Communication Architecture and Technologies for missions to Moon, Mars, and Beyond

Transcription

1 T T T T T T 1st Space Exploration Conference: Continuing the Voyage of Discovery 30 January - 1 February 2005, Orlando, Florida AIAA Communication Architecture and Technologies for missions to Moon, Mars, and Beyond * Tapan R. KulkarniT T, Avinash DharneT T and Daniele MortariT T Texas A&M University, College Station, TX, This paper proposes communication architecture to support data and command communication for future space missions to moon, Mars and beyond. It postulates a set of satellites placed in halo orbits at Earth-Moon, Sun-Earth and Sun-Mars Lagrangian points and a couple of relay satellites in a heliocentric earth orbit as the base of this architecture. This arrangement is robust to loss of service problems because of the redundancy built into the system. Also, the system significantly reduces the distances over which communication has to be achieved and thus lessens the demands on the communication hardware to be employed. This also results in power saving. The architecture is scalable and can be extended to missions farther out into space. As in the development of any infrastructure, some initial investment has to be made which however will provide returns in the form of simplifying communication procedures and protocols for the many space missions to come. I. Introduction n past, communication architecture of every spacecraft mission was developed independently and it was unrelated Iwith the communication architecture of some another mission. e.g. Earth-observing missions had communication architecture developed independently with respect to the ISS and Space Shuttle missions. Tracking and Data Relay Satellite System (TDRSS) was later implemented by NASA as a space network for manned missions and Earthobserving missions. Unfortunately, the costs associated with TDRSS services are high for most Earth-observing satellites and therefore these missions use new and modified old ground stations for obtaining their data. Communications for Mars and deep space missions were also developed independently from others and shared the use of the Deep Space Network (DSN). Future space missions (robotic) will need to operate autonomously by sensing the area near them in order to make decisions about where to go, how to go, what samples to measure, what 1 data to report and how to request and connect to the space communication network. Some robotic spacecraft may even need human intervention such as in assembling large space structures. Thus, there is a need of an advanced, compact and well-integrated communication architecture that enables reliable, multi-nodal and high data rate capabilities to provide continuous coverage of missions to Moon, Mars and beyond. So the principal objective of this paper is to investigate efficient and autonomous communication architecture and technologies to allow reliable communication from Earth to Moon, Mars and beyond. Minimum cost solutions, that are significantly more compact and efficient, will be identified among those fully complying with the network requirements and constraints. Dual-purpose architectures (e.g. communication, mapping), will be also investigated. The paper will focus on mission design that includes feasibility, planning, and maintenance for the optimal configurations, to improve connectivity with minimum costs. As for the communication architecture the costs are the mission cost, and the orbit and attitude control costs. A wide variety of requirements and constraints will be taken into consideration while proposing the solution scenarios, such as: 1. UService continuityu: If anyone of the communication nodes becomes inoperative (either, permanently or momentarily), then the communications are still guaranteed. 2. Uower efficiencyu: Minimize inter-node angles variations (to narrow the antennae FOV), minimize internode distances (to limit communication power), etc, 3. UTime efficiencyu: Minimize the overall distance (to limit communication times). * Graduate Student, Department of Aerospace Engineering, 3141 TAMU, AIAA Student Member. Graduate Student, Department of Mechanical Engineering, 3123 TAMU, AIAA Student Member. Associate rofessor, Department of Aerospace Engineering, 3141 TAMU, AIAA Member. 1 Copyright 2005 by the, Inc. All rights reserved.

2 The paper proposes to use a configuration in which spacecrafts are placed in halo orbits around Cislunar (LL1), Translunar (LL2), Sun-Mars L1 (SM L1), Sun-Mars L2 (SM L2) lagrangian point. For Earth-Moon Communication Architecture (ELCA) both the spacecrafts, at LL1 and LL2 are visible to each other at all times and the spacecraft at LL2 can map the far side of Moon2. For Earth-Mars Communication Architecture (EMCA) a potential three-node solution can be: one at SE-L2, one at SM-L1, and one at SM-L2. With two additional nodes, at relay points (R1 and R2, in heliocentric orbit) the nodes see one another at all times. Similarly, a configuration to planets beyond Mars can be constructed3. The proposed system will have a dramatic impact on reducing cost and optimizing the functionality of the configuration for planet surface navigation, as well as mapping the features of interest from orbit. II. Architecture, Technology and Orbits of spacecrafts for regions of interest A. Moon The lunar communication architecture (ELCA) for human and robotic mission includes two spacecrafts placed in halo orbits at the Cislunar (LL1) and Translunar (LL2) lagrangian points and lunar surface wireless local area network (WLAN). The radius and the period of the orbits can be adjusted in such a way that the spacecrafts in these halo orbits are visible to each other at all times. The spacecraft in halo orbit at LL2 can map far side of the Moon effectively i.e. 100% 24/7 coverage. We designed and ran a STK/Astrogator scenario in which the spacecraft is placed in halo orbit at LL2 and observed that the spacecraft comes close to approximately 4477 km from the lunar southern pole. The period of the halo orbit was 12 days. An impulsive maneuver takes place every time the spacecraft crosses Earth-Moon L2 Z-X plane with VBxB=0 km/s. A similar halo orbit was constructed at LL1 lagrangian point. These two spacecrafts are always visible to other spacecrafts in Earthorbit. The characteristics and requirements of node-to-node (LL1 to LL2) link options that were considered for Earth to Moon and Moon vicinity communications are as follows 4. Table 1: Characteristics and requirements of node-to-node link options for Earth to Moon and Moon vicinity communications Node-to-node link Data rate(mbps) Distance Technology Earth-Moon L1 (LL1) spacecraft to: Earth ground > ,000 km Ka-,X-bands Earth Orbit relay ,000 km Optical Moon science orbiter ,000 km Ka-,X-bands Moon human outpost ,000 km Optical, Ka Earth-Moon L2 (LL2) spacecraft to: Earth ground > ,000 km Ka-,X-bands Earth Orbit relay ,000 km Optical Moon science orbiter ,000 km Ka-,X-bands Moon human outpost ,000 km Optical, Ka Lunar Human outpost Wireless Local Area Network (WLAN) to: Other lunar surface entity at close range Other lunar surface entity at long range > m Ka-,X-,C-bands >50 50 km Ka-,X-,C-bands 2

3 B. Relay Spacecrafts Two additional Relay spacecrafts (R1 and R2) could be placed in heliocentric orbit coincident with Sun in such a way that the two spacecrafts and Earth are all separated by 120 from one another. In this way, one spacecraft leads and another one lags Earth by 120. Introduction of these relay spacecrafts reduces the length of communication link by about 47.6 %. The characteristics and requirements of node-to-node (R1 and R2 to Earth) link options that were considered for Earth to relay spacecraft communications are as follows 4. Table 2: Characteristics and requirements of node-to-node link options for Earth to Relay Spacecrafts communications Node-to-node link Data rate(mbps) Distance Technology Relay spacecrafts (R1 and R2) to: Earth ground >100 1 AU Optical Earth Orbit relay >100 1 AU Optical C. Mars The Martian communication architecture (EMCA) for human and robotic mission includes three spacecrafts,first two placed in halo orbits at the Sun-Mars L1 (SML1) and Sun-Mars L2 (SML2) lagrangian points and the remaining one can be in Mars-synchronous or Mars-polar orbit finally. Additionally there can also be a Mars surface wireless local area network (WLAN). Fig. 1 shows the configuration of spacecrafts in halo orbit at SML1 and at SML2 and polar orbit around Mars. To Sun To Relay in Earth Heliocentric Orbit SM L1 SM L2 Figure 1. Configuration of spacecrafts in halo orbit and polar orbit around Mars The two spacecrafts in halo orbit at SML1 and at SML2 are visible to each other at all times. A constant communication link is maintained with the spacecraft in polar orbit, which, in turn is in contact with the rover on the surface. Alternately, the rover can communicate with the two spacecrafts in halo orbit if there is any communication breakdown with the orbiter. The spacecraft in SML1 halo orbit communicates with the relay spacecrafts, which in turn communicate with spacecraft in halo orbit around Sun-Earth L2 (SE L2) lagrangian point. The spacecraft at SEL2 communicates with other Earth-orbiting spacecrafts. So, in this way, a communication link is established from Mars to Earth. The characteristics and requirements of node-to-node (SML1 and SML2 to Earth) link options that were considered for Earth to Mars communications are as follows 4. 3

4 Table 3: Characteristics and requirements of node-to-node link options for Earth to Moon and Moon vicinity communications Node-to-node link Data rate(mbps) Distance Technology SM L1 spacecraft to: Earth ground >1 2.5 AU Ka-,X-bands Sun-Earth L2 spacecraft > AU Optical Mars science orbiter ,000 km Optical, Ka Mars human outpost ,000 km Optical, Ka SM L2 spacecraft to: Earth ground >1 2.5 AU Ka-,X-bands Sun-Earth L2 spacecraft > AU Optical Mars science orbiter ,000 km Optical, Ka Mars human outpost ,000 km Optical, Ka Mars Human outpost Wireless Local Area Network (WLAN) to: Other Mars surface entity at close range Other Mars surface entity at long range > m Ka-,X-,C-bands >50 50 km Ka-,X-,C-bands We now discuss the scenarios and explain how communication will be maintained when Earth and Mars continue to revolve around Sun. Fig.2a shows a scenario when Earth and Mars are closest to each other and Fig.2b shows when Earth and Mars are furthest from each other. R2 R2 L2 L2 R1 R1 Figure 2. a. At closest distance b. At farthest distance From Fig.2a. it can be seen that Earth, R1 and R2 are at 120 from one another. When Earth and Mars are at closest distance, then communication will take place directly between the spacecrafts at SM L1 and SE L2. The bold orange line in Fig.2a shows that the communication between SEL2 and SML1 will be given higher priority and that between Earth and R1 and R2 will be given a lower priority (as shown by dashed blue lines). 4

5 Fig.2b shows a case when Earth and Mars are at farthest distance from each other. In such case, communication link will be established between SML1 spacecraft and either R1 or R2 (bold orange arrows). If R1 is chosen as a communication node, then the link will be completed as Mars rover, Mars polar satellite, SML1, R1, SE L2 and Earth-orbiting spacecraft. The communication between R1 and R2 can be given low priority (dashed blue line) in such a case. Alternately, if R1 deems failed or there is any breakdown at R1, then communication can be established through R2. Fig.3a shows a scenario when Mars is between R2 and Earth. In such case, a direct communication link will be established between SML1 and SEL2 since establishing a link with R2 will only increase the communication link length. So, the length of communication link is a critical parameter and shorter the link, the better. R2 R2 L2 L2 R1 R1 Figure 3. a. At midway distance between nodes b. Inter-node failure Fig. 3b shows a case when there is an inter-node failure (dotted blue line). In such a case, the communication will be re-established with the help of another relay satellite (R1 in this case). D. Beyond ( Jovian, Saturnian and beyond ) The relay system for Mars employs relay spacecrafts in Earth orbit. Similarly, the relay system developed for Jupiter uses relays in the heliocentric orbit coincident with the Martian orbit of the Sun. The Jovian system, however, does not employ the relays given in the Mars system. This is because of the fact that Jupiter is very far from the Mars orbit and the reduction in maximum distance by the inclusion of the relay spacecraft is very small. As in the Mars system the Jovian relay system includes relays in halo orbits at the L1 and L2 lagrangian points of the Sun-Jupiter system. Note that the distance from Jupiter to its L1 and L2 lagrangian points is rather large compared to the Martian L1/L2 points. This large distance between Jupiter and its L1/L2 points means that the entirety of the orbits of Jupiter s moons is easily covered by the halo relays. Additionally, the large distance from Jupiter to the halo relays will put them out of Jupiter s intense radiation belts. The concept for a relay system for Jupiter is given in Fig.4. Assuming that 10 Mbps rate can be achieved for the Jovian relay system, the gains provided by the use of Kaband system are used. Using the relay parameters for Earth-Orbit and Mars-orbit relays as following 5. Table 4: Relay parameters Earth-Orbit relay Mars-Orbit relay Frequency (GHz) Antenna diameter Transmit power (W) Receiver temp (K)

6 Also by assuming, that a data source transmitter (.i.e. an orbiter) near Jupiter with an aperture of 3 m and a 25 W transmitter, transmission from Jupiter to the relay spacecrafts in halo orbits at SJ L1 and at SJ L2 is at the Ka-band frequency of 32 GHz similar to the transmission from these relay spacecrafts to the relay spacecrafts in halo orbits at SM L1 and at SM L2 lagrangian points. Figure 4. Relay Network to cover Jovian system Based on Table 4 and above assumptions, the Jovian system link analysis results can are as follows 5. Table 5: Jovian system link analysis results Link Max range (AU) Min. data rate (Mbps) Jupiter to SJ L1 or SJ L SJ L1 or SJ L2 to Mars or relay spacecrafts in halo orbits at SM L1 and SM L2 Mars relay spacecrafts in halo orbits at SM L1 and SM L2 to Earth Orbit Between relay spacecrafts (R1 and R2) and spacecrafts in Earth-orbit (X-band) (X-band) A relay network for Saturnian system can be developed based on similar lines. 6

7 III. Cost Analysis NASA seeks to reduce the costs of designing, deploying and operating space exploration missions. So the keywords for space funding policy are Faster, better and cheaper. Cost analysis can be performed by evaluating the cost of developing, deploying and operating interplanetary, communication satellites 6. A. Development Costs The relay satellites R1 and R2 and all other spacecrafts in Mars-vicinity and Jovian vicinity can be classified both as interplanetary and communication spacecrafts. A common indicator of cost for spacecraft is its mass. The weighted average mass of communication spacecraft is approximately 1100 kg like Intelsat VII and the TDRS. Typically, for an interplanetary and communication spacecraft, mass includes antennas, communications equipment, other mission payload, and equipment such as thermal and attitude control subsystems. Adding the mass of propellant will give total mass of the spacecraft. For communication satellite, propellant mass is equal to the dry mass of the satellite. Assuming two 20 m antenna of mass 1500 kg, the mass of spacecraft comes to 4100 kg. For Cassini (interplanetary spacecraft), the propellant was 1.3 times dry mass of spacecraft. So the total mass comes to 9430 kg. Taking a clue from the cost of developing relay spacecrafts similar to the Lunar rospector ( at LL1 and at LL2), Genesis ( at SE L2), Mars Global Surveyor (Mars polar orbit and at SM L1 and at SM L2) and Galileo ( at SJ L1 and at SJ L2), the total development cost is approximately $ 7.8 billion for 10 spacecrafts. B. Deployment Costs Choice of a launch vehicle depends on factors like the mass and external dimensions of the spacecraft. Also, the launch vehicle itself can set constraints on spacecraft design in order to keep the spacecraft s mass and dimensions within the payload bay. So, taking an additional $ 50 million for each launch and for 10 launches, the deployment cost is $ 500 million. This further takes the total cost to $ 7.85 billion. C. Operation Costs Once in orbit, the spacecraft needs to be looked after, with someone available to monitor the spacecraft, usually on 24 hour basis. So operation cost is the cost spent to ensure that spacecraft performs the mission for which it was built. Operating costs cover a broad category of functions and can extend into decades. e.g. Voyager spacecrafts are still going on even 25 years after their launch. Operating cost for interplanetary spacecrafts vary by missions and over the period. e.g cost for Mission Operations and Data Analysis for Mars Observer totaled $40.5 million and was reduced to $ 34.3 million. Operating the DSN averages $ 34 million. Since all the relay spacecrafts are interplanetary spacecrafts that will supplement the DSN, the operating cost of $34 million per spacecraft can be assumed. Assuming this cost to remain constant over the lifetime of all relay satellites, total operation costs comes out to be $340 million. This takes the total cost of the mission to $ 8.19 billion. Although this is technically feasible, paying $ 8.19 billion seems a lot. But the returns (compact, autonomous, reliable, multi-nodal interplanetary communication architecture) may certainly outweigh the costs involved in long run. D. Cost reduction Spacecraft mass can be reduced by using lightweight components, alternative designs and advanced technologies, which, in return will reduce deployment cost and the total cost. Development cost can be reduced by reducing antenna size or reducing spacecraft mass or both. One alternative propulsion technology, electric propulsion is currently being evaluated for planetary missions. IV. Conclusion This paper proposes communication architecture to support data and command communication for future space missions to moon, Mars and beyond. This architecture is designed to be robust to communication link failures between the satellites and to provide a common underlying architecture on which future space missions can be planned. It is also designed to reduce the distances over which satellites have to communicate thus reducing the demands on communication hardware and power supply. This infrastructure, once built will simplify the communication issues in future space missions and thus make a return on the investment required to build it. 7

8 References 1 NASA, The Vision for Space Exploration, February Tapan R. Kulkarni, A mission to Earth-Moon L1 and L2 Lagrangian points, AIAA Region IV Student Conference, UT Arlington, April 01-03, Tapan R. Kulkarni and Daniele Mortari, Low energy interplanetary transfers using halo orbit hopping method with STK/Astrogator, AAS , 15th AAS/AIAA Space Flight Mechanics Meeting, Copper Mountain, Colorado, January 23-27, Kul Bhasin and Jeffrey Hayden, Developing Architectures and Technologies for an Evolvable NASA Space Communication Infrastructure, 22nd International Communications Satellite Systems Conference and Exhibit 2004 (ICSSC), Monterey, California, May 9-12, O. Scott Sands and Kul Bhasin, Relay Station Based Architectures and Technology for Space Missions to Outer lanets, 20th AIAA International Communication Satellite Systems Conference and Exhibit, Montreal, Canada, May Timothy G. Howard, An initial design assessment for a communications relay satellite to support the interplanetary information infrastructure, AIAA 16th International Communication Satellite Systems Conference, Washington D.C., Feb 25-29,