A Sustained Proximity Network for Multi-Mission Lunar Exploration

zyxw zyxwvu zyxw zyxwvu A Sustained Proximity Network for Multi-Mission Lunar Exploration Jason A. Soloff Goddard Space FIighr Center, Greenbelt, Maryland, 20771 Gary Noreen' and Leslie Deutsch: Jet Propulsion Luboratory, CaliforniaInstitute of Technology,Pasadena, California,91109 and zyxw zyxwvutsr zyxwv David Israel$ Goddord Space FIighr Center, Greenbelt, Maryland, 20771 Tbe Vision for Space Exploration calls for an aggressive sequence of robotic missions beginning in 2008 to prepare for a human return to the Moon by 2020, with the goal of establishing a sustained human presence beyond low Earth orbit A key enabler of exploration is reliable, available communication and navigation capabilities to snpport both human and robotic missions. An adaptable, sustainable communication and navigation architecture has been developed by Goddard Space Flight Center and the Jet Propulsion Laboratory to support human and robotic lonar exploration through the next two decades A key component of the architecture is scalable deployment, with the infrastrurture evolving as needs emerge, allowing NASA and its partner agencies to deploy an interoperable communication and navigation system in an evolutionary way, enabling cost effective, highly adaptable systems throughout the lunar exploration program. zyxwvut zyxwvutsr I. T Introduction HE Vision for Space Exploration' begins with a campaign of robotic precursor missions to the Moon. In response to this call, NASA created the Robotic Lunar Exploration Program (RLEP). RLEP consists of a multimission sequence of reconnaissance orbiters, landed elements and technology demonstrations whose combined mission is to characterize the lunar environment, prepare for a safe, assured human return. and validate the technology and operations concepts necessary for a sustained human presence beyond low Earth orbit. This paper describes a strawman network architecture to enable multi-mission operations and proximity crosssupport in the lunar environment. The concept builds on the lessons learned and successes of JPL's Mars Network by integrating proximity relay operations, interoperable protocols, and autonomous communications into the RLEP mission set. By designing individual RLEP missions with proximity networking in mind, a highly capable, available and reliable in-situ communicationnetwork can be built as the program evolves. YASA envisions launching robotic missions to the Moon on an approximately annual basis leadmg up to the human return to the Moon between 2015 and 2020. International space agencies have also identified aggressive robotic lunar exploration campaigns. The significant number of missions to the Moon over the next decade creates a unique "critical mass" that could be leveraged into an evolvable, sustainable lunar communications network, with each new mission adding new capabilities, extending and replenishing the existing network. This paper reviews lunar exploration requirements (both known and assumed) and the potential mission set that has been proposed to meet them. It then considers tbe communication and navigation requirements necessary to t ' ' Communication Systems Engineer, Microwave and Communication Systems Branch, Code 567, Member. Senior Engineer, Mission and Systems Architecture Section. MIS 301-170S, Senior Member. Manager, Architecture and Strategic Planning Ofice, Interplanetary Networks Directorate, MS 303-401, Member. Leader, Advanced Technology Development Group, Microwave and Communication Systems Branch, Code 567. 1 American Institute of Aeronautics and Astronautics zyxwvutsrqp zyxwvuts zyxw zyxwvu zyxwvutsrq zyxwvu zyxwvutsr zyxw zyxwv zyxwvu support these missions. Finally, an evolving proximity network implementation is described, discussing how the concept can extend and adapt to provide a continuous. sustained mission cross-support capability. II. The Robotic Lunar Expto~-atio~ Program To meet the first objective of the Vision for Space Exploration, NASA created the Robotic Lunar Exploration Program, the objectives of which are to perform science and measurements vital to lunar exploration, demonsfme key technologies and operations concepts, and deploy initial Mastructure to enable a sustained human presence at the Moon and beyond. RLEP is managed by the Goddard Space Flight Center and consists of a series of individual missions, the first of which is the Lunar Reconnaissance Orbiter (LRO) scheduled to launch in 2008. LRO will be followed by surface landed elements and additional reconnaissance missions to improve our knowledge of the Moon and the Cis-Lunar environment Follow-on RLEP missions are expected to prove technologies critical to the success of the Crew Exploration Vehicle and other Constellation systems, as well as pre-deploy infiwaumre to support extended d d o n human exploration of the Moon. The early science objectives of RLEP are derived h m the NASA Decadal Study and h m the input of the planetary science and space operations communities and include lunar topography, radiation environment, regolith composition, resource identification, and gravity mapping. The objectives of each RLEP mission are derived from the questions asked and answers learned during previous missions. If a mission detects fiom orbit the likely presence of resources, a later mission may be targeted to land at that site and directly sample the surface to provide “ground truth” measurements confirming the earlier discovery. In this way the program is responsive to new discoveries, new objectives, and emerging requirements of NASA’s exploration and science programs. LRO addresses the most pressing of these investigations. It is a multi-faceted mission, whose measurement set provides data products vital to exploration planning while filling large gaps in our scientific knowledge of the Moon. The measurements h m LRO’s selected instrument set consist of high resolution imagery and topogmphid measurement, radiation characterization, and resource identification. The mission will operate in a nominal 50 km lunar polar orbit in order to provide high fidelity science. The spacecraft will collect and return over 450 Gb/day of science data, delivering over 164 terabits during its one year primary mission. This data volume is unprecedented for a non-Earth science mission. Following LRO’s primary mission, the mission will enter a planned “extended mission” phase during which the.spacecrafl will be placed in a low maintenance orbit where it will perform additional science and likely function as an infhstructm resource for later missions. It is LRO’s extended mission, and the extended missions of other lunar orbiting spacerraft that provides the opportunity to leverage a local lunar proximity network. The second RLEP mission. to launch one year following LRO, is envisioned to consist of one or more landed elements to be deployed to the lunar surface from an orbiting host carrier. As previously mentioned these landers will target likely resource concentrations and provide in-situ measurements to confirm or disprove LRO’s findings. Multipurpose probes may also veri@ the lunar I missions will provide additional measurements in this manner, and eventually verify exploration technologies including power generation, and in-situ resource utilization. Missions throughout the RLEP sequence will reiy on high volume data retrieval and precision navigation. Landers and roving elements must be targeted to specific sites to c o n f i i measurements made from orbit. Eventual pre-deployed inhtructure must be delivered reliably to specific locations for 1atm zyxw Image from the ”VationolSpace Science Data Center 111. RLEP Use of Cross-Support The Moon’s topography, coupled with its tidally locked orbit about the Earth, creates unique locations near the poles that are in permanent shadow. Lunar scientists believe that these permanently shadowed regions are likely to contain trapped volatiles frozen into the regolith. Measurements performed by the Lunar Prospector and Clementine missionsv indicated the presence of resources including water ice in several craters near the lunar south pole4 (Figure 1). These resources may exist in significant quantities and provide a convenient resource for future human missions. 2 American Institute of Aeronautics and Astronautics zyxwvu zy zyxwvutsrqp zy zyxwvutsrq zyx zyxwvutsr zyxwvutsr The topography that protects these sites from solar illumination also hides them from view of the Earth. in order make in-situ measurements in permanently shadowed regions, it is necessary to find a non-line-of-sight method for communication between Earth and the landed surface probe. Proximity relay, in which one spacecraft serves as a CG~IIRIUS~&~XI intmCd.aii, ~ K W ~ ~ Ca iS e!egait solution to this problem. in proximity reiay operations, one vehicie, possibly masked by terrain as revealed at the Moon’s south pole by the Clementine’ mission (Figure 2), and therefore out of view of its intended recipient, transmits its data to a separate vehicle that is in view. A likely scenario is a landed probe communicating with an orbitii canier spacecraft. The receiving vehicle records and stores the data and later transmits it, along with its own data, to Earth or another intended destination. Transmission of commands to the masked vehicle is accomplished by initially transmitting them to the relay, which then delivers the commands to the intended user during a later line-of-sight contact period. Proximity relay operations, in which one mission provides cross-support to another, may provide the only realistic method for communication with probes deployed to permanently shadowed regions of the Moon. Figore 2 - Swth Pole Lunar Sorface Navigation will also play a key role in successful Imagefiom the National Space Science Data Center lunar robotic and human exploration. Initial robotic missions w ill need to determine their location to identie where measurements are made, allow for accurate maneuvering, and provide for the targeting and tenninal guidance of landed elements such as probes and rovers. The GPS constellation which provides this capability at Earth is not able to support lunar operations. Traditional direct-fkm-Earth Doppler and range measurement employed by systems such as NASA’s Deep Space Network (DSN) and Tracking and Data Relay Satellite System (TDRSS) can only provide navigation support to a single user per beam (with each beam requiring its own antenna asset) and cannot provide support to vehicles Operating on the far side of the Moon. Proximity communications could provide navigation support to these missions without occupying scarce Earth-based ground assets, as well as provide Capability on the far side. As RLEP missions grow in complexity, evolving to demonstrate key exploration technologies, and as more missions are undertaken by NASA and its inteI-IIatiOM1partners, the number of assets (landed probes, rovers and orbiting spacecraft) operating in the lunar environment will increase. Individual mission and campaign goals will likely require these vehicles to communicate to exchange data, coordinate actions and provide navigation information. The potential number of simultaneous vehicles, as in a distributed robotic surface explorer or an advanced in-situ resource utilization demonstration, may make direct-to-Earth communication impractical even with all mission assets in view due to the number of simultaneously supported communication links. In this case t5e most practical method of communication may be to establish a “local area network” consisting of the landed elements and one or more orbiting assets to act as a communication relay and router. Information moving between mission elements can then remain in the lunar environment (the local neiworkj while measurement data, engineering telemetry and operational command and control can be coordinated and distributed through the relay. This approach limits the operational complexity and cost of providing Earth-to-Moon communications for the mission. to zyxw zyxwv IV. Mars Network Concept for Mission Cross-Support NASA’s Science Mission Directorate has established a policy by which any mission to Mars having an expected on-orbit lifetime exceeding one year must carry a proximity relay capable communication system. This policy has established near Mars an ad-hoc relay network able to support advanced mission concepts. An excellent example of this policy’s success is the recent Mars Exploration Rover (MER) missions, Spirit and Opportunity, still collecting data on the Martian surface, w-hose communications were conducted primarily through pervious years’ Mars orbiters. 3 American Institute of Aeronautics and Astronautics zyxwvutsrq zyxwvut zyxwv zyxwvut zyx zyxwv zyxwv zyxwv zyxwvut Mars orbiters with relay radios are enabling exciting in-situ missions to the Red Planet. Spirit and Opportunity have been tracked by NASA’s Mars Global Surveyor (MGS) orbiter since their arrival at Mars and have relayed data to Earth through both Odyssey and MGS after landing on January 4th and 26th, respectively. Rovers have also communicated through the Mars Express spacecraft. NASA added relay radios to Mars Global Sweyor and Mars Odyssey specifically to provide relay services to other mksiogs. A c SY&, the rehy & i s z e ~ T . cf T a new m y d qn!oriig &er p h e i s : &io-@ ii big tern program, with each mission building on others to establish an interplanetary internet for the support of future missions. NASA will add Mars Reconnaissance Orbiter (MRO) to the Mars Network in 2005. MRO will carry the first Electra6, a firequency-agile soflware relay radio, and will have high rare X-band and Ka-band Direct-To-Earth links. While the Mars Network of science orbiters with relay radios has greatly improved our ability to navigate and communicate witb Marscraft (landers, rovers, aerobots, and orbiters in the vicinity of Mars),its capabilities are limited because it has been constructed by adding relay radios to orbiters optimized for science missions rather than for communications. NASA will augment the Mars Network in 2010 witb the Mars Telecomm~cations Orbiter (MTO), the first interplanetary spacecraft opthized for relay communications services. Figure 3 depicts the Mars Network prior to the addition of MTO to the constellation. MTO will be placed into a high orbit selected specifically for its relay mission. MTO will have the most advanced communications system ever put on an interplanetary spaceCraq with high performance X- and Ka-band links to Earth, high performance UHF and X-band relay links to other Mammft, and an experimental laser communications payload for Duect-ToEarth ( D E ) communications. MTO will dramatically increase both the data return !?om other missions sent to Mars and the amount of time and frequency Of Contacts to Mars missions, Figam 3 M a n lyctwo& Comef]ation fundamentally improving our ability to monitor and control Marscraft and leading to more flexible and reliable operations. The lessons learned at Mars with respect to operations and communication networking concepts can be directly applied to similar problems at the Moon and other destinations throughout the solar system. - V. Evolution of the Mars Network The Mars Network consists of Mars orbiters with radios capable of relaying communications to and from other Marscraft and ground Stations on Earth. The Mars Network currently consists of the NASA Mars Global Surveyor (MGS) and Mars Odyssey orbiters. The origin of the Mars Network can be traced to a French Mars balloon mission: Jacques Blamont of CNES proposed to send balloons to Mars on what eventually became the ill-fated Russian Mars -% spacecraft. To communicate with these balloons -which would have had no DTE capability - CNES developed the Mars Balloon Relay (MBR). A CNES MBR was first sent to Mars on another ill-fated spacecrafi in 1992: NASA’s Mars Observer. A CNES MBR successfully reached Mars on MGS in 1997. The international interest in relay communications at Mars led to the development of a space relay radio standard by the Consultative Committee on Space Data Standards (CCSDS): the Proximity-1 standard5. Proximity-1 is a flexible bidirectional protocol providing several operating modes and levels of service. While initially developed for Mars missions. it is intended to be used for other space missions as well. The CE 505 relay radio sent to Mars on the NASA Mars Odyssey orbiter in 2000 was the first to implement the CCSDS Proximity-1 protocol. This was followed by the MELACOM radio on Mars Express. MGS, Odyssey and MRO have nadir-pointed UHF Low Gain Antennas for relay communications. The plaiforms on which each of these LGAs is placed are shared with several scientific instruments, resulting in irregular, less than optimal performance paiterns. Mars Telecommunications Orbiter (MTO), to be launched in 2009, will have high zyxw 4 American Institute of Aeronautics and Astronautics zyxwvuts zyxwvuts zyx zyxwv zyxwv performance steered X-band and UHF relay antennas on a dedicated relay antenna platform. Because these antennas are steered. they will normally operate near their peak gain - unlike the UHF relay antennas on other Mars orbiters. The Mars Network evolved from an experiment, but has become a cornerstone of NASA’s deep space Ttre p h d approach to building the network, exp!o~-i~tion c z h new aibithg inissicin augrmfmg the network’s deployed capability has provided significant benefits for a modest additional investment in each mission. Figure 4 shows the deployment phasing ufthe existing and near-term tvfars Network ~ e T Q @ i ? 3 i r : Dl Q C 3 W M Figure 4 - Deployment Phasing of the Mars Network VI. As demonstnded through the zyxwv zyxw zyxwv X zyxw F L E 36 ‘C ‘t $2 Lunar Proximity Network Architecture Mars Network concept, ensuring interoperability between multiple missions requires both technological capability and standardization. P L has demonstrated initial success by standardizing on the CCSDS Proximity-1 protocol for relay applications on their deep space missions, and through the use of basic software radio hardware in the form of the Electra transceiver. Ensuring interoperability across the wider range of missions envisioned by the exploration program and Project Constellation requires a broader standard and more capable technology than has been thus far successfully demonstrated on Mars. Efforts in partnership between Goddard Space Flight Center, the Jet Propulsion Laboratory and the Glenn Research Center are underway to develop a common standard for a Space Telecommunications Radio System (STRS) and advanced, interoperable s o h e defined radios. STRS is an effort, supported by NASA Headquinters (Space Operations), that aims to establish the requirements and interoperability standards for a space capable software defined radio (SDR) architecture. By leveraging investment made in the DoD Joint Tactical Radio System (JTRS) program, STRS will provide to NASA a highly interoperable SDR architecture for use in manned and robotic Spacecraft, rovers*and infrastructureassets. Figure 5 - Conceptual Lunmr Communication Relay Network As depicted in Figure 5, a Lunar communication relay network can provide continuous coverage of any point on the surface of the moon. Simiiar in architecture to the Mars Network, or the terrestrial Iridium constellation, the network would consist of a fleet of zyxwvutsrqponm 5 American Institute of Aeronautics and Astronautics zyxwv zyxw zyx zyxwv zyxwv zyxwvutsrq zyxwvutsrqp zyxwvuts zyxwv zyxwvutsr relay spacecraft in circular or elliptical orbits. Proper phasing of the orbits can ensure that at least two relay spacecraft are visible from any surface location at any given time. The spacecraft can provide services along the lines of the Earth based TDRSS, including communication and navigation s u p p o ~providing a seamless operations ccncept ?XtwrnPar-=&t! md Lur?... SpXP. In addition to basic communication and navigation services, advances m networking protocols and standards --1. p s i b i e &e depiqrnmt of u k i * ~ - K k c xchh%ires n in s p c e . K e i g cammuni&oo s y s t e m envisioned by Goddard and JPL include rP and CCSDS packet routing, store-and-forward capabilities, and autonomous link detection and negotiation. By enabling relay providers to manage their own links through ad-hoc negotiations, increased network availability and operational flexibility is achieved These capabilities may prove very useful, especially in deployed sensor web concepts in which many small sensors are scattered over the surface and are responsible for their own self-orgsmizationand communication. With points of interest on the Moon located out of view of Earth, it is also d e s i l e to reduce SCtKduling and network coordination between relay assets, user spacecraft, and mission planners and flight controllers. By permitting the network to be self-organizing in an &hoc manner, similar to terrestrial 802.1 1 wireless networking, communication can be accomplished with a minimum of --based operator intervention. The idea of an ad-hoc commmication relay network is being investigated as a key element of the early Lunar Network in support of initial robotic exploration missions. Under this model, each robotic orbiter would cany with it a commlmication relay payload, and would become a network asset upon completion of its primary science or exploration mission. Missions originally placed into an appropriate orbit may assume their secondary relay role immediately. This is analogous to the Science Mission Directorate approach on Mars, and it would be a logical extension of the hfrastructure supporting robotic e?rploration as it can expand with time to serve eventual human missions as well. A - Direct to Moon @ThQ ranging when s m a m a t l is on “Fmnt Side” B - DTM ranging w k n subsatellite is visible - Relay mging to sFml%naR through subsarelllte &en missmn IS In the “Shielded Zone” c \ L t . Figure 6 - Lunar Far Side Relay Concept VII. Implementation of Lunar Proximity Network Both economic and physical factors demand that NASA accomplish as much as possible, and at the lowest possible mass for each mission. Economic in that the cost of sending mass to the Moon is significant (and nearly half of the mass sent to the Moon on any given mission is fuel). Physical in that the tidally locked rotation, and three-body gravity environment of cis-lunar space impacts both landed and orbital missions: landed missions to the far-side will never have direct views of Earth, while orbiting missions must make fiequent orbit maintenance burns to maintain altitude. A series of low maintenance, “from” orbits have been identified by Goddard and JPL into which a spacecrafi can be placed for its extended mission as a relay. For missions not in low maintenance orbits, such as LRO’s 50 lon science orbit, fkequent bums will be the norm. Each kilogram of spacecraft dry mass requires nearly a kilogram of fuel for a one year mission in a low lunar orbit. With mass (and fuel) at a premium, and an established operational need for relay communication in order to explore the far-side and many polar regions (as depicted in Figure 6), the argument for sharing resources through mission communication and navigation cross-support is strengthened. 6 American Institute of Aeronautics and Astronautics zyxwv zyxwvutsrq zyx zyxwvu zyxw zyxw zyxwvu zyx zyxwvu zyxwvu Each mission to the far-side (or into a location out of view of Earth such as a polar crater) requires relay communication to be successfid. To ensure success, it is advisable to provide redundancy in communication assets. On a single spacecraft mission, this would ordinarily be achieved by providing a secondary communication payload on the host relay spacecraft. This is in part the reliability approach taken wi?h the E l m Proximity Pay!& on the Mars Telecommunications Orbiter spacecraft. The probability of failure in both radios is unlikely, so the likelihood of a findonat relay mpahility is hi&-.. The psibi!i?y does exist, bo-x-vei, fii a s - j j m 2 fai:wc cxjicfusive of the communication system to disable both radios. In the lunar problem, however, the extra mass of the (hopefully) unused communication system comes at a premium in terms of fuel which translates to cost. If, instead of each mission providing its own redundancy and absorbing the associated cost, the mission is considered to be one of a series of mutually supportive missions. In this case, the entire program is treated as an integrated whole fiom the standpoint of communication and navigation infrastructure, and a new option becomes available. It is now possible to provide redundancy to ensure mission success by providing for redundant assets rather than d m h t components. That is, with the likelihood of failure of a radio being nearly the same between two spacecraR the likelihood that at least one of the two radios will be hctional is increased in comparison to that of a traditional dual String redundant mission. This argument emerges from the fact that the second radio, on a second independent spacecrafi is immune to a failure (not necessarily in the radio) on the first spacecraft. In addition to the increased redundancy for less mass that is achieved by employing a relay concept, the missions enjoy increased operati0~lflexibility. The more spacecraft that are capable of relay support, the more availabie communication contacts will occur. This allows mission planners to select more freely from the available contacts, or to use greater amounts of bandwidth to return increasing science measurement volumes to Earth, again increasing the benefit of a mission to science and exploration. It is possible to deploy proximity relay communication systems in several fonns: 1. A proximity relay communication system can be created by assembling appropriate off-the-shelf transmitters, receivers and digital interf8ces. These components are combined to provide an independent RF communication subsystem with its own interface to the spacecraft command and data handling system. This comes at the expense of increased component count, generally greater masses, and greater loading on the flight software. It can, however, be the least expensive option. 2. Relay optimized transceivers such as the P L Electra are designed to be entire relay communication subsystems in one component The Electra,and devices like it, combine a transmitter and receiver with a baseband processor to provide protocol and link management functions. The interface to the main spacecraft C&DH system involves command and data transfers, with system maintenance being handled internally. 3. A third option, enabled by reconfigurable communication technologies is to integrate relay functions into the primary spacecraft communication system. In this case, one communication system serves both the function of TT&C as well as relay. This option can provide the highest level of integration, but introduces additional complexity into the system. A final factor in establishing a proximity network is the need for interoperable modulation, frequency, channel coding, framing, and protocols. A key to the success of the Mars Network concept is the development and use of the CCSDS Proximity1 protocol, which calls for standard physical (frequency, modulation). link (coding, framing) interfaces, and basic network functionality in the form of reliable packet delivery. Over the Proximity-1 link spacerraft can then exchange data using reliable transfer protocols, such as CCSDS CFDP’,which allow assured delivery of large files. Vm. Conclusion Unique factors affect the design of lunar exploration missions. In order to achieve science and exploration objectives, it is necessary to deliver probes and other mission elements to areas of the Moon that are permanently cut off from communication with Earth. Transmission of commands to and retrieval of data from these missions require proximity relay communication cross-support mpability in another asset. To reduce the overall program cost, and increase each mission’s reliability, it is advisable for each long-duration orbiter to c a y a proximity relay capable communication system. Missions should rely on each other for support and redundancy rather than requiring specific missions to cany high reliability, high cost, redundant systems to ensure mission success in the event of a hilure. While requiring each long-term orbiter to provide single string relay support increases each mission’s costs,total costs to the program are decreased, while program reliability is increased, as is operational flexibility and science return. 7 American Institute of Aeronautics and Astronautics zyxwv zyxwvuts zyxwvu zyxwvutsrq zyxwvutsrqpo zyxwv zyxwvu zyxw zyxwv zyxwvutsrqp Acknowledgments The work described in this paper was carried out at NASA's Goddard Space Flight Center and at the Jet Propulsion Laboratory, California Institute of Technology under contract to NASA. References ' The I isionfor S p c e Erprorufion.National Aeronautics and Space Administration, NF-2O04-01-334-HQ. 2"Clementine Mission Information-. National Space Science Data Center. Goddard Space Flight Center, h ~ : / / n s s d c . g s f c . n ~ o v / D l a n e t a r v / c ihtml. e. %uuar Prospector Information". National Space Science Data Center. Goddard Space Flight Center. h t t D : N n s s d c . P s f c . n a s a r c o v ~ l .html. ~e~~~ 'W.C. Feldman, et. al., ''Fluxes of Fast and Epithermal Neutrons fi-01~1 Lunar Prospector:Evidence for Water Ice at the Lunar Poles," Science, Voi. 281, Issue 5382,4 September 1998, pp 14%1500. 5Praxim~y-ZSpnce Link Protocol, current version, ConsultativeCommittee on Space Data Systems, CCSDS 21 1.@El. "Edwards, C.,et ai-,T h e Electra Proximity Link Payload for Mars Relay," 54th International Astronautical Congress IAF03-Q.3.a-06.Bremen. Germany, October 2003. 'CCSDS File D e h e n Protocol, current version. Consultative Committee on Space Data Systems, CCSDS 727.0-B-2. 8 American Institute of Aeronautics and Astronautics