CONCEPT FOR CONTINUOUS INTER-PLANETARY COMMUNICATIONS

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CONCEPT FOR CONTINUOUS INTER-PLANETARY COMMUNICATIONSStevan M. Davidovich, Lockheed Martin, Western Development LaboratoriesJoel Whittington, Harris CorporationSunnyvale, California 94089then, satellites have become a major component y, satellites are an essential part of spaceexploration.AbstractA concept for inter-planetary communicationsis proposed. The concept employs three polar orbitingsatellites around the sun and a combination ofgeosynchronous and polar orbiting satellites aroundplanets of interest in the solar system. The key aspectof this concept is that it assures a continuouscommunication connection between two objects withinthe solar system, be it a spacecraft or a planet. Theorbital aspects of this concept are described, estimatesof the propagation delays are provided and a protocolfor exchanging and maintaining solar time is discussed.In addition, a RF communications link budgetassessment is made and the expected performancepresented. Performance areas where design tradesneed to be performed and a few enabling technologiesare briefly discussed.Inter-planetary exploration, be it Lunarhabitation, asteroid mining, Mars colonization orplanetary science/mapping missions of the solarsystem, will increase demands for inter-planetarycommunications.The movement of people andmaterial throughout the solar system will create theeconomic necessity for an information highway tomove data throughout the solar system in support ofinter-planetary exploration and exploitation.Thecommunication capabilities of this solar systeminformation highway need to be designed to offer; 1)continuous data, 2) reliable communications, 3) highbandwidth and 4) accommodate data, voice and video.As with most uncharted endeavors, it makessense to leverage off of existing technology andexplore enabling technologies that seem to offer themost promise. Today, the performance of satellitecommunication systems is very well known and aplethora of information exists on their analysis anddesign. This knowledge can be leveraged to build anew class of satellites for the solar system informationhighway.NomenclatureaABPSDd Semi-major axis, km Antenna aperture area, meters 2 Bits per second Aperture Diameter, meters Distance, meters or kilometers Orbital inclinatione Orbital eccentricityEIRP Effective Isotropic Radiated Power, dBWEi/N0 Bit Energy per Noise power density, dBf Frequency, hertzG Antenna gain in dBk Boltzman' s constant, 1.38 x 10·23 JoulefKelvin Implementation lossesL, Path LossP Power, wattsr radius distance of planet or orbitR Data rate, BPST Noise Temperature, Kelvin Time of periapsis passageµ Gravitational constants, km3/sec 2A Wavelength given by elf.Tl Antenna aperture efficiencyt:N Orbital velocity, km/secThis type of communication infrastructurewould greatly assist future space missions in the solarsystem. This paper will focus on the capability toprovide continuous communication services in directsupport of serious exploration of the solar system. Ourreasoning is based upon our current perception of whatis happening on Earth. On earth, the demand forcommunications has lead to the development andcontinuous expansion of the information highway.This expansion has included the transportation servicesindustry where today any truck, ship or airplane has theability to have access to wireless voice communicationand position determination services with either the USGPS system or it's Russian variant.IntroductionSatellite communications was born in the late1950s with the successful launch of SPUTNIK. SinceCopyright 1999 by the Space Studies Institute. All rights reserved213Today, space missions rely on their owncommunications systems to transmit and receive datafrom earth. NASA operates the Deep Space Network(DSN) that helps provide transmission of data betweenEarth and the various satellite control complexes, butthe space vehicle is still required to have enough powerto transmit to Earth, and always encounters outages

could be introduced around a planet to provide polarcap coverage.caused by solar or planetary obstructions. The recentfailure of Galileo's high gain antenna to deploy hasgreatly limited the amount of data that will be availableto Earth scientists over the life of the satellite mission.Large propagation delays, due to the size of thesolar system, cannot be avoided.This clearlyprecludes real-time interactive voice or video sessionsoutside the earth to lunar regions. Yet, the need forcontinuous, uninterrupted services of data, voice andvideo is important. Anyone monitoring the health andstatus of a space vehicle desires the ability to havecontinuous monitoring of spacecraft systems andpositions allows for quicker response to react toonboard anomalies. The recording of a spacecraft'sstate just prior to a major mishap would be ofsignificant help during an accident investigation sincethe concept of retrieving a flight data recorder (as usedin the aviation industry today) is not feasible in spaceexploration. As more humans venture into the solarsystem, these "safety of flight" issues become "safetyof life" issues. This means that service interruptionscaused by planetary and solar interference need to beminimized if not totally eliminated.Secondary reasons to develop and deploy asolar system information highway involves economicbenefits upon spacecraft designers and spacecraftoperators.By providing communication satellitesthroughout the solar system, satellite weight can bereduced since the spacecraft only needs to find it'snearest relay. The wireless industry has demonstratedits viability by providing customers with low-powerphones that interconnects them to anyone aroundworld. Mobile phones are currently interconnected toterrestrial base stations and in the near future spacebased networks such as Globalstar or Iridium.Figure 1 - Solar Polar ConstellationThe three solar polar planes are evenlyseparated by 120 degrees and have a semi-major axisdistance and eccentricity such that they could be easilylaunched from Earth and maintain a constant distancefrom the Earth. If needed, the relay satellite orbitcould be adjusted at either solar pole, which occursonce every six months, with a small velocity vectorchange to allow it to be captured in Earth orbit forretrieval or maintenance. This is a key aspect of theconstellation as will be explained later. It would takeeight months to fully deploy the solar polar orbitingconstellation.The satellites would be interconnected .withsatellite cross-links and would find the shortest pathsback to earth. These cross-links could use existingtechnologies and be either RF, optical or a combinationof both. This paper assumes current RF satellitecommunication technologies. Bandwidth could easilybe increased as new technologies are developed.The ConceptTheconceptproposestheuseofgeosynchronous satellites in planetary orbits to formplanetary communication networks to supportplanetary operations. We have tremendous civilianand military experience in the design, development,deployment and operation of geosynchronouscommunication satellites around Earth.Thisexperience can easily be applied to setting up similargeosynchronous communication constellations aroundother bodies in our solar system to provide planetarycommunications. These planetary constellations wouldbe interconnected to a network of three polar orbitingsatellites operating in the solar polar plane as shown inFigure 1. These polar orbiting satellites would formthe hub of an interplanetary communication network.In addition, various types of polar orbiting satellites214As already mentioned, the polar orbitingsatellites would pass by Earth every six months. Thisallows for possible maintenance, repair or retrievaloperations. With a fully deployed constellation ofthree satellite relays, there would be six earth flybysper year or a flyby every two months. In the event of amajor malfunction, replenishment satellites could beprepositioned in backup planes and activated if needed.Orbital CharacteristicsNewton's laws of motion lead Newton toaccurately assert that orbital motions are defined asellipses. Five independent parameters are needed tocompletely describe the size, shape and orientation ofan orbit. A sixth parameter is needed to predict the

location of an object along its orbital path at a specifiedtime. The classical set of five orbital parameters aredepicted in Figure 2 and are:1.2.3.4.5.4x:: xai - ,planetµ-km 3/sec2a, Semi-major axise, Eccentricityi, Inclinationn, Longitude of the ascending nodeco, Argument of periapsisThe sixth parameter is t, and is the time ofperiapsis tune2.23E 43.26E 53.99E 54.31E 4l.27E 83.80E 75.82E 66.90E 6CJ -rev/secl 59,40861,331Table 1 - Estimated Geosync DistancesThe values for Mercury and Venus are notreally valid since the distances are far in excess of theplanet's sphere of influence. The orbital perturbationsfrom the sun upon a satellite in these orbits wouldrequire an almost continuous fl V to keep it in ageosync orbit. The type of constellation best suited fora communication network around Venus and Mercuryis beyond the scope and intent of this paper, however,it might be doable with a solar sail. This is left as afuture topic to explore.Figure 2 - Orbital ElementsA special class of orbits occurs when theorbital eccentricity e 0 or is very close to 0. In thiscase the orbit is considered a circular orbit. The orbitvelocity in a perfect circular orbit can be calculated as- vcircular --reenter[l]where µ is the gravitational mass constant of the planetand r eenter is the distance to the center of the planet.'Circular or near-circular orbits are used in two specificapplications, which is of interest to us.The first is a geosynchronous orbit and isdefined as an orbit plane that is along the equator of aplanet and is at a distance such that a satellite's angularvelocity matches that of the planet. This is the primaryorbit type used by military and civilian communicationsatellites. The distance from the surface of a planet toits geosynchronous orbit is dependent upon the planets'gravitational mass constant (µ) and it's angular rate inrevolutions per second (co). Treating this as a twobody problem, the geosync distances, from the planets'surface, can be estimated by using equation [2] assummarized in Table 1.215The second type of circular orbit of interest isthe Polar orbit.It is defined as an orbit planperpendicular to a planet's equator. A special use ofpolar orbits around the Earth is used by the weatherforecasting community. Weather satellites are placedin a polar orbit with an inclination just slightly lessthen 90 degrees such that its orbit plane would precessaround the earth. This in tum provides the satellitewith 24-hour continuous solar power and visibility ofthe earth's weather systems. In our case, we use threepolar orbits around the Sun. The actual orbitalparameters of these relay satellites would be adjustedevery six months such that they would not make aclose approach to Earth. Close approaches to Earth\would create a large perturbation to the relay satellite'sorbit.The relay satellites would be launched directlyinto a Solar Polar orbit or into an Earth centered polarorbit and await a bum to send it into the proper solarpolar orbit. At a 200 km Earth polar orbit, the relaysatellite would be moving at about 7.78 km/sec and itsescape velocity is estimated at 11.01 km/sec. Since theEarth's escape velocity is less then what is needed tomaintain a circular orbit around the sun, 29.79 km/sec,only one major flv bum would have to be performed toget it into the proper solar polar orbit. The needed flvis around 22.01 km/sec. Once the relay satellite is at asolar pole it can perform a small flv to change its orbit

plane to maintain an adequate distance from Earth. Inthe event that the relay satellite needs to be retrieved,then it could perform a /iv at one of the solar poles andbe placed on an intercept course to earth to arrive in sixmonths.space delay (which is distance d times the speed oflight c) and 2) planetary delay. The sum of thesedelays yields an estimate of total delay.TotDly Planetldly Planet2dly (dxc)A large contribution to propagation delay is aresult of free space delay. The distances in the solarsystem make real-time interactive sessions impracticalin certain situations. Table 3 summarizes the range ofpropagation delays that could be expected throughoutthe solar system based on distances in Table 2.Inter-Planetary DistancesPlanetary motions are well understood andhave been for several centuries. Table l is an estimateof the range of distances between planets in our solarsystem. The largest distances between planets are inthe upper right area and the smallest in the lower left ofthe table.Delays in the relay satellites do exist but are sosmall and constant that they can be ignored. Relaydelay is simply a result of the repeater/router functions.Based upon current switch and router technology thesedelays range from .2 to 20 milliseconds.The estimated distances in Table 2 werecalculated with two simplifying assumptions. First,distance corrections due to orbit inclination wereignored because they were so small. For planets fromMars to Neptune the errors were in the hundredths of apercent. For Venus it was .175% and for Mercury itwas .746%. Secondly, due to the numerical approachused, distances calculated were based upon orbitsegment that divided a planets orbit into 3600segments or 1/10 of a degree. This assumptionproduced distance errors in the thousandths of apercent for all of the planets except Mercury, whichhad the largest error of 0.0367%.Planetary delays can be treated as a constantand are estimated as the average delay between ageosync satellite network and it's terminal location.This delay includes: 1) terminal processing delays, 2)satellite processing delays, if satellites are used in aplanetary network, and 3) atmospheric delays or transitdelays if data is being directed to an adjacent moon.Clearly, for planets such as Jupiter, its moons would beconsidered the planet's terminal location and delayswould be calculated accordingly.Satellite processing delays are very low andcan be estimated at 15 ms. Ground terminal delayshave typically been much more significant due tomultiplexer, switching, encrypting, routing anddecoding functions. These ground terminal delays canbe as high as 450 ms.Propagation DelaysUsing the estimated distances in Table 2, onecan estimate the range of propagation delays betweenplanets . A simplified model of propagation delaysconsiders only two types of delay. These are: 1) freeSunMercuryVenus1.09E 8SunMercuryEarthMarsJupiterMaximum Inter-Planetary Distances (km)SaturnUranusNeptune1.52E 82.49E 88.16E 8l.50E 93.03E 94.54E 92.17E 82.95E 88.62E 81.55E 93.08E 94.58E 99.23E 81.61E 93.14E 94.64E 99.63E 81.65E 93.18E 94.68E 91.71E 93.24E 94.74E 93.77E 95.31E 9VenusEarth1.47E 8Mars2.07E 81.37E 8Jupiter7.41E 86.71E 86.32E 8SaturnI.35E 9l.30E 91.26E 9Uranus2.74E 92.67E 92.63E 92.58E 9Neptune4.49E 94.42E 94.38E 94.34E 9[3]5.99E 94.24E 9Minimum Inter-Planetary Distances (km)Table 2 - Range of Distances throughout the Solar System216

SunMercurySunVenus363Maximum one way Inter-Planetary free-space Delay 891,0642,9545,23910,34815,363l tune14,98014,74714,61714,47314, 14812,83717,85220,13712,258Minimum one way Inter-Planetary free-space Delay (seconds)Table 3 - Range of 1-way free space delays throughout the Solar System without conjunctionsAtmospheric delays or transit delays are planetspecific. For Earth geosync operations they can beestimated at 119 ms. For Mars geosync operationsthey could be estimated to be 57 ms.Solar Radiation Interference and ConjunctionsSince the Sun is a strong source ofelectromagnetic energy,it causessignificantinterference to communications. As the Sun movesbetween two inter-planetary objects (planets orspacecraft) the ability to maintain communicationdegrades until it is no longer possible to operate.These periods of interference are also calledconjunctions. A recent example of solar conjunctionsis the Galileo mission. It spent over two years in orbitaround Jupiter. It experienced two conjunction periodsof about 17 days each. These periods were December11-28, 1995 and January 11-28, 19972 .Even though the apparent diameter of the sunfrom the Earth is 0.48 degrees, the diameter of solarradiation interference ranges from roughly 6 degrees ( 3 degrees from center) for a quiet sun and as high as 14degrees ( 7 degrees from center) for an active sun.Using this, one can construct a sphere, centered in themiddle of the solar system that is used to estimate .periods of conjunctions. The sphere would have aradius ranging from a low of 7 .86 x 106 km for a quietsun (3 degrees from center) to 18.06 x 106 km for anactive sun (7 degrees from center). For this analysis,we shall be using the worst case estimates, since ourobjective is to assure continuous communication.Since planetary orbits are not perfectlycircular, the actilal angular magnitude of interferencevaries. Table 4 summarizes the worst case range ofangular interference by planet. It is important to note217that for all planets, except Mercury, the differencebetween the maximum and minimum angular range ofinterference is less then Y2 degree.MaxMinMercury42.87 Deg29.01 DegVenus19.08 Deg18.83 DegEarth14.00 Deg13.54 DegMars9.99 Deg8.29 DegJupiter2.79 Deg2.54 DegSaturn1.54 Deg1.38 DegUranus0.76 Deg0.68 DegNeptune0.46 Deg0.46 DegTable 4 - Angular Range of InterferenceFigure 3 - Solar ConjunctionThe angles in Table 4 provide the basis for asimple method to estimate the availability ofcommunication between two planets based upon solarconjunctions. Availability is a dimensionless, realnumber between 0 and l, where "O" means no

availability and "1" means constant availability. As isobserved in Figure 3, the planet nearest to the suncauses the largest conjunction.Since the solar system is in constant motionwith angular velocities that are not changing that fast,the availability can be estimated by subtracting thefraction of the orbit that is in conjunction from 1. b. .ConjuctionAngleA vaz 1a z1zty 1-------360[ ]4In addition, the sun acts a white noise jamrnerof ground surface terminals when the Sun is alignedwith the downlink terminal beam. This alignmenthappens twice a year around the equinoxes for geosyncsatellites. During this period, around the equinox, theground terminal's receiving system is saturated withthe sun's radio signal for short periods each day. Wewill call this Geosync jamming. The period ofdisruption is also based upon solar activity. Worst caseoutages for a quiet sun can be as long as 23 minutesand for an active sun it can be as high as 55 minutes.Assuming 10 hours a year of outages due to this typeof conjunction yields an availability contribution of0.998859 defined as Ag. An overall availability, Ao.can be derived using the model of series systems 3 asfollows:[5]By applying the above relationship to theangles listed in Table 3, we can estimate theavailability ranges due to solar conjunctions. Theresults, A ' are summarized in Table 4.Planetary Interference (Conjunctions)effect is small, it is relevant since our objective is toachieve continuous connections.These planetaryconjunctions can be grouped into two sets: l)conjunctions caused by a planet's satellite moons and2) conjunctions caused by other planets.Detailed calculations of planetary conjunctionsis beyond the intended scope of this concept paper, b

communications has lead to the development and continuous expansion of the information highway. This expansion has included the transportation services industry where today any truck, ship or airplane has the ability to have access to wireless voice communication and position determination services with either the US

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