A stylus plural styli or styluses is a writing utensil. The word is also used for a computer accessory PDAs. It usually refers to a narrow elongated staff, similar to a modern ballpoint pen. Many styluses are heavily curved to be held more easily.Styli were first used by the ancient Mesopotamians in order to write in cuneiform, usually made out of reeds that grew on the sides of the Tigris and Euphrates rivers and in marshes and down to Egypt where the Egyptians used styluses from sliced reeds with sharp points. Cuneiform was entirely based on the wedgeshaped mark that the end of a cut reed made when pushed into a clay tablet, hence the name cuneiform from Latin cuneus = wedge. Styli were used from classical times until the nineteenth century to write on wax tablets tabulae, which were used for various purposes, from secretaries' notes to recording accounts. Some waxtablets have been preserved in waterlogged deposits, for example in the Roman fort at Vindolanda on Hadrian's Wall. One end of such styli was pointed for writing and the other was flattened into a broad shape for erasing.Styli are used in various arts and crafts still. Example situations rubbing off dry transfer letters, tracing designs onto a new surface with carbon paper, and hand embossing. Styli are also used to engrave into materials like metal or clay.In the sound recording industry, a stylus is a phonograph or gramophone needle used to play back sound on gramophone records, as well as to record the sound indentations on the master record.Several technologies were used to record the sounds, beginning with wax cylinders. The harder the material used, the harder the stylus had to be. The latter stylus for vinyl records were made out of Sapphire or diamond.
Today, the term stylus often refers to an input method usually used in PDAs, graphics tablets, Tablet PCs, and UMPCs. In this method, a stylus that secretes no ink touches a touch screen instead of a finger to avoid getting the natural oil from one's hands on the screen. Styli are also used with the Nintendo DS handheld gaming device, which has two screens, the bottom one being touchsensitive.A stylus may also be used to scribe a recording into smoked foil or glass. In various instruments this method may be used instead of a pen for recording as it has the advantage of being able to operate over a wide temperature range, does not clog or dry prematurely, and has very small friction in comparison to other methods. These characteristics were useful in certain types of early seismographs and in recording barographs used in determining sailplane altitude records.The sharpest stylus possible has a single atom at its tip. Such styli are used in scanning tunneling microscopes. Romans, for writing upon wax tablets.The spelling was influenced by the Greek word st???? meaning column or pillar. According to the 1875 London Dictionary of Greek & Roman Antiquities a Stilus is an object tapering like an architectural column a metal instrument resembling a pencil in size and shape, used for writing or recording impressions upon waxed tablets. It signifies
The Global Positioning System GPS is the only fully functional Global Navigation Satellite System GNSS. Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time. Other similar systems are the Russian GLONASS incomplete as of 2007, the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India.Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it came to the budget for the GPS program.The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year,including the replacement of ageing satellites, and research and development.
A typical GPS receiver calculates its position using the signals from four or more GPS satellites. Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. In other words, the receiver uses four measurements to solve for 4 variables x, y, z, and t. These values are then turned into more userfriendly forms, such as latitude/longitude or location on a map, then displayed to the user.Following the shootdown of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good. Since then, GPS has become a widely used aid to navigation worldwide, and a useful tool for mapmaking, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks. Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message, parameters to calculate the location of the satellite the ephemeris, and the general system health the almanac. The signals travel at a known speed the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry see trilateration. Although four satellites are required for normal operation, fewer may be needed in some special cases. For example, if one variable is already known for example, a seagoing ship knows its altitude is 0, a receiver can determine its position using only three satellites. Also, in practice, receivers use additional clues doppler shift of satellite signals, last known position, dead reckoning, inertial navigation, and so on to give degraded answers when fewer than four satellites are visible.
The current GPS consists of three major segments. These are the space segment SS, a control segment CS, and a user segment US.The space segment SS comprises the orbiting GPS satellites, or Space Vehicles SV in GPS parlance. The GPS design originally called for 24 SVs, 8 each in three circular orbital planes, but this was modified to 6 planes with 4 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. Orbiting at an altitude of approximately 20,200 kilometers 12,600 miles or 10,900 nautical miles orbital radius of 26,600 km 16,500 mi or 14,400 NM, each SV makes two complete orbits each sidereal day.The ground track of each satellite therefore repeats each sidereal day. This was very helpful during development, since even with just 4 satellites, correct alignment means all 4 are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.As of September 2007, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.
The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National GeospatialIntelligence Agency NGA.The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2d Space Operations Squadron 2 SOPS of the United States Air Force USAF. 2 SOPS contacts each GPS satellite regularly with a navigational update using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs. These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked 'unhealthy', so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again. Even if just one satellite is maneuvered at a time, this implies at least five satellites must be visible to be sure of getting data from four.
The user's GPS receiver is the user segment US of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiverprocessors, and a highlystable 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 2006, receivers typically have between twelve and twenty channels.GPS receivers may include an input for differential corrections, using the RTCM SC104 format. This is typically in the form of a RS232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even lowcost units commonly include Wide Area Augmentation System WAAS receivers. Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 is a newer and less widely adopted protocol. Both are proprietary and controlled by the USbased National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the timeofday, GPS week number and satellite health information all transmitted in the first part of the message, an ephemeris transmitted in the second part of the message and an almanac later part of the message. The messages are sent in frames, each taking 30 seconds to transmit 1500 bits. The first 6 seconds of every frame contains data describing the satellite clock and its relationship to GPS system time. The next 12 seconds contain the ephemeris data, giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in nonnominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds worst case before it is received, due to the low data transmission rate. The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time UTC. A new part of the almanac is received for the last 12 seconds in each 30 second frame. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the entire almanac from a single satellite. The almanac serves several purposes. The first is to assist in the acquisition of satellites at powerup by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process.
Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. The second purpose is for relating time derived from the GPS system called GPS time to the international time standard of UTC.Finally, the almanac allows a single frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual frequency receivers. However it is often better than no correction since ionospheric error is the largest error source for a single frequency GPS receiver. An important thing to note about navigation data is that each satellite transmits only its own ephemeris, but transmits an almanac for all satellites.Each satellite transmits its navigation message with at least two distinct spread spectrum codes the Coarse / Acquisition C/A code, which is freely available to the public, and the Precise P code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudorandom PRN code at 1.023 million chips/sec so that it repeats every millisecond. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The Pcode is a 10.23 megachip/sec PRN code that repeats only every week. When the antispoofing mode is on, as it is in normal operation, the P code is encrypted by the Ycode to produce the PY code, which can only be decrypted by units with a valid decryption key. Both the C/A and PY codes impart the precise timeofday to the user.
To start off, the receiver picks which C/A codes to listen for by PRN number, based on the almanac information it has previously acquired. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern, then measures the received time for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number, referenced to its local clock, starting at the same time the satellite sent it. It then computes the offset to the local clock that generates the maximum correlation. This offset is the time delay from the satellite to the receiver, as told by the receiver's clock. Since the PRN repeats every millisecond, this offset is precise but ambiguous, and the ambiguity is resolved by looking at the data bits, which are sent at 50 Hz 20 ms and aligned with the PRN code.This data is used to solve for x,y,z and t. Many mathematical techniques can be used. The following description shows a straightforward iterative way, but receivers use more sophisticated methods.Next, the orbital position data, or ephemeris, from the Navigation Message is then downloaded to calculate the satellite's precise position. A moresensitive receiver will potentially acquire the ephemeris data more quickly than a lesssensitive receiver, especially in a noisy environment.17 Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. Receivers can substitute altitude for one satellite, which the GPS receiver translates to a pseudorange measured from the center of the Earth.
When pseudoranges have been determined for four satellites, a guess of the receiver's location is calculated. Dividing the speed of light by the distance adjustment required to make the pseudoranges come as close as possible to intersecting results in a guess of the difference between UTC and the time indicated by the receiver's onboard clock. With each combination of four satellites, a geometric dilution of precision GDOP vector is calculated, based on the relative sky positions of the satellites used. As more satellites are picked up, pseudoranges from more combinations of four satellites can be processed to add more guesses to the location and clock offset.The receiver then determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a specific coordinate system, e.g. latitude/longitude, using the WGS 84 geodetic datum or a local system specific to a country. There are many other alternatives and improvements to this process. If at least 4 satellites are visible, for example, the receiver can eliminate time from the equations by computing only time differences, then solving for position as the intersection of hyperboloids. Also, with a full constellation and modern receivers, more than 4 satellites can be seen and received at once. Then all satellite data can be weighted by GDOP, signal to noise, path length through the ionosphere, and other accuracy concerns, and then used in a least squares fit to find a solution. In this case the residuals also gives an estimate of the errors. Finally, results from other positioning systems such as GLONASS or the upcoming Galileo can be used in the fit, or used to doublecheck the result. By design, these systems use the same bands, so much of the receiver circuitry can be shared, though the decoding is different.