Dedy Darwin

Global Positioning System (GPS)

Artist's conception of GPS Block II-F satellite in orbit

The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver. In addition to GPS other systems are in use or under development. The Russian GLObal NAvigation Satellite System (GLONASS) was for use by the Russian military only until 2007. There are also the planned Chinese Compass navigation system and Galileo positioning system of the European Union (EU). GPS was created and realized by the U.S. Department of Defense (USDOD) and was originally run with 24 satellites. It was established in 1973 to overcome the limitations of previous navigation systems.

Antenna

GPS consists of three parts: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.

The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service (see GPS navigation devices).

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).

Space segment

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes, but this was modified to six planes with four satellites each. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which, of course, sum to 360 degrees.

Constellation GPS

Control segment

The control segment is composed of

a master control station (MCS),
an alternate master control station,
four dedicated ground antennas and
six dedicated monitor stations

The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.

User segment

GPS Receiver

The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels.^[update]

Mayan Civilization

The Maya is a Mesoamerican civilization, noted for the only known fully developed written language pre-Columbian Americas, as well as its art, architecture, and mathematical and astronomical systems. Initially established during the Pre-Classic period (c. 2000 BC to 250 AD), according to the Mesoamerican chronology, many Maya cities reached their highest state of development during the Classic period (c. 250 AD to 900 AD), and continued throughout the Post-Classic period until the arrival of the Spanish.

The Maya civilization shares many features with other Mesoamerican civilizations due to the high degree of interaction and cultural diffusion that characterized the region. Advances such as writing, epigraphy, and the calendar did not originate with the Maya; however, their civilization fully developed them. Maya influence can be detected from Honduras, Guatemala, Northern El Salvador Mexico, more than 1000 km (625 miles) from the Maya area. Many outside influences are found in Maya art and architecture, which are thought to result from trade and cultural exchange rather than direct external conquest. and to as far as central.

The Maya peoples never disappeared, neither at the time of the Classic period decline nor with the arrival of the Spanish conquistadores and the subsequent Spanish colonization of the Americas. Today, the Maya and their descendants form sizable populations throughout the Maya area and maintain a distinctive set of traditions and beliefs that are the result of the merger of pre-Columbian and post-Conquest ideas and cultures. Many Mayan languages continue to be spoken as primary languages today; the Rabinal Achí, a play written in the Achi' language, was declared a Masterpiece of the Oral and Intangible Heritage of Humanity by UNESCO in 2005.