Satellites, GPS & atomic clock

2016. május 4.

Rozgonyi Kristóf

Phisical foundations of IT tools,

2016.05.04

Brief historical introduction

• The Space Race began on August 2, 1955,when the Soviet Union responded to theUS announcement four days earlier ofintent to launch artificial satellites.

• The first artificial satellite was Sputnik 1,launched by the Soviet Union on October4, 1957.

• Explorer 1 was the United States’ firstartificial satellite, launched on January 31,1958.

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Brief historical introduction

• William Guier and George Weiffenbach decided tomonitor Sputnik’s radio transmissions.

• Because of the Doppler effect, they could pinpointwhere the satellite was along its orbit.

• A year later they investigated the inverse problem –pinpointing the user’s location, given that of thesatellite.

• The first satellite navigation system, TRANSIT,used by the United States Navy, was firstsuccessfully tested in 1960. (It used a constellationof five satellites.)

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The early concept of GPS

• The TRANSIT system was too slow for the Air Force operation, thusthe research continued with the Timation (Time Navigation) satellites.

• The first launched in 1967, and with the third one in 1974 carrying thefirst atomic clock into orbit.

• In 1973, a meeting at the Pentagon discussed the creation of aDefense Navigation Satellite System (DNSS).

• Later the DNSS program was named Navstar, or Navigation SystemUsing Timing and Ranging.

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The construction of GPS

• In the early stage Navstar used already orbitingsatellites.

• Eleven "Block I" prototype satellites werelaunched between 1978 and 1985.

• The "Block II" satellites were the first fullscale operational GPS satellites, was firstlaunched in 1989.

• By December 1993, GPS achieved initialoperational capability, indicating a fullconstellation of 24 satellites.

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The construction of GPS

• Full Operational Capability was declared by AirForce Space Command in April 1995.

• In 2004, Qualcomm announced successful testsof assisted GPS for mobile phones.

• The latest upgrade of the system was "BlockIIF" with twelve satellites, launched between2010 and 2016.

• The next generation "Block III" satellites areproject to launch in 2017.

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Politics and GPS

• In the early stage the system was only available forthe US military.

• After a Korean aircraft was accidentally shot downin 1983, Ronald Reagan issued a directive makingGPS freely available for civilian use.

• Although the signal was publicly available, it wasintentionally degraded, because the highest qualitysignal was reserved for military use.

• The US government decrypt the signal at 2000 toprovide the same precision to civilians that wasafforded to the military.

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Main satellite orbits

• Yet 72 GPS navigation satellites have been launched, 31 are operational(at full constellation 24 satellite are in operation)

• Three main types ofEarth-close satellite orbits:

• Low Earth Orbit (LEO):100-1500 km

• Middle Earth Orbit(MEO): 5000-10000 km

• Geo-Synchro-Orbit (GSO):36000 km

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GPS orbits

• The satellite system is made up six orbital planes with four satelliteseach. (the angular difference between satellites in each orbit is 30,105, 120, and 105 degrees apart)

• The six orbit planes have approximately 55◦ inclination (tilt relative tothe Earth’s equator) and are separated by 60◦ right ascension of theascending node (angle along the equator from a reference point to theorbit’s intersection)

• Orbital radius of approximately 26600 km , thus the orbital period is11 hours and 58 minutes so that the satellites pass over the samelocations or almost the same locations every day.

• GPS orbits gif

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Operation Control Segment

• The Operation Control Segment (OCS) serves as the control segmentof record. It provides the operational capability and keeps the GPSsystem operational and performing within specification.

• OCS contains eleventracking and monitoringstations all over the globe.

• Contains a control stationand ground antennas inorder to communicate withthe satellites regularly.

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GPS satellite frequencies

Band Frequency Description

L1 1575.42 MHz C/A and P(Y) codes.

L2 1227.60 MHz P(Y) code.

L3 1381.05 MHz Used for nuclear detonation(NUDET) detection.

L4 1379.913 MHz Being studied for additionalionospheric correction.

L5 1176.45 MHz Civilian safety-of-life(SoL) signal.

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GPS signal encryption

• The relevant bands are L1 & L2, where the low-bitrate message datais encoded with a high-rate pseudo-random (PRN) sequence that isdifferent for each satellite.

• Coarse Acquisition code (C/A): for civilian use, transmits data at 1.023million chips per second.

• Precision code (P(Y)): or U.S. military use, transmits at 10.23 millionchips per second.

• A chip is a pulse of a pseudo random code, thus the signal will be themultiplication of the cip and the bit.

• To decrypt the signal you need to know the pseudo random code (theGold code) of the satellite.

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GPS signal decryption

• Because all of the satellite signals are modulated onto the same carrierfrequency, the signals must be separated after demodulation.

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GPS message

• Each GPS satellite continuously broadcasts a navigation message at arate of 50 bits per second.

• Each message takes 750 seconds to complete, and contain twenty-fiveframe, each contain five subframe:

Subframe Description

1 Satellite clock, GPS time relationship.

2-3 Ephemeris (precise satellite orbit).

4-5 Almanac component(satellite network synopsis, error correction).

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Coordinate system

• The International Celestial Reference Frame (ICRF) is a quasi-inertialframe of reference centered at the barycenter of the Solar System,defined by the measured positions of 212 extragalactic sources.

• ICRF is now the standardreference frame used todefine the positions of theplanets (including theEarth)

• ICRF is the realization ofInternational CelestialReference System (ICRS).

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Universal Time

• Coordinated Universal Time (UTC), is the primary time standard bywhich the world regulates clocks and time.

• UTC is defined by various synchronized atomic clocks and at irregularintervals to compensate for the slowing of Earth’s rotation.

• GPS use the Universal Time (UT), that is not corrected to match therotation of the Earth, thus the UT does not contain leap seconds.

• The GPS navigation message includes the difference between GPStime and UTC.

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GPS reciver position

• To calculate the receiver’sposition three conditionsmust be satisfied:

• Clocks need to besynchronized

• Reciver have to know thesatellites positions

• Reciver have to connect atleast to four satellites

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GPS problems

• The main problem is timekeeping:

• We need to measure time very precisely.

• We have to use atomic clocks.

• We have to make corrections for relativistic effects.

• For measure position very precisely, other ground stations andavailable satellites required.

• The system is sensitive to solar flares.

• Owned and run by the US army.

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The history of the atomic clock

• Lord Kelvin suggested to use atomic transitions tomeasure time in 1879.

• The first atomic clock, based on the transition ofthe cesium-133 atom, was built by Louis Essen in1995.

• This led to the internationally agreed definition ofthe latest SI second being based on atomic time.

• In 2004, NIST scientists demonstrated a chip-scaleatomic clock.

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Physical background

• The definition makes the cesium oscillator the primary standard fortime and frequency measurements, called the cesium standard.

• Cesium is an ideal material for atomic clocks, thus it is solid at roomtemperature and have only one electron on its valence shell.

• The actual time-reference of an atomic clock based on the hyperfinetransition of the cesium.

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Physical background

• The core of the atomic clock is a tunable microwave cavity containinggas.

• We measure the photons (or the lack of photons) from the inducedemissions (induced absorption).

• With the help of thedetector’s feedback signalwe can keep the microwavecavity at resonancefrequency.

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Modern atomic clocks

• Modern clocks try to correct the unwantedside-effects, and uses advanced techniques.

• The modern standards use laser cooling,magneto-optical traps and precision laserspectroscopy.

• Furthermore the modern clocks use othermaterials to increase the accuracy.

• Current accuracy: 1.5× 10−16 s

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GPS time synchronization

• Every satellite has at least two onboard cesium atomic clocks.

• At each control stations there is also an atomic clock.

• The clock not synchronized to each otherbut the UT "absolute" time.

• We have to make the relativisticcorrections for the satellite’s clocks:

• Special relativistic correction, for thespeed of the satellite.

• General relativistic correction, for theEarth mass.

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Accurate GPS positioning

• Differential GPS (DGPS) require oneknown ground station, thus the errorsoccurred by the atmosphere can beeliminate.

• The geodesical accuration, uses twoground station, and the absolute positionof one station is known.

• With this corrections, mm size positioningcan be reached.

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Future of the GPS, alternatives

• 32 more satellites are planned with NASA’s new atomic clock: DeepSpace Atomic Clock (2016).

• Alternative systems: GLONASS (Russian), Galileo (Europa), Beidou-2(China).

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