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Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.
Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !
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The BeiDou Navigation System (simplified Chinese: 北斗导航系统; traditional Chinese: 北斗導航系統; pinyin: Běidǒu dǎoháng xìtǒng) or BeiDou (Compass) Navigation Satellite System (simplified Chinese: 北斗卫星导航系统; traditional Chinese: 北斗衛星導航系統; pinyin: Běidǒu wèixīng dǎoháng xìtǒng) is a project by China to develop an independent satellite navigation system. It may refer to either one or both generations of the Chinese navigation system.
The first BeiDou system, officially called BeiDou Satellite Navigation Experimental System, or known as BeiDou-1, consists of three satellites and offers limited coverage and applications. It has been offering navigation services, mainly for customers in China and neighboring regions, since 2000.
The second generation of the system, known as Compass or BeiDou-2, which will be a global satellite navigation system consisting of 35 satellites, is still under construction as of May 2012. It became operational in China in December 2011, with 10 satellites in use. It is planned to offer services to customers in the Asia-Pacific region by 2012, and to global customers upon its completion in 2020.
The BeiDou Navigation System is named after the Big Dipper constellation, which is known in Chinese as Běidǒu. The name literally means "Northern Dipper", the name given by Chinese astronomers to the seven brightest stars of Ursa Major or 'the Great Bear' constellation. Historically, this set of stars was used in navigation to locate the North Star Polaris. As such, BeiDou also serves as a metaphor for the purpose of the satellite navigation system.
The first two satellites, BeiDou-1A was launched on 30 October 2000, BeiDou-1B followed on 20 December 2000. The third satellite BeiDou-1C (as backup satellite), was put into orbit on 25 May 2003. The successful launch of BeiDou-1C also meant the establishment of the BeiDou-1 navigation system.
It followed that in February 2007, the fourth and also the last satellite of BeiDou-1 system, the BeiDou-1D (sometimes called BeiDou-2A, serving as a backup satellite), was sent up into space. It was reported that the satellite had suffered from a control system malfunction but was then fully restored.
In April 2007, the first satellite of BeiDou-2, namely Compass-M1 (to validate frequencies for the BeiDou-2 constellation) was successfully put into its working orbit. The second BeiDou-2 constellation satellite Compass-G2 was launched on 15 April 2009. The third satellite (Compass-G1) was carried into its orbit by LM-3C on January 17, 2010. On 2 June 2010, the fourth satellite was launched successfully into orbit. The fifth orbiter was launched into space by LM-3I carrier rocket from Xichang Satellite Launch Center on August 1, 2010. Three months later, on November 1, 2010, the sixth satellite was sent into orbit by LM-3C. Another satellite, the Beidou-2/Compass IGSO-5 (fifth inclined geosynchonous orbit) satellite, was launched from the Xichang Satellite Launch Center by a Long March-3A on December 1, 2011 (UTC).
On January 15, 2010 the official website of the BeiDou Navigation Satellite System went online.
In September 2003, China intended to join the European Galileo positioning system project and was to invest €230 million (USD296 million, GBP160 million) in Galileo over the next few years. It's believed that China's "BeiDou" navigation system would then only be used by its armed forces. In October 2004, China officially joined the Galileo project by signing the Agreement on the Cooperation in the Galileo Program between the "Galileo Joint Undertaking" (GJU) and the "National Remote Sensing Centre of China" (NRSCC). Based on the Sino-European Cooperation Agreement on Galileo program, China Galileo Industries (CGI), the prime contractor of the China’s involvement in Galileo programs was founded in December 2004. By April 2006, eleven cooperation projects within the Galileo framework had been signed between China and EU.
BeiDou-1 is an experimental regional navigation system, which consists of four satellites (three working satellites and one backup satellite). The satellites themselves were based on the Chinese DFH-3 geostationary communications satellite and had a launch weight of 1,000 kilograms (2,200 pounds) each.
Unlike the American GPS, Russian GLONASS, and European Galileo systems, which use medium Earth orbit satellites, BeiDou-1 uses satellites in geostationary orbit. This means that the system does not require a large constellation of satellites, but it also limits the coverage to areas on Earth where the satellites are visible. The area that can be serviced is from longitude 70°E to 140°E and from latitude 5°N to 55°N. A frequency of the system is 2491.75 MHz. 
The first satellite, BeiDou-1A was sent into its orbit on October 31, 2000. The second satellite, BeiDou-1B was successfully launched on December 21, 2000. The last satellite of the constellation, BeiDou-1C was carried into its orbit position on May 25, 2003, this launch also completed the construction of the experimental system.
To calculate a position, the following procedure is used:
In 2007, the official Xinhua News Agency reported that the resolution of the BeiDou system was as high as 0.5 metres, considerably better than unaided GPS. With the existing user terminals appears that the calibrated accuracy is 20m (100m, uncalibrated).
The terminal can communicate with the ground station by sending and receiving short messages.
As of 2008, one BeiDou-1 terminal costs about CN¥20,000RMB (US$2,929), almost 10 times the price of GPS counterpart. It's said that the reason why is the terminal so expensive is due to "using expensive imported Chips"，but China seemed to have found replacement and the price could lower to less than CN¥1,000RMB. By the China High-Tech Fair ELEXCON 2009 (November 16–21, 2009) in Shenzhen, China, a terminal solution costing no more than CN¥3,000RMB was presented.
BeiDou-2 is not an extension to the existing BeiDou-1, but will rather supersede it outright. The new system will be a constellation of 35 satellites, which include 5 geostationary orbit satellites for backward compatibility with BeiDou-1, and 30 non-geostationary satellites (27 in medium earth orbit and 3 in inclined geostationary orbit), that will offer complete coverage of the globe. There will be two levels of service provided; free service to civilians and licensed service to Chinese government and military users:
The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe. The ranging signals are based on the CDMA principle and have complex structure typical of Galileo or modernized GPS. Similar to the other GNSS, there will be two levels of positioning service: open and restricted (military). The public service shall be available globally to general users. When all the currently planned GNSS systems are deployed, the users will benefit from the use of a total constellation of 75+ satellites, which will significantly improve all the aspects of positioning, especially availability of the signals in so-called urban canyons. The general designer of Compass navigation system is Sun Jiadong, who is also the general designer of its predecessor, Beidou navigation system.
Frequencies for Compass are allocated in four bands: E1, E2, E5B, and E6 and overlap with Galileo. The fact of overlapping could be convenient from the point of view of the receiver design, but on the other hand raises the issues of inter-system interference, especially within E1 and E2 bands, which are allocated for Galileo's publicly-regulated service. However, under International Telecommunications Union (ITU) policies, the first nation to start broadcasting in a specific frequency will have priority to that frequency, and any subsequent users will be required to obtain permission prior to using that frequency, and otherwise ensure that their broadcasts do not interfere with the original nation's broadcasts. It now appears that Chinese Compass satellites will start transmitting in the E1, E2, E5B, and E6 bands before Europe's Galileo satellites and thus have primary rights to these frequency ranges.
Although little was officially announced by Chinese authorities about the signals of the new system, the launch of the first Compass satellite permitted independent researchers not only to study general characteristics of the signals but even to build a Compass receiver.
Compass-M1 is an experimental satellite launched for signal testing and validation and for the frequency filing on 14 April 2007. The role of Compass-M1 for Compass is similar to the role of the GIOVE satellites for the Galileo system. The orbit of Compass-M1 is nearly circular, has an altitude of 21,150 km and an inclination of 55.5 degrees.
Compass-M1 transmits in 3 bands: E2, E5B, and E6. In each frequency band two coherent sub-signals have been detected with a phase shift of 90 degrees (in quadrature). These signal components are further referred to as "I" and "Q". The "I" components have shorter codes and are likely to be intended for the open service. The "Q" components have much longer codes, are more interference resistive, and are probably intended for the restricted service.
The investigation of the transmitted signals started immediately after the launch of Compass -M1 on 14 April 2007. Soon after in June 2007, engineers at CNES reported the spectrum and structure of the signals. A month later, researchers from Stanford University reported the complete decoding of the “I” signals components. The knowledge of the codes allowed a group of engineers at Septentrio to build the COMPASS receiver and report tracking and multipath characteristics of the “I” signals on E2 and E5B.
|Code modulation||BPSK(2)||BPSK(2)||BPSK(2)||BPSK(10)||BPSK(10)||BPSK (10)||BPSK (1)|
|Carrier frequency, MHz||1561.098||1561.098||1207.14||1207.14||1268.52||1268.52||1575.42|
|Chip rate, Mchips/sec||2.046||2.046||2.046||10.230||10.230||10.230||1.023|
|Code period, chips||2046||??||2046||??||10230||??||1023|
|Code period, msec||1.0||>400||1.0||>160||1.0||>160||1.0|
|Navigation frames, sec||6||??||6||??||??||??||6|
|Navigation sub-frames, sec||30||??||30||??||??||??||30|
|Navigation period, min||12.0||??||12.0||??||??||??||12.5|
Characteristics of the "I" signals on E2 and E5B are generally similar to the civilian codes of GPS (L1-CA and L2C), but Compass signals have somewhat greater power. The notation of Compass signals used in this page follows the naming of the frequency bands and agrees with the notation used in the American literature on the subject, but the notation used by the Chinese seems to be different and is quoted in the first row of the table.
|Mission||Date||Name||Launch center||Launch vehicle||Bus||Orbit|
|07-32||2007-04-13||Compass-M1||Xichang||CZ-3C||DFH-3||MEO ~21,500 km|
|07-40||2010-07-31||Compass-IGSO1||Xichang||CZ-3A||DFH-3||HEO ~36,000 km|
|07-45||2010-12-17||Compass-IGSO2||Xichang||CZ-3A||DFH-3||HEO ~36,000 km|
|07-46||2011-04-10||Compass-IGSO3||Xichang||CZ-3A||DFH-3||HEO ~36,000 km|
|07-49||2011-07-27||Compass-IGSO4||Xichang||CZ-3A||DFH-3||HEO ~36,000 km|
|07-51||2011-12-01||Compass-IGSO5||Xichang||CZ-3A||DFH-3||HEO ~36,000 km|
In December 2011, the system went into operation on a trial basis. It has started providing navigation, positioning and timing data to China and the neighbouring area for free from 27 December. During this trial run, Compass will offer positioning accuracy to within 25 meters, but the precision will improve as more satellites are launched. Upon the system's officially launch, it pledged to offer general users positioning information accurate to the nearest 10 m, measure speeds within 0.2 m per second, and provide signals for clock synchronisation accurate to 0.02 microseconds.
It is planned that BeiDou-2 system will commence offering services for the Asia-Pacific region by the end of 2012; The global navigation system should be finished by 2020.
As of April 2012, 13 satellites for BeiDou-2 have been launched. According to a Xinhua news report, "[t]he basic structure of the Beidou system has now been established, and engineers are now conducting comprehensive system test and evaluation. The system will provide test-run services of positioning, navigation and time for China and the neighboring areas before the end of this year, according to the authorities." The system became operational in the China region in December 2011.
|4/14/2007||LM-3A||Compass-M1||MEO ~21,500 km||Testing only||BeiDou-2 (Compass)|
|4/15/2009||LM-3C||Compass-G2||Drifting||No|
|1/17/2010||LM-3C||Compass-G1||GEO 144.5°E ||Yes|
|8/1/2010||LM-3A||Compass-IGSO1||118°E incl 55°||Yes|
|12/18/2010||LM-3A||Compass-IGSO2||118°E incl 55°||Yes|
|04/10/2011||LM-3A||Compass-IGSO3||118°E incl 55°, 200~35,991 km||Yes|
|07/26/2011||LM-3A||Compass-IGSO4||35695 x 35865 km incl 55.2 deg long: 80 to 112 deg E||Yes|
|12/02/2011||LM-3A||Compass-IGSO5||35712 x 35873 km incl 55.2 deg long: 79 to 110 deg E||Yes|