LADEE (Lunar Atmosphere and Dust Environment Explorer)
LADEE is a lunar science orbiter mission under development at NASA to address the goals of the NRC (National Research Council) decadal surveys and the SCEM (Scientific Context for Exploration of the Moon) report to study the pristine state of the lunar atmosphere and dust environment prior to significant human activities.
The goal of the LADEE mission is to determine the composition of the lunar atmosphere and investigate the processes that control its distribution and variability, including sources, sinks, and surface interactions. LADEE will also determine whether dust is present in the lunar exosphere, and reveal the processes that contribute to its sources and variability. These investigations are relevant in the understanding of surface boundary exospheres and dust processes throughout the solar system, address questions regarding the origin and evolution of lunar volatiles, and have potential implications for future exploration activities. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
The top-level programmatic and science requirements for the LADEE project are designed to accomplish the following mission objectives:
• Determine the composition of the lunar atmosphere and investigate the processes that control its distribution and variability, including sources, sinks, and surface interactions.
• Characterize the lunar exospheric dust environment and measure any spatial and temporal variability and impacts on the lunar atmosphere
• Demonstrate that Lunar Laser Com Demonstration (LLCD) can operate at high data rates from lunar distances
• Create a low-cost reusable spacecraft architecture that can meet the needs of certain planetary science missions
• Demonstrate the capability of the Minotaur V as a launch vehicle for planetary missions.
- On August 2, 2011, the LADEE project passed the MCDR (Mission Critical Design Review). 11)
- In November 2012, NASA/ARC completed the initial electromagnetic interference tests of LADEE. 12)
- In June 2013, the LADEE spacecraft arrived at NASA's Wallops Flight Facility to begin final processing for its trip to the moon. 13)
The LADEE spacecraft bus design was derived from the MCSB (Modular Common Spacecraft Bus), architecture developed at NASA/ARC (Ames Research Center) from 2006-2008. The MCSB is a small, low-cost spacecraft designed to deliver scientifically and technically useful payloads to a variety of locations, including LEO (Low Earth Orbit), lunar orbit and lunar surface, Earth-Moon Lagrange points, and NEOs (Near Earth Objects). 14)
The spacecraft bus is a lightweight carbon composite structure designed to accommodate launch loads and provide attenuation of impact loads. It is also designed for ease of manufacturing and assembly. The modularity of the design is intended not only for multiple mission configurations but also parallelism in development and assembly. The system-level components were drawn from low-cost flight-proven product lines.
For LADEE, the spacecraft bus modules consist of (Figure 1): (1) the Radiator Module, which carries the avionics, electrical system, and attitude sensors, (2) the Bus Module, (3) the Payload Module, which carries the two largest instruments, (4) the Extension Module, which houses the propulsion system, and (5) the Propulsion Module.
Figure 1: Illustration of the LADEE orbiter (right) and the bus modules (left), image credit: NASA/ARC
Figure 2: Top view of the radiator assembly (image credit: NASA/ARC)
Figure 3: Bottom view of the radiator assembly (image credit: NASA/ARC)
One prominent characteristic of the bus design is that the solar arrays are body-mounted and fixed. While this arrangement reduces the available power, it eliminates deployment and articulation mechanisms, which eliminates several failure modes. It also ensures power production in almost any attitude, which enables very robust safe modes. This process also allows spacecraft attitude to be used for thermal control, and eliminates a hot and a cold side, which minimizes the need for heater power. The spacecraft can be flown in either a spinner or 3-axis control mode.
Figure 4 shows the LADEE system block diagram. The flight avionics packages consists of a commercially available 8-slot 3U cPCI (Compact Peripheral Component Interface) integrated avionics system providing the following functions:
• Command & Data Handling avionics
• Power distribution
• Solar array and battery charge management
• Pyrotechnic actuation.
Propulsion system: The bipropellant propulsion system consists of a main thruster, six attitude control thrusters, two fuel tanks, two oxidizer tanks, two pressurant tanks, an ordnance valve driver box, and associated tubing and cabling. A separate electronics box in the Propulsion Module handles the valve driver actuation. - The propulsion system is being developed at SS/L (Space Systems/Loral) of Palo Alto, CA. NASA awarded a contract to SS/L in December 2009. 15)
The ACS (Attitude Control Subsystem) includes six 22 N thrusters using MMH (Monomethylhydrazine) propellant, with MON-3 oxidizer. The 6 thrusters are mounted in two canted pairs beneath the spacecraft’s lower deck.
EPS (Electrical Power Subsystem): The EPS consists of an array of body-fixed solar panels, connected to batteries through the solar array control card within the avionics chassis. The body-fixed array design minimizes articulation on the spacecraft bus.
Figure 4: Functional block diagram of the LADEE spacecraft (image credit: NASA/ARC)
Figure 5: Configuration of the payload module (image credit: NASA/ARC)
RF communications: Use of a modular design. The radio has separate receiver, transmitter and IHPA modules and produces 5 W of RF transmitter power with flexible transmission power modes. An omnidirectional/medium gain antenna design of ARC is used to achieve omni-directional coverage with a smaller area of medium gain response.
The LADEE spacecraft has a launch mass of ~ 383 kg, the bus diameter is 117 cm, power of ~ 295 W and a mission duration of ~160 days (30 days to travel to the moon, 30 days for checkout and 100 days for science operations).
Figure 6: Photo of the integrated LADEE spacecraft at NASA/ARC (image credit: NASA, Ref. 7)
Figure 7: LLCD (Lunar Laser Communication Demonstration) components integrated onto the LADEE spacecraft (image credit: NASA) 16)
Launch: The LADEE spacecraft was launched on September 7, 2013 (UTC 3:37:00) on a Minotaur-5 vehicle of OSC (a 5-stage converted Peacekeeper missile). This represented the maiden flight of the Minotaur-V rocket. The launch site was the commercial MARS (Mid-Atlantic Regional Spaceport) facility on Wallops Island, VA. 17) 18) 19)
Lunar orbit: Low-altitude (50 km) retrograde equatorial orbit.
LADEE will spend ~60 days reaching nominal lunar orbit and checking out systems before its 100 day science mission starts (it will take 30 days to enter lunar orbit). The nominal science orbit will be a near-circular (about 50 km) retrograde equatorial orbit with a period of 113 minutes. The periselene will be over the sunrise terminator. 20)
Figure 8: Illustration of the LADEE mission phases (image credit: NASA, Ref. 7)
Figure 9: Artist's rendition of of the LADEE spacecraft in lunar orbit (image credit: NSAS/ARC)
• January 31, 2014: NASA's LADEE observatory has been approved for a 28-day mission extension. The spacecraft is now expected to impact the lunar surface on or around April 21, 2014, depending on the final trajectory. The extension provides an opportunity for the satellite to gather an additional full lunar cycle worth of very low-altitude data to help scientists unravel the mysteries of the moon’s atmosphere. 21)
• On January 15, LROC (Lunar Reconnaissance Orbiter Camera) of orbital colleague LRO (Lunar Reconnaissance Orbiter) snapped a picture of LADEE. LADEE is in an equatorial orbit (east-to-west) while LRO is in a polar orbit (south-to-north). The two spacecraft are occasionally very close and on Jan. 15, 2014, the two came within 9 km of each other. As LROC is a pushbroom imager, it builds up an image one line at a time, so catching a target as small and fast as LADEE is tricky. 22)
• On January 5, 2014, the LADEE spacecraft completed 1000 lunar orbits. LADEE continues to perform splendidly. In the evening of Jan. 19, LADEE executed a near-perfect periapsis-raising maneuver, one of a series of maneuvers designed to maintain LADEE’s science orbit within the desired altitude range. Without these maneuvers, LADEE’s orbit would decay in a matter of days, resulting in premature impact on the lunar surface. 23)
• Dec. 27, 2013: The completion of the 30-day LLCD (Lunar Laser Communication Demonstration) mission has revealed that the possibility of expanding broadband capabilities in space using laser communications is as bright as expected. 24)
- For example, LLCD demonstrated error-free communications during broad daylight, including operating when the moon was to within three degrees of the sun as seen from Earth. LLCD also demonstrated error-free communications when the moon was low on the horizon, less than 4 degrees, as seen from the ground station, which also demonstrated that wind and atmospheric turbulence did not significantly impact the system. LLCD was even able to communicate through thin clouds, an unexpected bonus.
- Operationally, LLCD demonstrated the ability to download data from the LADEE spacecraft itself. The project was able to download LADEE's entire stored science and spacecraft data (1 GB) in less than five minutes, which was only limited to the 40 Mbit/s connection to that data within LADEE. Using LADEE's onboard radio system would take several days to complete a download of the same stored data. Additionally, LLCD was to prove the integrity of laser technology to send not only error-free data but also uncorrupted commands and telemetry or monitoring messages to and from the spacecraft over the laser link.
- LLCD also demonstrated the ability to "hand-off" the laser connection from one ground station to another, just as a cellphone does a hand-off from one cell tower to another. An additional achievement was the ability to operate LLCD without using LADEE's radio at all. The project was able to program LADEE to awaken the LLCD space terminal and have it automatically point and communicate to the ground station at a specific time without radio commands. This demonstrates that this technology could serve as the primary communications system for future NASA missions.
• Dec. 11, 2013: LADEE completed its OLM-3 (Orbit Lowering Maneuver-3) on Nov. 10, and its OLM-4 on Nov. 20, 2013. During the commissioning phase when ground controllers were checking out the instruments and spacecraft, LADEE was orbiting the moon 250 km above the surface. Now LADEE is in its science operations orbit. All three science instruments are taking data in their planned sequence. LADEE’s new orbit makes its closest approach to the moon’s surface (or periapsis altitude) between 20 and 50 km, and farthest point (or apoapsis altitude) between 75 and 150 km. 25)
- The payload instrument measurements changed when LADEE swooped down to 50 km above the moon’s surface after OLM-3 on Nov. 10. At that low vantage point, NMS (Neutral Mass Spectrometer) was able to detect Argon-40 for the first time, and see its distinctive variation across the lunar dawn. Argon-40, a noble gas, has an atomic mass 10 times that of helium, and tends to stay closer to the lunar surface. The rate that LDEX (Lunar Dust Experiment) was sensing lunar dust at high altitudes (approximately one dust grain per minute) suddenly increased several-fold at 50 km. And things got even better after OLM-4 brought LADEE into its science orbit.
• Nov. 22, 2013: LADEE has completed the commissioning phase, and now is ready to begin the mission’s primary science phase. 26)
After the successful OLM-3 (Orbit Lowering Maneuver) on Nov. 10, LADEE was in an elliptic pre-science orbit. The first six days in this orbit were dedicated to completing the science instrument commissioning, doing opportunistic science measurements in coordination with NASA’s LRO (Lunar Reconnaissance Orbiter) spacecraft, and taking measurements of the impact of the Leonids meteor shower on the lunar environment.
• On Nov. 20, 2013, the LADEE spacecraft successfully entered its planned orbit around the moon's equator — a unique position allowing the small probe to make frequent passes from lunar day to lunar night. This will provide a full scope of the changes and processes occurring within the moon's tenuous atmosphere. 27)
- LADEE now orbits the moon about every two hours in the altitude range of 12-60 km above the moon's surface. For about 100 days, the spacecraft will gather detailed information about the structure and composition of the thin lunar atmosphere and determine whether dust is being lofted into the lunar sky.
• On Oct. 26, 2013, ESA's OGS (Optical Ground Station) on the island of Tenerife has received laser signals of LLCD over a distance of ~400, 000 km from NASA’s latest Moon orbiter, LADEE. The data were delivered many times faster (40 Mbit/s) than possible with traditional radio waves, marking a significant breakthrough in space communications. With the first two communication passes with LADEE on 26 October and six more to 29 October, the ESA team on Tenerife are tweaking the station hardware – especially for the uplink – and improving procedures. 28)
- The contact with Tenerife came just days after LADEE made history on October 18 in the first-ever laser transmission from lunar orbit, picked up by a NASA station at White Sands, New Mexico, USA. The craft is also transmitting to a third station, at NASA’s Jet Propulsion Laboratory in California. - During the coming weeks, ESA engineers will test uplink communications at 20 Mbit/s and obtain accurate ‘time-of-travel’ measurements to be used for calculating the spacecraft’s orbit.
• On October 18, 2013, the LLCD (Lunar Laser Communication Demonstration) has made history using a pulsed laser beam to transmit data from lunar orbit to Earth at a record-breaking download rate of 622 Mbit/s. LLCD is NASA's first system for two-way communication using a laser instead of radio waves. It also has demonstrated an error-free data upload rate of 20 Mbit/s transmitted from the primary ground station in New Mexico to the spacecraft currently orbiting the moon. 29) 30) 31)
Figure 10: Historic demonstration proves laser communication possible (image credit: NASA)
• Oct. 17, 2013: During the NASA shutdown period (general government shutdown Oct. 1-17, 2013), the LADEE mission continued to perform its critical maneuvers and capture into the commissioning orbit around the moon. The trajectory correction maneuver (TCM-1) was completed on Oct. 1, and set the spacecraft to rendezvous with the moon on Oct. 6. The Neutral mass Spectrometer (NMS) cap ejection on Oct. 3 was successful. The first LOI-1 (Lunar Orbit Insertion) maneuver (lasting ~4 minutes) on Oct. 6 was very accurate, and required no course adjustments afterward. This is an impressive performance of the propulsion system, given the size of the LOI-1 burn. The maneuver put the spacecraft into a 24 hour elliptical lunar orbit. 32)
- The LOI-2 maneuver on Oct. 9,2013 also was very accurate, putting LADEE into a 4-hour elliptic lunar orbit.
- The third and final LOI-3 burn occurred on Oct. 12, and put the spacecraft into the 2 hour commissioning orbit (roughly 235 km x 250 km). The LADEE spacecraft commissioning activities are now complete, and the instrument commissioning activities have begun.
- The LDEX (Lunar Dust Expreriment) and UVS (Ultraviolet Spectrometer) aliveness activities were completed successfully on Oct. 16, 2013 with both instrument covers deployed. These instrument cover deployments were the last remaining planned critical events for the mission. All critical maneuvers and all instrument cover deployments are completed at this point. The science instrument commissioning and lasercom primary experiment will be conducted through mid-November, at which point the spacecraft will start to drop down to the lower lunar science orbit.
• Sept. 25, 2013: The LADEE observatory continues the phase of the mission where it is cruising on its way to the moon. LADEE currently is in its third and final elliptic orbit around Earth – performing phasing loops. After the final perigee pass around Earth on Oct. 1, 2013, LADEE will travel to the point at which it will be captured around the moon using an initial LOI-1 (Lunar Orbit Insertion) burn of the onboard main engine. After that, LADEE will be in lunar orbit. 33)
- The major activities accomplished since the first perigree maneuver (PM-1) on Sept. 13, 2013, involved instrument checkouts with their covers closed. These tests ensure that the instruments are operational, and survived the stresses of launch. All three science instruments, as well as the laser communication experiment, successfully completed their tests and look healthy.
• Sept. 13, 2013: The LADEE observatory has completed the checkout phase of the mission, and is now in the cruise phase on the way to the moon. The project is currently in elliptic orbits around Earth, called phasing loops, and will continue with two more of these elliptical orbits until LADEE is captured around the moon using an initial LOI-1 (Lunar Orbit Insertion-1) burn on Oct. 6, 2013. After that the spacecraft is in lunar orbit. 34)
Sensor complement: (NMS, UVS, LDEX, LLCD)
LADEE employs a high-heritage science instrument payload, including a neutral mass spectrometer, an ultraviolet spectrometer, and an in-situ dust sensor. In addition to these science instruments, LADEE will also carry a laser communications system technology demonstration.
NMS (Neutral Mass Spectrometer):
The NMS instrument is based on a similar instrument on NASA's CoNTour (Comet Nucleus Tour) mission with a launch on July 3, 2002 and the SAM (Sample Analysis at Mars) instrument developed for the Mars Science Laboratory (MSL). The NMS uses a high sensitivity quadrupole mass spectrometer with a 150 Dalton range and unit mass resolution. The NMS is a NASA/GSFC instrument. Participating organizations are: the University of Michigan/Space Physics Research Lab, Battel Engineering, AMU Engineering, and Nolan Engineering.
Although to date only He, Ar-40, K, Na and Rn-222 have been firmly identified in the lunar exosphere and arise from the solar wind (He), the lunar regolith (K and Na) and the lunar interior (Ar-40, Rn-222), upper limits have been set for a large number of other species. The LADEE NMS observations will determine the abundance of several species and substantially lower the present upper limits for many others. Additionally, LADEE NMS will observe the spatial distribution and temporal variability of species which condense at nighttime and show peak concentrations at the dawn terminator (e.g. Ar-40), possible episodic release from the lunar interior, and the results of sputtering or desorption processes from the regolith. 35)
• High-sensitivity quadrupole mass spectrometer, mass range 1-150 Dalton and unit mass resolution
• At 50 km lunar altitude or lower can detect helium, argon and other species
• UHV (Ultra High Vacuum) materials and processing used in the fabrication of NMS yield a substantial improvement over background instrument noise from Apollo era instruments, corresponding increase in sensitivity of the measurement.
• The sensitivity is necessary to adequately measure the low density atmosphere of the moon.
• Closed source species: He, Ar, non-reactive neutrals
• Open source species: neutrals and ions
• Mass range: 2 - 150 Dalton
• Mass resolution: unit mass resolution over entire range
• Sensitivity: 10-2 (counts per second) / (particles cm-3)
• Instrument mass: 11.3 kg, envelope: 43.2 cm x 24.5 cm x 37.0 cm, power: 34.4 W average, data rate: 3.5 kbit/s.
The elements of the LADEE NMS are shown in Figure 11. 36) Its ion dual ion source, hyperbolic quadrupole rod assembly, and vacuum housing consist of refurbished elements of the engineering unit NGIMS (Neutral Gas and Ion Mass Spectrometer ) developed for the CONTOUR mission. This mass spectrometer is a similar design to the Cassini INMS (Ion and Neutral Gas Mass Spectrometer) designed and developed at NASA Goddard as a facility instrument for this mission to the moons of Saturn.
Figure 11: Illustration of the NMS elements (image credit: NASA)
Legend to Figure 11: The top image shows the NMS elements. The bottom model shows BOC (Break-Off Cap), ELEC (Electronics box), S (Ion Source Region), Q (Quadrruple Region), D (Detector), RF (Radio Frequency).
With a new ion/electron optical design the sensitivity of the NMS has been improved over the INMS by more than an order of magnitude to optimize measurements in the low density lunar exosphere. The NMS closed source is suitable for species that do not adsorb on ion source surfaces such as He and Ar. A RAM density enhancement is realized in this source and the measurement of these inert noble gas species is not impacted by the multiple wall collisions in this source prior to ionization. On the other hand, the open source is required for a search for species such as metal atoms that are surface reactive. In this source the atomic or molecular species are ionized and focused into the quadrupole analyzer without wall collisions.
NMS calibration approach on the vacuum chamber: The NMS calibration will enable the response of the NMS to lunar gas to be established so that the detector counts measured in lunar orbit can be converted into ion source densities for the open source and into neutral flux into the closed source. The NMS was calibrated on a thermal gas ultra-high vacuum chamber pumped by magnetically levitated turbomolecular pumps. Both the chamber and the mass spectrometer were baked for several days prior to calibration. High purity N2 and the noble gases He, Ne, Ar, Kr, and Xe were utilized either as single gases or as mixtures. The flow out of the vacuum chamber was limited by an iris valve to minimize density gradients within the vacuum chamber. A NIST tracable Bayard Alpert gauge measured pressure. The mass spectrometer was operated with first with GSE (Ground Support Equipment) electronics to optimize lens focusing and to establish the response of the detector signal to variations in lens voltages and then moved to the NMS flight electronics.
NMS calibration after separation from the vacuum chamber: The NMS sensor incorporates a passive chemical getter that can remove the active gases. After the chamber calibration described in the preceeding paragraph had been completed a mixture of noble gases He, Ar, Kr, and Xe was prepared and several static mode mass spectrometer scans carried out where the only pump utilized was the the getter. The sensitiveity of the closed source established for various noble gas isotopes from one of these runs is shown in Figure 12.
Figure 12: Sensitivity values established with one of the calibration runs for the closed source for each of the redundant detectors are shown for the noble gas isotopes (image credit: NASA)
Legend to Figure 12: The sensitivity plotted on the vertical axis is in units of (counts/second)/(particle/cc) after normalization using ionization cross-sections to that of N2 to illustrate the differences between the low (21Ne through136Xe) and high frequency (4He through 20Ne) FR regions.
UVS (Ultraviolet/Visible Spectrometer):
The UVS instrument is a next-generation, high-reliability version of the LCROSS UV-VIS spectrometer, spanning 230-810 nm wavelength, with high (<1 nm) spectral resolution. UVS will also perform dust occultation measurements via a solar viewer optics system. The objective is to examine exospheric emissions and a broader continuum of forward or backward scattered light from dust down to ~10 nm in size. In occultation mode, UVS will study dust distributions down to ~300 m above the surface. The UVS is a NASA/ARC instrument. Participating organizations are: Aurora Design & Technology, and Visioneering, LLC. 37) 38)
Figure 13: Photo of the UVS instrument (image credit: NASA/ARC)
LADEE mission measurement concept:
• UVS includes UV-VIS spectrometer, telescope, solar diffuser, & bifurcated optical fiber
• UVS observations consists of limb and occultation measurements
• Limb observations measure the lunar atmosphere, & also measure limb dust by measuring back- or forward-scattered sunlight
• Solar occultation observations measure lunar atmospheric dust extinction from 0 to 50 km.
• In Limb mode measures atmospheric species including: K, Na, Al, Si, Ca, Li, OH, H2O
• By combining long integration times, UVS measures each specie to < current upper limits
• In limb mode measures dust (via scatter) at concentrations as low as 10-4 per cc for r=100 nm size particles
• In occultation mode, UVS measures dust (via extinction) at concentrations as low as 10-4 per cc for r=100 nm size particles down 300 m in altitude.
• The UVS instrument has a mass of 3.98 kg and an average power consumption of 1$ W.
Figure 14: Predicted SNR (Signal-to-Noise Ratio), image credit: NASA
UVS has two means of observing: a Limb Telescope and a SOV (Solar Occultation Viewer). The Limb Telescope is pointed just above the surface of the moon, at its limb, looking for emission by exosphere gasses and the scattering of sunlight by dust grains. For a typical orbit, limb observation will be centered on the morning and evening terminators and local noon. During each of these activities the LADEE spacecraft will keep the telescope pointed no more than 20 km above the lunar surface. Each limb “stare” is accompanied by a “nod” during which the telescope field of view is nodded down to below the lunar limb,so that the moon is entirely in the FOV (Field of View) of the telescope, and then up to an altitude of 50 km. The “nod” will help to resolve any altitude variations in gasses or dust and provides a calibration point for zodiacal light and light scattered into the telescope from the surface of the moon.
The SOV allows UVS to stare at the sun, monitoring it as it rises or sets across the lunar limb. The advantage of a solar occultation observation is the very high signal-to-noise achieved (SNR>500) in the instrument for very short (<20 ms) integration times. The high SNR and rapid sample rate allows UVS to search for extinction due to dust at very low altitudes above the lunar surface (<1.5 km).
LDEX (Lunar Dust Experiment):
LDEX is designed to map the spatial and temporal variability of the dust size and density distributions in the lunar environment. LDEX is an impact ionization dust detector, sensing dust impacts in situ at LADEE orbital altitudes of 50 km and below, with a particle size range of between 100 nm and 5 µm. Dust particle impacts on a large hemispherical target create electron and ion pairs. The latter are focused and accelerated in an electric field and detected at a microchannel plate. The detector area of LDEX is ~ 0.01 m2. 39) 40) 41)
LDEX has been designed and developed at LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado, Boulder, CO. Participating organizations are: Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany; University of Stuttgart, and the Institute for Geosciences, Heidelberg University, Heidelberg, Germany.
• LDEX measures the mass of individual dust grains with m ≥ 1.7 x 10-16 kg (radius rg ≥ 0.3 µm) for impact speeds ~ 1.7 km/s
• Also measures the collective current due to grains below the threshold for individual detection, enabling the search for dust grains with rg ~ 0.1 µm over the terminators.
LDEX has a mass of 3.6 kg (with margin), size: 15 cm x 15 cm x 20 cm, power of 6.11 W (peak) and 3.8 W in operations, the data rate is 1 kbit/s (64 Mbit/day).
Figure 15: Schematic view of the LDEX measurement concept (image credit: NASA, University of Colorado)
Of particular interest is to verify from orbit the presence of water ice in the permanently shadowed lunar craters.
Figure 16: Cutaway view of the LDEX instrument (image credit: NASA, University of Colorado)
Legend to Figure 16: The target hemisphere is segmented to accommodate impact detection to determine the mass and speed of dust grains (top segment) and a time-of-flight setup for chemical analysis (bottom segment).
Figure 17: Photo of the LDEX engineering model (image credit: NASA, University of Colorado)
LDEX aboard the LADEE spacecraft can collect a large number of samples from a greater part of the entire surface for analysis. It thus combines the advantages of a remote sensing instrument and a lander. The instrument is especially sensitive to the metallic compounds of minerals and any species which easily form ions (e.g. water).
LLCD (Lunar Laser Communications Demonstration):
LLCD is a collaborative project of MIT/LL (Massachusetts Institute of Technology/Lincoln Laboratory) and NASA/GSFC. The goal is to demonstrate and validate duplex optical communications from a lunar orbiting spacecraft to an Earth-based ground receiver. The requirements call for data rates of up to 620 Mbit/s in downlink and up to 20 Mbit/s in uplink. In addition, two-way time-of-flight measurements are needed with the potential to perform ranging with sub-centimeter accuracy. 42) 43) 44) 45) 46) 47)
LLCD is a secondary payload on the LADEE mission. LLCD will demonstrate:
• Photon counting pulse position modulation
• Inertial stabilization
• Integrating an optical communications terminal to a spacecraft
• Link operations from lunar orbit
• Scalable array ground receiver.
Optical communications offers many potential benefits for future deep space missions. Increased aperture gains and reduced diffraction losses at optical wavelengths, as compared to radio-frequency (RF) wavelengths, enable higher data rate forward and return links with reduced size, weight, and power (SWAP) burden on a spacecraft and smaller ground terminals on the Earth. Moreover, the 10's of THz of available spectrum in the optical bands is unregulated, and, commercially-available optical transmitter and receiver technologies provide for efficient utilization of >40 GHz of optical bandwidth, enabling higher data rate links that are much more photon efficient than those that might be achieved using the narrower bandwidths available for RF links. For these reasons, NASA has identified optical communications as an important technology for the future of deep space communications. The LLCD (Lunar Laser Communications Demonstration) on LADEE is intended to be a first step toward developing an operational optical communications capability for future manned and robotic missions.
The LLCD comprises 3-elements:
• LLST (Lunar Lasercom Space Terminal)
• LLGT (Lunar Lasercom Ground Terminal)
• LLOC (Lunar Lasercom Operations Center).
Each of these elements is being designed, built, tested, and operated by MIT Lincoln Laboratory. The LLCD project is being managed by NASA's Goddard Space Flight Center. The LLST payload will be operated for a total of 16 days during the 1 month commissioning phase which precedes the science phase. During the commissioning phase, the spacecraft will be in a ~2 hour orbit at an altitude of approximately 250 km above the lunar surface. Power limitations and thermal considerations on the spacecraft limit the duration of LLST operations to about 15 minutes per orbital pass. Since the ground terminal must be in view for communications operations to take place, each day will provide opportunities for 3-5 laser communications orbits (Ref. 42).
Overview of LLST (Lunar Lasercom Space Terminal):
The LLST instrumentation is comprised of three modules (Figure 18): an optical module, a modem module, and a controller electronics module. The optical module is situated on the exterior of the LADEE spacecraft payload module, while the modem and controller electronics modules are mounted on the interior of the spacecraft. The LLST payload has a mass of ~30 kg and operates at a power in the range of ~50-140 W.
The optical module is a Cassegrain telescope with a 10 cm aperture on a two-axis gimbal that enables optical link operation over a wide range of spacecraft orientations. The telescope and backend optics are inertially stabilized using MIRU (Magnetohydrodynamic Inertial Reference Unit) that rejects high-frequency disturbances from the spacecraft interface (MIRU was built by the ATA (Applied Technology Associates) Corporation of Albuquerque, NM). A wide field-of-view InGaAs quadrant detector is used for spatial acquisition and coarse tracking of the optical uplink. Transmit and receive signals are coupled to and from the telescope via optical fibers. These fibers are mounted to piezoelectric actuators that are used to provide a point-ahead capability between the transmit and receive fibers and low-bandwidth fine spatial tracking of the optical uplink signal.
Figure 18: The LLST instrumentation: a) the optical module, b) the modem module, and c) the controller electronics module (image credit: MIT, NASA)
The optical transmitter and receiver are contained in the modem module, which is fiber coupled to the optical module. Digital electronics in the modem aggregate the various downlink data sources (including science data from the LADEE spacecraft, high-rate LLST telemetry, and loopback of the optical uplink) and encode the data using a high-efficiency half-rate code. The encoded data are modulated onto an optical carrier using a high-bandwidth pulse-position modulation format (slot rates up to 5 GHz) and amplified to 0.5 W average power in an EDFA (Erbium-Doped Fiber Amplifier) prior to transmission on the downlink. An optically preamplified direct-detection receiver based on a low-noise EDFA is used for the uplink receiver. A near-optimum hard-decision pulse-position modulation demodulator based on a previously demonstrated binary PPM demodulator6 demodulates the uplink waveform prior to decoding of the uplink signals in a high-density SRAM-based field programmable gate array.
The controller electronics module, a custom avionics module based on a single-board computer, provides closed-loop control of the various actuators in the optical module. It also provides command and telemetry interfaces for the LLST payload to the LADEE spacecraft and configures and controls the modem.
Figure 19: Illustration of LLST components on the LADEE spacecraft (image credit: MIT, NASA)
Overview of the modem module (Ref. 43): The modem design (Figure 20) consists of four vertically-stacked slices: the power, digital, analog and EO (Electro-Optic) slices. The advantage of the modular approach is that each slice can be built and tested in parallel which reduces integration and test time of the flight modem assembly and allows for design flexibility to address evolving requirements as the spacecraft design matures. All four slices are connected with external electrical cables and the transmit and receive fibers are run from the EO slice to the optical module. The total mass of the modem is ~10.96 kg and it occupies a volume of ~26 liter. The modem consumes ~78 W during normal operations.
For the optical downlink, the modem aggregates the various data sources on the spacecraft, encodes, interleaves and frames the data, modulates the data onto the downlink optical carrier and amplifies the optical signal prior to transmission. There are three sources of data for the downlink: high-rate LLST telemetry from the controller electronics (up to 5 Mbit/s), data received on the optical uplink (up to 20 Mbit/s), and science data from the LADEE spacecraft (up to 40 Mbit/s). The data source aggregation and downlink encoding is performed in an SRAM-based field programmable gate array (FPGA) in the modem digital slice.
The encoder consists of a ½-rate serially concatenated PPM (Pulse Position Modulation) turbo code which is computed using a simple convolutional encoder and performs within 1.5 dB of the theoretical channel capacity. PPM is used for the downlink because it is orthogonal with high power efficiency in the transmitter and is well-matched to the superconducting photon counting detectors in the ground terminal.
The energy efficiency of the optical downlink is -4.6 dB detected photons/bit or 25-100 µJ/bit at channel capacity and before modulation of the data, interleaving is used to mitigate the atmospheric effects on the downlink channel. The specific modulation used is a 16-ary PPM format with achievable user data rates from 39 – 620 Mbit/s which corresponds to PPM slot rates of 0.311 – 5 GHz. The wide range of available data rates enables demonstration of the optical link in a wide range of atmospheric conditions.
The 620 Mbit/s link has been specified to offer up to 16 multiplexed subchannels, where each individual subchannel provides a data rate of 38.55Mbit/s, resulting in a maximum aggregate data rate of roughly 620 Mbit/s. In order to achieve high link reliability, the proposed link is based on NASA’s capacity-approaching modulation and coding scheme, which comprises a serial concatenation of an inner accumulate PPM (Pulse Position Modulation) and an outer convolutional code SCPPM (Serially Concatenated PPM). 48)
Figure 20: Overview of the LLST modem module components (image credit: MIT, NASA)
The modulation in the downlink occurs in the EO slice which utilizes a MOPA (Master Oscillator Power Amplifier) architecture. The master oscillator consists of a continuous wave DFB laser at ~ 1550 nm and the electrical to optical data conversion occurs in a LiNbO3 modulator. The optical amplification of the downlink data to a 0.5 W average power is achieved with a double-pass two-stage polarization-maintaining EDFA design. Each gain stage is pumped with two external grating stabilized 976 nm pump lasers. To optimize the power efficiency of the modem pairs of pump lasers are connected electrically in series.
Modem uplink performance: The uplink receiver is an optically pre-amplified direct detection receiver, as shown in Figure 21. The design consists of a two-stage polarization insensitive EDFA for high gain (> 40 dB) and low noise figure (< 4 dB) performance.
Figure 21: Illustration of the receive EDFA design with the expected optical input average powers (image credit: MIT, NASA)
The expected average optical powers at the fiber input to the receiver range from -58 to -78 dBm. After the second gain stage a narrow-band 10 GHz FBG (Fiber Bragg Grating) optical filter is used to reduce spontaneous-emission noise in the signal incident on the tracking and communications photodetectors. Because of the 40ºC temperature swing in the modem, the peak wavelength shift in the FBG’s is expected to be ~ ± 20 GHz and this assumes a Δλ/ΔT of ~ 1 GHz for the FBG’s. Because of this large peak wavelength shift, temperature stabilization in the FBG’s is required.
Table 1: Overview of LLCD performance parameters
Overview of LLGT (Lunar Lasercom Ground Terminal):
The LLGT (Figure 22) consists of an array of transceiver and receiver telescopes and a control room. The telescope arrays are used to demonstrate a scalable and cost-effective approach for providing large-aperture transmitters and receivers. They also provide spatial diversity which helps mitigate the deleterious effects of atmospheric turbulence on the optical uplink and downlink signals.
Four 15 cm refractive telescopes are used for the optical uplink and four 40 cm reflective telescopes are used as collectors for the optical downlink. Each telescope is fiber-coupled to the control room where the optical transmitters and receivers reside. All 8 telescopes are mounted on a single elevation over an azimuth gimbal. The gimbal provides coarse pointing for all of the telescopes with near-hemispherical coverage. The back-end optics for each telescope include a focal plane array and a fast-steering mirror for high-bandwidth tracking of the optical downlink and correction of any relative pointing biases between the telescopes on the gimbal. While mounting all of the telescopes on a single gimbal is not required for the arrayed transmitter/receiver concept, it simplifies the process of time-aligning the transmitted and received waveforms over the range of pointing required to support the optical links. The telescopes are housed in a fiberglass environmental enclosure which maintains a suitable environment for their operation.
Figure 22: Schematic layout of the LLGT functions (image credit: MIT, NASA)
The control room houses all of the electronics to control the gimbal and telescopes as well as the ground terminal modem electronics and optics. Four EDFA-based 10 W optical transmitters generate the pulse-position modulated signals for the optical uplink. Each transmitter is fiber coupled to one of the transceiver telescopes via a polarization-maintaining single-mode fiber.
The wavelengths of each of the transmitters are slightly detuned to allow for low-power-penalty non-coherent combining of the signals at the space-terminal receiver. The optical downlink receiver is based on photon-counting superconducting nanowire arrays. These detectors operate at cryogenic temperatures to provide very high photon detection efficiencies, and have been previously used to demonstrate high data rate optical communications with receiver efficiencies exceeding 1 bit / detected photon.
In order to achieve good coupling efficiency in the telescopes in the presence of atmospheric turbulence while preserving the polarization of the downlink signal, a custom multi-mode polarization-maintaining fiber is used to couple the receiver telescopes to the SNDAs. Custom FPGA-based digital electronics in the control room interface to the various data sources and destinations, generate the waveforms for the optical uplink, and demodulate and decode the detected signal on the optical downlink.
The LADEE mission will utilize the MOC (Mission Operations Center) located at NASA/ARC ( Ames Research Center) in Moffett Field, California. The project will use the SOC (Science Operations Center) located at Goddard Space Flight Center (GSFC) in Greenbelt, MD. The primary ground station, the transportable LLGT (Lunar Lasercom Ground Terminal), will be located at the WSC (White Sands Complex) near Las Cruces, New Mexico.
The data from the ground station(s) will be routed to the MOC at ARC for processing, distribution and data storage/archiving. Science and instrument data, along with processed spacecraft health and safety data, will be transmitted to the GSFC SOC. The SOC will perform instrument data processing and scientific analysis of instrument data. Figure 23 shows a schematic representation of the mission system architecture and command and data flow within it (Ref. 14).
Figure 23: Overview of the mission system architecture (image credit: NASA)
Figure 24: Alternate pictorial view of the LADEE ground segment (image credit: NASA) 49)
Ground terminal receiver:
Tight size/weight/power constraints on the satellite payload (the average optical transmit power is < 1 W), in addition to the large R2 link loss due to the approximately 400 thousand kilometer separation between the earthbound ground terminal and lunar satellite, necessitate efficient usage of the optical signal appearing at the ground terminal receiver. Some constraints on the collection optics for LLCD are that: 50)
1) they be simple and relatively compact and transportable to support relocation on short notice — this feature initially provided flexibility in postponing the final ground terminal site selection in order to optimize mission link availability depending on the expected seasonal cloud coverage at launch time.
2) they mitigate communications performance degradation on days with strong atmospheric turbulence.
The receiver collection optics consist of four 40 cm Ø telescopes mounted to a gimbal and aligned to a common boresight; the use of multiple telescopes provides spatial diversity to reduce the severity of atmospheric-induced fades; the use of a common, fixed bore-sight eliminates the need for dynamic, orientation-dependent deskew and synchronization procedures to time align signals across the telescope array. A clamshell enclosure protects these telescopes from the weather during periods of inactivity.
Figure 25: Block diagram of the LLCD downlink (image credit: MIT/LL)
Light from each telescope is guided by a weakly polarization-maintaining multimode optical fiber (MMPMF), and lens coupled onto a 14 µm diameter array of four interleaved superconducting nanowire single-photon detectors (SNSPD) — this passive and relatively simple multimode scheme maintains high coupling efficiency in the presence of strong turbulence without the need for adaptive optics. Each of the total sixteen detector elements is independently biased and cooled to 2.6 K inside a closed-cycle cryostat to achieve detection efficiencies of ~60% per nanowire as measured from the input to the MMPMF.
Fast reset times (~15 ns) and small timing jitter (~60 ps FWHM) allow the SNSPD-based photon counting receiver to achieve high count rates with relatively few detector elements when compared to other technologies with similar detection efficiency. The nanowire geometry seen in Figure 25, which resembles a wire grid, results in a 3-4 dB reduction in detection efficiency for fields polarized perpendicular to the wires; thus, polarization alignment between the multimode fiber and detector sample is required.
The output electrical pulses from the detectors are conditioned first by cryogenic GaAs HEMT amplifiers packaged in proximity to the detectors on the sample mount at 2.6 K, and then subsequently by two additional stages of amplification at room temperature, outside of the refrigerator. The amplified waveforms pass through a one bit analog-to-digital converter (i.e., comparator) whose discrimination threshold is determined empirically to maximize SNR.
External optical signal reception:
ESA (European Space Agency) is modifying its OGS (Optical Ground Station) equipment to participate in the LLCD (Lunar Laser Communications Demonstration) experiments. The OGS is located at the Observatorio del Teide (OT) on Tenerife island, Spain. The OGS has been used to commission and test laser communication terminals, such as those on the ARTEMIS, OICETS, TerraSAR-X and NFIRE satellites. It is now being prepared to test the LCTs onboard the Alphasat, EDRS-A and EDRS-C satellites. 51)
Table 2: Summary of LLCD operational parameters for the communication link with the OGS
ESA's OGS will also use the laser to communicate with NASA's LADEE spacecraft orbiting the moon. ESA's OGS is being upgraded with a complementary unit and, together with two US ground terminals, will relay data at unprecedented rates using infrared light beams at a wavelength similar to that used in fiber-optic cables on Earth. 52)
The testing took place in July 2013 at a Zurich, Switzerland, facility owned by ESA’s industrial partner RUAG and made use of a new detector and decoding system, a ranging system and a transmitter.
A NASA team, supported by the MIT/LL (Massachusetts Institute of Technology/ Lincoln Laboratory) and the Jet Propulsion Laboratory, brought over their laser terminal simulator, while ESA together with RUAG and Axcon of Denmark set up the European equipment to test compatibility between the two sets of hardware.
The first laser link-up with LADEE is expected to be attempted four weeks after launch, around mid-October 2013.
Figure 26: Photo of the OGS telescope in the open dome (image credit: ESA)
Figure 27: Photo of the OGS (Optical Ground Station) on Tenerife (image credit: ESA)
Legend to Figure 27: ESA's OGS is 2400 m above sea level on the volcanic island of Tenerife. Visible green laser beams are used for stabilizing the sending and receiving telescopes on the two islands. The invisible infrared single photons used for quantum teleportation are sent from the neighbouring island La Palma and received by the 1 m telescope located under the dome of the OGS. Initial experiments with entangled photons were performed in 2007, but teleportation of quantum states could only be achieved in 2012 by improving the performance of the setup.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.