Laser Communications Relay Demonstrations

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1 Laser Communications Relay Demonstrations Vishesh Shrivastava Department of Computer Science & Engineering KLS Gogte Institute of Technology Belagavi, India Contact No Abstract This is a report that explains about NASA's Laser Communications Relay Demonstration Project (LCRD).LCRD will fly two optical communications stations on a Loral commercial communications satellite in GEO orbit to communicate with 2 ground stations. LCRD is a joint project between NASA s Goddard Space Flight Centre (GSFC), California Institute of Technology (JPL), the Jet Propulsion Laboratory and the Massachusetts Institute of Technology Lincoln Laboratory (MIT LL). LCRD will operate for at least two years in GEO, and exhibit how optical communications can meet NASA s growing demand for higher data rates and be a path discoverer for providing optical services on NASA s Data Relay Satellite and Next Generation Tracking. The optical communications potential of Laser Communication will allow it to serve as a developmental testbed. LCRD technology is going to be trialled by NASA within the next 4 years in order to test communications in near-earth and deep-space robotic and human missions. This report reviews the exploratory designs and mission concept for the flight and ground optical segments, and gives report on preliminary outcomes from several trade studies conducted during this project. Shweta Gharshi Department of Computer Science & Engineering KLS Gogte Institute of Technology Belagavi, India Contact No shwetagharshi155@gmail.com earlier days. NASA is taking several steps to jump off on a new era of communication and Information technology. Optical communications (or laser communication) is one of those next step in communications technology that will help NASA to commence more complicated assignments in the future that, when compared to traditional RF systems, need much higher data rates or decreased size, mass and power overload on the spacecraft. Index Terms optical communications, GEO orbit, Data Relay Satellite, Ground Optical Segments. I. Introduction NASA has relied completely on radio frequency (RF)-based communications as the only possible medium for exchanging data between a mission and a spacecraft since its inception in 1958.The information from an exploration or a scientific discovery has to get back to Earth, and then the more data that can also be sent to the spacecraft. Nowadays several missions are demanding communication with higher data rates than the Revolutionizing the way in which NASA communicates mission-critical information, Laser Communications will use lasers to transmit and encode the data. Laser communications will allow communications rates 10 to 100 times quicker than RF-based communication. The Laser light s Wavelength is shorter than radio waves and minimize the area the energy spreads as it navigates through space. With a shorter wavelength there is much more bandwidth available for an optical signal. NASA must coordinate with its national and international associates to ensure use of proper frequency bands that avoid interference during transmissions. The rise in bandwidth will empower NASA to conduct with no spectrum allocation 654

2 constraints. This technology will permit spacecraft to send high-resolution science data across the solar system to scientists on Earth and facilitate researchers to study other planets at the same level of extent as they study our own.space laser communications will also permit missions to use bandwidth-hungry instruments, such as synthetic aperture radar, hyperspectral imagers and other instruments with high data rate requirements and high definition. II. LCRD: Key Mission Facts The LCRD mission is NASA s first, long duration optical communications mission. The project will benefit mature concepts and deliver technologies suitable to both near-earth and deep space communication network missions. The investigation will enable a variety of robust, exploration missions and future missions, providing a higher data rate and transferring more accurate navigation capabilities with reduced power, weight and size requirements. Laser Communication Relay Demonstration will demonstrate some optical communications technologies and concepts of operations, and advanced networking technologies applicable to Deep Space missions. LCRD is the next step in NASA finally providing an optical communications service on the Data Relay Satellites and Next Generation Tracking. Figure-Overview of LCRD Architecture III. Leveraging NASA s Lunar Laser Communication Demonstration NASA recently developed the Lunar Laser Communication Demonstration (LLCD). It was NASA s First High Rate Space Lasercom which was launched in August 2013 as a secondary payload on the Lunar Atmosphere and Dust Environment Explorer (LADEE). The LLCD is a short duration optical communications experiment between the Earth and the Moon. 655

3 Real-time optical relay from Ground Station 1 On Earth through the GEO spacecraft to another Ground Station 2 on Earth Pulse Position Modulations appropriate for deep space communications and other different power limited users like small Near Earth missions Differential PSK Modulations Appropriate for Near Earth high data rate communications Demonstration of various mission scenarios through spacecraft imitations at the Earth ground station Performance demonstrations a n d t est i n g of coding, network layer and link layer protocols over optical links over an orbiting tested. LLCD will ably demonstrate the following: Pulse Position Modulation Photon counting on the downlink Inertial stabilization High-efficiency reception and transmission of Pulse Position Modulation Very low power, size and weight 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 LLCD will prove the practicability of optical communications. It will not provide the crucial operational knowledge to permit optical communications to support mission critical communications on future missions due to the very limited operating time (potentially less than 16 hours over the life of the mission).in order to make optical communications useful to future projects, long mission life space terminals must be developed, advanced and proven. There needs to be a demonstration of handovers between multiple ground sites. Laser Communication s flight payload will have 2 optical communications ground stations on Earth and 2 optical communications terminals in space to allow the mission to demonstrate: IV. The Flight Payload The LCRD flight payload consists of 2 individual optical communications terminals and a High Speed Electronics unit to interface to the spacecraft host and 2 terminals. The Laser Communication Relay Demonstration Flight Payload will fly on a Loral Commercial Communications Satellite as a hosted payload. The flight payload will be flown on a GEO spacecraft. The major subsystems are: 2 optical communications modules (heads) 2 optical module controllers 2 modems capable of supporting or handling both Pulse Position Modulation (PPM) and Differential Phase Shift Keying (DPSK) Ver y high Speed Electronics to interconnect the two Optical modules, perform network and data processing, and to interface to the host spacecraft. High rate bi-directional communications between Geostationary Earth Orbit (GEO) and Earth 656

4 Figure 1- LCRD flight payload block diagram approximately 2 milliradians. This detector is used both for detection of a scanned uplink signal and also used as a tracking sensor for initial pull-in of the signal. The telescope is mounted to 2-axis gimbal and is stabilized via a magneto hydrodynamic inertial reference unit (MIRU) Angle-rate sensors in the MIRU detect angular disturbances which are then rejected by using voice-coil actuators for inertial stabilization of the telescope. Optical fibres couple the optical module to the modems from where transmitted optical waveforms are processed and Control for each optical module and its corresponding modems is provided by a controller. Each optical module is held and secured during launch with a cover and onetime launch latch. An optical communications terminal on LCRD consists of an optical module, PPM modem, a DPSK modem, and an optical module controller. V. The Flight Optical Communications Module Each of the 2 optical communications terminals to be flown on the GEO orbit spacecraft will receive and transmit the optical signals. During the transmission, the fundamental functions of GEO optical communications terminal are to efficiently produce the optical power that can have the data modulated or harmonized onto it; format, encode and interleave approaching electronic data; adjust the optical beam with this data; transmit and amplify this optical power through efficient optics; and aim the very thin beam at the ground station on earth, despite platform vibrations distortions and motions. During the acceptance, the GEO optical communications terminal must provide a collector which is large enough to capture adequate power to support the data rate; couple this light onto efficient detectors, low noise while reducing the coupled background light; and perform synchronization, deinterleaving, decoding and demodulation of received waveform. Each optical module, shown in Figure 2, is a 4-inch reflective telescope that generates a ~15 microra dian downlink beam. The optical module also houses a spatial acquisition detector which is a normal simple quadrant detector, with a field of glimpse of Figure 2- Inertially Stabilized Optical Module LCRD Optical Terminal configuration:- Wavelength: 1550 nm Mass: 69 Kg Power: 130 W DC Data Rates: Gbps using DPSK and 622 Mbps using PPM Optical Transmit Power: 0.5 W Telescope Diameter: 108 mm MIT Lincoln Laboratory Design VI. Flight Modem There exist some dissimilarity between the technological approaches to access optical communications particularly designed for Near-Earth missions versus deep space missions 657

5 due to the immensely differing ranges and data rates for Near-Earth against deep space missions. Photon counting and Pulse Position Modulation (PPM) has been recognized as technique of choice for deep space missions and Differential Phase Shift Keying (DPSK) is the modern preferred choice for Near-Earth mission. LCRD will support Differential Phase Shift Keying (DPSK) which has far better fading tolerance and sensitivity as compared to simply onoff-keying and It also supports communications when the Sun is in the field of view because of the use of a singlemode receiver (received light is coupled into a single-mode optical fibre which supply as a spatial filter) and optical band pass filtering. LCRD leverages a MIT LL previously designed DPSK modem as a cost effective way to provide a DPSK signal. LCRD will both receive and transmit data at an (uncoded) rate from 72 Mbps to 2.88 Gbps. The DPSK modem employs identical signalling for both uplink and downlink directions. A sequence of fixed duration pulses at a 2.88 GHz clock rate is generated by a DPSK transmitter. A bit is encoded in the phase difference between two consecutive pulses. Demodulation is accomplished with a single Mach- Zehnder optical interferometer unconcerned of data rate, the clock rate remains fixed and the EDFA exaggerate the optical signal to 0.5-W average power level. The DPSK receiver has an optical filter and an optical preamplifier stage, at which point the light is split between a clock recovery unit and the communications receiver.the modems instead support a relay architecture where up- and down-link errors are corrected together in a decoder located at the destination ground station. LCRD will also support pulse position modulation (PPM) making use of the same modem that supports DPSK. Figure -Benefits of Optical Communication VII. The Ground Segment The LCRD Ground Segment is comprised of the 2 optical communications ground stations and LCRD Mission Operations Centre (LMOC).Scheduling, control and command of the LCRD payload and the ground stations is performed by LMOC. The earth ground station must provide 3 functions when communicating with one of the 2 optical communications terminals on the GEO spacecraft. They are: It transmit a signal to the GEO space terminal It Receive the communications signal from the GEO space terminal Transmission of an uplink beacon beam so that the GEO space terminal points to the correct location on the Earth VIII. LCRD Ground Station 1 JPL will setup its OCTL (Optical Communications Telescope Laboratory) at Table Mountain, CA, so that it can be used as GS-1 of the demonstration. 658

6 Figure 5 - Lunar Lasercom Ground Terminal will be enhanced with Adaptive Optics and the ability to receive and demodulate a DPSK signal Technologies used in OCTL:- Instruments to monitor or oversees sky and atmospheric conditions System to support safe laser beam propagation through navigable air and near-earth space Adaptive optics correction of atmospheric turbulence effects Remote ground station control capability Deep space (PPM) and near-earth (DPSK) modulation formats Network protocols to ensure or guarantee data distribution and delivery IX. LCRD Ground Station 2 MIT Lincoln Laboratory constructed and is building the Lunar Lasercom Ground Terminal (LLGT) [9] for NASA s Lunar Laser Communications Demonstration (LLCD). The LLGT, shown in Figure5, will be refurbished and enhanced to serve as Ground Station 2 for LCRD. A conclusive summary of the LLGT, as designed for LLCD, follows below. The primary enhancements for LCRD will be an adaptive optics system to couple received light into single mode fibre (to support the DPSK signal), and further development of the single photon detectors (to support the PPM signal), including the development of more robust and scalable optical packaging, cabling, and readout electronics. X.ACKNOWLEDGEMENT We have taken efforts in this report. However, it would not have been possible without the kind support and help of many individuals and organizations. We would like to extend our sincere thanks to all of them. We would like to express our gratitude towards our parents & member of KLS Gogte Institute of Technology, Belgaum for their kind co-operation and encouragement which help us in completion of this paper. Our thanks and appreciations also go to our friends in developing the paper and people who have willingly helped me out with their abilities. XI. Conclusion The Laser Communication optical communications terminal leverages previous NASA and MIT Lincoln Laboratory designs The Laser Communication optical module is based on the Lunar Laser Communications Demonstration (LLCD) optical module The Pulse Position Modulation modem is normally based on the LLCD design The Differential PSK modem is based on a DPSK modem designed by MIT Lincoln Laboratory Laser Communication will provide an on orbit platform to test new international standards for both Near Earth and Deep Space missions for future interoperability Laser Communication includes technology development and demonstrations beyond the optical physical link Delay Tolerant Networking (DTN) protocols will be incorporated on board at target data rates applicable to Mars 659

7 or L1/L2 relay trunk lines NASA can go from these demonstrations to providing an operational optical communications service on the Next Generation Tracking and Data Relay Satellites. REFERENCES 1.Wang, J.P.; Magliocco, R.J.; Spellmeyer, N.W.; Rao, H.; Kochhar, R.; Caplan, D.O.; Hamilton, S.A.;, "A consolidated multi-rate burst-mode DPSK transmitter using a single Mach- Zehnder modulator," Optical Fiber Communication Conference and Exposition (OFC/NFOEC), 2011 and the National Fiber Optic Engineers Conference, vol., no., pp.1-3, 6-10 March Caplan, D.O.; Rao, H.; Wang, J.P.; Boroson, D.M.; Carney, J. J.; Fletcher, A.S.; Hamilton, S.A.; Kochhar, R.; Magliocco, R.J.; Murphy, R.; Norvig, M.; Robinson, B.S.; Schulein, R.T.; Spellmeyer, N.W.;, "Ultra-wide-range multi-rate DPSK laser communications," Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS), 2010 Conference on, vol., no., pp.1-2, May Shoup, R.; List, N.; Fletcher, A.; Royster, T.;, "Using DVB- S2 over asymmetric heterogeneous optical to radio frequency satellite links," MILITARY COMMUNICATIONS CONFERENCE, MILCOM 2010, vol., no., pp , Oct Nov K. E. Wilson, J. Wu, N. Page, M. Srinivasan, The JPL Optical Communications Telescope Laboratory (OCTL), Test Bed For the Future Optical Deep Space Network JPL, Telecommunications and Data Acquisition 660