NASA Facts Space Navigation

8/7/2019 NASA Facts Space Navigation

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A N E DUCA T I O NA L P UB L I CA T I O N OF T H ENA T I O NA L A E RO NA UT I CS A ND S P A CE A DM I N I S T RA T I O N

NF-37/ 12-67

SDase NavigaIio~OBJECTIVE: MARSOn earth man moves between points whichremain fixed in relation to one another. But in th esofar system all bodies are in continuous motion.Soon man will walk on the moon, and not longthereafter he will journey to the planets; man willnavigate great distances in space. He will be ableto navigate there with a precision unfamiliar tomost earth-bo und navigators. Yet the tools andtechniques which will be used will seem familiar,

Space navigation is essentially the same as naviga-tion on earth: a process of finding out where youare with respect to where you want to be.If the objective is Mars, for example, the astro-nauts will not move directly toward the planet butalong a curving pa th toward another point in space.Here they will rendezvous with M ars. They mustarrive at that point in space a t the same t ime theplanet Mars arrives.Though the spacecraft will be traveling thou-

Figure 1-The sextant-for centuries a tool for mariners, in the form shown here-has been adapted for use inspace navigation. See "The Sextant."


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sands of miles per hour, it will seem to the men onboard that it is han ging motionless i n space. Thestars will appear to be fixed in their positions.This is a fortunate circumstance of nature; it willallow the astronauts to determine the spacecraftsposition.The receding earth will not remain behind butwill be moving ahead imperceptibly. The pointfrom which the astronauts left the earth is nowonly a point in space.In the trajectory designed to encounter Mars,the astronauts will be in orbit around the sun.ESCAPING THE EARTH

In our solar system each planet is held in itsorbit by its velocity balanced against the sunsgravitational force. The inner planets, where thesunsi force is greatest, travel at greater speedsthan the outer planets, where the force is weaker.If a spacecraft is to leave the earth (Figure 2)for another planet, it must increase or decreasethe speed imparted to it by the orbiting earth. Yeteither way this velocity change is a fraction of theorbital speed of the earth, which is about 66 thou-sand miles per hour.To reach a planet closer to the sun (Venus, forexample), a spacecraft must escape earth in adirection opposite to earths travel. This reducesits velocity and allows the sun to pull it inward sotha t its orbita l path wi ll intersect tha t of the planet.With correct timing, the inner planet will be in thevicinity of the intersecting point when the space-craft gets there. If they dont collide, the space-craft will continue in an elliptical orbit around thesun forever.On the other hand, to reach a planet fartherfrom the sun, a spacecraft must escape earth inthe same direction as its movement around thesun (Figure 3). Its greater velocity throws thespacecraft outward, bu t the suns gravitation grad-ually slows it down and places it into an elongatedorbit that intersects that of the outer planet. Here

again accurate timing is essential to insure thatits arrival at the intersecting point coincides withthat of the planet. (Figure 4).COURSE ALTERATIONS

It is velocity change which alters the course ofan orbiti ng body.The velocity imparted by earth also puts thespacecraft into the ecliptic plane (the plane of theearths orbit).It would take a considerable amount of energyto get out of that plane. But fortunately all planetsmove in planes very close to the ecliptic and theyall o rbit in the same direction.Now, if on a journey to Mars the spacecraft isinjected in to its trajectory i n the same directionas the earths travel around the sun, its greatervelocity will throw it outward towards Mars.But is the injection velocity correct? The paththe spacecraft should follow hasbut 100% accuracy cannot be remost sophisticated flight programsfor inaccuracies in injection or prcise orbits of the earth and theOn this trip, a discrepancy iat earth of less than 1/10 of 1will cause the craft to miss Marof a mil lion miles.The additional propellant andcorrect so large an error nearvoyage might be prohibitive. Tprobably fail. The error must beas possible.Errors in trajectory can be corring up, or slowing down, the samount of correction needed iscomparing the actual trajectory wtended.The actual trajectory is detelishing the position of the spaintervals of time. This requirmeasurements.

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Figure 2-If the spacecraft is to leave the earth, it mustincrease or decrease the speed imparted to it by theorbiting earth.

Figure 3-To reach a planet farther from the sun, a space-craft must escape earth in the same direction as its move-ment around the sun.

~ 0 ~ ~ ~ 0 ~ ~the actual, mechanical procedureused to steer a vehicle along a path, or tomaintain its attitude in a specific orientationin space.ic: the apparent annual path of the sunthe stars, as seen from e arth, projec tedonto the celestial sphere; the intersection ofthe earths orbit with the celestial sphere.

lane: the orbital plane of Earth.68:the information required by a vehi-make it follow a prescribed path orfulfi l l a particular objective.

avigation: the process of determining vehicleposition and velocity in some known frame ofreference.: the path of a body in space when thepath is closed and repetitive.

r a j e ~ t 0 r ~ :the path of a body in space withmore or less specific initial and end points.

Figure 4-Accurate timing is essential to insure arrival atthe planet. Figure 5 -The astronauts use thei r sextant to tracklandmarks.3


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Figure 6-The angle defines a cone of position. some unavoidable'error . . .

Figure 8-. . . position is determined within a footballshaped volume. Figure 9-Intersectio n of ellipsoid and cone establish esposition more closely.

Figurelo-The astronauts know approximately how far theyare from the desired trajectory.4

Figure 11-Repeated sightings reduce the amount ofuncertainty.


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THE SEXTANTFundamental to al l navigation is the abil i ty tomeasure angles.For this, astronauts use a device which has beenused in navigating for over two hundred years:the sextant. (See Figure 1, Page 1.)The angle between a planet and a known starcan be measured precisely with a sextant. Thisangle is recorded in a computer along with theexact tim e of sighting. The angle defines a coneof position because that angle could have beensighted from any point on the surface of the cone.(Figure 6).But one sighting is not suff ic ient to tel l theastronauts where they are. They do know they arenear the ir desired trajectory. Their approximatespeed, elapsed time, and the limits of possibleerror locate them within a football-shaped-volumewhich navigators call the estimated ellipsoid ofposition. (Figures 7 and 8 . )The volume where this ellipsoid intersects thecone establishes the crafts position more closely;they are somewhere in the area where the two coin-cide (Figure 9). From this the astronauts knowapproximately how far they are from the desiredtrajectory. (Figure 10)Repeated sightings will be made along the wayto reduce the amount of uncertainty, until theactual trajectory is determined within allowablelimits. (Figure 111)When the actual trajectory is determined tohave deviated too far from the desired one, a mid-course correction must be made to attain a cor-rected trajectory.LANDING ON A PLANETThroughout the flight, earth-based trackinginformation will be used to correct or supplementon-boa rd navigation. Bu t as the spacecraft nearsits target position, m illions of miles from earthstracking stations, the astronauts will have to relyalmost entirely upon the on-board navigationalequipment because they need a precision fargreater than that available from earth data.

Furthermore, any back-up information wouldtake 1 2 minutes to travel the 13 4 mil l ion milesthat separate the astronauts from earth.The exact location and speed of the craft mustbe determined with respect to the planet, usingsextant sightings. The sightings wi ll be made be-tween the target plane t and known stars. Then thefinal velocity changes that will achieve a precise

parking orbit around the planet can be calculated.These velocity changes enable the spacecraft toapproach the planet from the proper distance andgo into parking orbit. The astronauts will then usethe ir sextant to tra ck landmarks. (Figure 5 ) Thisconfirms their parking orbit, and determines itscharacteristics which permit the m to plan a descenttrajectory.To navigate their return to earth the astronautswill reverse the above procedure.

GUIDANCE OF UNMANNED SPACECRAFTDeep space tracking equipment and methodsare already so accurate that unmanned craft canbe guided to the moon and nearby planets withremarkable accuracy.For example, Mariner II, on its mission of flyingpast Venus, traversed one hundred and eightymillion miles of space and passed Venus 21,648miles from the planet; 10,000 miles had beenplanned .This is like shooting at a moving target from apla tform that is moving and rotating, at a range ofone mile and h itt ing the target four and one quarterinches from deadcenter.Later, Ranger VI1 hit the moon within eight milesof its aimin g point. An inch and a half miss at onemile.Ranger IX came closer, impacting only two-and-three-quarter miles from its target center. Mariner

Figure 12-Surveyor spacecraft.5


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IV, traveling three hundred and twenty-five millio n The time-keeping and time-recording devicesmiles, swept past Mars within two thousand miles which are essential to space navigation are now soof its aiming point. Surveyors 1 an d Ill soft-landed precise tha t they have an accuracy of one ten-on the moon within a few miles of target centers. billio nth of a second.Lunar Orbiters have been placed in precise orb its Even though navigational instruments havearound the moon, as has the Interplanetary Ex-plorer XXXV. These historic space Slights are greattechnological achievements. They are skillfuldemonstrations of the reliability and accuracy ofearth-based tracking techniques.NEW TOOLS

There are new tools which have been developedto solve the unique problems introduced by thethird dimension and immense vastness of space.Basically these tools consist of:a. Information gathering devices, both optical andb. Information processing devices (computers);c. Time-ke eping devices.Manned space navigation and guidance demanda great variety of information gathering devices.They include:a. Sextants or other optical instrume nts to m easureangles between celestial bodies;b. On-board radar and other electronic equipmentfor determining altitude and velocity relative to

the target planet, when near the planet;c. Earth-based radio and radar for precise distanceand velocity measurements;d. Inertial sensing and measurement equipment toprovide a reference frame for positioning thevehicle during mid-course correction and togauge the extent of the correction.The information processing devices includecomputers (both on-board and earth-based) thatuse stored information as well as information fur-nished them by the measuring and sensing devices.The devices process at h igh speed the large numberof involved calculations necessary for space navi-gation. They can solve in seconds problems whichwo uld' take an experienced navigator an entirevoyage to calculate. Information processing devicesalso include certain elements of the deep spacecommunication systems that carry informationbetween spacecraft and earth.

electronic;

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Figure 13-A navigation panel for space flight.

reached high degrees of reliability and accuracy,more effort is being directed toward simplifyingequipment and methods, aimed at solving the con-siderable problems of landing on a planet with anaccuracy of 10 miles.In manned flight, studies are under way to deter-mine how on-board and earth-based systems canbest work together to reduce on-board equipmentas much as possible. Knowledge is being improvedconstantly about the planets' masses, their orbitsand distances from the sun, so that trajectoriescan be computed more accurately.Trajectory equations are being developed whichcan be solved by simplified computers, which willstill provide navigational data to the degree ofreliability needed for manned flight to the planets.Knowledge is augmented by missions likeGemini; in these p ractice m issions man is gainingexperience in measuring angles, distances andrelative velocities, and in determining the changesneeded to attain specific trajectories.


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APOLLO MOON FLIGHTSystems development in the most ambitiousspace project to date-the Apollo Moon Flight-iswell under way.The guidance and navigation station aboard theApollo is equipped with scanning telescope, sextant,digital computer and an inertial measurement unit.The Apollo astronaut can maneuver the spacecraftdirectly by the use of manually operated controlsor can select programs in the computer which w illinitia te automatic maneuvers.This complete system permits all navigationfunctions for the lunar journey to be performedon board and independent of earth-based trackingor com puting facilit ies, if necessary.The navigator will make repeated sightings toconfirm his position and trajectory.However, throughout the entire lunar trip he willhave earth-based tracking information availablefor use in his own computer.

APOLLO ON-BOARD EQUIPMENTThis is how the on-board equipment works:a. The line of sight of the scanning telescope isfixed to the spacecraft. Looking through thisscope, the navigator acquires both the earth an dthe selected star;

b. He maneuvers the spacecraft until the earthlandmark is centered at zero. He rotates the tele-scope until the reticle falls across the star; thisalso turns the sextant;c. The navigator notes the angle of the star and

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sets the-sextant at that angle. Now, lookingthrough the sextant, both images appear en-larged and superimposed. When they are linedup, the mark button is pushed. This feedsprecise time and angle information into thecomputer;The computer also receives earth-based track ingdata which, together with the observed data,keeps the navigator informed of his currentposition and actual trajectory;The navigator instructs the computer to makethe necessary trajectory correction. The com-puter determines the exact amount and directionof thrust needed to accomplish this correction;

f. The inertial measurement unit furnishes a senseof direction t o the computer, enabling the space-craft to be oriented correctly. The rocket is fired.The inertial measurement unit measures theacceleration, and feeds this data to the computer;g. When the desired speed change is accomplished,

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the computer cuts the engine. Now the space-craft is on its corrected course. More than onecorrection may be needed. As the moon is ap-proached sextant readings may be made betweenknown lunar landmarks and appropriate starsto determine the final course correction;Both on-board and earth-based computers deter-mine independently what manuever is requiredto place the Apollo craft into lunar orbit;The familiar landmark trackin g technique is thenused to confirm the parking orbit, putt ing manfor the f irst t ime in a posit ion to land on anextraterrestrial surface.The navigational experience gained i n the Apolloprogram will contribute information needed fortrip s of greater complexity.

EARLY ASTRONOMERS:THE COURSES OF THE PLANETSMan has observed the movements of the sun,moon, and planets for hundreds of years.In the 15th century Nicolaus Copernicus led th eway to modern mans perception of the solar sys-tem: the earth revolves around the sun along withthe other planets.In the next century, the astronomer Tycho Brahemade his painstaking determinations of the loca-tions of the planets. His remarkably accurate ob-servations preceded the telescope.In the 17 th century, Johannes Kepler, after yearsof patient calculation, discovered the laws thatgovern the movements of the planets. Using TychoBrahes observations, he demonstrated that aplanet does not travel i n an exact circular orb itaround the sun but in a slightly elliptical one, withthe sun at one focus of the ellipse. This was thefoundation for his first law: the orb its of the planetsare ellipses, w ith the sun at a comm on focus.His second law is that each planet revolves sothat the line joining it to t he sun sweeps over equal

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areas in equal intervals of time.His third law, or discovery, is tha t the squares ofthe periods of any two planets are in the sameproportion as the cubes of their mean distancesfrom the sun.Keplers laws are used today to calculate thetrajectories of space probes.Galileo Galilei was the first man to peer at theheavens throu gh an optica l telescope. He saw tha tthere are worlds and satellites beyond earth. Henoted the craters of the moon, the moons of Jupiter,and the phases of Venus.

Interest now began to shift from the courses ofthe planets to studies of the forces controlling theplanets.In the next generation Isaac Newton wrote hislaws of motion, which use both Keplers laws andGalileos work on mechanics. Newtons laws state:1.A body either remains at rest or moves withconstant speed in a straight line provided it isnot acted upon by any external force.2.A force acting on a body causes it to acceleratein the direction of the force, the accelerationbeing directly proportional to the force andinversely proportional to the mass of the body.3. Every action is accompanied by an equal andopposite reaction.By means of his laws of motion and by mathemati-cal reasoning, Newton succeeded in reducingKeplers geometrical description of the planetarysystem to a single comprehensive law:The gravitational force exerted between two bodiesis proportional to the product of their massesdivided by the square of the distance betweentheir centers of gravity.

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0Newtons law of gravitation provided the key fo runderstanding planetary motion; men could viewthe solar system as a vast machine, with predicta-ble parts and movements. For example, Neptune

and Pluto were calculated to exist by Newtonslaws, and they were duly discovered.From that time onward mans capability to navi-gate in space waited only upon development oftechnology.

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