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International Atomic Time

International Atomic Time

Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. It is a weighted average of the time kept by about 300 atomic clocks (including a large number of caesium atomic clocks) in over 50 national laboratories worldwide. It has been available since 1955, and became the international standard on which UTC is based on January 1, 1972, as decided by the 14th General Conference on Weights and Measures (CGPM). The International Bureau of Weights and Measures is in charge of the realization of TAI. The highest precision realization of TAI times can only be determined retrospectively, as the timescale is defined by periodic comparisons among its participating atomic clocks. However, these corrections are usually only needed for applications that require nanosecond-scale accuracy. Most time service users use realtime estimates of TAI provided by atomic clocks that have been previously referenced to the composite timescale. GPS is a commonly-used realtime source of time traceable back to TAI. Coordinated Universal Time (UTC) is the basis for legal time worldwide, and always differs from TAI by an integral number of seconds. In mid 2005, UTC was behind TAI by 32 seconds. The difference is due to an initial ten second offset on 1 January 1972 when UTC was established and leap seconds, which have been periodically inserted into UTC since the first on 30 June 1972 due to slight irregularities in Earth's rate of rotation. While TAI is a continuous and stable timescale, UTC has intentional discontinuities to keep it from drifting more than 0.9 seconds from UT1, a timescale defined by the Earth's rotation. Roughly speaking, solar noon (the time at which the sun is directly overhead) would drift away from 12:00:00 without leap second corrections. UT1 is computed by the International Earth Rotation and Reference Systems Service (IERS). TAI was defined such that TAI = UT1 on January 1 1958. Because UTC is a discontinuous timescale, it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval. Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that can not handle leap seconds.

External links

- [http://www.bipm.fr/enus/5_Scientific/c_time/time_1.html Bureau International des Poids et Mesures]
- [http://hpiers.obspm.fr IERS website]
- [http://www.boulder.nist.gov/timefreq/general/faq.htm NIST Time and Frequency FAQs] Category:Time scales ja:国際原子時

Time scale

A time scale specifies divisions (scale) of time. A time standard is a specification of either the rate at which time passes, or points in time, or both. In music, Curtis Roads (2001, p.3-4) distinguishes nine time scales of music: #Infinite: literally infinite, such as the length of sine waves in classical Fourier analysis, #Supra: months, years, decades, and centuries; everything above the level of #Macro: "overall musical architecture or form" or the level of the individual piece; minutes, hours, or even days, #Meso: "Divisions of form" including movements, sections, phrases; seconds and minutes, #Sound object (Schaeffer 1959, 1977): "a basic unit of musical structure" and a generalization of note (Xenakis' ministructural time scale); fraction of a second to several seconds, #Micro: "sound particles" (see granular synthesis) down to the threshold of audible perception; thousands to millionths of seconds, #Sample: sample (music), measured as are samples in millionths of a second or microseconds, #Subsample: changes "too brief to properly recorded or perceived", billionths of a second, nanosecond, or less, and #Infinitesimal: literally "infinitely brief" such as delta functions. Music may, however, exist "outside" of time when structured through "principles whose defnitions does not imply a temporal order", including scales and many other precompositional techniques, musical instruments, and aleatoric music. Examples such as sound installations in which the order of the sound is determined by, for example, a listeners movement through the system, are thus placed in time. (ibid, p.38) See also: duration, rhythm, timescapes, and geologic timescale.

Source

- Roads, Curtis (2001). Microsound. MIT. ISBN 0262182157. Category:Time scales Category:Rhythm

Caesium

Caesium (cesium in the United States) pronounced /ˈsiːziəm/; is a chemical element in the periodic table that has the symbol Cs and atomic number 55. It is a soft silvery-gold alkali metal which is one of at least three metals that are liquid at or near room temperature. This element is most notably used in atomic clocks. The variant spelling cesium is sometimes used, especially in North American English, but caesium is the spelling used by the IUPAC, although since 1993 it has recognized cesium as a variant as well.

Notable characteristics

The electromagnetic spectrum of caesium has two bright lines in the blue part of the spectrum along with several other lines in the red, yellow, and green. This metal is silvery gold in color and is both soft and ductile. Caesium is also the most electropositive and most alkaline of the stable chemical elements and also has the least ionization potential of all the elements, except for francium. Caesium is the least abundant of the five non-radioactive alkali metals. (Technically, francium is the least common alkali metal, but since it is highly radioactive with less than 30 grams in the entire earth at one time, its abundance can be considered zero in practical terms.) Along with gallium and mercury, caesium is among the only metals that are liquid at or near room temperature. Caesium reacts explosively in cold water and also reacts with ice at temperatures above −116°C. Caesium hydroxide (CsOH) is a very strong base and will rapidly etch the surface of glass.

Applications

Caesium is most notably used in atomic clocks, which are accurate to seconds in many thousands of years. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium. SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the caesium-133 atom.
- Cs-134 has been used in hydrology as a measure of caesium output by the nuclear power industry. This isotope is used because, while it is less prevalent than either Cs-133 or Cs-137, Cs-134 can be produced solely by nuclear reactions. Cs-135 has also been used in this function.
- Like other group 1 elements, caesium has a great affinity for oxygen and is used as a "getter" in vacuum tubes.
- This metal is also used in photoelectric cells.
- In addition, caesium is used as a catalyst in the hydrogenation of certain organic compounds.
- Radioactive isotopes of caesium are used in the medical field to treat certain types of cancer.
- Caesium fluoride is widely used in organic chemistry as a base and as a source of anhydrous fluoride ion.
- Caesium vapor is used in many common magnetometers.
- Because of their high density, Caesium chloride solutions are commonly used in molecular biology for density gradient ultracentrifugation, primarily for the isolation of nucleic acids from biological samples. More recently this metal has been used in ion propulsion systems.

History

Caesium (Latin caesius meaning "sky blue") was spectroscopically discovered by Robert Bunsen and Gustav Kirchhoff in 1860 in mineral water from Dürkheim, Germany. Its identification was based upon the bright blue lines in its spectrum and it was the first element discovered by spectrum analysis. The first caesium metal was produced in 1881. Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical applications.

Occurrence

1881 An alkali metal, caesium occurs in lepidolite, pollucite (hydrated silicate of aluminium and caesium) and within other sources. One of the world's most significant and rich sources of this metal is located at Bernic Lake in Manitoba. The deposits there are estimated to contain 300,000 metric tons of pollucite at an average of 20% caesium. It can be isolated by electrolysis of fused cyanide and in a number of other ways. Exceptionally pure and gas-free caesium can be made by the thermal decomposition of caesium azide. The primary compounds of caesium are its chloride and its nitrate. The price of caesium in 1997 was about $US 30 per gram.

Isotopes

Caesium has at least 39 known isotopes which is more than any other element, except francium. The atomic masses of these isotopes range from 112 to 151. Even though this element has the largest number of isotopes, it has only one naturally occurring stable isotope, Cs-133, the other isotopes (except for the isotopes noted on this page) have half-lives from a few days to fractions of a second. The radiogenic isotope Cs-137 has been used in hydrologic studies, analogous to the use of H-3 (tritium). Cs-137 is produced from the detonation of nuclear weapons and is produced in nuclear power plants, and notably from the 1986 Chernobyl meltdown. Beginning in 1954 with the commencement of nuclear testing, Cs-137 was released into the atmosphere where it is absorbed readily into solution. Once Cs-137 enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes can be estimated as a function of time.

Precautions

All alkaline metals are highly reactive. Caesium, being one of the heavier alkaline metals, is highly explosive in cold water; and can even break the bath that it is being contained. Caesium should be considered highly toxic. Some of its radioisotopes are even more toxic. Caesium hydroxide is an extremely strong base, and can attack glass.

References

- [http://periodic.lanl.gov/elements/55.html Los Alamos National Laboratory - Cesium]

External links

- [http://www.webelements.com/webelements/elements/text/Cs/index.html WebElements.com – Caesium]
- [http://www.cs.rochester.edu/users/faculty/nelson/cesium/cesium_faq.html FAQ from alt.cesium newsgroup] Category:Chemical elements Category:Alkali metals ko:세슘 ja:セシウム th:ซีเซียม

UTC

:For alternate uses of UTC see UTC (disambiguation) Coordinated Universal Time or UTC, also sometimes referred to as "Zulu time" or Z, is an atomic realization of Universal Time (UT) or Greenwich Mean Time, the astronomical basis for civil time. Time zones around the world are expressed as positive and negative offsets from UT. UTC differs by an integral number of seconds from International Atomic Time (TAI), as measured by atomic clocks and a fractional number of seconds from UT. UTC is a hybrid time scale: the rate of UTC is based on atomic frequency standards but the epoch of UTC is synchronized to remain close to astronomical UT. The Earth's rotation is very slowly decelerating (due to braking action of the tides), hence the mean solar day has increased since TAI was introduced on 1 January 1958 (under another name). For this reason, UT is 'slower' than TAI. As of 1 January 1999, TAI was ahead of UTC by 32 seconds, consisting of a 10-second offset introduced on 1 January 1972 to account for all variations between 1958 and 1971, plus an additional 22 leap seconds introduced between 1972 and 1998. UTC is maintained within 0.9 s of UT1 (UT1 is one of three precise definitions of UT); leap seconds are added (or, theoretically, subtracted) at the end of any UTC month as necessary. The primary dates for leap second adjustments are at the end of the day on June 30 and December 31. The secondary dates, which to date have been unused, are March 31 and September 30. To date, all such adjustments – the first in 1972 – have been positive and applied on dates June 30 or December 31, where an additive leap second is designated as 23:59:60. The announcement of leap seconds is made by the International Earth Rotation and Reference Systems Service (IERS), based on precise astronomical forecasts of the Earth's rotation. Historically, one leap second has been required every one to two years. However a leap second has not been required since 1998, as the deceleration of the Earth's rotation slowed temporarily in the past seven years. The IERS announced in July 2005 that the next leap second will be on 31 December 2005. For most practical and legal-trade purposes, the fractional difference between UTC and UT (or GMT) is inconsequentially small, and for this reason UTC is colloquially called GMT sometimes, even if this is not technically correct.

Proposal to redefine UTC and abolish leap seconds

There is a proposal to redefine UTC and abolish leap seconds, such that sundials would slowly get further out-of-sync with civil time. See Leap second for more information.

General information

"UTC" is not a true acronym; it is a variant of Universal Time, UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It [http://www.boulder.nist.gov/timefreq/general/misc.htm#Anchor-14550 may be regarded] as a compromise between the English acronym "CUT" and the French acronym "TUC" (temps universel coordonné). It is sometimes erroneously expanded into "Universal Time Code".

Converting Universal Time
to Standard (Winter) Local Times
Hawaii-Aleutian Standard Time	UTC -10
Alaska Standard Time	UTC -9
Pacific Standard Time	UTC -8
Mountain Standard Time	UTC -7
Central Standard Time	UTC -6
Eastern Standard Time	UTC -5
Atlantic Standard Time	UTC -4
Greenwich Mean Time/Western European Time	UTC
Central European Time	UTC +1
Eastern European Time	UTC +2
Moscow Time	UTC +3
Indian Standard Time	UTC +5:30
Singapore Standard Time, Hong Kong Time, Australian Western Standard Time, Chinese Standard Time	UTC +8
Japan Standard Time, Korea Standard Time	UTC +9
Australian Central Standard Time	UTC +9:30
Australian Eastern Standard Time	UTC +10
New Zealand Standard Time	UTC +12

For more, see time zone.

International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus. This is done under the auspices of the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). However, local clusters of atomic clocks are sufficient for accuracy to within a few tens of nanoseconds. UTC is the time system used for many Internet and World Wide Web standards. In particular, the Network Time Protocol, designed to synchronize the clocks of many computers over the Internet (usually to that of a known accurate atomic clock), uses UTC. As indicated in the standards, it is convenient to include the UTC date too. The UT time zone is sometimes denoted by the letter Z since the equivalent nautical time zone (GMT) has been denoted by Z since about 1950, and by a "zone description" of zero hours since 1920. See Time zone history. Since the NATO phonetic alphabet and radio-amateur word for Z is "Zulu", UT is sometimes known as Zulu time.

Amateur Radio

Those who transmit on the amateur radio bands often log the time of their radio contacts in UTC, as transmissions can go worldwide on some frequencies. In the past, the FCC required all amateur radio operators in the United States of America to log their radio conversations. While maintaining a record of radio transmissions is no longer required in the USA, many American amateur radio operators still choose to maintain a log expressing the time of their transmissions in UTC, due to the world wide reach of ham radio.

References

- ITU-R Recommendation TF.460-4: Standard-frequency and time-signal emissions. International Telecommunication Union. (Annex I of this document contains the official definition of UTC.)
- Dennis D. McCarthy: "Astronomical Time". Proc. IEEE, Vol. 79, No. 7, July 1991, pp. 915-920.
- Nelson, McCarthy, et al.: "[http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf The leap second: its history and possible future]" (381 KB PDF file), Metrologia, Vol. 38, pp. 509–529, 2001.
- David W. Allan, Neil Ashby, Clifford C. Hodge: The Science of Timekeeping. Hewlett Packard Application Note 1289, 1997.

External links

- [http://www.bipm.org/en/scientific/tai/time_server.html Bureau International des Poids et Mesures UTC/TAI Time Server]
- [http://www.time.gov/ The official U.S. time]
- [http://www.worldtimeserver.com/ World Time Server - any location, any time]
- [http://www.thetimenow.com/ thetimeNOW - Current time in all time zones]
- [http://aa.usno.navy.mil/faq/docs/UT.html United States Naval Observatory - What is Universal Time?]
- [http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat International Earth Rotation Service Leap Second Updates]
- [http://www.qsl.net/zl1bpu/micro/CLOCK/ Make your own UTC /Local time hardware clock]
- [http://www.w3.org/TR/NOTE-datetime W3C Specification about UTC Date and Time] and IETF Internet standard RFC 3339
- [http://www.grc.nasa.gov/WWW/MAEL/ag/zulu.htm Zulu Time]
- [http://www.hko.gov.hk/gts/time/worldtime2.htm Hong Kong Time by Hong Kong Observatory] Category:Time scales als:UTC ko:협정 세계시 zh-min-nan:UTC ja:協定世界時 nb:UTC simple:Coordinated Universal Time th:เวลาพิกัดสากล

1972

1972 (MCMLXXII) was a leap year that started on a Saturday.

Events

- International year of the book

January

- January 2 - The Pierre Hotel Heist - Six men rob the safety deposit boxes of the Pierre Hotel in New York City. Loot is at least $4 million.
- January 4 - Rose Heilbron becomes the first woman judge at the Old Bailey in London.
- January 5 - President of the United States Richard Nixon orders the development of a space shuttle program.
- January 4 - Kurt Waldheim becomes the Secretary General of the United Nations.
- January 7 - Iberian Airlines passenger planes crashes into an 800' peak on island of Ibiza - 104 dead.
- January 9 - Howard Hughes speaks by telephone to denounce Clifford Irving's supposed biography about him.
- January 9 - RMS Queen Elizabeth is destroyed by fire (Hong Kong harbor).
- January 11 - East Pakistan becomes independent with the name Bangladesh.
- January 14 - King Frederick IX of Denmark dies - his daughter Queen Margaret II of Denmark ascends to the throne at January 16.
- January 19 - Libertarian enclave Minerva on a platform in the South Pacific, sponsored by the Phoenix Foundation, declares independence. Soon neighboring Tonga annexes the area and dismantles the platform
- January 22 - Denmark, Ireland and the United Kingdom join the EEC.
- January 23 - New Delhi bootlegger sells wood alcohol to a wedding party - 100 dead
- January 24 - Japanese soldier Shoichi Yokoi is discovered in Guam. He had spent 28 years in the jungle.
- January 25 - Shirley Chisholm, the first African American woman elected to US Congress, announces her candidacy for US president.
- January 26 - Yugoslavian air stewardress Vesna Vulovic is the only survivor when her plane crashes in Czechoslovakia. She survives after falling about 30,000' in the tail section of the aircraft.
- January 28 - Richard Chanfray claims he is Count of St Germain on French television.
- January 30 - Bloody Sunday - the British Army kills 13 unarmed Roman Catholic civil rights marchers in Derry, Ireland.
- January 30 - Pakistan withdraws from the British Commonwealth.
- January 31 - King Mahendra of Nepal dies, becoming the second king to die that month, and is succeeded by his son, Birendra.

February

- February 1 - First scientific hand-held calculator (HP-35) introduced (price $395).
- February 2 - A bomb explodes in British Yacht Club in West Berlin. Only casualty is Irwin Beelitz, a German boat builder. Movement 2 June announces it is in support of Irish Republican Army.
- February 2 - Anti-British riots throughout Ireland take place. The British Embassy in Dublin is burned to the ground as are several British owned businesses.
- February 3 - The Winter Olympics begin in Sapporo, Japan.
- February 4 - Mariner 9 sends pictures from Mars.
- February 5 - US airlines begin mandatory inspection of passengers and baggage.
- February 5 - Bob Douglas becomes the first African American elected to the Basketball Hall of Fame.
- February 9 - The British government declares a state of emergency over a miners' strike.
- February 15 - President of Ecuador José María Velasco Ibarra is deposed for the fourth time.
- February 15 - Phonorecords granted U.S. Federal copyright protection for the first time.
- February 17 - Sales of the Volkswagen Beetle model exceed those of Ford Model-T (15 million).
- February 18 - The California Supreme Court invalidates the state's death penalty and commutes the sentences of all death row inmates to life in prison.
- February 21-February 27 - President Richard M. Nixon makes an unprecedented eight-day visit to the People's Republic of China and meets with Mao Zedong.
- February 21 - The Soviet unmanned spaceship Luna 20 lands on the Moon.
- February 22 - IRA bomb in Aldershot - 7 dead.
- February 23 - Angela Davis is released from jail.
- February 23 - A Lufthansa plane is hijacked and taken to Aden. Passengers are released after a ransom of 16 million D-marks is agreed.
- February 24 - North Vietnamese negotiators walk out of the peace talks in Paris to protest US air raids.
- February 26 - A coal sludge spill kills 125 in Buffalo Creek.
- February 26 - Luna 20 comes back to Earth with a cargo of moon rocks.

March

- March 1 - Thai province Yasothon created after being split off from the Ubon Ratchathani Province.
- March 1 - British 14-year-old schoolboy Timothy Davey is sentenced in Turkey for "conspiring to sell cannabis."
- March 1 - The Club of Rome publishes report "Boundaries on the Growth."
- March 2 - Launch of the Pioneer 10 spacecraft.
- March 2 - Jean-Bedel Bokassa becomes the president of the Central African Republic.
- March 3 - Sculpted figures of Jefferson Davis, Robert E. Lee, and Stonewall Jackson are completed at Stone Mountain, Georgia.
- March 4 - Libya and the Soviet Union sign a cooperation treaty.
- March 5 - Greek composer Mikis Theodorakis leaves the Greek Communist Party.
- March 13 - The United Kingdom and the People's Republic of China elevate diplomatic exchanges to the ambassadorial level after 22 years.
- March 13 - Clifford Irving admits to a New York court that he had fabricated Howard Hughes "autobiography."
- March 16 - The first building of the Pruitt-Igoe housing development is destroyed.
- March 19 - India and Bangladesh sign a friendship treaty.
- March 24 - To prevent further unionist misrule, Britain takes over direct rule of Northern Ireland.
- March 26 - 19 climbers on Mount Fuji die in an avalanche.
- March 30 - Vietnam War: The Eastertide Offensive begins after North Vietnamese forces cross into the Demilitarized Zone (DMZ) of South Vietnam.

April

- April 3 - First call was made with a cell phone (cellular phone) in New York.
- April 7 - US Mafioso Joe Gallo shot in Umberto's Clam House in Little Italy.
- April 10 - The USA and the Soviet Union join some 70 nations in signing an agreement to ban biological warfare.
- April 10 - A 7.0 Richter scale earthquake kills 1/5 of the population of Iranian province of Fars.
- April 13 - The Universal Postal Union decides to recognize the People's Republic of China as the only legitimate Chinese representative, effectively expelling the Republic of China administering Taiwan.
- April 16 - Apollo 16 launched.
- April 16 - Vietnam War: Nguyen Hue Offensive – Prompted by the North Vietnamese offensive, the United States resumes bombing of Hanoi and Haiphong.
- April 18 - The Roland Corporation is founded in Osaka.
- April 22 - Sylvia Cook and John Fairfax have rowed across the Pacific.
- April 27 - Constructive Vote of No Confidence against German Chancellor Willy Brandt fails under obscure circumstances.
- April 29 - The fourth anniversary of the Broadway musical Hair is celebrated with a free concert at a Central Park bandshell, followed by dinner at the Four Seasons. There, thirteen Black Panther protesters and the show's co-author, Jim Rado, are arrested for disturbing the peace and marijuana use.

May

- May 5 - An Alitalia DC-8 crashes west of Palermo, Sicily – 115 dead.
- May 13 - Fire in a nightclub atop the Sennichi department store in Osaka, Japan – 115 dead.
- May 15 - Governor George Wallace of Alabama is shot by Arthur Herman Bremer at a Laurel, Maryland political rally.
- May 17 - The closing notice is posted for the Broadway musical Hair.
- May 18 - Four troopers of both SAS and SBS are parachuted onto the HMS Queen Elizabeth II, 1000 miles off Britain in the Atlantic, after a bomb threat and demand for ransom. It turns out to be bogus.
- May 19 - Three out of six bombs explode in the Springer Press building in Hamburg, Germany - 17 injured. The Red Army Faction claims responsibility.
- May 21 - In Rome, Laszlo Toth attacks Michelangelo's Pieta statue with a sledgehammer shouting that he is Jesus Christ
- May 22 - Earthquake lasting 20 seconds destroys most of Bingol, Turkey - more than 1000 dead, 10.000 made homeless
- May 22 - Ceylon becomes the republic of Sri Lanka under prime minister Sirimavo Bandaranaike when its new constitution is ratified.
- May 24 - A RAF bomb explodes in the Campbell Barracks of the US Army Supreme European Command in Heidelberg. Two US soldiers dead.
- May 26 - Richard Nixon and Leonid Brezhnev sign SALT I treaty in Moscow (including Anti-Ballistic Missile Treaty; also other agreements were made).
- May 26 - Willandra National Park is established in Australia.
- May 30 - The Angry Brigade goes on trial.
- May 30 – 3 members of Japanese Red Army kill 24 and injure 100 in Lod Airport, Israel.

June

- June - Iraq nationalizes the Iraq Petroleum Company.
- June 2 - Andreas Baader, Jan-Carl Raspe, Holger Meins and some other members of Red Army Faction are arrested in Frankfurt am Main after a shootout.
- June 3 - Sally Priesand becomes the first female US rabbi.
- June 4 - Angela Davis found not guilty of murder.
- June 14 - June 23 - Hurricane Agnes kills 117 in US east coast.
- June 15 - Ulrike Meinhof and Gerhard Müller of Red Army Faction are arrested in a teacher's apartment in Langenhagen, West Germany.
- June 17 - Watergate scandal: Five White House operatives are arrested for burglarizing the offices of the Democratic National Committee.
- June 17 - Return of Okinawa from United States' control to Japan.
- June 17 - Chilean president Salvador Allende forms a new government.
- June 18 - West Germany beat the Soviet Union 3-0 to win Euro 72.
- June 23 - Watergate Scandal: U.S. President Richard M. Nixon and White House chief of staff H. R. Haldeman are taped talking about using the Central Intelligence Agency to obstruct the Federal Bureau of Investigation's investigation into the Watergate break-ins.
- June 25 - Juan Peron is elected president of Argentina.
- June 26 - Nolan Bushnell and Ted Dabney found Atari.
- June 28 - US president Nixon announces that no new draftees will be sent to Vietnam.
- June 29 - Supreme Court of the United States rules that the death penalty is unconstitutional.

July

- July 1 - The Broadway production of the musical Hair closes after 1,752 performances.
- July 2 - Following Pakistan's surrender to India in the Indo-Pakistani War of 1971, both nations sign the historic Simla Agreement agreeing to settle their disputes bilaterally.
- July 4 - The first Rainbow Gathering held in Colorado.
- July 8 - The USA sells grain to the Soviet Union for $750 million.
- July 10 - A stampede of elephants kills 24 in the Chandka Forest in India.
- July 15 - The Pruitt-Igoe housing development is demolished.
- July 18 - Anwar Sadat expels 20.000 soviet advisors from Egypt
- July 21 - Bloody Friday — 22 bombs explode in Belfast, Ireland. 9 people were killed and a further 130 seriously injured.
- July 23 - The United States launches LANDSAT 1, first Earth-resources satellite.
- July 25 - US Health officials admit that blacks were used as guinea pigs in a syphilis experiment.
- July 29 - National dock strike begins in Britain.

August

- August 4 - Arthur Bremer jailed for 63 years for shooting George Wallace.
- August 4 - Dictator Idi Amin declares that Uganda will expel 50,000 Asians with British passports to Britain within three months.
- August 11-August 12 - Last US ground troops withdrawn from Vietnam.
- August 16 - The Royal Moroccan Air Force mistakenly fires upon, but fails to bring down, Hassan II of Morocco's plane while he was traveling back to Rabat.
- August 23 - R.J (Dick) Hamer replaces Henry Boltie As Victorian Premier.
- August 28 - Prince William of Gloucester dies in an air crash.

September

- September 1 - Bobby Fischer defeats Boris Spassky in a chess match at Reykjavik, Iceland, and becomes the first American chess champion (see Match of the Century).
- September 5-September 6 - Munich Massacre: Eleven Israeli athletes at the Summer Olympic Games in Munich are killed after eight members of the Arab terrorist group Black September invade the Olympic Village; five guerillas and one policeman are also killed in a failed hostage rescue.
- September 14 - West Germany and Poland renew diplomatic relations.
- September 17 - Uganda announces that there are Tanzanian troops in its territory.
- September 17 - M
- A
- S
- H debuts on CBS.
- September 19 - Parcel bomb sent to Israeli Embassy in London kills one diplomat.
- September 21 - Philippine president Ferdinand Marcos issued Proclamation No. 1081 placing the entire country under martial law.
- September 25 - Norwegian EC referendum, 1972 - the people of Norway reject membership into the European Economic Commission.
- September 27 - Joint Communique of the Government of Japan and the Government of the People's Republic of China.
- September 28 - The goal heard round the world. Canada wins the summit series with a goal by Paul Henderson.
- September 29 - Sino-Japanese relations: Japan normalized diplomatic relations with the People's Republic of China after breaking official ties with the Republic of China.

October

- October 1 - First publication reporting the production of a recombinant DNA molecule, marking the birth of modern molecular biology methodology. :: Jackson, David A.; Symons, Robert H.; and Berg, Paul. (1972). [http://www.pnas.org/cgi/content/abstract/69/10/2904 Biochemical Method for Inserting New Genetic Information into DNA of Simian Virus 40: Circular SV40 DNA Molecules Containing Lambda Phage Genes and the Galactose Operon of Escherichia coli]. Proceedings of the National Academy of Sciences (USA) 69(10), 2904-2909.
- October 2 - Denmark joins the EEC. The Faroe Islands stay out.
- October 5 - The United Reformed Church is founded out of the Congregational and Presbyterian Churches.
- October 6 - Train crash in Saltillo, Mexico – 208 dead.
- October 12 - En route to her station in the Gulf of Tonkin, a racial brawl involving more than 100 sailors breaks out aboard the United States Navy aircraft carrier USS Kitty Hawk. Nearly 50 sailors are injured.
- October 13 - A Fairchild passenger plane transporting a rugby union team crashes at about 14,000' in the Andes mountain range, near the Argentina/Chile border. Sixteen of the survivors are found alive December 20 but they have had to resort to cannibalism to survive (see Uruguayan Air Force Flight 571).
- October 16 - A plane carrying US congressman Hale Boggs of Louisiana and three other men vanishes in Alaska. The wreckage has never been found, despite a massive search at the time.
- October 16 - Rainbow, a British television programme for children, debuts.
- October 16 - Rioting inmates of the Maze prison cause a fire that destroys most of the camp
- October 17 - Queen Elizabeth II of the United Kingdom visits Yugoslavia.
- October 25 - First female FBI agents hired.
- October 25 - Belgian cyclist Eddy Merckx sets a new world hour record in Mexico City.
- October 29 - The Black September group hijacks a Lufthansa Boeing 727 over Turkey and demands the release of three of their comrades still held for the massacre of Israeli athletes at the Olympic games
- October 30 - US President Richard Nixon approves legislation to increase Social Security spending by US$5.3 billion.

November

- November ? - At a scientific meeting in Honolulu, over [http://bancroft.berkeley.edu/ROHO/projects/biosci/symposium/cohen/text.html corned beef sandwiches], Herbert Boyer and Stanley N. Cohen conceived the concept of recombinant DNA. They published their results in November 1973 in PNAS. Separately in 1972, Paul Berg also recombined DNA in a test tube. Recombinant DNA technology has dramatically changed the field of biological sciences, especially biotechnology, and opened the door to genetically modified organisms.
- November 5 - Group of Amerindians occupies the Bureau of Indian Affairs.
- November 7 - U.S. presidential election, 1972: Republican incumbent Richard Nixon defeats Democratic Senator George McGovern (the election had the lowest voter turnout since 1948 with only 55 percent of the electorate voting).
- November 11 - Vietnam War: Vietnamization - The United States Army turns over the massive Long Binh military base to South Vietnam.
- November 14 - The Dow Jones Industrial Average closes above 1,000 (1,003.16) for the first time.
- November 16 - The United Nations Educational, Scientific and Cultural Organization adopts the [http://whc.unesco.org/pg.cfm?cid=182 Convention Concerning the Protection of the World Cultural and Natural Heritage].
- November 17 - Juan Perón returns to Argentina.
- November 22 - Vietnam War: The United States loses its first B-52 Stratofortress of the war.
- November 30 - Vietnam War: White House Press Secretary Ron Ziegler tells the press that there will be no more public announcements concerning American troop withdrawals from Vietnam due to the fact that troop levels are now down to 27,000.

December

- December 2 - Gough Whitlam becomes the first Labour Party Prime Minister of Australia for 23 years. He is famously sworn in on the election night and his first action using executive power is to withdraw all Australian personnel from the Vietnam War.
- December 7 - PIRA kidnaps Jean McConville in Belfast.
- December 7 - Apollo 17, the last manned mission to the moon, is launched.
- December 7 - Imelda Marcos is stabbed and seriously wounded by an assailant; her bodyguards shoot him.
- December 15 - The Commonwealth of Australia ordains equal pay to women.
- December 21 - East Germany and West Germany recognize each other.
- December 21 - ZANLA troopers attack Altera Farm in north-east Rhodesia
- December 22 - 6.25 Richter scale earthquake in Managua, the capital of Nicaragua – over 12,000 dead. President Somoza is later accused of pocketing millions of dollars worth of foreign aid.
- December 22 - Australia establishes diplomatic relations with China and West Germany.
- December 23 - Earthquake in Nicaragua kills 5000-10.000 in the capital Managua
- December 28 - The bones of Martin Bormann identified in Berlin.
- December 29 - An Eastern Air Lines Lockheed L-1011 crashes into the Everglades in Florida, killing 99 of 163 onboard.

Unknown dates

- Prime minister of Sweden, Olof Palme compares the American bombings of North Vietnam to Nazi massacres. The US breaks diplomatic contact with Sweden.
- The last major epidemic of smallpox in Europe breaks out in Yugoslavia.
- The United Kingdom begin to train Special Air Service for anti-terrorist duties.
- Steve Jobs graduates from Homestead High School and enrolls in Reed College in Portland, Oregon but drops out after one semester.
- Kim Sung-il becomes president of North Korea.
- The Japanese government begins building a railway tunnel between Honshu and Hokkaido.
- Stephen Hawking is confined to a wheelchair due to motor neuron disease.
- The Bureau of Alcohol, Tobacco and Firearms becomes independent from IRS.
- The "tea house" Mellow Yellow opens on the Amstel River in Amsterdam, pioneering the legal sale of marijuana in the Netherlands.
- The Aboriginal Tent Embassy founded on the lawn of Parliament House in Canberra.
- First women admitted to Dartmouth College.
- Colombian looters find Ciudad Perdida but keep it a secret until government reveals it 1975.
- Frank Serpico exposes corruption in New York City police.
- Vietnam War veteran Richard McCoy hijacks a United Airlines jet and extorts $500,000 – he is later captured.
- The Yellow River dries up for the first time in known history.
- Somalian language gets a written form.
- Assassination of Zanzibar's leader Sheik Abeid Karume.
- Tamil United Front, pro-Tamil organization, founded.
- Worship of Norse gods officially approved in Iceland.
- Women are allowed to compete in the Boston Marathon for the first time.
- The Second Cod War between UK and Iceland.
- First use of the term Hadean.
- Robert Wood Johnson Foundation established.
- The first Ruby Tuesday(resturant) founded.

Births

January-March

- January 2 - Taye Diggs, American actor
- January 12 - Espen Knutsen, Norwegian hockey player
- January 17 - Ken Hirai, Japanese singer and songwriter
- January 18 - Mike Lieberthal, baseball player
- January 23 - Marcel Wouda, Dutch swimmer
- February 1 - Yoshi DeHerrera, American television personality
- February 2 - Klára Dobrev, wife of Hungarian Prime Minister Ferenc Gyurcsány
- February 4 - Giovanni Silva De Oliveira, Brazilian footballer
- February 14 - Drew Bledsoe, American football player
- February 15 - Jaromir Jagr, Czech hockey player
- February 16 - Jerome Bettis, American football player
- February 17 - Billie Joe Armstrong, American musician (Green Day)
- February 17 - Philippe Candeloro, French figure skater
- February 21 - Seo Taiji, Korean musician
- February 24 - Richard Chelimo, Kenyan athlete (d. 2001)
- February 29 - Antonio Sabato Jr., Italian actor
- March 6 - Shaquille O'Neal, American basketball player
- March 10 - Takashi Fujii (Matthew Minami), Japanese television performer
- March 10 - Matt Kenseth, American race car driver
- March 10 - Eugene Roshal, Russian-born computer programmer
- March 15 - Mark Hoppus, American musician (Blink 182)
- March 17 - Mia Hamm, American soccer player
- March 20 - Alexander Kapranos, English singer and guitarist (Franz Ferdinand (band))
- March 22 - Shawn Bradley, American basketball player
- March 22 - Elvis Stojko, Canadian figure skater
- March 23 - Judith Godrèche, French actress
- March 27 - Jimmy Floyd Hasselbaink, Dutch footballer

April-June

- April 3 - Jennie Garth, American actress
- April 4 - Adam Clayton Powell Jr., American politician
- April 11 - Jason Varitek, baseball player
- April 13 - Fiona McSwein, Founder of Juice Associates
- April 17 - Tony Boselli, American football player
- April 17 - Jennifer Garner, American actress
- April 17 - Muttiah Muralitharan, Sri Lankan cricketer
- April 19 - Rivaldo, Brazilian footballer
- April 24 - Chipper Jones, baseball player
- May 2 - The Rock, American professional wrestler and actor
- May 4 - Mike Dirnt, American musician (Green Day)
- May 10 - Radosław Majdan, Polish goalkeeper
- May 20 - Busta Rhymes, American musician and actor
- May 21 - The Notorious B.I.G., American musician (d. 1997)
- May 28 - Michael Boogerd, Dutch cyclist
- May 30 - Manny Ramirez, baseball player
- June 4 - Derian Hatcher, American hockey player
- June 5 - Justin Smith, American drummer, The Seeds
- June 7 - Karl Urban, New Zealand actor
- June 15 - Andy Pettitte, baseball player
- June 19 - Brian McBride, American soccer player
- June 21 - Irene van Dyk, South African-born netball player
- June 23 - Zinédine Zidane, French footballer
- June 25 - Carlos Delgado, baseball player
- June 29 - Samantha Smith, American activist (d. 1985)

July-September

- July 3 - Asha Gill, English-born television host
- July 8 - Saurav Ganguly, Indian cricketer
- July 2 - Wayne Brady, American actor and comedian
- July 7 - Lisa Leslie, American Basketball player
- July 27 - Jill Arrington, American football reporter
- July 28 - Elizabeth Berkley, American actress
- August 6 - Geri Halliwell, English musician (Spice Girls)
- August 11 - Jonathon Prandi, American model and actor
- August 14 - Ed O'Bannon, American basketball player
- August 15 - Ben Affleck, American actor
- August 23 - Dave Chappelle, American actor and comedian
- August 25 - Marvin Harrison, American football player
- August 30 - Cameron Diaz, American actress
- August 30 - Pavel Nedved, Czech footballer
- September 2 - Sergei Zholtok, Russian hockey player (d. 2004)
- September 8 - Lisa Kennedy, American disc jockey and political satirist
- September 10 - Ghada Shouaa, Syrian athlete
- September 12 - Jason Statham, English actor
- September 17 - Bobby Lee, American comedian
- September 21 - Liam Gallagher, British singer (

General Conference on Weights and Measures

The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). It is one of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention du Mètre (Metre Convention) of 1875. It meets in Paris every four to six years. In 2002 the CGPM represented 51 member states and ten further associate members ⁽¹⁾. As of 2005, the number of associates has grown to 17.

CGPM Meetings

- 1^st (1889) - kilogram defined as mass of the international prototype kilogram (IPK) made of platinum-iridium and kept at the International Bureau of Weights and Measures (Bureau international des poids et mesures), Sèvres, France. International prototype metre sanctioned.
- 2^nd (1897)
- 3^rd (1901) - Litre redefined as volume of 1 kg of water. Clarified that kilograms are units of mass, "standard weight" defined, standard acceleration of gravity defined endorsing use of grams force and making them well-defined.
- 4^th (1907) - carat = 200 mg adopted.
- 5^th (1913) - International Temperature Scale proposed.
- 6^th (1921) - Metre Convention revised.
- 7^th (1927) - Consultative Committee for Electricity (CCE) created.
- 8 ^th (1933) - need for absolute electrical unit identified.
- 9^th (1948) - ampere, coulomb, farad, henry, joule, newton, ohm, volt, watt, weber defined. Chose degree Celsius from among the three names then in use. Lowercase l adopted as symbol for litre. Both the comma and dot on a line are accepted as decimal marker symbols. Symbols for the stere and second changed [http://www1.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=9&RES=7].
- 10^th (1954) - kelvin, standard atmosphere defined. International System of Units (metre, kilogram, second, ampere, kelvin, candela) began.
- 11^th (1960) - metre redefined in terms of wavelengths of light. Hertz, lumen, lux, tesla adopted. New metric system given the official symbol SI for Système International d'Unités, the "modernized metric system". Prefixes pico-, nano-, micro-, mega-, giga- and tera- confirmed.
- 12 ^th (1964) - original definition of litre = 1 dm³ restored. atto- and femto- prefixes.
- 13^th (1967) - second redefined as duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at a temperature of 0 K. Degree Kelvin renamed kelvin. Candela redefined.
- 14^th (1971) - new SI base unit mole defined. Pascal, siemens approved.
- 15^th (1975) - peta- and exa- prefixes. Gray and becquerel radiological units.
- 16^th (1979) - candela, sievert defined. Both l and L provisionally allowed as symbols for litre.
- 17^th (1983) - metre redefined in terms of the speed of light, but keeps same length.
- 18^th (1987) - conventional values adopted for Josephson constant, K_J, and von Klitzing constant, R_K, preparing the way for alternate definitions of the ampere and kilogram.
- 19^th (1991) - new prefixes yocto-, zepto-, zetta- and yotta-.
- 20^th (1995) - supplementary SI units (radian and steradian) become derived units.
- 21^st (1999) - new SI derived unit, the katal = mole per second, for the expression of catalytic activity.
- 22^nd (2003) - both the comma and dot on a line are reaffirmed as decimal marker symbols [http://www1.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=22&RES=10].

References

⁽¹⁾ [http://www1.bipm.org/en/convention/member_states/ CGPM Member States] Conférence générale des poids et mesures ko:국제도량형총회 ja:国際度量衡総会

Global Positioning System

:GPS redirects here. For other uses of the acronym GPS, see GPS (disambiguation). GPS (disambiguation) have been launched since 1978. ]] The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR GPS - Navigation Signal Timing and Ranging Global Positioning System), is a satellite navigation system used for determining one's precise location and providing a highly accurate time reference almost anywhere on Earth or in Earth orbit. It uses an intermediate circular orbit (ICO) satellite constellation of at least 24 satellites. The precision of the GPS signal itself is about 20 meters (65 ft). Using differential GPS and other error-correcting techniques, the precision can be improved to about 10 cm (4 in). The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. The GPS system is divided into three segments: space, control and user. The space segment comprises the GPS satellite constellation. The control segment comprises ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading data for transmission by the satellites. The user segment consists of GPS receivers used for both military and civilian applications. A GPS receiver decodes time signal transmissions from multiple satellites and calculates its position by trilateration. Daily management of the GPS satellite constellation is conducted by the 2d Space Operations Squadron at Schriever Air Force Base. The cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites. The first GPS satellite was launched in February 1978, and the most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989.

Technical description

1989 The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were initially manufactured by Rockwell and now manufactured by Lockheed Martin. The first was launched in February 1978, and the most recent was launched September 25 2005. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometres (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories. The receiver does not need a precise clock, but does need a clock with good short-term stability and the ability to receive signals from four satellites in order to determine its own latitude, longitude, elevation, and the precise time. The receiver computes the distance to each of the four satellites from the difference between local time and the time the satellite signals were sent (this distance is called a pseudorange). It then decodes the satellites’ locations from their radio signals and an internal database. The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. Because the receiver does not have a very precise clock it cannot compute the time delays. However, it can measure with high precision the differences between the times when the various messages were received. This yields 3 hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver. This is why at least four satellites are needed: fewer than 4 satellites yield 2 hyperboloids, whose intersection is a curve; it is impossible to know where the receiver is located along the curve without supplemental information, such as elevation. If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude). When there are n > 4 satellites, the n-1 hyperboloids should, assuming a perfect model and measurements, intersect on a single point. In reality, the surfaces rarely intersect, because of various errors. The question of finding the point P can be reformulated into finding its three coordinates as well as n numbers r_i such that for all i, PS_i-r_i is close to zero, and the various r_i-r_j are close to C.Δ_ij where C is the speed of light and Δ_ij are the time differences between signals i and j. For instance, a least squares method may be used to find an optimal solution. In practice, GPS calculations are more complex (repeat measurements, etc.). There are several causes: The initial local time is a guess due to the relatively imprecise clock of the receiver, the radio signals move more slowly as they pass through the ionosphere, and the receiver may be moving. To counteract these variables, the receiver then applies an offset to the local time (and therefore to the spheres' radii) so that the spheres finally do intersect in one point. Once the receiver is roughly localized, most receivers mathematically correct for the ionospheric delay, which is least when the satellite is directly overhead and becomes greater toward the horizon, as more of the ionosphere is traversed by the satellite signal. Since it is common for the receiver to be moving, some receivers attempt to fit the spheres to a directed line segment. The receiver contains a mathematical model to account for these influences, and the satellites also broadcast some related information which helps the receiver in estimating the correct speed of propagation. High-end receiver/antenna systems make use of both L1 and L2 frequencies to aid in the determination of atmospheric delays. Because certain delay sources, such as the ionosphere, affect the speed of radio waves based on their frequencies, dual frequency receivers can actually measure the effects on the signals. In order to measure the time delay between satellite and receiver, the satellite sends a repeating 1,023 bit long pseudo random sequence; the receiver knows the seed of the sequence, constructs an identical sequence and shifts it until the two sequences match. Different satellites use different sequences, which lets them all broadcast on the same frequencies while still allowing receivers to distinguish between satellites. This is an application of Code Division Multiple Access, or CDMA. Several frequencies make up the GPS electromagnetic spectrum:
- L1 (1575.42 MHz):
Carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted precision P(Y) code.
- L2 (1227.60 MHz):
Usually carries only the P(Y) code. The encryption keys required to directly use the P(Y) code are tightly controlled by the U.S. government and are generally provided only for military use. The keys are changed on a daily basis. In spite of not having the P(Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal (in a round-about manner) to increase accuracy and remove error caused by the ionosphere.
- L3 (1381.05 MHz):
Carries the signal for the GPS constellation's alternative role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events.
- L4 (1841.40 MHz):
Being studied for additional ionospheric correction.
- L5 (1176.45 MHz):
Proposed for use as a civilian safety-of-life signal. A minor detail is that the atomic clocks on the satellites are set to "GPS time", which is the number of seconds since 04:00:00 (4 A.M.), January 6, 1980. It is ahead of UTC because it does not follow leap seconds. Receivers thus apply a clock correction factor (which is periodically transmitted along with the other data), and optionally adjust for a local time zone in order to display the correct time. The clocks on the satellites are also affected by both special and general relativity, which causes them to run at a slightly slower rate than do clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, which is corrected by electronics on each satellite. This offset is a dramatic proof of the special theory of relativity in a real-world system, as it is exactly that predicted by the theory, within the limits of accuracy of measurement. The inspiration for the GPS system came when the Soviets launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, due to the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion. It was only a small leap of logic to realize that the converse was also true; if the satellite's position was known then they could identify their own position on Earth. Doppler effect

Sources of GPS measurement errors

Ideally, GPS receivers would easily be able to convert the C/A and P(Y)-code measurements into accurate positions. However, a system with such complexity leaves many openings for errors to affect the measurements. The following are several causes of error in GPS measurements.

Clocks

Both GPS satellites and receivers are prone to timing errors. Ground stations throughout the world monitor the satellites to ensure that their atomic clocks are kept synchronized. Receiver clock errors depend upon the oscillator provided within the unit. However, they can be calculated and then eliminated once the receiver is tracking at least four satellites.

Ionosphere

The Ionosphere is one of the leading causes of GPS error. The speed of light varies due to atmospheric conditions. As a result, errors greater than 10 meters may arise. To compensate for these errors, the second frequency band L2 was provided. By comparing the phase difference between the L1 and L2 signals, the error caused by the ionosphere can be calculated and eliminated.

Multipath

The antenna receives not only direct GPS signals, but also multipath signals: reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). For long delay multipath signals, the receiver itself can filter the signals out. A variety of receiver techniques, most notably Narrow Correlator spacing, have been developed to mitigate multipath error contributions to pseudorange measurements. For shorter delay multipath signals that result from reflections from the ground, special antenna features may be used such as a ground plane, or a choke ring antenna. Shorter multipath signals from ground reflections can often be very close to the direct signals, and can greatly reduce precision.

Selective Availability

In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military, its allies and a few others, mostly government users. However, on May 1, 2000, then US President Bill Clinton announced that this "Selective Availability" would be [http://www.ostp.gov/html/0053_2.html turned off] by 2006, allowing all users to enjoy nearly the same level of access, with a precision of position determination of less than 20 meters. SA is turned off completely already.

Techniques to improve GPS accuracy

The accuracy of GPS can be improved in a number of ways:
- Using a network of fixed ground based reference stations. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. This method is called Differential GPS or DGPS. DGPS was especially useful when GPS was still degraded (via the "Selective Availability" described above), since DGPS could nevertheless provide 5–10 metre accuracy. The DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.
- Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition (JDAM).
- The Wide Area Augmentation System (WAAS). This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays, individual satellite clock drift, and suchlike. Although only a few WAAS satellites are currently available as of 2004, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility). The current WAAS system only works for North America (where the reference stations are located), and due to the satellite location the system is only generally usable in the eastern and western coastal regions. However, variants of the WAAS system are being developed in Europe (EGNOS, the Euro Geostationary Navigation Overlay Service), and Japan (MSAS, the Multi-Functional Satellite Augmentation System), which are virtually identical to WAAS.
- A Local Area Augmentation System (LAAS). This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fifty kilometre radius around the transmitter.
- A Carrier-Phase Enhancement (CPGPS). This technique utilizes the 1.575 GHz L1 carrier wave to act as a sort of clock signal, resolving ambiguity caused by variations in the location of the pulse transition (logic 1-0 or 0-1) of the C/A PRN signal. The problem arises from the fact that the transition from 0-1 or 1-0 on the C/A signal is not instantaneous, it takes a non-zero amount of time, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. A successful correlation could be defined in a number of various places along the rising/falling edge of the pulse, which imparts timing errors. CPGPS solves this problem by using the L1 carrier, which has a period 1/1000 that of the C/A bit width, to define the transition point instead. The phase difference error in the normal GPS amounts to a 2-3 m ambiguity. CPGPS working to within 1% of perfect transition matching can achieve 3 mm ambiguity; in reality, CPGPS coupled with DGPS normally realizes 20-30 cm accuracy.
- Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.
- Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, accurate determination of range signal can be resolved to an accuracy of less than 10 centimetres. This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differenti