will be capable to not only simulate the GEO relay scenario, but to also insert data relays for some deep space link scenarios.
LCRD will not just demonstrate technology. It will be the first instantiation of an optical link service provider. The two optical links will be capable of supporting the realtime relay from a satellite user to Earth or even between two user platforms. The system will incorporate service interfaces for LCRD users to schedule their links, transmit and receive their data terrestrially, and receive in-event support status. Experiments will include executing timelines that include multiple orbiting users with different data rates and service requirements. NASA will be able to develop the specifications for a future communications architecture, based on experience and validated models. Trade studies to define system requirements will be able to use LCRD data to substantiate values and claims. Future users of optical communications will also be able to see an operational system, in order to understand how the services will enable their missions and set their expectations for the network support. It is anticipated that some of the experiment time will be used to support experiments defined by commercial operators, in order to address their particular interests.
Though it is expected that Industry would provide the future relay flight hardware, the NASA experience in procuring, integrating, testing, and operating the flight terminal will inform the procurement activities of future systems. NASA will be more capable to develop the specifications and manage system deliveries. The technology, knowledge, and experience will all be shared with Industry and will improve the design proposals.
The experience as a hosted payload will benefit both NASA and commercial operators. NASA will get to experience the full lifecycle from concept development through operations and will have a better understanding of all of the benefits and possible limitations of a hosted payload approach. Commercial satellite communications operators will also receive insight into what the opportunity to host a NASA relay communications capability would entail. Perhaps more importantly, commercial operators will see optical communications demonstrated between Earth stations and a GEO relay and be able to start planning their own future architectures incorporating the technology.
Overall LCRD Operations
LCRD systems are distributed across several facilities to support experiments and demonstrations. LCRD consists of the LCRD Mission Operations Center at WSC (White Sands Complex), two OGSs located in California and Hawaii, a partial Mission Operations Center (to support experimenters and the engineering team) located at GSFC, and the flight payload in geostationary orbit. The facilities and their locations are shown in Figure 15.
Figure 15: LCRD operational asset map (image credit: NASA)
Control of all activities during LCRD will take place from the LMOC (LCRD Mission Operations Center) at WSC. The LMOC is connected with all other segments, and communicates with the two ground stations using high capacity connections. Connection to the space segment will be provided either through one of the ground stations, or through a lower capacity connection to the host spacecraft's HMOC (Mission Operations Center) and then to the LCRD flight payload by RF link.
The LMOC will provide services such as:
• Planning and scheduling
• Status Monitoring
• Reporting and Accountability.
The mission operations for the spacecraft and the optical communications demonstration are intimately intertwined. The unique nature of the demonstration is that there is a path to and from the spacecraft that is outside the usual RF connection. Commands for the payload can be sent via either the optical uplink or via the Host Spacecraft RF uplink. There are two paths for getting engineering data (health and status), again via optical or RF. The LMOC coordinates all optical communications activities and provides an interface to the spacecraft operations.
Due to the vagaries of weather and atmospheric conditions, operations strategies for mitigation of these effects will be explored. One possibility would be to have multiple terminals within the same beam simultaneously receive the same data to guarantee getting through to at least one terminal at a reasonably high percentage of the time. On the other hand, buffering and retransmission strategies can be used to downlink the data to single geographically (and meteorologically) diverse stations in a form of temporal diversity.
The ground stations will have the capability to simulate both user spacecraft and user MOC data systems. This will allow the demonstration of high data rate scenarios without the requirement for high data rate connections external to the ground stations. The simulators will also allow multiple user and user-type scenarios. The LCRD payload itself will also include the ability to simulate user spacecraft data and multiple relay user spacecraft data systems.
The system will be continuously operating, as much as possible, over the two year mission. The system will either be configured to be demonstrating or testing a specific DTE (Direct-to-Earth) scenario, relay scenario, or be continually characterizing the optical channel and hardware. The DTE and relay scenarios will emulate different user and relay locations, orbits, and/or trajectories.
Flight Payload Operations
The LCRD flight payload has the following operating modes as defined in Figure 16:
1) Off/Survival Mode: In this mode the LCRD Payload power is removed and only the survival heaters are powered from the Host Spacecraft. The Payload will launch in this mode and remain there until the first power-on event.
2) Payload System Initialization Mode: In this mode, the Payload is powered on, but not yet ready to support normal operations. The Payload will enter this mode in one of two cases: Payload initial power up after launch or Payload return to power after Off/Survival mode.
3) Payload Self-Test/Calibration Mode: In this mode, the Payload will have at least the SSU and one OST (Optical Space Terminal) powered on. The second OST may be either powered off or powered on. This mode is used to execute predefined self-test functions within the Payload.
4) Payload Operational Mode: In this mode, the Payload has been powered up and initial configuration established such that the Payload can communicate with the LMOC via the Host interface, can attempt to acquire an optical communication links with OGSs and/or User platforms, and once a link is established provide forward and return service relays and Direct Uplinks/Downlinks.
Figure 16: LCRD payload mode transition diagram (image credit: NASA)
Ground Segment Operations
The ground segment consists primarily of the LMOC and two OGSs. The LMOC coordinates all LCRD operations activities, provides the mission data store, provides an interface for User service planning and scheduling, and provides the experiment operations center. The OGSs provide a User MOC interface, User MOC and user platform simulators, and the optical link from the Earth to the Payload. The LMOC will operate 24 hours per day, seven day per week to receive Payload, OGS, and Host Mission telemetry. The LMOC and OGSs will initially be staffed 8 hours per day, 5 days per week after on-orbit Payload checkout to conduct experiments and support User services. The ground segment will be staffed for certain 24 hour period during the mission to support longer duration experiment operations.
Routine operations will be performed continuously during staffed operations periods. The main operations tasks will be performed from the LMOC at WSC and include:
• Monitor payload and ground system telemetry for proper execution of commands
• Monitor payload and ground system telemetry for limit violations or changes in state that may be a precursor to limit violations
• Monitor payload and ground system telemetry for anomalies that threaten health and safety, and to take appropriate action to ensure health and safety in the event of a system anomaly
• Monitor the atmospheric conditions at each ground station
• Monitor link status for each optical link and the host spacecraft RF link
• Monitor quality of service for each user service
• Respond to user service requests and support requests from the ExpO team and external users
• Perform planned and unplanned ground station handover and ground system reconfiguration operations when required
• Execute performance data monitoring functions and data storage in support of ExpO team analysis
• Implement scheduling change requests from the ExpO team and external users
• Perform routine housekeeping functions including data store back up, periodic payload and ground calibration functions, clock management, data correlation, preventive system maintenance and other planned maintenance and repair activities.
• Contact engineering, ground system, or supervisory personnel in the event of a significant anomaly.
Each LCRD OGS will be staffed during the same period as the LMOC to support routine operations which include:
• Provide laser safety functions
• Monitor and control ground station local equipment
• Monitor the atmospheric conditions at the ground station
• Monitor link status for the ground station optical link
• Monitor quality of service for each User service provided at the ground station
• Support diagnostic procedures when degraded performance is detected
• Perform routine housekeeping functions including data store back up, ground station optical calibration functions, clock management, preventive system maintenance and other planned maintenance activities
• Contact engineering, ground system, or supervisory personnel in the event of a significant anomaly.
PAT (Pointing, Acquisition, and Tracking) Concept of Operations
There are two PAT operations cases for LCRD. The first is the establishment of optical links between the payload and an OGS. This is the simplest case given the relative stability of the host spacecraft orbit and the fixed location of the ground stations on the Earth's surface. This case will generally be applied to at least one optical terminal on the payload for normal operations. It may be applied to both optical terminals during demonstration and experiment activities and may include an outside user ground station not under LCRD control. The second case is PAT operations for a moving user platform. This could include an airborne platform or space-borne Low Earth Orbit (LEO) platform (up to 1000 km altitude). The most complex case for LCRD will be PAT associated with a LEO platform due to relative movement of the LEO platform with regard to the payload. This case may be applied to at least one optical terminal on the payload to support a user forward and/or return relay service. It may be applied to both optical terminals during demonstration and experiment activities to support user-to-user relays.
The LCRD payload, each LCRD OGS, and user platforms will follow a predefined acquisition sequence. The LMOC will coordinate the initial acquisition sequence between the payload and a ground station element and/or the payload and a user platform. A series of states are passed through during the acquisition process. Each state represents an intermediate step that is the result of a prior action. The diagram illustrating nominal acquisition sequence flow from state to state is shown in Figure 17. In this diagram states evolve from left to right during the course of the acquisition.
Figure 17: Nominal LCRD acquisition flow (image credit: NASA)
The LCRD Payload Optical Space Terminal states are:
• S1 - LCRD Optical Space Terminal initialized but not illuminated by the beacon or uplink beam
• S2 - LCRD Optical Space Terminal illuminated by the beacon
• S3 - LCRD Optical Space Terminal illuminated by the uplink beam
• S4 - LCRD Optical Space Terminal receiving data frames.
The Ground Stations or User Platform states are:
• T1 - Ground Station or User Platform initialized but not illuminated by the downlink
• T2 - Ground Station or User Platform illuminated by the (optionally dithered) downlink beam
• T3 - Ground Station or User Platform synchronized clock to downlink beam stream
• T4 - Ground Station or User Platform receiving data frames.
Once initial acquisition has been established between an OGS and the payload or the payload and a user platform, maintaining the communications link is handled by the payload and the OGS or the payload and the user without interaction from the LMOC. The payload and OGSs may use a SLPA (Closed Loop Point Ahead) algorithm built into the optical link to maintain accurate pointing. The link between the payload and a user platform is required to use CLPA. The User platform is required to apply Doppler compensation to maintain the link between the Payload and the User platform.
Optical Trunk Line Concept of Operations
LCRD provides the capability to multiplex user service data and payload command and telemetry data using the same optical link. This is referred to as an "optical trunk line" and is illustrated in Figure 18. The trunk line concept allows multiple users, types of data, and data destinations to be multiplexed together on the optical link at the optical frame level. The forward data trunk line is demultiplexed at the LCRD payload, and user data or commands routed to the correct user platform (or executed by the payload in the case of command). The return trunk line data frames get demultiplexed on the ground; User data sent to the appropriate user MOC, and telemetry is sent to the LCRD MOC.
The optical trunk line can support a slot rate of 2.88 Gbit/s in each direction for any modulation or data rate. The optical frame rate depends upon the modulation (DPSK or PPM), configured link dead time, and number of Q-repeats. The transmit side modem sends a continuous stream of frames. LCRD supports multiplexing of multiple data streams over a single optical link in both directions: user service data frames, payload command and telemetry frames, and idle fill frames (sent whenever there is no user data or payload commands or telemetry). This is achieved by the use of dynamic TDMA (Time Division Multiple Access), with the frames sent "on demand", in the order that they are received by the modem.
Figure 18: Optical trunk line (image credit: NASA)
Ground Station Handover Concept of Operations
Under certain conditions, the optical link between the payload and an OGS may be handed over to another OGS. A handover between ground stations may only occur if there is an available ground station to receive the handed over optical link. Some conditions that may drive this scenario are weather, laser safety issues, planned or unplanned maintenance, and laser operation blackout periods. Handovers are a highly manual procedure with no autonomy built into the payload. All handover operations must be coordinated through the LMOC, whether planned or unplanned. The general sequence of events is the same for both planned and unplanned handover scenarios. In this context, "OGS-A" is the station handing over, "OGS-B" is the receiving ground station. Assume OGS-B is fully functional with locally nominal atmospheric conditions.
1) User(s) and LCRD assets notified of impending ground station handover if possible.
2) OGS-B brought to operational/hot stand by state. This state means being configured to begin the acquisition sequence with the Payload and in the same optical link configuration (data rate an modulation) and ground system configuration as OGS-A for user services and in-band command/telemetry.
3) User services and/or in-band command/telemetry over the optical link terminated at OGS-A and the Payload.
4) Users establish forward/return data link with OGS-B and terminate with OGS-A.
5) Payload is configured via the host spacecraft RF link to point an OST at OGS-B and is ready to begin acquisition.
6) OGS-B and the payload complete acquisition sequence and establish optical communications link.
7) User services and/or in-band command/telemetry resume through OGS-B.
Figure 19 illustrates the pre-handover configuration and the post-handover configuration. During this scenario, the optical link with the user platform does not have to be terminated and should remain established through the second OST if possible to expedite resumption of service. The user will lose any data transmitted during the ground station handover. The user will be notified ahead of time if possible to mitigate data loss if the user mission desires.
Figure 19: Optical ground station handover (image credit: NASA)
LCRD is designed to support demonstrations and experiments in optical communications and optical communication networking. The LCRD Principle Investigator will lead a team to define demonstrations and experiments to be executed using LCRD mission assets. As the mission progresses, new demonstrations and experiments will be defined and executed using the base system functions or system enhancements. LCRD will support simulation of:
1) User-GEO-Ground relay
2) User-to-User Relay
3) Near Earth and Deep Space Relay
4) Direct Uplink/Downlink
5) Optical trunk line handovers
6) Forward/return link handovers.
LCRD will enable the following capabilities required to support experiments and demonstrations:
1) User MOC Simulators (UMSs) and User Platform Simulators (UPSs)
2) Data collection and tools for analysis of data and link characteristics
3) Simulation of various latencies in data transmission and receipt
4) Variations in scheduled service data rates and combinations of service data rates
5) Data rate variations in response to deteriorating link conditions
6) Variations in user traffic loading relative to allocated service data rates
7) Simulation of weather and atmospheric effects
8) Simulation of different line-of-site distances
9) Variations in relay asset availability.
Laser transmitter power reduction from Ground Stations, off-pointing the payload optical space terminal or attenuating the signal at the Ground Station may be used to simulate the effects of the channel on the weak signals returned from deep space distances. The relay may also use different modulation or data rates on each leg to simulate collection of data from multiple sources at lower data rates, which is then combined and relayed to a Ground Station.
Relay tests in each Near-Earth or deep space scenario will attempt to simulate differences in latency, maximum data rates, line-of-site and scheduled link availability, and other required tuning to accurately reflect different scenarios. LCRD will enable the implementation of tools required to support these scenarios over the course of the mission. The LCRD system will enable evaluation of protocols such as DTN ( Disruption Tolerant Networking), which are meant to alleviate the related communications issues.
The LMOC is responsible for coordination of LCRD network activities. The LMOC is the focal point for planning the use of LCRD resources for functions including experiment operations, user service support, payload operations, sustaining engineering activities, and maintenance activities. The LMOC will be responsible for coordinating the configuration of LMOC subsystems, the LCRD payload, and OGS support.
The LMOC will produce three schedule types covering different timescales: the Mission Timeline, the Active Schedule, and the Operational Schedule. The Mission Timeline will be a human-readable, high-level, long-term operations schedule incorporating long-range planning inputs. Its purpose will be to allow each ground element to plan their operator staffing schedules, to arrange periods of extended operations (outside of typical operating times), and to identify potential scheduling conflicts well in advance to facilitate negotiating solutions. Inputs will include scheduled LMOC downtimes, availability schedules for each OGS, availability of external interface systems, high-level experiment plans, and LCRD User Service Level Agreements.
The Active Schedule will be the medium-term planning product consisting of the conflict-free, constraint checked, and optimized schedule of committed operational services and all LCRD activities necessary to support those services. Its purpose will be to represent the configuration of all LCRD assets and support schedules or constraints from external entities to ensure a consistent configuration that will enable execution of planned user services. Inputs will include the current Mission Timeline, updates to scheduled LMOC downtimes, updates to availability schedules for each OGS, availability of external interfaces, predictive avoidance constraints from the Laser Clearing House, detailed experiment plans, and user service requests. Mission Planning and Scheduling will generate an updated Active Schedule a minimum of once per week for LCRD operations covering the next fourteen 24-hour operational days.
Mission Planning and Scheduling will generate two additional types of products for external systems based on the Active Schedule. The first will be a predictive avoidance request message covering the next few days of laser activity by the LCRD payload optical terminal that will be sent to the Laser Clearing House. This will be used by Laser Clearing House to generate a Predictive Avoidance file for LCRD. The second product for an external system will be a high-level activity schedule containing times the LCRD payload will operate and any operational events that may impact host spacecraft flight or ground operations. The LMOC will send this schedule to the Host Mission Operations Center as a planning aid for the host spacecraft mission.
The Operational Schedule will be the short-term schedule provided to the LMOC subsystems to be executed for asset configuration and control. This schedule is generated once before every "operational day", which is defined to be the current 24-hour period beginning at 00:00:00 UTC. It will facilitate timing and coordinating the configuration of all assets to support user services and experiments. This schedule may be regenerated at any time during an operational day for unplanned OGS handovers, unplanned maintenance or downtime events for the LMOC, LCRD payload, each OGS, or external support assets, changes to experiment or demonstration support plans, change requests from users, or any other reason.
An OSE (Operational Schedule Excerpt) will be generated from the full Operational Schedule and distributed to each asset required to support the schedule for the current operational day. The OSE consists of the subset of the full Operational Schedule that pertains specifically to that asset, will be executable by that asset's local control system, and will contain any information required to execute the schedule. The appropriate versions will be sent to the LMOC Service Management subsystem, Payload T&C System, and each OGS. An Operational Schedule for
LCRD will ingest planning aids and information from external sources for use in conflict resolution and optimization of the Active Schedule and Operational Schedule. These products include the Predictive Avoidance file from the Laser Clearing House, planning aids from the host spacecraft operator, and flight dynamic information from LCRD users.
Planning for a given time period will begin with the process of adding that period to the Mission Timeline. This will facilitate long-term planning, principally resolving issues of staffing and asset downtime, and will be largely a manual process carried out by the mission planner. In the medium term the mission planner will add activities for the period to the active schedule. This will allow a more detailed representation of planned activities in software and conflict resolution and optimization of the schedule. Near the start of a given operational day, the Mission Planning and Scheduling function will output the Operational Schedule covering that operational day based on the current Active Schedule (Ref. 9).
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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 (email@example.com).