- Email: [email protected]
Lunar Laser Communication Demonstration operations architecture,
Author's Accepted Manuscript
Lunar laser communication demonstration operations Architecture Farzana I. Khatri, Bryan S. Robinson, Marilyn D. Semprucci, Don M. Boroson
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PII: DOI: Reference:
S0094-5765(15)00038-7 http://dx.doi.org/10.1016/j.actaastro.2015.01.023 AA5336
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Acta Astronautica
Received date: 3 November 2014 Revised date: 6 January 2015 Accepted date: 30 January 2015 Cite this article as: Farzana I. Khatri, Bryan S. Robinson, Marilyn D. Semprucci, Don M. Boroson, Lunar laser communication demonstration operations Architecture, Acta Astronautica, http://dx.doi.org/10.1016/j.actaastro.2015.01.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
65th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by the International Astronautical Federation. All rights reserved.
Lunar Laser Communication Demonstration Operations Architecture Farzana I. Khatri, Bryan S. Robinson, Marilyn D. Semprucci, and Don M. Boroson MIT Lincoln Laboratory, U.S.A. Email: [email protected] Abstract Radio waves have been the standard method for deep-space communications since the earliest days of space exploration. However, the recent success of the Lunar Laser Communications Demonstration (LLCD) program will clearly revolutionize the way data is sent and received from deep space. LLCD demonstrated record-breaking optical up/downlinks between Earth and the Lunar Lasercom Space Terminal (LLST) payload on NASA’s Lunar Atmosphere Environment Explorer (LADEE) satellite orbiting the Moon. A space-to-ground optical downlink as fast as 622 Mbps was demonstrated as well as a ground-to-space uplink as fast as 20 Mbps. The LLCD operations architecture was designed to support a wide range of operations conditions, multiple ground terminals with varying designs and capabilities, short contact times including energy and thermal constraints, and limited viewing opportunities. This paper will explore the operations architecture used for the LLCD as well as present ideas on how best to make future laser communications operations routine and suitable for wide-scale deployment.
Highlights •
Lasercom offers enormous SWaP benefits along with data rate improvements which will benefit future space missions.
•
LLCD demonstrated the potential for a future operational lasercom system.
•
Operational lasercom is poised to become a reality within the next decade.
Keywords Lasercom; Operations; LADEE; LLCD I. INTRODUCTION NASA has been developing free space laser communications (lasercom) technologies to support the high data rates required for future deep-space science, exploration, and human missions. Free space lasercom can potentially offer a tremendous Size, Weight, and Power (SWaP) advantage over Radio Frequency (RF) systems due to its short wavelength. A major first step towards an operational deep-space lasercom system occurred during mid-October to mid-November 2013 when NASA’s Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time a duplex lasercom link between a satellite in lunar orbit and multiple ground stations on Earth [1][2]. The LLCD system consisted of a space terminal, the Lunar Lasercom Space Terminal (LLST) [3][4], and a primary ground terminal, the Lunar Lasercom Ground Terminal (LLGT) [5], a transportable system stationed at White Sands, NM for the mission. The space terminal was a payload on the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite [6]. Additional ground stations at Table Mountain, CA [7] and Tenerife, Canary Islands, Spain [8] were also employed during the demonstration. The entire demonstration was coordinated This work is sponsored by National Aeronautics and Space Administration under Air Force Contract #FA872105-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.
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and run from the Lunar Lasercom Operations Center (LLOC) at MIT Lincoln Laboratory in Lexington, MA. The LLST, LLGT, LLOC, and overall LLCD system were all designed, built, and operated by teams from the MIT Lincoln Laboratory. The LLCD program was overseen by NASA Goddard Space Flight Center, and the LADEE satellite was designed, built, and operated by the NASA Ames Research Center (ARC). In this paper, we briefly review the LLCD system and goals; we will then examine operational considerations that have been traditionally perceived as concerns for lasercom systems. The LLCD operations architecture provided solutions to these concerns and we will describe those solutions. In addition, we will discuss ideas on how best to make lasercom an operational reality for the future. II. SYSTEM OVERVIEW The high level goals for the LLCD technology demonstration were to help make lasercom a reality for future NASA science and exploration missions, to perform a detailed, integrated design for a capable end-to-end system, and to demonstrate many of the major functions required by future operational lasercom missions. The system requirements for the program were to reliably demonstrate: pointing, acquisition, and tracking with narrow optical beams; up to 622 Mbps optical downlink and up to 20 Mbps optical uplink, the sending of uplink optical commands and downlinking of telemetry plus science data. These objectives were to be accomplished during the variety of atmospheric and link geometry conditions that occurred over the duration of the mission. LLCD was highly successful and achieved all its goals, including demonstrating the major functions required for a future operational system. In this section, we will briefly review the major sub-systems involved in the demonstration. II.I. Lunar Lasercom Space Terminal (LLST) The Lunar Lasercom Space Terminal (LLST) as installed on the LADEE satellite is shown in Figure 1. The LLST consisted of three parts: the Optical Module (OM), the Modem Module (MM), and the Controller Electronics (CE). The OM consisted of a 10-cm reflective telescope that produced a nearly diffraction-limited 15-µm beam. It also contained a wide field-of-view acquisition sensor that detected an optical uplink beacon. The OM was connected to the MM via two optical fibers. The first fiber was the transmit fiber which contained the 0.5-Watt downlink beam, modulated using a 16-ary Pulse Position Modulated (PPM) waveform with selectable data rates from 39 Mbps to 622 Mbps. The second fiber was the uplink receive fiber which sent the received optical signal to a receiver which demodulated the 4-ary PPM uplink at either 10 or 20 Mbps.
Figure 1: The Lunar Lasercom Space Terminal (LLST) installed on the LADEE satellite. The control and telemetry/command interfaces for the OM and MM were provided by the CE. There was also a 40 Mbps data interface between the LADEE data buffer and the downlink side of the MM, as well as data connections from the MM to the CE. II.II. LLCD Ground Terminals The LLCD program originally employed a single ground station, built specifically for LLCD by MIT Lincoln Laboratory (the LLGT) [5]. However, given the limited planned operations schedule (~16 days during one month during which to successfully demonstrate LLCD) two additional ground terminals were added late in the program as
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alternate terminals. The additional ground stations increased operations time as well as provided a backup in the case of heavy cloud cover at the primary terminal. Both alternate terminals were existing lasercom terminals that were retrofitted to be compatible with LLCD. The ground station at Table Mountain, the Lunar Lasercom OCTL Terminal (LLOT) [7], was designed and operated by NASA’s Jet Propulsion Laboratory (JPL). The ground station at Tenerife, the Lunar Lasercom Optical Ground Station (LLOGS) [8], was designed and operated by the European Space Agency (ESA). The three ground terminals will be described briefly in this section. II.II.I. Lunar Lasercom Ground Terminal (LLGT)
Figure 2: The Lunar Lasercom Ground Terminal (LLGT) at White Sands, NM. The LLGT dome is 4-m tall. Also shown in the background are 19-m RF dishes used for NASA’s Tracking and Data Relay System (TDRS). Note the TDRS dishes are not part of the system and are some distance away. LLCD’s primary ground terminal, the LLGT, is shown in Figure 2 [5]. The LLGT was designed and built at MIT Lincoln Laboratory in Lexington, MA. It was designed to be transportable; the LLGT was transported, re-assembled, and tested once it was moved to its final location at White Sands, NM. The LLGT consisted of four commercial 40cm telescopes to receive the downlink communications at 39, 78, 155, 311, and 622 Mbps. The ground receiver was custom-built at MIT Lincoln Laboratory and consisted of four superconducting nanowire quad detector arrays [9]. The ground electronics were capable of real-time data transmission and delivery. The LLGT also housed four custom-built 15-cm telescopes to transmit the uplink beacon and 10- or 20-Mbps uplink communications signals. All the telescopes were mounted on a single gimbal housed in an environmentallycontrolled enclosure. Adjacent to this enclosure was a 40-ft trailer containing the local operations center, electronics, and opto-electronics. The LLGT operations trailer was connected via a 40 Mbps ground data network (T3 line) directly to the Lunar Lasercom Operations Center (LLOC) in Lexington, MA. During operations, there were two engineers staffed at the LLGT. II.II.II. Lunar Lasercom OCTL Terminal (LLOT) NASA Jet Propulsion Lab’s (JPL) Optical Communications Telescope Laboratory (OCTL), located on Table Mountain near Wrightwood, CA was one of two secondary ground terminals for LLCD [7]. This terminal consisted of a 1-m telescope for transmit/receive. The ground receiver was a tungsten silicide (WSi) superconducting nanowire single photon detector (SNSPD) array and supported 39 and 78 Mbps downlink communications rates. A softwarebased receiver was used to process the downlink communication data after the lasercom operations pass was over. The LLOT transmitted an uplink beacon for acquisition, but uplink communications was not implemented. II.II.III. Lunar Lasercom Optical Ground Station (LLOGS) The European Space Agency (ESA)’s Optical Ground Station (OGS) located at the Observatorio del Teide on Tenerife, Canary Islands, Spain was the other secondary ground terminal for LLCD [8]. This terminal consisted of a 1-m telescope for the downlink receiver with three individual 40 mm telescopes for the uplink beacon and communications transmitter. The primary ground receiver was a commercial photon counting detector from Hamamatsu which supported 39 Mbps downlink rate. A software-based receiver was used to post-process the downlink communication signal.
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II.III. Lunar Lasercom Operations Center (LLOC)
Figure 3:: Lunar Lasercom Operations Center view of space terminal control and visualization portion of LLOC.
Figure 4:: Block diagram of LLCD system architecture. LLOGS=Lunar Lasercom Optical Ground System, LLOT=Lunar Lasercom OCTL Terminal, LLGT=Lunar Lasercom Ground Terminal, LLOC=Lunar Lasercom Operations Center, DS DSN=Deep N=Deep Space Network, MOC=Mission Operations Center, SOC=Science Operations Center, ESA=European Space Agency, JPL=NASA Jet Propulsion Laboratory, MITLL=MIT Lincoln Laboratory, ARC=Ames Research Center, GSFC=NASA Goddard Space Flight Center. A view of the Lunar Lasercom Operations Center (LLOC) is shown in Figure 3.The .The signal, command, and telemetry flow from/to the LLOC is shown in Figure 4.. The LLOC had a direct duplex data connection from the LLGT facility at White Sands to enable high rate (optical) data forwarding and reception. There was also a lower rate connection to the Science ience Operations Center (SOC) at NASA GSFC through which LADEE spacecraft telemetry was received and LLST commands/telemetry were sent/received. Nearly full command and control of the LLGT was possible from the LLOC. Real-time time telemetry from all three gro ground und terminals was processed and displayed in the LLOC. Verbal information channels called “voice loops” were used in the LLOC to communicate real-time real by voice to/from the Mission Operations Center (MOC), SOC, and all three ground terminals. All telemetry that came into the LLOC as well as all commands that went out of the LLOC were archived. III. LLCD OPERATIONS PLANNING AND ACTIVIT ACTIVITIES The 16 days of LLCD operations were spread over one month by breaking them up into four “Four Lunar Day Blocks” (4LDB). Each 4LDB consisted of four Lunar days (defined roughly as the period of time when the Moon was visible at all 3 ground terminals) where each successive day started about 45 minutes later due to the shift in Moonrise time. Throughout each Lunar day during the 4LDB, 88-10 10 of the ~ 2 hour LADEE passes were dedicated to LLCD operations. The LADEE MOC and SOC were staffed to support LLCD for 19 19-hours hours during each Lunar day to allow for operations at all three ground terminals. The four primary 4LDB’s for LLCD took place over a 5-week 5
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period during October and November 2013, specifically during: October 17-21, October 26-29, November 2-5, and November 17-19.
Figure 5: Output of LLCD Operations planning tool showing 4-Lunar Day Block, Days vs. Moon Elevation. LLOGS=Lunar Lasercom Optical Ground System, LLGT=Lunar Lasercom Ground Terminal, LLOT=Lunar Lasercom OCTL Terminal. Block planning for the LLCD 4LDB occurred during the week prior to the first 4LDB and subsequently during the off times, between the 4LDBs. Several days prior to the start of the 4LDB, the LADEE planning team released a proposed plan for the 4LDB. Given the proposed spacecraft activities, the LLCD team determined how to best accomplish their goals for the upcoming block. A visual MATLAB-based LLCD planning tool was developed to aid in block planning. A sample output of the tool is shown in Figure 5. The tool output shows the satellite (Moon) elevation angle as a function of days for each of the three ground terminals as well as indications of day or night operation. Considerations that were taken into account during block planning were: ground terminal visibility and elevation angles, Sun angles, and space terminal activities. Once agreement was reached, the activity plan was finalized by the LADEE planning team and prepared for upload to the spacecraft at the start of the 4LDB. Day planning activities took place about 10-12 hours prior to each LLCD operations day. During this time, primary/backup ground terminals were selected for each lasercom operations pass where more than one ground terminal was visible. A notional timeline of a typical LLCD operations pass with the LLGT terminal is shown in Table 1. This table gives a sense of the major activities taking place when establishing the lasercom link. The LLCD system was able to operate for up to 30-40 minutes per 2-hour LADEE pass; the exact operating time was determined by considerations such as thermal issues, energy and battery conditions (depending on Sun illumination), and LLST operating modes used during the lasercom operations pass. Table 1: Notional LLCD operations timeline for a lasercom operations pass. T is the power on request time of the LLST. Time (min) Location Activity T-30 Primary/backup ground Star calibration of telescope, warm up of all terminals equipment and lasers T-15 LLOC Weather-based selection of ground terminal T-5 Primary ground Uplink beacon powered on and pointed at terminal (LLGT) LADEE T LLOC Power on sequence requested for LLST’s CE and MM begins T+1 LLST CE powered on, LLST telemetry begins to flow, gimbal begins to slew to point at LLGT T+1.1 LLST MM powered on T+1.5 LLST Beacon uplink from LLGT acquired T+2 LLST MM programming complete T+2.1 LLST/LLGT Error free bi-directional link established T+29 LLOC Power down sequence requested for LLST’s CE and MM T+30 LLOC Power down complete, LLST telemetry ceases Note that during the third week of the mission, an automated LLCD operations script was implemented and employed. This script was programmed to run at a specified time during the 4LDB and performed power on,
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pointing, acquisition, bi-directional lasercom link establishment, and power down. During passes that used the automated script, the only manual step required in the sequence above was the calibration of the ground telescope. IV. LASERCOM OPERATIONAL CONSIDERATIONS, LLCD SOLUTIONS, AND FUTURE CONSIDERATIONS The high-level goal of LLCD was to prove out a capable end-to-end integrated lasercom system design. In order to meet this goal, a key objective was to show that LLCD could operate reliably and easily under various conditions. Any communications system, RF or optical, has operational considerations. For example, all near-earth and deepspace links are governed by orbital positions of the transmitter(s) and receiver(s) involved. There are several operational considerations which have been traditionally perceived as concerns for lasercom systems which the LLCD aimed to address during operations. These operational considerations, how they were addressed by the LLCD, as well as future considerations are discussed in this section. IV.I. Earth’s Atmosphere and Clouds If ground-based receivers are employed in a lasercom system (as they were in LLCD), dealing with the Earth’s atmosphere becomes an operational consideration. Atmospheric effects can be separated by whether or not a CloudFree Line of Sight (CFLOS) is available. Since lasercom does not work without CFLOS, cloud blockage is treated as a separate consideration. IV.I.I. Earth’s Atmosphere with CFLOS Not unlike an RF communications system, an operational lasercom system must be able to operate with variations in received power. Received power variations are caused by orbital positions of the transmitter and receiver. For any Earth-based terminals, the received power variations are also caused by the atmosphere. Two key causes of receive power variation are atmospheric absorption and turbulence [10]. Both these effects vary with elevation angle and weather conditions.
Figure 6: A typical model prediction of one month of uplink fiber power received at LLST for various (best, nominal, and worst) atmospheric conditions at LLGT is shown. During the design phase of LLCD, before any hardware was built, we developed a detailed model of the lasercom link. Figure 6 shows a prediction we made of uplink communications power received in LLST’s optical fiber over the course of a typical month of operations, assuming a CFLOS. The analysis methodology we used was similar in to what was discussed in Ref [10]. Best, nominal, and worst atmospheric conditions (shown by the different colors of the plot in Figure 6) used for the LLGT took into account atmospheric absorption, turbulence, and orbital positions of the terminals. We found the power variation at the receive fiber to be between -65 and -60 dBm (1 dBm = 1 mW optical power). Figure 7 shows actual data we took for a few minutes of one LLST-LLGT lasercom operations pass on October 19, 2013. These results were typical of the lasercom links established during LLCD. The fiber power (red trace) falls between -65 and -60 dBm, just as we predicted. Not unlike an RF communications system, the LLCD could accommodate power variations by allowing for variable data rates. As mentioned earlier, the LLGT-LLST uplink could operate at either 10 or 20 Mbps to accommodate variation in the uplink receive power. Similarly, the LLST-LLGT downlink could operate at any of 39, 78, 155, 311, and 622 Mbps. The lower data rates could also be used in the event of thin cirrus clouds which can further attenuate optical signals.
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A future operational lasercom system will likely require a “knob” on data rate (like LLCD) to accommodate different operating conditions. To make the system even more robust, a software control system to optimize the data rate based on received signal and noise levels could be implemented.
Figure 7: Uplink power transmitted from LLGT measured at LLCD for 1 pass. The red trace is the fiber received power which is used for uplink communications. The blue trace is the acquisition detector power. IV.I.II. Clouds Since clouds are unavoidable for an Earth-based ground receiver, being able to predict and address cloud-related issues in real-time is an important aspect of an operational lasercom system. For LLCD, we used two main methods to assess ground terminal ground cloud cover. The first tool we used was developed and implemented by Northrop-Grumman to predict and evaluate cloud cover for the ground terminals [14]. Second, for each lasercom operations pass, we selected a primary and backup ground terminal. If there was thick cloud cover at the primary ground station, the backup ground station was selected. This decision was made in about 15 minutes prior to the start of the lasercom operations pass, based on real-time sky camera views (see Figure 8). Once the ground terminal selection was made, we had two additional methods to address LLCD operations in a cloudy environment. For intermittent clouds during a lasercom operations pass, we demonstrated use of a delay or disruption tolerant network [11]. The second approach demonstrated during LLCD, which works well if the ground station in use suddenly becomes clouded during a lasercom operations pass, is a mid-pass station handover.
Figure 8: All-sky camera views from LLGT at White Sands, NM. Left side shows CFLOS, right side shows clouded over. Any operational lasercom system of the future with Earth-based terminals will be required to address the cloud issue. If the mission requires real-time data downlinks, multiple ground stations can be used. The space terminal can then automatically attempt acquisition with all ground stations in view until a link is established. If the link goes down mid-pass, the system could move on to another ground station for a mid-pass handover. Ground stations can be automated to stand down in the event of a clouded out sky camera. A system to provide continuous downlink data relay throughout the process would need to be developed. For a mission that does not require real-time data
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downloads, the delay/disruption tolerant network can be employed. This ensures the data will eventually be downlinked. IV.II. Pointing Narrow Beams Lasercom systems are incredibly efficient when it comes to delivering power. The advantage gained from this is very low SWaP terminals; this comes at the cost of pointing and stabilizing the very narrow beam. Beams for lasercom systems can be as small as a few microradians (depending on the aperture’s size) and, hence, are more difficult to point than RF communication beams. Typical spacecraft pointing using star trackers is on the order of a few milliradians, which is adequate for RF, but not quite good enough to quickly and blindly point an optical beam to a ground station. To facilitate pointing for narrow beams and expedite the acquisition process, LLCD employed a few simple techniques described here. On the LLST, calculated ephemeris files for each ground terminal were provided weekly (at the start of each 4LDB) by the LADEE flight dynamics team and uploaded to the LLST. These files were used along with 1-second attitude updates from LADEE to point the LLST during lasercom operations. An inertial stabilization system provides local disturbance rejection. These systems facilitated a wide field-of-view (~ 2 mrad) quadrant detector to be used for uplink spatial acquisition as well as coarse tracking of the uplink beam. On the ground, pointing files calculated by the LADEE flight dynamics team were also distributed weekly to each location. The LLGT mount model was developed using star calibrations for known-bright stars. The LLGT pointing was checked with stars just prior to each lasercom operations pass. In addition, the uplink acquisition beacon from the ground terminal was diverged from diffraction-limited to increase the likelihood of hitting the LLST. The LLGT (and the other terminals) were also capable of scanning beams to cover a larger uncertainty. With these systems in place, we found that lasercom links between the LLST and LLGT always came up automatically and nearly instantaneously (other than the expected delay due to the round trip light propagation between the two terminals). In the future, lasercom systems can employ methods similar to those developed and shown during LLCD to alleviate any concerns about pointing narrow beams. IV.III. Predictive Avoidance and Aircraft Avoidance Unlike RF systems, lasercom frequencies are located in an unregulated part of the electromagnetic spectrum. However, laser transmission from the ground is regulated in the United States to avoid accidental irradiation of spacecraft or aircraft. The U.S. Air Force’s Laser Clearing House (LCH) regulates ground-based laser transmission by identifying Predictive Avoidance (PA) timing windows during which transmitted lasers could possibly damage sensitive spacecraft [12]. The U.S. Federal Aviation Agency (FAA) regulates potential laser interactions with aircraft. If the ground station is not placed in a no-fly zone, airplane sensors can be used during operations to help control laser shuttering for aircraft avoidance [13]. Since ground-based lasercom systems generally employ an uplink beacon laser to aid in acquisition between the space terminal and the ground terminal, spacecraft and aircraft avoidance are likely to be operational considerations. During LLCD, compatibility with LCH regulations was actively addressed by the LLGT and the LLOT. Both terminals were equipped with rules for dealing with PA events during lasercom link operations. Since the LLOT used the already established OCTL telescope, it had a LabView-based computer program which read the PA file and automatically shuttered the laser when needed. The LLGT employed a manual 2-person shutdown procedure based on PA times. During LLCD operations, LCH provided PA times 24 hours in advance for both LLGT and LLOT. LCH predicted PA events using a spatial window defined by the center Moon +/- a keep out zone of about 0.5o. Despite the large spatial window used for PA calculations, we found that PA events were quite short and infrequent. For the LLGT, we found the mean and median PA durations were 77 and 14 seconds, respectively. There were about 10 events per 24-hour day, on average. If a short PA event occurred during a lasercom operations pass, we were able to automatically reacquire and continue operations with the LLGT. For longer PA events, a mid-pass handover offered an operational solution although we did not encounter such an event during LLCD. For aircraft avoidance, LLCD employed two methods. First, there was no need for real-time aircraft avoidance at LLGT since it was in a no-fly zone. However, since the LLOT was not in a no-fly zone, it had a camera-based system for aircraft avoidance. As a side note, there was no requirement for either spacecraft or aircraft avoidance for the European Space Agency system, LLOGS. In the future, the FAA regulations may be relaxed for aircraft avoidance considerations since uplink beacon powers for nearer-Earth missions are likely to be quite small. Furthermore, as pointing capabilities of lasercom
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terminals are better understood by the LCH, they may reduce the size of their spatial windows used to calculate PA events. Since it is likely that some sort of disruption to operational lasercom links will always occur, solutions like automatic re-acquisition, mid-pass station handover, and delay / disruption tolerant networking can all be used to mitigate the issue. IV.IV. Staffing During LLCD, we found that it was possible to operate the entire system with a very small staff, even initially. Further, once our automated LLCD operations script was in place, there was very little to do in the LLOC. Initially, there were three official operations staff roles in the LLOC: the Lasercom Test Director (LTD), the Space Terminal Operator (STO), and the Ground Terminal Liaison (GTL). Unit engineers were kept on hand to quickly address any issues that arose. The primary responsibility of the LTD was to conduct tests as planned in a way that ensured LLCD mission objectives were met. In addition, the LTD made the weather-based ground terminal selection prior to each lasercom operations pass, as well as decided the power on/off times. The LTD monitored the energy consumption and dictated the power off time. The LTD also selected the downlink data rate based on weather, ground terminal capabilities, and elevation angle. The STO operated and monitored the overall health and safety of the LLST. The STO controlled the LLST via a GUI which consisted of local scripts that initiated multi-step macros residing on the LLST. The same GUI could be used to send commands to the LLST over RF or optical. The STO also implemented any data rate or other mode changes requested by the LTD. The GTL worked with the three ground terminals to ensure they were standing by and prepared for possible operations. Once the ground terminal was selected for a particular lasercom operations pass, the GTL was in charge of informing the ground terminal teams of everything that was occurring in the LLOC including power on, status, data rates, and power off. The GTL reported any relevant information back to the LLOC. By the third week of LLCD operations, the automated LLCD operations script was in place which automatically performed power on, pointing, acquisition, bi-directional lasercom link establishment, and power down of the LLST. At that point, we found that one person in the LLOC could easily monitor and report progress of the lasercom link on the voice loop. Each ground terminal was staffed as necessary to operate the telescopes and modems at that location. Ground terminals were nominally staffed whenever the moon was up at that location, unless weather or other events precluded such. For a future operational lasercom system, it could be envisioned that a single trained individual would be able to operate the entire link, including the ground terminal(s). The ground terminals could have on-site staff “on call” to perform any required on-site tasks such as cleaning telescopes or maintenance. It is also not inconceivable to run the entire system autonomously. V. CONCLUSIONS Lasercom offers enormous SWaP benefits along with significant improvements in data rates, both of which will be beneficial to future near-earth and deep-space missions. While the LLCD mission was quite short (16 Lunar days of operations spread over 1 month), all the major perceived hurdles of lasercom were surmounted including atmosphere and cloud issues, pointing and acquisition, and predictive avoidance. Furthermore, LLCD demonstrated a near turn-key system with no more than a handful of staff. After the huge success of the LLCD mission, operational lasercom is poised to become reality within the next decade. ACKNOWLEDGEMENTS The authors would like to thank the additional members of the LLCD Operations Team: Jamie Burnside, Pat Cable, Steve Constantine, Cathy DeVoe, Matt Grein, John Guineau, Robert Lafon, Jan Kansky, Dan Murphy, Bob Schulein, and Matt Willis. The authors would like to additionally thank: the LLOT team (Abi Biswas, Joe Kovalik, Malcolm Wright), the LLOGS team (Zoran Sodnik, Igor Zayer, Marco Lanucara), the LADEE Team from NASA ARC (Rusty Hunt, Mike Logan, Vanessa Kuroda, Rich Bielawski, Butler Hine, Howard Cannon, John Bresina, Brandon Owens, Rick Elphic, Mark Shirley, Matt Dortenzio, Ken Galal, Lisa Policastri), the NASA GSFC Team (Paul Swenson, Rich Hoffman, Andrew Menas, Tiffany Navas, Cory Heiges, Bob Caffrey, Bob Hanna, Jennifer Sager Cosgrove, Steve Kreisler, Steve Hillenius, Dave Israel, Greg Menke), our Mission Manager Don Cornwell, as well as the many others who spent late nights working on this system.
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REFERENCES [1] D. M. Boroson, B. S. Robinson, D.V. Murphy, D. A. Burianek, F. Khatri, J. M. Kovalik, Z. Sodnik, D. M. Cornwell, “Overview and Results of the Lunar Laser Communication Demonstration,” Proceedings of SPIE, v.8971, 2014. [2] B. S. Robinson, D. M. Boroson, D. M. Burianek, D. V. Murphy, F. I. Khatri, J. W. Burnside, J. E. Kansky, A. Biswas, Z. Sodnik, D. M. Cornwell, “The NASA Lunar Laser Communication Demonstration – Successful High-Rate Laser Communications to and from the Moon,” SpaceOps 2014. [3] K. E. Nevin, K. B. Doyle, and A. D. Pillsbury, “Optomechanical Design and Analysis for the LLCD Space Terminal Telescope,” Proceedings of SPIE, v. 8127, 2011. [4] J. W. Burnside, S. D. Conrad, A. D. Pillsbury, and C. E. DeVoe, “Design of an inertially stabilized telescope for the LLCD,” Proceedings of SPIE v. 7923, 2011. [5] D. V. Murphy, J. E. Kansky, M. E. Grein, R. T. Schulein, M. M. Willis, and R. E. Lafon, “LLCD Operations using the Lunar Lasercom Ground Terminal,” Proceedings of SPIE, v. 8971, 2014. [6] E. Grun, M. Horanyi, and Z. Sternovsky, “The Lunar Dust Environment,” Planetary and Space Science, v. 49, n. 14, pp. 1672-1680, 2011. [7] A. Biswas, J. M. Kovalik, M. W. Wright, W. T. Roberts, M. K. Cheng, K. J. Quirk, M. Srinivasan, M. D. Shaw, K. M. Birnbaum, “LLCD Operations using the Optical Communications Telescope Laboratory (OCTL),” Proceedings of SPIE, v. 8971, 2014 [8] Z. Sodnik, H. Smit, M. Sans, I. Zayer, M. Lanucara, I. Montilla, A. Alonso, “LLCD Operations using the Lunar Lasercom OGS Terminal,” Proceedings of SPIE, v. 8971, 2014. [9] M. E. Grein, A. J. Kerman, E. A. Dauler, O. Shatrovoy, R. J. Molnar, D. Rosenberg, J. Yoon, D. E. DeVoe, D. V. Murphy, B. S. Robinson, and D. M. Boroson, “Design of a Ground-based Optical Receiver for the Lunar Laser Communications Demonstration,” International Conference on Space Optical Systems and Applications, p. 78-82, 2011. [10] F. I. Khatri and A. Biswas, “Signal and Background Levels for the Mars Laser Communications Demonstration (MLCD),” 2005 Digest of the LEOS Summer Topical Meetings, p. 25-6, 2005. [11] D. J. Israel, F. Davis, and J. Marquart, “A DTN-based multiple access fast forward service for the NASA space network,” Proceedings of the 2011 IEEE International Conference on Space Mission Challenges for Information Technology, p. 61-5, 2001. [12] S. Alfano, R. Burns, D. Pohlen, and G. Wirsig, “Predictive Avoidance for Ground-Based Laser Illumination,” Journal of Spacecraft and Rockets, Vol. 37, No. 1 (2000). [13] A. Biswas, W. T. Roberts, J. M. Kovalik, and M. W. Wright, “OCTL Laser Beam Transmission Interruptions due to Aircraft and Predictive Avoidance,” IPN Progress Report 42-191 (2012). [14] R. Alliss and B. Felton, “The mitigation of Cloud Impacts on Free Space Optical Communications,” Proceedings of the SPIE, v. 8380, 2012.
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Lunar laser communication demonstration operations Architecture Farzana I. Khatri, Bryan S. Robinson, Marilyn D. Semprucci, Don M. Boroson
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Received date: 3 November 2014 Revised date: 6 January 2015 Accepted date: 30 January 2015 Cite this article as: Farzana I. Khatri, Bryan S. Robinson, Marilyn D. Semprucci, Don M. Boroson, Lunar laser communication demonstration operations Architecture, Acta Astronautica, http://dx.doi.org/10.1016/j.actaastro.2015.01.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
65th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by the International Astronautical Federation. All rights reserved.
Lunar Laser Communication Demonstration Operations Architecture Farzana I. Khatri, Bryan S. Robinson, Marilyn D. Semprucci, and Don M. Boroson MIT Lincoln Laboratory, U.S.A. Email: [email protected] Abstract Radio waves have been the standard method for deep-space communications since the earliest days of space exploration. However, the recent success of the Lunar Laser Communications Demonstration (LLCD) program will clearly revolutionize the way data is sent and received from deep space. LLCD demonstrated record-breaking optical up/downlinks between Earth and the Lunar Lasercom Space Terminal (LLST) payload on NASA’s Lunar Atmosphere Environment Explorer (LADEE) satellite orbiting the Moon. A space-to-ground optical downlink as fast as 622 Mbps was demonstrated as well as a ground-to-space uplink as fast as 20 Mbps. The LLCD operations architecture was designed to support a wide range of operations conditions, multiple ground terminals with varying designs and capabilities, short contact times including energy and thermal constraints, and limited viewing opportunities. This paper will explore the operations architecture used for the LLCD as well as present ideas on how best to make future laser communications operations routine and suitable for wide-scale deployment.
Highlights •
Lasercom offers enormous SWaP benefits along with data rate improvements which will benefit future space missions.
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LLCD demonstrated the potential for a future operational lasercom system.
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Operational lasercom is poised to become a reality within the next decade.
Keywords Lasercom; Operations; LADEE; LLCD I. INTRODUCTION NASA has been developing free space laser communications (lasercom) technologies to support the high data rates required for future deep-space science, exploration, and human missions. Free space lasercom can potentially offer a tremendous Size, Weight, and Power (SWaP) advantage over Radio Frequency (RF) systems due to its short wavelength. A major first step towards an operational deep-space lasercom system occurred during mid-October to mid-November 2013 when NASA’s Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time a duplex lasercom link between a satellite in lunar orbit and multiple ground stations on Earth [1][2]. The LLCD system consisted of a space terminal, the Lunar Lasercom Space Terminal (LLST) [3][4], and a primary ground terminal, the Lunar Lasercom Ground Terminal (LLGT) [5], a transportable system stationed at White Sands, NM for the mission. The space terminal was a payload on the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite [6]. Additional ground stations at Table Mountain, CA [7] and Tenerife, Canary Islands, Spain [8] were also employed during the demonstration. The entire demonstration was coordinated This work is sponsored by National Aeronautics and Space Administration under Air Force Contract #FA872105-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.
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and run from the Lunar Lasercom Operations Center (LLOC) at MIT Lincoln Laboratory in Lexington, MA. The LLST, LLGT, LLOC, and overall LLCD system were all designed, built, and operated by teams from the MIT Lincoln Laboratory. The LLCD program was overseen by NASA Goddard Space Flight Center, and the LADEE satellite was designed, built, and operated by the NASA Ames Research Center (ARC). In this paper, we briefly review the LLCD system and goals; we will then examine operational considerations that have been traditionally perceived as concerns for lasercom systems. The LLCD operations architecture provided solutions to these concerns and we will describe those solutions. In addition, we will discuss ideas on how best to make lasercom an operational reality for the future. II. SYSTEM OVERVIEW The high level goals for the LLCD technology demonstration were to help make lasercom a reality for future NASA science and exploration missions, to perform a detailed, integrated design for a capable end-to-end system, and to demonstrate many of the major functions required by future operational lasercom missions. The system requirements for the program were to reliably demonstrate: pointing, acquisition, and tracking with narrow optical beams; up to 622 Mbps optical downlink and up to 20 Mbps optical uplink, the sending of uplink optical commands and downlinking of telemetry plus science data. These objectives were to be accomplished during the variety of atmospheric and link geometry conditions that occurred over the duration of the mission. LLCD was highly successful and achieved all its goals, including demonstrating the major functions required for a future operational system. In this section, we will briefly review the major sub-systems involved in the demonstration. II.I. Lunar Lasercom Space Terminal (LLST) The Lunar Lasercom Space Terminal (LLST) as installed on the LADEE satellite is shown in Figure 1. The LLST consisted of three parts: the Optical Module (OM), the Modem Module (MM), and the Controller Electronics (CE). The OM consisted of a 10-cm reflective telescope that produced a nearly diffraction-limited 15-µm beam. It also contained a wide field-of-view acquisition sensor that detected an optical uplink beacon. The OM was connected to the MM via two optical fibers. The first fiber was the transmit fiber which contained the 0.5-Watt downlink beam, modulated using a 16-ary Pulse Position Modulated (PPM) waveform with selectable data rates from 39 Mbps to 622 Mbps. The second fiber was the uplink receive fiber which sent the received optical signal to a receiver which demodulated the 4-ary PPM uplink at either 10 or 20 Mbps.
Figure 1: The Lunar Lasercom Space Terminal (LLST) installed on the LADEE satellite. The control and telemetry/command interfaces for the OM and MM were provided by the CE. There was also a 40 Mbps data interface between the LADEE data buffer and the downlink side of the MM, as well as data connections from the MM to the CE. II.II. LLCD Ground Terminals The LLCD program originally employed a single ground station, built specifically for LLCD by MIT Lincoln Laboratory (the LLGT) [5]. However, given the limited planned operations schedule (~16 days during one month during which to successfully demonstrate LLCD) two additional ground terminals were added late in the program as
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alternate terminals. The additional ground stations increased operations time as well as provided a backup in the case of heavy cloud cover at the primary terminal. Both alternate terminals were existing lasercom terminals that were retrofitted to be compatible with LLCD. The ground station at Table Mountain, the Lunar Lasercom OCTL Terminal (LLOT) [7], was designed and operated by NASA’s Jet Propulsion Laboratory (JPL). The ground station at Tenerife, the Lunar Lasercom Optical Ground Station (LLOGS) [8], was designed and operated by the European Space Agency (ESA). The three ground terminals will be described briefly in this section. II.II.I. Lunar Lasercom Ground Terminal (LLGT)
Figure 2: The Lunar Lasercom Ground Terminal (LLGT) at White Sands, NM. The LLGT dome is 4-m tall. Also shown in the background are 19-m RF dishes used for NASA’s Tracking and Data Relay System (TDRS). Note the TDRS dishes are not part of the system and are some distance away. LLCD’s primary ground terminal, the LLGT, is shown in Figure 2 [5]. The LLGT was designed and built at MIT Lincoln Laboratory in Lexington, MA. It was designed to be transportable; the LLGT was transported, re-assembled, and tested once it was moved to its final location at White Sands, NM. The LLGT consisted of four commercial 40cm telescopes to receive the downlink communications at 39, 78, 155, 311, and 622 Mbps. The ground receiver was custom-built at MIT Lincoln Laboratory and consisted of four superconducting nanowire quad detector arrays [9]. The ground electronics were capable of real-time data transmission and delivery. The LLGT also housed four custom-built 15-cm telescopes to transmit the uplink beacon and 10- or 20-Mbps uplink communications signals. All the telescopes were mounted on a single gimbal housed in an environmentallycontrolled enclosure. Adjacent to this enclosure was a 40-ft trailer containing the local operations center, electronics, and opto-electronics. The LLGT operations trailer was connected via a 40 Mbps ground data network (T3 line) directly to the Lunar Lasercom Operations Center (LLOC) in Lexington, MA. During operations, there were two engineers staffed at the LLGT. II.II.II. Lunar Lasercom OCTL Terminal (LLOT) NASA Jet Propulsion Lab’s (JPL) Optical Communications Telescope Laboratory (OCTL), located on Table Mountain near Wrightwood, CA was one of two secondary ground terminals for LLCD [7]. This terminal consisted of a 1-m telescope for transmit/receive. The ground receiver was a tungsten silicide (WSi) superconducting nanowire single photon detector (SNSPD) array and supported 39 and 78 Mbps downlink communications rates. A softwarebased receiver was used to process the downlink communication data after the lasercom operations pass was over. The LLOT transmitted an uplink beacon for acquisition, but uplink communications was not implemented. II.II.III. Lunar Lasercom Optical Ground Station (LLOGS) The European Space Agency (ESA)’s Optical Ground Station (OGS) located at the Observatorio del Teide on Tenerife, Canary Islands, Spain was the other secondary ground terminal for LLCD [8]. This terminal consisted of a 1-m telescope for the downlink receiver with three individual 40 mm telescopes for the uplink beacon and communications transmitter. The primary ground receiver was a commercial photon counting detector from Hamamatsu which supported 39 Mbps downlink rate. A software-based receiver was used to post-process the downlink communication signal.
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II.III. Lunar Lasercom Operations Center (LLOC)
Figure 3:: Lunar Lasercom Operations Center view of space terminal control and visualization portion of LLOC.
Figure 4:: Block diagram of LLCD system architecture. LLOGS=Lunar Lasercom Optical Ground System, LLOT=Lunar Lasercom OCTL Terminal, LLGT=Lunar Lasercom Ground Terminal, LLOC=Lunar Lasercom Operations Center, DS DSN=Deep N=Deep Space Network, MOC=Mission Operations Center, SOC=Science Operations Center, ESA=European Space Agency, JPL=NASA Jet Propulsion Laboratory, MITLL=MIT Lincoln Laboratory, ARC=Ames Research Center, GSFC=NASA Goddard Space Flight Center. A view of the Lunar Lasercom Operations Center (LLOC) is shown in Figure 3.The .The signal, command, and telemetry flow from/to the LLOC is shown in Figure 4.. The LLOC had a direct duplex data connection from the LLGT facility at White Sands to enable high rate (optical) data forwarding and reception. There was also a lower rate connection to the Science ience Operations Center (SOC) at NASA GSFC through which LADEE spacecraft telemetry was received and LLST commands/telemetry were sent/received. Nearly full command and control of the LLGT was possible from the LLOC. Real-time time telemetry from all three gro ground und terminals was processed and displayed in the LLOC. Verbal information channels called “voice loops” were used in the LLOC to communicate real-time real by voice to/from the Mission Operations Center (MOC), SOC, and all three ground terminals. All telemetry that came into the LLOC as well as all commands that went out of the LLOC were archived. III. LLCD OPERATIONS PLANNING AND ACTIVIT ACTIVITIES The 16 days of LLCD operations were spread over one month by breaking them up into four “Four Lunar Day Blocks” (4LDB). Each 4LDB consisted of four Lunar days (defined roughly as the period of time when the Moon was visible at all 3 ground terminals) where each successive day started about 45 minutes later due to the shift in Moonrise time. Throughout each Lunar day during the 4LDB, 88-10 10 of the ~ 2 hour LADEE passes were dedicated to LLCD operations. The LADEE MOC and SOC were staffed to support LLCD for 19 19-hours hours during each Lunar day to allow for operations at all three ground terminals. The four primary 4LDB’s for LLCD took place over a 5-week 5
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period during October and November 2013, specifically during: October 17-21, October 26-29, November 2-5, and November 17-19.
Figure 5: Output of LLCD Operations planning tool showing 4-Lunar Day Block, Days vs. Moon Elevation. LLOGS=Lunar Lasercom Optical Ground System, LLGT=Lunar Lasercom Ground Terminal, LLOT=Lunar Lasercom OCTL Terminal. Block planning for the LLCD 4LDB occurred during the week prior to the first 4LDB and subsequently during the off times, between the 4LDBs. Several days prior to the start of the 4LDB, the LADEE planning team released a proposed plan for the 4LDB. Given the proposed spacecraft activities, the LLCD team determined how to best accomplish their goals for the upcoming block. A visual MATLAB-based LLCD planning tool was developed to aid in block planning. A sample output of the tool is shown in Figure 5. The tool output shows the satellite (Moon) elevation angle as a function of days for each of the three ground terminals as well as indications of day or night operation. Considerations that were taken into account during block planning were: ground terminal visibility and elevation angles, Sun angles, and space terminal activities. Once agreement was reached, the activity plan was finalized by the LADEE planning team and prepared for upload to the spacecraft at the start of the 4LDB. Day planning activities took place about 10-12 hours prior to each LLCD operations day. During this time, primary/backup ground terminals were selected for each lasercom operations pass where more than one ground terminal was visible. A notional timeline of a typical LLCD operations pass with the LLGT terminal is shown in Table 1. This table gives a sense of the major activities taking place when establishing the lasercom link. The LLCD system was able to operate for up to 30-40 minutes per 2-hour LADEE pass; the exact operating time was determined by considerations such as thermal issues, energy and battery conditions (depending on Sun illumination), and LLST operating modes used during the lasercom operations pass. Table 1: Notional LLCD operations timeline for a lasercom operations pass. T is the power on request time of the LLST. Time (min) Location Activity T-30 Primary/backup ground Star calibration of telescope, warm up of all terminals equipment and lasers T-15 LLOC Weather-based selection of ground terminal T-5 Primary ground Uplink beacon powered on and pointed at terminal (LLGT) LADEE T LLOC Power on sequence requested for LLST’s CE and MM begins T+1 LLST CE powered on, LLST telemetry begins to flow, gimbal begins to slew to point at LLGT T+1.1 LLST MM powered on T+1.5 LLST Beacon uplink from LLGT acquired T+2 LLST MM programming complete T+2.1 LLST/LLGT Error free bi-directional link established T+29 LLOC Power down sequence requested for LLST’s CE and MM T+30 LLOC Power down complete, LLST telemetry ceases Note that during the third week of the mission, an automated LLCD operations script was implemented and employed. This script was programmed to run at a specified time during the 4LDB and performed power on,
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pointing, acquisition, bi-directional lasercom link establishment, and power down. During passes that used the automated script, the only manual step required in the sequence above was the calibration of the ground telescope. IV. LASERCOM OPERATIONAL CONSIDERATIONS, LLCD SOLUTIONS, AND FUTURE CONSIDERATIONS The high-level goal of LLCD was to prove out a capable end-to-end integrated lasercom system design. In order to meet this goal, a key objective was to show that LLCD could operate reliably and easily under various conditions. Any communications system, RF or optical, has operational considerations. For example, all near-earth and deepspace links are governed by orbital positions of the transmitter(s) and receiver(s) involved. There are several operational considerations which have been traditionally perceived as concerns for lasercom systems which the LLCD aimed to address during operations. These operational considerations, how they were addressed by the LLCD, as well as future considerations are discussed in this section. IV.I. Earth’s Atmosphere and Clouds If ground-based receivers are employed in a lasercom system (as they were in LLCD), dealing with the Earth’s atmosphere becomes an operational consideration. Atmospheric effects can be separated by whether or not a CloudFree Line of Sight (CFLOS) is available. Since lasercom does not work without CFLOS, cloud blockage is treated as a separate consideration. IV.I.I. Earth’s Atmosphere with CFLOS Not unlike an RF communications system, an operational lasercom system must be able to operate with variations in received power. Received power variations are caused by orbital positions of the transmitter and receiver. For any Earth-based terminals, the received power variations are also caused by the atmosphere. Two key causes of receive power variation are atmospheric absorption and turbulence [10]. Both these effects vary with elevation angle and weather conditions.
Figure 6: A typical model prediction of one month of uplink fiber power received at LLST for various (best, nominal, and worst) atmospheric conditions at LLGT is shown. During the design phase of LLCD, before any hardware was built, we developed a detailed model of the lasercom link. Figure 6 shows a prediction we made of uplink communications power received in LLST’s optical fiber over the course of a typical month of operations, assuming a CFLOS. The analysis methodology we used was similar in to what was discussed in Ref [10]. Best, nominal, and worst atmospheric conditions (shown by the different colors of the plot in Figure 6) used for the LLGT took into account atmospheric absorption, turbulence, and orbital positions of the terminals. We found the power variation at the receive fiber to be between -65 and -60 dBm (1 dBm = 1 mW optical power). Figure 7 shows actual data we took for a few minutes of one LLST-LLGT lasercom operations pass on October 19, 2013. These results were typical of the lasercom links established during LLCD. The fiber power (red trace) falls between -65 and -60 dBm, just as we predicted. Not unlike an RF communications system, the LLCD could accommodate power variations by allowing for variable data rates. As mentioned earlier, the LLGT-LLST uplink could operate at either 10 or 20 Mbps to accommodate variation in the uplink receive power. Similarly, the LLST-LLGT downlink could operate at any of 39, 78, 155, 311, and 622 Mbps. The lower data rates could also be used in the event of thin cirrus clouds which can further attenuate optical signals.
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A future operational lasercom system will likely require a “knob” on data rate (like LLCD) to accommodate different operating conditions. To make the system even more robust, a software control system to optimize the data rate based on received signal and noise levels could be implemented.
Figure 7: Uplink power transmitted from LLGT measured at LLCD for 1 pass. The red trace is the fiber received power which is used for uplink communications. The blue trace is the acquisition detector power. IV.I.II. Clouds Since clouds are unavoidable for an Earth-based ground receiver, being able to predict and address cloud-related issues in real-time is an important aspect of an operational lasercom system. For LLCD, we used two main methods to assess ground terminal ground cloud cover. The first tool we used was developed and implemented by Northrop-Grumman to predict and evaluate cloud cover for the ground terminals [14]. Second, for each lasercom operations pass, we selected a primary and backup ground terminal. If there was thick cloud cover at the primary ground station, the backup ground station was selected. This decision was made in about 15 minutes prior to the start of the lasercom operations pass, based on real-time sky camera views (see Figure 8). Once the ground terminal selection was made, we had two additional methods to address LLCD operations in a cloudy environment. For intermittent clouds during a lasercom operations pass, we demonstrated use of a delay or disruption tolerant network [11]. The second approach demonstrated during LLCD, which works well if the ground station in use suddenly becomes clouded during a lasercom operations pass, is a mid-pass station handover.
Figure 8: All-sky camera views from LLGT at White Sands, NM. Left side shows CFLOS, right side shows clouded over. Any operational lasercom system of the future with Earth-based terminals will be required to address the cloud issue. If the mission requires real-time data downlinks, multiple ground stations can be used. The space terminal can then automatically attempt acquisition with all ground stations in view until a link is established. If the link goes down mid-pass, the system could move on to another ground station for a mid-pass handover. Ground stations can be automated to stand down in the event of a clouded out sky camera. A system to provide continuous downlink data relay throughout the process would need to be developed. For a mission that does not require real-time data
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downloads, the delay/disruption tolerant network can be employed. This ensures the data will eventually be downlinked. IV.II. Pointing Narrow Beams Lasercom systems are incredibly efficient when it comes to delivering power. The advantage gained from this is very low SWaP terminals; this comes at the cost of pointing and stabilizing the very narrow beam. Beams for lasercom systems can be as small as a few microradians (depending on the aperture’s size) and, hence, are more difficult to point than RF communication beams. Typical spacecraft pointing using star trackers is on the order of a few milliradians, which is adequate for RF, but not quite good enough to quickly and blindly point an optical beam to a ground station. To facilitate pointing for narrow beams and expedite the acquisition process, LLCD employed a few simple techniques described here. On the LLST, calculated ephemeris files for each ground terminal were provided weekly (at the start of each 4LDB) by the LADEE flight dynamics team and uploaded to the LLST. These files were used along with 1-second attitude updates from LADEE to point the LLST during lasercom operations. An inertial stabilization system provides local disturbance rejection. These systems facilitated a wide field-of-view (~ 2 mrad) quadrant detector to be used for uplink spatial acquisition as well as coarse tracking of the uplink beam. On the ground, pointing files calculated by the LADEE flight dynamics team were also distributed weekly to each location. The LLGT mount model was developed using star calibrations for known-bright stars. The LLGT pointing was checked with stars just prior to each lasercom operations pass. In addition, the uplink acquisition beacon from the ground terminal was diverged from diffraction-limited to increase the likelihood of hitting the LLST. The LLGT (and the other terminals) were also capable of scanning beams to cover a larger uncertainty. With these systems in place, we found that lasercom links between the LLST and LLGT always came up automatically and nearly instantaneously (other than the expected delay due to the round trip light propagation between the two terminals). In the future, lasercom systems can employ methods similar to those developed and shown during LLCD to alleviate any concerns about pointing narrow beams. IV.III. Predictive Avoidance and Aircraft Avoidance Unlike RF systems, lasercom frequencies are located in an unregulated part of the electromagnetic spectrum. However, laser transmission from the ground is regulated in the United States to avoid accidental irradiation of spacecraft or aircraft. The U.S. Air Force’s Laser Clearing House (LCH) regulates ground-based laser transmission by identifying Predictive Avoidance (PA) timing windows during which transmitted lasers could possibly damage sensitive spacecraft [12]. The U.S. Federal Aviation Agency (FAA) regulates potential laser interactions with aircraft. If the ground station is not placed in a no-fly zone, airplane sensors can be used during operations to help control laser shuttering for aircraft avoidance [13]. Since ground-based lasercom systems generally employ an uplink beacon laser to aid in acquisition between the space terminal and the ground terminal, spacecraft and aircraft avoidance are likely to be operational considerations. During LLCD, compatibility with LCH regulations was actively addressed by the LLGT and the LLOT. Both terminals were equipped with rules for dealing with PA events during lasercom link operations. Since the LLOT used the already established OCTL telescope, it had a LabView-based computer program which read the PA file and automatically shuttered the laser when needed. The LLGT employed a manual 2-person shutdown procedure based on PA times. During LLCD operations, LCH provided PA times 24 hours in advance for both LLGT and LLOT. LCH predicted PA events using a spatial window defined by the center Moon +/- a keep out zone of about 0.5o. Despite the large spatial window used for PA calculations, we found that PA events were quite short and infrequent. For the LLGT, we found the mean and median PA durations were 77 and 14 seconds, respectively. There were about 10 events per 24-hour day, on average. If a short PA event occurred during a lasercom operations pass, we were able to automatically reacquire and continue operations with the LLGT. For longer PA events, a mid-pass handover offered an operational solution although we did not encounter such an event during LLCD. For aircraft avoidance, LLCD employed two methods. First, there was no need for real-time aircraft avoidance at LLGT since it was in a no-fly zone. However, since the LLOT was not in a no-fly zone, it had a camera-based system for aircraft avoidance. As a side note, there was no requirement for either spacecraft or aircraft avoidance for the European Space Agency system, LLOGS. In the future, the FAA regulations may be relaxed for aircraft avoidance considerations since uplink beacon powers for nearer-Earth missions are likely to be quite small. Furthermore, as pointing capabilities of lasercom
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terminals are better understood by the LCH, they may reduce the size of their spatial windows used to calculate PA events. Since it is likely that some sort of disruption to operational lasercom links will always occur, solutions like automatic re-acquisition, mid-pass station handover, and delay / disruption tolerant networking can all be used to mitigate the issue. IV.IV. Staffing During LLCD, we found that it was possible to operate the entire system with a very small staff, even initially. Further, once our automated LLCD operations script was in place, there was very little to do in the LLOC. Initially, there were three official operations staff roles in the LLOC: the Lasercom Test Director (LTD), the Space Terminal Operator (STO), and the Ground Terminal Liaison (GTL). Unit engineers were kept on hand to quickly address any issues that arose. The primary responsibility of the LTD was to conduct tests as planned in a way that ensured LLCD mission objectives were met. In addition, the LTD made the weather-based ground terminal selection prior to each lasercom operations pass, as well as decided the power on/off times. The LTD monitored the energy consumption and dictated the power off time. The LTD also selected the downlink data rate based on weather, ground terminal capabilities, and elevation angle. The STO operated and monitored the overall health and safety of the LLST. The STO controlled the LLST via a GUI which consisted of local scripts that initiated multi-step macros residing on the LLST. The same GUI could be used to send commands to the LLST over RF or optical. The STO also implemented any data rate or other mode changes requested by the LTD. The GTL worked with the three ground terminals to ensure they were standing by and prepared for possible operations. Once the ground terminal was selected for a particular lasercom operations pass, the GTL was in charge of informing the ground terminal teams of everything that was occurring in the LLOC including power on, status, data rates, and power off. The GTL reported any relevant information back to the LLOC. By the third week of LLCD operations, the automated LLCD operations script was in place which automatically performed power on, pointing, acquisition, bi-directional lasercom link establishment, and power down of the LLST. At that point, we found that one person in the LLOC could easily monitor and report progress of the lasercom link on the voice loop. Each ground terminal was staffed as necessary to operate the telescopes and modems at that location. Ground terminals were nominally staffed whenever the moon was up at that location, unless weather or other events precluded such. For a future operational lasercom system, it could be envisioned that a single trained individual would be able to operate the entire link, including the ground terminal(s). The ground terminals could have on-site staff “on call” to perform any required on-site tasks such as cleaning telescopes or maintenance. It is also not inconceivable to run the entire system autonomously. V. CONCLUSIONS Lasercom offers enormous SWaP benefits along with significant improvements in data rates, both of which will be beneficial to future near-earth and deep-space missions. While the LLCD mission was quite short (16 Lunar days of operations spread over 1 month), all the major perceived hurdles of lasercom were surmounted including atmosphere and cloud issues, pointing and acquisition, and predictive avoidance. Furthermore, LLCD demonstrated a near turn-key system with no more than a handful of staff. After the huge success of the LLCD mission, operational lasercom is poised to become reality within the next decade. ACKNOWLEDGEMENTS The authors would like to thank the additional members of the LLCD Operations Team: Jamie Burnside, Pat Cable, Steve Constantine, Cathy DeVoe, Matt Grein, John Guineau, Robert Lafon, Jan Kansky, Dan Murphy, Bob Schulein, and Matt Willis. The authors would like to additionally thank: the LLOT team (Abi Biswas, Joe Kovalik, Malcolm Wright), the LLOGS team (Zoran Sodnik, Igor Zayer, Marco Lanucara), the LADEE Team from NASA ARC (Rusty Hunt, Mike Logan, Vanessa Kuroda, Rich Bielawski, Butler Hine, Howard Cannon, John Bresina, Brandon Owens, Rick Elphic, Mark Shirley, Matt Dortenzio, Ken Galal, Lisa Policastri), the NASA GSFC Team (Paul Swenson, Rich Hoffman, Andrew Menas, Tiffany Navas, Cory Heiges, Bob Caffrey, Bob Hanna, Jennifer Sager Cosgrove, Steve Kreisler, Steve Hillenius, Dave Israel, Greg Menke), our Mission Manager Don Cornwell, as well as the many others who spent late nights working on this system.
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REFERENCES [1] D. M. Boroson, B. S. Robinson, D.V. Murphy, D. A. Burianek, F. Khatri, J. M. Kovalik, Z. Sodnik, D. M. Cornwell, “Overview and Results of the Lunar Laser Communication Demonstration,” Proceedings of SPIE, v.8971, 2014. [2] B. S. Robinson, D. M. Boroson, D. M. Burianek, D. V. Murphy, F. I. Khatri, J. W. Burnside, J. E. Kansky, A. Biswas, Z. Sodnik, D. M. Cornwell, “The NASA Lunar Laser Communication Demonstration – Successful High-Rate Laser Communications to and from the Moon,” SpaceOps 2014. [3] K. E. Nevin, K. B. Doyle, and A. D. Pillsbury, “Optomechanical Design and Analysis for the LLCD Space Terminal Telescope,” Proceedings of SPIE, v. 8127, 2011. [4] J. W. Burnside, S. D. Conrad, A. D. Pillsbury, and C. E. DeVoe, “Design of an inertially stabilized telescope for the LLCD,” Proceedings of SPIE v. 7923, 2011. [5] D. V. Murphy, J. E. Kansky, M. E. Grein, R. T. Schulein, M. M. Willis, and R. E. Lafon, “LLCD Operations using the Lunar Lasercom Ground Terminal,” Proceedings of SPIE, v. 8971, 2014. [6] E. Grun, M. Horanyi, and Z. Sternovsky, “The Lunar Dust Environment,” Planetary and Space Science, v. 49, n. 14, pp. 1672-1680, 2011. [7] A. Biswas, J. M. Kovalik, M. W. Wright, W. T. Roberts, M. K. Cheng, K. J. Quirk, M. Srinivasan, M. D. Shaw, K. M. Birnbaum, “LLCD Operations using the Optical Communications Telescope Laboratory (OCTL),” Proceedings of SPIE, v. 8971, 2014 [8] Z. Sodnik, H. Smit, M. Sans, I. Zayer, M. Lanucara, I. Montilla, A. Alonso, “LLCD Operations using the Lunar Lasercom OGS Terminal,” Proceedings of SPIE, v. 8971, 2014. [9] M. E. Grein, A. J. Kerman, E. A. Dauler, O. Shatrovoy, R. J. Molnar, D. Rosenberg, J. Yoon, D. E. DeVoe, D. V. Murphy, B. S. Robinson, and D. M. Boroson, “Design of a Ground-based Optical Receiver for the Lunar Laser Communications Demonstration,” International Conference on Space Optical Systems and Applications, p. 78-82, 2011. [10] F. I. Khatri and A. Biswas, “Signal and Background Levels for the Mars Laser Communications Demonstration (MLCD),” 2005 Digest of the LEOS Summer Topical Meetings, p. 25-6, 2005. [11] D. J. Israel, F. Davis, and J. Marquart, “A DTN-based multiple access fast forward service for the NASA space network,” Proceedings of the 2011 IEEE International Conference on Space Mission Challenges for Information Technology, p. 61-5, 2001. [12] S. Alfano, R. Burns, D. Pohlen, and G. Wirsig, “Predictive Avoidance for Ground-Based Laser Illumination,” Journal of Spacecraft and Rockets, Vol. 37, No. 1 (2000). [13] A. Biswas, W. T. Roberts, J. M. Kovalik, and M. W. Wright, “OCTL Laser Beam Transmission Interruptions due to Aircraft and Predictive Avoidance,” IPN Progress Report 42-191 (2012). [14] R. Alliss and B. Felton, “The mitigation of Cloud Impacts on Free Space Optical Communications,” Proceedings of the SPIE, v. 8380, 2012.
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