GPS System
* GPS satellites around the earth 2x a day
* This satellite transmits signals to the earth
* Signal is used to calculate the position
* GPS distinguish the transmitted time to calculate position
* This time is calculated as the distance from several GPS satellites to calculate positions on earth and its surface, including the exosphere
Basic GPS Work
* Must have at least 3 GPS satellites to calculate the 2D position and movement.
* With 4 satellites, our GPS position to calculate the 3D position (latitude, longitude and altitude).
* With the position information, GPS can calculate other data such as: speed, direction, trajectory, distance, distance to destination, sunrise and sunset and others.
The accuracy of GPS Devices
* GPS typically has 12 channels in parallel.
* Invoice atmosphere can reduce accuracy.
* GPS for your flight to achieve accuracy up to + / - 15 meters.
* WAAS (Wide Area Augmentation System) to improve the accuracy up to + / - 3-8 meters.
* No special equipment or extra fees to get a WAAS signal, during the country to install the WAAS ground / satellite koresi.
* Medium Differential GPS (DGPS) can improve the accuracy up to + / - 3-5 meters.
* DGPS consists of devices that receive and retransmit signals to correct the position, the tool is used for the flight, at Halim Airport is 2 units to improve the accuracy of DGPS.
* For this correction we have to have a differential GPS beacon receiver and antenna, as in the GPS295 where we can set the frequency of these beacons.
Reference Map
* In general reference map yangdigunakan particular flight is WGS84.
* WGS84 is the constant reference that is used for modeling the earth consisting of primary and secondary data
* Primary data is the oval shape of the earth, a circular rotating speed and the earth are included in the reference ellipse
* The secondary data is the data model Earth's gravity.
* All data navigation (air), standardized with the WGS 84 standard to meet the requirements of RNAV (Radio Navigation) to meet the global reference.
GPS Satellite
* The first GPS satellite was launched in 1978.
* 24 satellites in achieved in 1994, now has more than 30 GPS satellites above the earth berorbit us.
* Age of Satellite average 10 years, after that no replacement / routine maintenance.
* Weight Satellites approximately + / - 2.000 pounds (nearly 1 ton)
* The width of the solar panel antenna + / - 17 feet (+ / - 5 meters).
* Power transmission is <= 50 watts.
* Position orbit at an altitude of + / - 12.000 miles above the earth's surface.
* Speed 7.000 mph home range.
* GPS satellites using solar energy (sunlight), but provided a backup battery to avoid the Total Solar Eclipse.
* Power used to maintain its orbit is a small rocket.
GPS Signal
* There are 2 GPS Signals L1 & L2 signal
* L1 works on 1575 MHz frequency on the UHF wave band.
* Moving straight direct (line of sight) through clouds, glass and plastic.
* What is inhibiting transmission of solid objects such as: buildings, trees, mountains, etc..
* There are three pieces of information on the GPS signal:
* Pseudorandom code (ID code): is the information transmitted to the receiver that the units we receive a satellite signal like on the page shown by the bar chart BAR
* Ephemeris data: is the signal strength data and time information
* Almanac data: is the info about where the actual satellite location that shows the position of satellites in the GPS satellite status page.
Sources of Error
* The delay of the reflection of the ionosphere and the troposphere: a decline in accuracy resulting from the delay time when the signal current through this layer, but the GPS can be corrected by assuming an average error factor.
* Error of signal reflections: this happens if the GPS signal berpantul through objects such as buildings or mountains before he received our unit.
* Error Time of our unit: Accuracy of time / hour of our units are not as precise Atomic clock in GPS satellites (GPS using Atomic Clock). For that there is little error time.
* Orbital errors - known as ephemeris errors, this happens if there is a shift from the orbit / reports from the satellite to its position.
* Number of satellites received: Add a lot of the received signal plus high accuracy, Banugnan, mountain, electronic interference, even a shade tree can reduce accuracy.
* Position relative from the Satellite / disturbance of the hypotenuse: this happens if the satellite position is located at the corner of a very wide or very close or almost coincide with one another so that the calculation accuracy is reduced.
* The decrease degradation regulated by the U.S. defense department / SA (Selective Availability): this is done to avoid military use accuracy in special cases, and even military use / adjust the orbit of which focus on specific areas such as apda perangteluk, SA has been waived, because of civil aviation in particular objected to the end of May 2000, the government abolished this SA for civil aviation has a better accuracy.
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