Future In-Space Operations
Overview for Future In-Space Operations October 2013 Bernard Edwards Chief, Communications Systems Engineer NASA Goddard Space Flight Center Mission Statement The Laser Communications Relay Demonstration (LCRD) will demonstrate optical communications relay services between GEO and Earth over an extended period, and thereby gain the knowledge and experience base that will enable NASA to design, procure, and operate cost-effective future optical communications systems and relay networks. LCRD is the next step in NASA eventually providing an opticalcommunications service on the Next Generation Tracking and Data Relay Satellites LCRD Payload and Host Spacecraft
Mission Overview LCRD Payloadand Host Spacecraft LCRD Flight Payload 2 Optical Relay Terminals 10.8 cm aperture 0.5 W transmitter Space Switching Unit 1244 Mbps DPSK 311 Mbps 16-PPM 1244 Mbps DPSK 311 Mbps 16-PPM Mission Concept Orbit: Geosynchronous Longitude TBD between 162W to 63W 2 years mission operations 2 operational GEO Optical Relay Terminals 2 operational Optical Earth Terminals Optical relay services provided Ability to support a LEO User Hosted Payload Launch Date: Dec 2017 Table Mountain, CA White Sands, NM LCRD Ground Station 1 1 m transmit and receive aperture 20 W transmitter LCRD Ground Station 2 15 cm transmit aperture 20 W transmitter 40 cm receive aperture NASA Optical Communication Technology Strategy
2013 2017 2020 2025 Near Earth Flight Terminal LLCD Technology Transfer Commercialized LADEE Demo GEO Demo LCRD LEO Demo Near Earth Missions Commercialization Optical Module DPSK Modem Controller Electronics Deep Space Flight Terminal Candidate Deep Space Host Demo Mission Other Deep Space Missions Key DOT Technology Identification & Development SCaN Optical Ground Infrastructure Optical Comm Ground Stations (LLGT, OCTL, Tenerife) LCRD SCaN Operational Optical Ground Stations Added as Mission Needs Require (including International Space Agency Sites) Technology Investment and Development Stabilization Detectors Vibration Systems Engineering Laser Power/Life Pointing SNSPD arrays, photon counting space receiver, ground receiver detection array, NAF APD/nanowire det., COTS quadrant spatial-acquisition detectors Mini FOG Spacecraft disturbance rejection platform, piezo-based point-ahead mechanism CFLOS Analysis, Optical Comm Cross Support Low-noise laser PPM Laser Transmitter Flexured Gimbal Mount Leveraging the Lunar Laser Communications Demonstration (LLCD)
NASAs first high rate space laser communications demonstration Space terminal integrated on the Lunar Atmosphere and Dust Environment Explorer (LADEE) Launched on 6 September 2013 from Wallops Island on Minotaur V Completed 1 month transfer (possible lasercomm ops) 1 month lasercomm 400,000 km 250 km lunar orbit 3 months science 50 km orbit 3 science Payloads Neutral Mass Spectrometer UV Spectrometer Lunar Dust Experiment LLCD Flight Hardware Optical Module Designed and fabricated by MIT LL Inertially-stabilized 2-axis gimbal Fiber-coupled to Modem transmit (Tx) and receive (Rx) Modem Module (MM) Designed and fabricated by MIT LL Pulse Position Modulation Only Digital encoding/decoding electronics,1550 nm fiber Tx and Rx Controller Electronics Built by Broad Reach Engineering for OM, MM control Telemetry & Command (T&C) interface to S/C All Modules Interconnected via electrical cables and optical fibers LLCD Provides the Foundation for LCRD
Lunar Lasercom Space Terminal Modem Module Lunar Lasercom Ground Terminal DL 622 Mbps UL 20 Mbps White Sands, NM Controller Electronics 1.55 um band LADEE Spacecraft DL > 38 Mbps Optical Module DL > 38 Mbps UL > 10 Mbps Tenerife Table Mtn, CA Lunar Lasercom Optical Ground System (ESA) Lunar Lasercom OCTL Terminal (JPL) LCRD will leverage designs and hardware from LLCD, with modifications to satisfy mission requirements. LCRD Design Reference Mission
User 1 S/C User n S/C GS-1 GS-m User 1 MOC User k MOC LMOC Active optical link Future optical link Terrestrial Internet Protocol Network Simultaneous multiple real-time user support and multiple store & forward user support multiplexed on single trunkline Different user services: frame, DTN, Scheduled and Unscheduled Ground Station handovers Number of Users, Mission Operations Centers (MOCs), and Payloads scalable Emulation of different relay and user location and orbits by the insertion of delays and disconnections in the data paths LCRD Baseline Hosted on a Space Systems/LoralCommercial Communications Satellite Flight Payload Two MIT LL designed Optical Modules (OM) Two Integrated Modems that can support bothDifferential Phase Shift Keying (DPSK) andPulse Position Modulation (PPM) Two OM Controllers that interface with the HostS/C Space Switching Unit to interconnect the twoIntegrated Modems and perform dataprocessing Two Optical Communications Ground Stations Upgraded JPL OCTL (Table Mountain, CA) Upgraded LLCD LLGT (White Sands, NM) LCRD Mission Operations Center (LMOC) Connected to the two Optical CommunicationsGround Stations Connected to Host S/C MOC LCRD GS and Optical Space Terminal Location
161W 112W 63W OST Possible Location GEO Locations were chosen to ensure at least 20 above horizon for both Ground Stations LCRD Mission Architecture
LCRD Payloadand Host Spacecraft LCRD Flight Payload W (PPM/DPSK) DPSK at Gbps PPM at 311 Mbps 1550 nm band 1550 nm band 4x UL Transceivers 4x DL Receivers Environmental enclosure surrounding UL and DL telescopes Host Spacecraft RF Link Chiller for cooling trailer and telescopes Converted 40-ft ISO container housing controls, modems, and operator console 18-ft Clamshell weather cover Table Mountain, CA Host Mission Ops Center (HMOC) White Sands, NM LCRD Optical Ground System (LOGS) - OCTL Based on Lunar Lasercom Ground Terminal (LLGT) NISN NISN NISN LCRD Ground Station-1 1 20 W (PPM/DPSK) LCRD Ground Station-2 W (PPM/DPSK) 40 cm (PPM/DPSK) LCRD Mission Ops Center (LMOC) NASA GSFC Relay Optical Link Relay Link Features:
Coding/Interleaving at the link edges Rate DVB-S2 codec (LDPC) 1 second of interleaving for atmospheric fading mitigation Data can be relayed or looped back PPM or DPSK can be chosen independently on each leg OST-1 OST-2 Codec/ Interleave Modem Optics Atmosphere Free Space Optics Modem Space Switching Unit Modem Optics Free Space Atmosphere Optics Modem Codec/ Interleave GS-1 LCRD Payload GS-2 Bus and Payload Overview
Bus Overview Existing SS/L commercial satellite bus LCRD package is located on the S/C Earth deck, similar to a typical North panel extension The enclosure North-facing surface is the main radiator with Optical Solar Reflectors Secondary LCRD radiator panel is on the South side Star trackers located on the top of the enclosure for optimal registration with OMs Star Tracker Optical Module CE Modem A Switch Modem B Radiator (back view) Equipment Panel & Radiator 1 2 1 2 Payload Hardware Overview
Integrated Modem (qty 2) 0.5 W transmitter; optically pre-amplified receiver DPSK and PPM modulation 27 kg, 130 W Supports Tx and Rx frame processing No on-board coding and interleaving Optical Module (qty 2) Gimbaled telescope (elevation over azimuth) 12 half-angle Field of Regard 10.8 cm aperture, 14 kg Local inertial sensor stabilization Space Switching Unit (qty 1) Flexible interconnect between modems to support independent communication links High speed frame switching/routing Command and telemetry processor Controller Electronics (CE) (qty 2) OM control/monitoring Interface to Host Spacecraft 7 kg, 151 W Flight Payload Functional Diagram
Space Switching Unit Frame Switching Command & Telemetry Processing Controller Electronics 1 Integrated Modem 1 Integrated Modem 2 Controller Electronics 1 Host S/C 1553 Host S/C 1553 Optical Data & Frame Processing Optical Data & Frame Processing Host S/C 1 PPS Host S/C Interface Load Drivers Load Drivers Host S/C Interface Host S/C 1 PPS Receiver Transmitter Transmitter Receiver Sensor Processing PAT Processing Sensor Processing PAT Processing fiber fiber All these elements involved in optical communication and influence link budget Optical Module 1 Optical Module 2 Optical Telescope Pointing & Jitter Control Optical Telescope Pointing & Jitter Control To & From Ground or LEO Terminals To & From Ground or LEO Terminals SpaceWire Downlink communication signal High Speed Serial Uplink communication signal Analog Uplink acquisition beacon signal Two Ground Stations JPL will upgrade the JPL Optical Communications Telescope Laboratory (OCTL) to form the LCRD Optical Ground Stations (LOGS) This is a single large telescope design Adaptive Optics and support for DPSK will be added LCRD will upgrade the Lunar Laser Communications Demonstration (LLCD) Ground Terminal developed by MIT Lincoln Laboratory This is an array of small telescopes with a photon counter for PPM Both stations will have atmospheric monitoring capability to validate optical link performance models over a variety of atmospheric and background conditions Ground Station Components
Upgrade of JPLs OCTL Upgrade of LLGT 20 W transmit power 1 meter transmit/receive aperture 40 cm receive aperture; 15 cm transmit aperture Identical equipment for atmospheric monitoring Receive adaptive optics Receive adaptive optics and uplink tip/tilt correction Identical Ground Modem, Codec, and Amplifier systems for DPSK and PPM Wide angle beacon for initial acquisition Scanning beacon for initial acquisition Laser safety system for aircraft avoidance Operation in restricted flight airspace Legacy array of superconducting nanowire single photon detectors DPSK Modulation/Demodulation
In the DPSK system, each slot contains an optical pulse with phase = 0 or . Data carried as a relative phase difference between adjacent pulses. DPSK Transmitter The average power-limited transmitter allows peak power gain for rate fall-back via burst mode operation. In DPSK pulse is not self consistent (like in OOK) it needs reference neighbouring pulse. Because neighbouring pulse serves as a LO the requirement for transmitter and receiver coherences is drastically relaxed comparing to the coherent systems. At the DPSK receiver, the original sequence is demodulated using a fiber delay-line interferometer to compare the phase of adjacent pulses. DPSK Receiver PPM Signaling For PPM, the binary message is encoded in which of M=16 slots contains a signal pulse. Optical modulation accomplished with the same hardware that implements burst-mode DPSK, with the applied phase irrelevant for PPM PPM Signaling PPM demodulation is accomplished by comparing the received power in each slot with a (controllable) threshold value Uses the same pre-amplifier and optical filter as the DPSK receiver, but by-passes the delay-line interferometer PPM modulation suits to be efficient for energy starved, but BW not limited links. PPM requires an accurate knowledge of the pulse position; therefore imposing strict requirements on clock jitter requirements. threshold PPM Receiver Line of Sight and CFLOS The first consideration in link establishment is whether a line of sight between the source and destination exists. Free space laser communications through Earths atmosphere is nearly impossible in the presence of most types of clouds. Typical clouds have deep optical fades and therefore it is not feasible to include enough margin in the link budget to prevent a link outage. Key parameter when analyzing free space laser communications through the atmosphere is the probability of a cloud-free line of sight (CFLOS) channel. A mitigation technique ensuring a high likelihood of a CFLOS between the source and destination is needed to maximize the transfer of data and overall availability of the network. Using several laser communications terminals on the relay spacecraft, each with its own dedicated ground station, to simultaneously transmit the same data to multiple locations on Earth A single laser communications terminal in space can utilize multiple ground stations that are geographically diverse, such that there is a high probability of CFLOS to a ground station from the spacecraft at any given point in time. Storing data until communications with a ground station can be initiated Having a dual RF / laser communications systems onboard the spacecraft. NASA has studied various concepts and architecture for a future laser communications network.The analysis indicates ground segment solutions are possible for all scenarios, but usually require multiple, geographically diverse ground stations in view of the spacecraft. Network Availability A ground station is considered available for communication when it has a CFLOS at an elevation angle to the spacecraft terminal of approximately 20 or more. The network is available for communication when at least one of its sites is available. Typical meteorological patterns cause the cloud cover at stations within a few hundred kilometers of each other to be correlated. Stations within the network should be placed far enough apart to minimize these correlations May lead to the selection of a station that has a lower CFLOS than sites not selected, but is less correlated with other network sites. Having local weather and atmospheric instrumentation at each site and making a simple cloud forecast can significantly reduce the amount of time the space laser communications terminal requires to re-point and acquire with a new ground station. In addition to outages or blockages due to weather, a laser communications link also has to be safe and may have times when transmissions are not allowed. Optical Communications Network Operations Center (NOC)
In order to provide all of this flexibility for users, the relay network operations center must assume the responsibility for the user data flows. The NOC must now keep an accounting of the user data in transit within the provider system (onboard the relay or within a ground station). Any handovers or outages that require retransmissions or rerouting within the provider network must all be managed by the NOC transparently to the users. The NOC must also be able to provide the necessary insight to resolve any lost data issues reported by users. The LCRD Mission Operations Center (MOC) acts as a future NOC in the demonstration Essential Experiments and Demonstrations
Experiments will begin immediately following launch and Payloadcheckout During the first six months, the highest priority experiments willdemonstrate technology readiness for the next generation TDRS infusiontarget Laser Communications Link and Atmospheric Characterization Earth-Based Relay (Next Generation TDRS) The remaining mission time will continue the essential experiments tocollect additional data and also include: Development of operations efficiency (handover strategies, more autonomousops, etc.) Planetary/Near-Earth Relay scenarios (additional delays, reduced data rates,non-continuous trunkline visibility) Low Earth Orbit (LEO) - real or simulated 23 SCaNs Optical Communications Strategy for Near Earth
SCaN has made a considerable investment in the 10 cm optical module design being used on both the Lunar Laser Communications Demonstration (LLCD) and the Laser Communications Relay Demonstration (LCRD) In the optical module there are minor differences between the two The major difference is in the modem (DPSK at Gbps for LCRD and PPM at 622 Mbps for LLCD) SCaN would like to re-use that design as much as possible: Future Low Earth Orbit (LEO) compatible terminal Future lunar missions (far side exploration) Next Generation TDRS (perhaps with an upgraded higher rate modem) For missions deeper in the solar system, SCaN has made a limited investment in the Deep Space Optical Terminal (DOT) concept being worked on at JPL 24 Summary The LCRD optical communications terminal leverages previous work done for NASA With a demonstration life of at least two years, LCRD will provide the necessary operational experience to guide NASA in developing an architecture and concept of operations for a worldwide network Unlike other architectures, it will demonstrate optical to optical data relay LCRD will provide an on orbit platform to test new international standards for future interoperability LCRD includes technology development and demonstrations beyond the optical physical link NASA is looking forward to flying the LCRD Flight Payload as a hosted payload on a commercial communications satellite NASA can go from this demonstration to providing an operational optical communications service on the Next Generation Tracking and Data Relay Satellites