cryogenic rocket

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Cryogenic rocket engine

From Wikipedia, the free encyclopedia

Vulcainengine of Ariane 5rocket.

RL-10is an early example of cryogenic rocket engine.

A cryogenic rocket engineis a rocket enginethat uses a cryogenic fuelor oxidizer, that is, itsfuel or oxidizer (or both) are gases liquefied and stored at very low temperatures.[1]Notably,these engines were one of the main factors ofNASA's success in reaching the Moon by theSaturn Vrocket.[1]

During World War II, when powerful rocket engines were first considered by the German,American and Soviet engineers independently, all discovered that rocket engines need high massflow rateof both oxidizer and fuel to generate a sufficient thrust. At that time oxygen and lowmolecular weight hydrocarbons were used as oxidizer and fuel pair. At room temperature andpressure, both are in gaseous state. Hypothetically, if propellants had been stored as pressurizedgases, the size and mass of fuel tanks themselves would severely decrease rocket efficiency.Therefore, to get the required mass flow rate, the only option was to cool the propellants down tocryogenictemperatures (below 150 C, 238 F), converting them to liquidform. Hence, all
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cryogenic rocket engines are also, by definition, either liquid-propellant rocket enginesor hybridrocket engines.[2]

Various cryogenic fuel-oxidizer combinations have been tried, but the combination of liquidhydrogen (LH2) fuel and the liquid oxygen (LOX) oxidizer is one of the most widely used. [1][3]

Both components are easily and cheaply available, and when burned have one of the highestenthalpyreleases by combustion,[4]producing specific impulseup to 450s (effective exhaustvelocity4.4km/s).

Contents

1 Construction

2 LOX+LH2 rocket engines by government agency

3 References

4 External links

Construction

The major components of a cryogenic rocket engine are the combustion chamber(thrustchamber),pyrotechnic initiator, fuel injector, fuel cryopumps, oxidizer cryopumps, gas turbine,cryo valves, regulators, the fuel tanks, and rocket engine nozzle. In terms of feeding propellantsto the combustion chamber, cryogenic rocket engines (or, generally, all liquid-propellantengines) are eitherpressure-fedorpump-fed, and pump-fed engines work in either a gas-

generator cycle, a staged-combustion cycle, or an expander cycle.

The cryopumps are always turbopumpspowered by a flow of fuel through gas turbines. Lookingat this aspect, engines can be differentiated into a main flow or a bypass flow configuration. Inthe main flow design, all the pumped fuel is fed through the gas turbines, and in the end injectedto the combustion chamber. In the bypass configuration, the fuel flow is split; the main part goesdirectly to the combustion chamber to generate thrust, while only a small amount of the fuel goesto the turbine.[citation needed]

LOX+LH2 rocket engines by government agency

Currently, six governments have successfully developed and deployed cryogenic rocket engines:

India

CE-7.5[5]

CE-20
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Overview and history

The Hungarian-Britishphysicist Dennis Gabor(in Hungarian: Gbor Dnes),[1][2]was awardedtheNobel Prize in Physicsin 1971 "for his invention and development of the holographicmethod".[3]His work, done in the late 1940s, built on pioneering work in the field of X-ray

microscopy by other scientists including Mieczysaw Wolfkein 1920 and WL Braggin 1939.[4]The discovery was an unexpected result of research into improving electron microscopesat theBritish Thomson-Houston(BTH) Company in Rugby, England, and the company filed a patentin December 1947 (patent GB685286). The technique as originally invented is still used inelectron microscopy, where it is known as electron holography, but optical holography did notreally advance until the development of the laserin 1960. The word holographycomes from theGreekwords (hlos; "whole") and (graph; "writing" or "drawing").

Horizontal symmetric text, by Dieter Jung

The development of the laserenabled the first practical optical holograms that recorded 3Dobjects to be made in 1962 by Yuri Denisyukin the Soviet Union[5]and by Emmett LeithandJuris Upatnieksat the University of Michigan, USA.[6]Early holograms used silver halidephotographic emulsions as the recording medium. They were not very efficient as the producedgrating absorbed much of the incident light. Various methods of converting the variation in

transmission to a variation in refractive index (known as "bleaching") were developed whichenabled much more efficient holograms to be produced.[7][8][9]

Several types of holograms can be made. Transmission holograms, such as those produced byLeith and Upatnieks, are viewed by shining laser light through them and looking at thereconstructed image from the side of the hologram opposite the source.[10]A later refinement, the"rainbow transmission" hologram, allows more convenient illumination by white light ratherthan by lasers.[11]Rainbow holograms are commonly used for security and authentication, forexample, on credit cards and product packaging.[12]

Another kind of common hologram, the reflectionor Denisyuk hologram, can also be viewed

using a white-light illumination source on the same side of the hologram as the viewer and is thetype of hologram normally seen in holographic displays. They are also capable of multicolour-image reproduction.[13]

Specular holographyis a related technique for making three-dimensional images by controllingthe motion of specularities on a two-dimensional surface.[14]It works by reflectively orrefractively manipulating bundles of light rays, whereas Gabor-style holography works bydiffractively reconstructing wavefronts.
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Most holograms produced are of static objects but systems for displaying changing scenes on aholographic volumetric displayare now being developed.[15][16][17]

Holograms can also be used to store, retrieve, and process information optically.[18]

In its early days, holography required high-power expensive lasers, but nowadays, mass-produced low-cost semi-conductor or diode lasers, such as those found in millions of DVDrecordersand used in other common applications, can be used to make holograms and have madeholography much more accessible to low-budget researchers, artists and dedicated hobbyists.

It was thought that it would be possible to use X-rays to make holograms of very small objectsand view them using visible light[citation needed]. Today, holograms with x-rays are generated byusing synchrotronsor x-ray free-electron lasersas radiation sources and pixelated detectors suchas CCDsas recording medium.[19]The reconstruction is then retrieved via computation. Due tothe shorter wavelength of x-rayscompared to visible light, this approach allows to image objectswith higher spatial resolution.[20]As free-electron laserscan provide ultrashort and x-ray pulses

in the range of femtosecondswhich are intense and coherent, x-ray holography has been used tocapture ultrafast dynamic processes.[21][22][23]

How holography works

Recording a hologram
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Reconstructing a hologram

Close-up photograph of a hologram's surface. The object in the hologram is a toy van. It is no morepossible to discern the subject of a hologram from this pattern than it is to identify what music has beenrecorded by looking at a CDsurface. Note that the hologram is described by the speckle pattern, ratherthan the "wavy" line pattern.

Holography is a technique that enables a light field, which is generally the product of a lightsource scattered off objects, to be recorded and later reconstructed when the original light field isno longer present, due to the absence of the original objects.[24]Holography can be thought of assomewhat similar to sound recording, whereby a sound field created by vibrating matter likemusical instrumentsor vocal cords, is encoded in such a way that it can be reproduced later,

without the presence of the original vibrating matter.

Laser

Holograms are recorded using a flash of light that illuminates a scene and then imprints on arecording medium, much in the way a photograph is recorded. In addition, however, part of thelight beam must be shone directly onto the recording medium - this second light beam is knownas the reference beam. A hologram requires a laseras the sole light source. Lasers can be
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precisely controlled and have a fixed wavelength, unlike sunlight or light from conventionalsources, which contain many different wavelengths. To prevent external light from interfering,holograms are usually taken in darkness, or in low level light of a different color from the laserlight used in making the hologram. Holography requires a specific exposuretime (just likephotography), which can be controlled using a shutter, or by electronically timing the laser.

Apparatus

A hologram can be made by shining part of the light beam directly onto the recording medium,and the other part onto the object in such a way that some of the scattered light falls onto therecording medium.

A more flexible arrangement for recording a hologram requires the laser beam to be aimedthrough a series of elements that change it in different ways. The first element is a beam splitterthat divides the beam into two identical beams, each aimed in different directions:

One beam (known as the illuminationor object beam) is spread using lensesand directed onto thescene using mirrors. Some of the light scattered (reflected) from the scene then falls onto therecording medium.

The second beam (known as the reference beam) is also spread through the use of lenses, but isdirected so that it doesn't come in contact with the scene, and instead travels directly onto therecording medium.

Several different materials can be used as the recording medium. One of the most common is afilm very similar tophotographic film(silver halidephotographic emulsion), but with a muchhigher concentration of light-reactive grains, making it capable of the much higher resolutionthat holograms require. A layer of this recording medium (e.g. silver halide) is attached to a

transparent substrate, which is commonly glass, but may also be plastic.

Process

When the two laser beams reach the recording medium, their light waves, intersect and interferewith each other. It is this interference pattern that is imprinted on the recording medium. Thepattern itself is seemingly random, as it represents the way in which the scene's light interferedwith the original light source but not the original light source itself. The interference patterncan be considered an encodedversion of the scene, requiring a particular key the original lightsource in order to view its contents.

This missing key is provided later by shining a laser, identical to the one used to record thehologram, onto the developed film. When this beam illuminates the hologram, it is diffractedbythe hologram's surface pattern. This produces a light field identical to the one originallyproduced by the scene and scattered onto the hologram. The image this effect produces in aperson's retinais known as a virtual image.
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Holography vs. photography

Holography may be better understood via an examination of its differences from ordinaryphotography:

A hologram represents a recording of information regarding the light that came from the originalscene as scattered in a range of directions rather than from only one direction, as in a photograph.This allows the scene to be viewed from a range of different angles, as if it were still present.

A photograph can be recorded using normal light sources (sunlight or electric lighting) whereas alaser is required to record a hologram.

A lens is required in photography to record the image, whereas in holography, the light from theobject is scattered directly onto the recording medium.

A holographic recording requires a second light beam (the reference beam) to be directed onto therecording medium.

A photograph can be viewed in a wide range of lighting conditions, whereas holograms can onlybe viewed with very specific forms of illumination.

When a photograph is cut in half, each piece shows half of the scene. When a hologram is cut inhalf, the whole scene can still be seen in each piece. This is because, whereas each point in aphotographonly represents light scattered from a single point in the scene, each pointon aholographic recording includes information about light scattered from every pointin the scene. Itcan be thought of as viewing a street outside a house through a 4ft x 4ft window, then through a2ft x 2ft window. One can see all of the same things through the smaller window (by moving thehead to change the viewing angle), but the viewer can see more at oncethrough the 4ft window.

A photograph is a two-dimensional representation that can only reproduce a rudimentary three-dimensional effect, whereas the reproduced viewing range of a hologram adds many more depthperception cuesthat were present in the original scene. These cues are recognized by the humanbrainand translated into the same perception of a three-dimensional image as when the originalscene might have been viewed.

A photograph clearly maps out the light field of the original scene. The developed hologram'ssurface consists of a very fine, seemingly random pattern, which appears to bear no relationshipto the scene it recorded.

Physics of holography

For a better understanding of the process, it is necessary to understand interferenceanddiffraction. Interference occurs when one or more wavefrontsare superimposed. Diffractionoccurs whenever a wavefront encounters an object. The process of producing a holographicreconstruction is explained below purely in terms of interference and diffraction. It is somewhatsimplified but is accurate enough to provide an understanding of how the holographic processworks.
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For those unfamiliar with these concepts, it is worthwhile to read the respective articles beforereading further in this article.

Plane wavefronts

A diffraction gratingis a structure with a repeating pattern. A simple example is a metal platewith slits cut at regular intervals. A light wave incident on a grating is split into several waves;the direction of these diffracted waves is determined by the grating spacing and the wavelengthof the light.

A simple hologram can be made by superimposing twoplane wavesfrom the same light sourceon a holographic recording medium. The two waves interfere giving a straight line fringe patternwhose intensity varies sinusoidally across the medium. The spacing of the fringe pattern isdetermined by the angle between the two waves, and on the wavelength of the light.

The recorded light pattern is a diffraction grating. When it is illuminated by only one of the

waves used to create it, it can be shown that one of the diffracted waves emerges at the sameangle as that at which the second wave was originally incident so that the second wave has been'reconstructed'. Thus, the recorded light pattern is a holographic recording as defined above.

Point sources

Sinusoidal zone plate

If the recording medium is illuminated with a point source and a normally incident plane wave,the resulting pattern is a sinusoidal zone platewhich acts as a negative Fresnel lenswhose focallength is equal to the separation of the point source and the recording plane.

When a plane wavefront illuminates a negative lens, it is expanded into a wave which appears todiverge from the focal point of the lens. Thus, when the recorded pattern is illuminated with the

original plane wave, some of the light is diffracted into a diverging beam equivalent to theoriginal plane wave; a holographic recording of the point source has been created.

When the plane wave is incident at a non-normal angle, the pattern formed is more complex butstill acts as a negative lens provided it is illuminated at the original angle.
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Complex objects

To record a hologram of a complex object, a laser beam is first split into two separate beams oflight. One beam illuminates the object, which then scatters light onto the recording medium.According to diffractiontheory, each point in the object acts as a point source of light so the

recording medium can be considered to be illuminated by a set of point sources located atvarying distances from the medium.

The second (reference) beam illuminates the recording medium directly. Each point source waveinterferes with the reference beam, giving rise to its own sinusoidal zone plate in the recordingmedium. The resulting pattern is the sum of all these 'zone plates' which combine to produce arandom (speckle) pattern as in the photograph above.

When the hologram is illuminated by the original reference beam, each of the individual zoneplates reconstructs the object wave which produced it, and these individual wavefronts addtogether to reconstruct the whole of the object beam. The viewer perceives a wavefront that is

identical to the wavefront scattered from the object onto the recording medium, so that it appearsto him or her that the object is still in place even if it has been removed. This image is known asa "virtual" image, as it is generated even though the object is no longer there.

Mathematical model

A single-frequency light wave can be modelled by a complex numberU, which represents theelectricor magnetic fieldof the light wave. The amplitudeandphaseof the light are representedby the absolute valueand angleof the complex number. The object and reference waves at anypoint in the holographic system are given by UOand UR. The combined beam is given by UO+UR. The energy of the combined beams is proportional to the square of magnitude of thecombined waves as:

If a photographic plate is exposed to the two beams and then developed, its transmittance, T, isproportional to the light energy that was incident on the plate and is given by

where kis a constant.

When the developed plate is illuminated by the reference beam, the light transmitted through theplate, UHis equal to the transmittance Tmultiplied by the reference beam amplitude UR, giving

It can be seen that UHhas four terms, each representing a light beam emerging from thehologram. The first of these is proportional to UO. This is the reconstructed object beam whichenables a viewer to 'see' the original object even when it is no longer present in the field of view.
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The second and third beams are modified versions of the reference beam. The fourth term isknown as the "conjugate object beam". It has the reverse curvature to the object beam itself andforms a real imageof the object in the space beyond the holographic plate.

When the reference and object beams are incident on the holographic recording medium at

significantly different angles, the virtual, real and reference wavefronts all emerge at differentangles, enabling the reconstructed object to be seen clearly.

Recording a hologram

Items required

An optical table being used to make a hologram

To make a hologram, the following are required:

a suitable object or set of objects

a suitable laser beam

part of the laser beam to be directed so that it illuminates the object (the object beam) and anotherpart so that it illuminates the recording medium directly (the reference beam), enabling the

reference beam and the light which is scattered from the object onto the recording medium toform an interference pattern

a recording medium which converts this interference pattern into an optical element whichmodifies either the amplitude or the phase of an incident light beam according to the intensity ofthe interference pattern.

an environment which provides sufficient mechanical and thermal stability that the interferencepattern is stable during the time in which the interference pattern is recorded[25]

These requirements are inter-related, and it is essential to understand the nature of opticalinterference to see this. Interferenceis the variation in intensitywhich can occur when two lightwavesare superimposed. The intensity of the maxima exceeds the sum of the individualintensities of the two beams, and the intensity at the minima is less than this and may be zero.The interference pattern maps the relative phase between the two waves, and any change in therelative phases causes the interference pattern to move across the field of view. If the relativephase of the two waves changes by one cycle, then the pattern drifts by one whole fringe. Onephase cycle corresponds to a change in the relative distances travelled by the two beams of onewavelength. Since the wavelength of light is of the order of 0.5m, it can be seen that very smallchanges in the optical paths travelled by either of the beams in the holographic recording system
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lead to movement of the interference pattern which is the holographic recording. Such changescan be caused by relative movements of any of the optical components or the object itself, andalso by local changes in air-temperature. It is essential that any such changes are significantlyless than the wavelength of light if a clear well-defined recording of the interference is to becreated.

The exposure time required to record the hologram depends on the laser power available, on theparticular medium used and on the size and nature of the object(s) to be recorded, just as inconventional photography. This determines the stability requirements. Exposure times of severalminutes are typical when using quite powerful gas lasers and silver halide emulsions. All theelements within the optical system have to be stable to fractions of a m over that period. It ispossible to make holograms of much less stable objects by using apulsed laserwhich produces alarge amount of energy in a very short time (s or less). [26]These systems have been used toproduce holograms of live people. A holographic portrait of Dennis Gabor was produced in 1971using a pulsed ruby laser.[27][28]

Thus, the laser power, recording medium sensitivity, recording time and mechanical and thermalstability requirements are all interlinked. Generally, the smaller the object, the more compact theoptical layout, so that the stability requirements are significantly less than when makingholograms of large objects.

Another very important laser parameter is its coherence.[29]This can be envisaged by consideringa laser producing a sine wave whose frequency drifts over time; the coherence length can then beconsidered to be the distance over which it maintains a single frequency. This is importantbecause two waves of different frequencies do not produce a stable interference pattern. Thecoherence length of the laser determines the depth of field which can be recorded in the scene. Agood holography laser will typically have a coherence length of several meters, ample for a deep

hologram.

The objects that form the scene must, in general, have optically rough surfaces so that theyscatter light over a wide range of angles. A specularly reflecting (or shiny) surface reflects thelight in only one direction at each point on its surface, so in general, most of the light will not beincident on the recording medium. A hologram of a shiny object can be made by locating it veryclose to the recording plate.[30]

Hologram classifications

There are three important properties of a hologram which are defined in this section. A givenhologram will have one or other of each of these three properties, e.g. an amplitude modulatedthin transmission hologram, or a phase modulated, volume reflection hologram.

mplitude and phase modulation holograms

An amplitude modulation hologram is one where the amplitude of light diffracted by thehologram is proportional to the intensity of the recorded light. A straightforward example of thisisphotographic emulsionon a transparent substrate. The emulsion is exposed to the interferencepattern, and is subsequently developed giving a transmittance which varies with the intensity of
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the pattern - the more light that fell on the plate at a given point, the darker the developed plate atthat point.

A phase hologram is made by changing either the thickness or the refractive indexof the materialin proportion to the intensity of the holographic interference pattern. This is aphase gratingand

it can be shown that when such a plate is illuminated by the original reference beam, itreconstructs the original object wavefront. The efficiency (i.e. the fraction of the illuminatedbeam which is converted to reconstructed object beam) is greater for phase than for amplitudemodulated holograms.

Thin holograms and thick (volume) holograms

A thin hologram is one where the thickness of the recording medium is much less than thespacing of the interference fringes which make up the holographic recording.

A thick or volume hologramis one where the thickness of the recording medium is greater than

the spacing of the interference pattern. The recorded hologram is now a three dimensionalstructure, and it can be shown that incident light is diffracted by the grating only at a particularangle, known as the Bragg angle.[31]If the hologram is illuminated with a light source incident atthe original reference beam angle but a broad spectrum of wavelengths; reconstruction occursonly at the wavelength of the original laser used. If the angle of illumination is changed,reconstruction will occur at a different wavelength and the colour of the re-constructed scenechanges. A volume hologram effectively acts as a colour filter.

Transmission and reflection holograms

A transmission hologram is one where the object and reference beams are incident on therecording medium from the same side. In practice, several more mirrors may be used to directthe beams in the required directions.

Normally, transmission holograms can only be reconstructed using a laser or a quasi-monochromatic source, but a particular type of transmission hologram, known as a rainbowhologram, can be viewed with white light.

In a reflection hologram, the object and reference beams are incident on the plate from oppositesides of the plate. The reconstructed object is then viewed from the same side of the plate as thatat which the re-constructing beam is incident.

Only volume holograms can be used to make reflection holograms, as only a very low intensitydiffracted beam would be reflected by a thin hologram.

Gallery of full-color reflection holograms of mineral specimens
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Hologram of Elbaite on Quartz

Hologram of Tanzanite on Matrix

Hologram of Tourmaline on Quartz

Hologram of Amethyst on Quartz

Holographic recording media

The recording medium has to convert the original interference pattern into an optical elementthat modifies either the amplitudeor thephaseof an incident light beam in proportion to theintensity of the original light field.

The recording medium should be able to resolve fully all the fringes arising from interferencebetween object and reference beam. These fringe spacings can range from tens of micrometerstoless than one micrometer, i.e. spatial frequencies ranging from a few hundred to several thousandcycles/mm, and ideally, the recording medium should have a response which is flat over thisrange. If the response of the medium to these spatial frequencies is low, the diffraction efficiencyof the hologram will be poor, and a dim image will be obtained. Standard photographic film hasa very low or even zero response at the frequencies involved and cannot be used to make ahologram - see, for example, Kodak's professional black and white film [32]whose resolutionstarts falling off at 20 lines/mm it is unlikely that any reconstructed beam could be obtainedusing this film.
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If the response is not flat over the range of spatial frequencies in the interference pattern, then theresolution of the reconstructed image may also be degraded.[33][34]

The table below shows the principal materials used for holographic recording. Note that these donot include the materials used in the mass replicationof an existing hologram, which are

discussed in the next section. The resolution limit given in the table indicates the maximalnumber of interference lines/mm of the gratings. The required exposure, expressed as millijoules(mJ) of photon energy impacting the surface area, is for a long exposure time. Short exposuretimes (less than 1/1000 of a second, such as with a pulsed laser) require much higher exposureenergies, due to reciprocity failure.

The Global Positioning System(GPS) is a space-based satellite navigationsystem that provideslocation and time information in all weather conditions, anywhere on or near the Earth wherethere is an unobstructed line of sight to four or more GPS satellites.[1]The system providescritical capabilities to military, civil and commercial users around the world. It is maintained bythe United States government and is freely accessible to anyone with a GPS receiver.

The GPS project was developed in 1973 to overcome the limitations of previous navigationsystems,[2]integrating ideas from several predecessors, including a number of classifiedengineering design studies from the 1960s. GPS was created and realized by the U.S.Department of Defense(DoD) and was originally run with 24satellites. It became fullyoperational in 1995. Bradford Parkinson, Roger L. Easton, and Ivan A. Gettingare credited withinventing it.

Advances in technology and new demands on the existing system have now led to efforts tomodernize the GPS system and implement the next generation of GPS IIIsatellites and NextGeneration Operational Control System (OCX).[3]Announcements from Vice President Al Gore

and the White Housein 1998 initiated these changes. In 2000, the U.S. Congressauthorized themodernization effort, GPS III.

In addition to GPS, other systems are in use or under development. The Russian GlobalNavigation Satellite System (GLONASS) was developed contemporaneously with GPS, butsuffered from incomplete coverage of the globe until the mid-2000s.[4]There are also the plannedEuropean Union Galileo positioning system, India's Indian Regional Navigation SatelliteSystem, and the Chinese Beidou Navigation Satellite System.

Contents

1 History

o 1.1 Predecessors

o 1.2 Development

o 1.3 Timeline and modernization
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o 1.4 Awards

2 Basic concept of GPS

o 2.1 Fundamentals

o 2.2 More detailed description

o 2.3 User-satellite geometry

o 2.4 Receiver in continuous operation

o 2.5 Non-navigation applications

3 Structure

o 3.1 Space segment

o 3.2 Control segment

o 3.3 User segment

4 Applications

o 4.1 Civilian

4.1.1 Restrictions on civilian use

o 4.2 Military

5 Communication

o 5.1 Message format

o 5.2 Satellite frequencies

o 5.3 Demodulation and decoding

6 Navigation equations

o 6.1 Problem description

o 6.2 Least squares solution method

o 6.3 Closed-form solution methods (Bancroft, etc.)

7 Error sources and analysis
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8 Accuracy enhancement and surveying

o 8.1 Augmentation

o 8.2 Precise monitoring

o 8.3 Timekeeping

8.3.1 Leap seconds

8.3.2 Accuracy

8.3.3 Format

o 8.4 Carrier phase tracking (surveying)

9 Regulatory spectrum issues concerning GPS receivers

10 Other systems

11 See also

12 Notes

13 References

14 Further reading

15 External links

History

The design of GPS is based partly on similar ground-based radio-navigation systems, such asLORANand the Decca Navigator, developed in the early 1940s and used by the British RoyalNavy during World War II.

Predecessors

In 1956, the German-American physicist Friedwardt Winterberg[5]proposed a test of general

relativity- detecting time slowing in a strong gravitationalfield using accurate atomic clocksplaced in orbit inside artificial satellites. Calculations using general relativity determined that theclocks on the GPS satellites would be seen by Earth's observers to run 38 microseconds fasterper day (than those on Earth), and this was corrected for in the design of GPS.[6]

The Soviet Unionlaunched the first man-made satellite, Sputnik, in 1957. Two Americanphysicists, William Guier and George Weiffenbach, at Johns Hopkins's Applied PhysicsLaboratory(APL), decided to monitor Sputnik's radio transmissions.[7]Within hours they
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realized that, because of the Doppler effect, they could pinpoint where the satellite was along itsorbit. The Director of the APL gave them access to their UNIVACto do the heavy calculationsrequired. The next spring, Frank McClure, the deputy director of the APL, asked Guier andWeiffenbach to investigate the inverse problem pinpointing the user's location given that ofthe satellite. (At the time, the Navy was developing the submarine-launched Polarismissile,

which required them to know the submarine's location.) This led them and APL to develop theTransitsystem.[8]In 1959, ARPA (renamed DARPAin 1972) also played a role inTransit.[9][10][11]

Official logo for NAVSTAR GPS

Emblem of the 50th Space Wing

The first satellite navigation system, Transit, used by the United States Navy, was firstsuccessfully tested in 1960.[12]It used a constellationof five satellites and could provide anavigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timationsatellite that proved the ability to place accurate clocks in space, a technology required by GPS.

In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signaltransmission from pairs of stations,[13]became the first worldwide radio navigation system.Limitations of these systems drove the need for a more universal navigation solution with greateraccuracy.

While there were wide needs for accurate navigation in military and civilian sectors, almost noneof those was seen as justification for the billions of dollars it would cost in research,development, deployment, and operation for a constellation of navigation satellites. During theCold Wararms race, the nuclear threat to the existence of the United States was the one need thatdid justify this cost in the view of the United States Congress. This deterrent effect is why GPSwas funded. It is also the reason for the ultra secrecy at that time. The nuclear triadconsisted of

the United States Navy's submarine-launched ballistic missiles(SLBMs) along with UnitedStates Air Force(USAF) strategic bombers and intercontinental ballistic missiles(ICBMs).Considered vital to the nuclear-deterrence posture, accurate determination of the SLBM launchposition was a force multiplier.

Precise navigation would enable United States submarinesto get an accurate fix of theirpositions before they launched their SLBMs.[14]The USAF, with two thirds of the nuclear triad,also had requirements for a more accurate and reliable navigation system. The Navy and Air
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Force were developing their own technologies in parallel to solve what was essentially the sameproblem. To increase the survivability of ICBMs, there was a proposal to use mobile launchplatforms (such as Russian SS-24and SS-25) and so the need to fix the launch position hadsimilarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System forAccurate ICBM Control) that was essentially a 3-DLORAN. A follow-on study, Project57, wasworked in 1963 and it was "in this study that the GPS concept was born". That same year, theconcept was pursued as Project621B, which had "many of the attributes that you now see inGPS"[15]and promised increased accuracy for Air Force bombers as well as ICBMs. Updatesfrom the Navy Transit system were too slow for the high speeds of Air Force operation. TheNaval Research Laboratory continued advancements with their Timation (Time Navigation)satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clockinto orbit.[16]

Another important predecessor to GPS came from a different branch of the United States

military. In 1964, the United States Armyorbited its first Sequential Collation of Range(SECOR) satellite used for geodetic surveying.[17]The SECOR system included three ground-based transmitters from known locations that would send signals to the satellite transponder inorbit. A fourth ground-based station, at an undetermined position, could then use those signals tofix its location precisely. The last SECOR satellite was launched in 1969. [18]Decades later,during the early years of GPS, civilian surveying became one of the first fields to make use ofthe new technology, because surveyors could reap benefits of signals from the less-than-complete GPS constellation years before it was declared operational. GPS can be thought of asan evolution of the SECOR system where the ground-based transmitters have been migrated intoorbit.

Development

With these parallel developments in the 1960s, it was realized that a superior system could bedeveloped by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in amulti-service program.

During Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagondiscussed the creation of aDefense Navigation Satellite System (DNSS). It was at this meetingthat "the real synthesis that became GPS was created." Later that year, the DNSS program wasnamedNavstar, or Navigation System Using Timing and Ranging.[19]With the individualsatellites being associated with the name Navstar (as with the predecessors Transit andTimation), a more fully encompassing name was used to identify the constellation of Navstarsatellites,Navstar-GPS, which was later shortened simply to GPS. [20]Ten "Block I" prototypesatellites were launched between 1978 and 1985 (with one prototype being destroyed in a launchfailure).[21]

After Korean Air Lines Flight 007, a Boeing 747carrying 269 people, was shot down in 1983after straying into the USSR'sprohibited airspace,[22]in the vicinity of Sakhalinand MoneronIslands, President Ronald Reaganissued a directive making GPS freely available for civilian use,
http://en.wikipedia.org/wiki/Ronald_Reaganhttp://en.wikipedia.org/wiki/Ronald_Reaganhttp://en.wikipedia.org/wiki/Ronald_Reaganhttp://en.wikipedia.org/wiki/Moneron_Islandhttp://en.wikipedia.org/wiki/Moneron_Islandhttp://en.wikipedia.org/wiki/Moneron_Islandhttp://en.wikipedia.org/wiki/Sakhalinhttp://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-22http://en.wikipedia.org/wiki/Prohibited_airspacehttp://en.wikipedia.org/wiki/Prohibited_airspacehttp://en.wikipedia.org/wiki/Prohibited_airspacehttp://en.wikipedia.org/wiki/Boeing_747http://en.wikipedia.org/wiki/Boeing_747http://en.wikipedia.org/wiki/Boeing_747http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007http://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-ieee2008-21http://en.wikipedia.org/wiki/GPS_Block_Ihttp://en.wikipedia.org/wiki/GPS_Block_Ihttp://en.wikipedia.org/wiki/GPS_Block_Ihttp://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-20http://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-19http://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-18http://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-17http://en.wikipedia.org/wiki/SECORhttp://en.wikipedia.org/wiki/United_States_Armyhttp://en.wikipedia.org/wiki/United_States_Armyhttp://en.wikipedia.org/wiki/United_States_Armyhttp://en.wikipedia.org/wiki/United_States_Armyhttp://en.wikipedia.org/wiki/United_States_Armyhttp://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-16http://en.wikipedia.org/wiki/Global_Positioning_System#cite_note-15http://en.wikipedia.org/wiki/LORANhttp://en.wikipedia.org/wiki/SS-25http://en.wikipedia.org/wiki/SS-24


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once it was sufficiently developed, as a common good.[23]The first satellite was launched in1989, and the 24thsatellite was launched in 1994. The GPS program cost at this point, notincluding the cost of the user equipment, but including the costs of the satellite launches, hasbeen estimated to be about USD$5 billion (then-year dollars).[24]Roger L. Eastonis widelycredited as the primary inventor of GPS.

Initially, the highest quality signal was reserved for military use, and the signal available forcivilian use was intentionally degraded (Selective Availability). This changed with President BillClintonordering Selective Availability to be turned off at midnight May 1, 2000, improving theprecision of civilian GPS from 100 to 20 meters (328 to 66ft)[citation needed]. The executive ordersigned in 1996 to turn off Selective Availability in 2000 was proposed by the U.S. Secretary ofDefense, William Perry, because of the widespread growth of differential GPSservices toimprove civilian accuracy and eliminate the U.S. military advantage. Moreover, the U.S. militarywas actively developing technologies to deny GPS service to potential adversaries on a regionalbasis.[25]

Over the last decade, the U.S. has implemented several improvements to the GPS service,including new signals for civil use and increased accuracy and integrity for all users, all whilemaintaining compatibility with existing GPS equipment.

GPS modernization[26]has now become an ongoing initiative to upgrade the Global PositioningSystem with new capabilities to meet growing military, civil, and commercial needs. Theprogram is being implemented through a series of satellite acquisitions, including GPS Block IIIand the Next Generation Operational Control System (OCX). The U.S. Government continues toimprove the GPS space and ground segments to increase performance and accuracy.

GPS is owned and operated by the United States Government as a national resource. Department

of Defense (DoD) is the steward of GPS.Interagency GPS Executive Board (IGEB)oversawGPS policy matters from 1996 to 2004. After that the National Space-Based Positioning,Navigation and Timing Executive Committee was established by presidential directive in 2004 toadvise and coordinate federal departments and agencies on matters concerning the GPS andrelate

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