GPS (Global Positioning System)

Understanding GPS

GPS (Global Positioning System) is a system of navigation and positioning satellite owned and managed by the United States. The system is designed to provide position and velocity of three-dimensional as well as information about the time, continuously around the world without time-dependent and weather, for many people simultaneously. Currently GPS is widely used around the world in various fields of application that requires information about the position, velocity, acceleration time or throughput. GPS can provide position information with accuracy varying from a few millimeters (zero-order) up to tens of meters.
GPS capability

Some GPS capabilities, among others, can provide information about the position, velocity, and time quickly, accurately, cheaply, anywhere in the world without depending on the weather. It should be noted that GPS is the only navigation system or positioning systems in some of this century which has such powerful capabilities. The accuracy of GPS can achieve accuracy of position a few mm to some cm / s for speed and accuracy of a few nanoseconds for time accuracy. Positioning accuracy obtained will depend on several factors: the positioning method, satellite geometry, the level of data accuracy, and data processing methods.

Products supplied GPS

In general, GPS is a product of the position, velocity, and time. In addition there are several other products such as acceleration, azimuth, attitude parameters, the TEC (Total Electron Content), WVC (Water Vapour Content), Polar motion parameters, as well as some products that need to be combined with external information from other systems, products include high ortometrik , undulasi geoid and vertical deflection.

Composer segments GPS System

In general there are three segments in the segment of the GPS system control system, the satellite segment and user segment.

GPS satellites can be analogous to the space station, equipped with antennas to transmit and receive wave signals. These signals are then received by a GPS receiver on / near the surface of the earth, and is used to determine position information, speed, and time. Besides the GPS satellites are also equipped to control the satellite attitude. The GPS satellites can be divided over several generations, namely: block I, block II, IIA blocks, block IIR and IIF block. Until April 1999 there were 8 block II satellites, 18 satellites and 1 block II A II R satellites are operational blocks.

In general, the segment control system serves to control and monitor satellite operations and ensure that the satellite is working properly

User segment consists of GPS satellite users everywhere. In this case the GPS signal receiver (GPS receiver) is required to receive and process signals from GPS satellites to be used in the determination of position, velocity and time. The main components of a GPS receiver in general is an antenna with a pre-amplifier, the RF section with the identifier signal and the signal processor, micro processor for controlling the receiver, the data sampling and data processing (navigation solution), precision oscillator, power supply, unit command and display , and memory as well as data recorder

The principle of positioning with GPS

The principle of positioning with GPS is using the method of resection of the distance, where distance measurements carried out simultaneously to multiple satellites that have known coordinates. On GPS measurements, each epoknya has four parameters that must be determined: ie 3 parameters the coordinates X, Y, Z or L, B, h and a parameter error due to time ketidaksinkronan clock oscillator in the satellite with a clock in the GPS receiver. Therefore, a minimum required distance measurements to four satellites.

Type tool (Receiver) GPS

There are three types of GPS devices, with each providing a level of accuracy (position) is different. The first type is the type of GPS Navigation (Handheld, Handy GPS). Type nagivasi price is quite cheap, about 1-4 million dollars, but the accuracy of the current position given the new can reach 3 to 6 meters. The second type of tool is a single-frequency geodetic type (type mapping), commonly used in survey and mapping that takes a position accuracy of about an inch up to several decimetre. Last type is type of geodetic dual frequency which can provide position accuracy up to millimeters. This type is commonly used for precise positioning applications such as web development point of control, deformation surveys, and geodinamika. Price geodetic type receivers are quite expensive, reaching hundreds of millions of dollars for one unit.

Signal and Bias on GPS

GPS emits two signals of frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz). L1 signal is modulated with two pseudo-random signal that is the code P (Protected) code and C / A (coarse / Acquisition). L2 signal carries only the code P. Each satellite transmits a unique code so that the receiver (GPS receiver) can identify the signals from each satellite. At the feature "Anti-Spoofing" is enabled, then the P code will be encrypted and subsequently known as the code P (Y) or Y-code

When the signal through the atmospheric layers, then the signals will be disturbed by the content of the atmosphere. The amount of interference is called bias. Bias signals are mainly composed of two kinds of bias ionosphere and troposphere bias. This bias must be taken into account (modeled or estimated or perform differencing technique for the differential method with a baseline distance is not too long) to get the coordinates of the final solution with good accuracy. If a bias is ignored then it can provide position error up to order m.

Error Source in GPS

In the GPS system there are some errors system components that will affect the accuracy of the results obtained position. Mistakes are for example satellite orbit errors, satellite clock errors, receiver clock errors, antenna phase center errors, and multipath. Other things also have accompanying imaging system errors such as effects, and noise. These errors can be eliminated one by using data differencing techniques.

Method of positioning with GPS

Method of determining GPS position with the first two, the absolute method, and the differential method. Each method can then be done by real time or post-processing. If the object is stationary then the determined position is called static methods. Conversely, if the specified object moves its position, then the method is called kinematic. Further more detail we will find methods such as tuition, DGPS, RTK, GPS Survey, Rapid static, pseudo-kinematic, and stop and go, and there are still some other methods.

The position accuracy obtained from the GPS System

For civilian applications, GPS provides the positioning accuracy in a fairly broad spectrum, ranging from meters to millimeters. Prior to May 2000 (SA on) the accuracy of GPS positioning with an absolute method of data psedorange reach 30-100 meters. Then after SA off accuracy improved to 3-6 meters. Meanwhile DGPS technique provides 1-2 meter accuracy, and precision engineering provide 1-5 centimeter RTK. For positions with millimeter accuracy provided by GPS survey techniques with GPS equipment the type of geodetic dual frequency and specific data processing strategy.

Applications of GPS Technology

GPS (Global Positioning System) satellite navigation system is the most popular and most widely applied in the world today, both on land, sea, air and space. Besides military applications, GPS applications areas are quite rampant today include mapping surveys, geodinamika, geodesy, geology, geophysics, transportation and navigation, deformation monitoring, agriculture, forestry, and even sports and recreation. In Indonesia alone, the use of GPS has been started since a few years ago and continues to grow until now, both in volume and type of application

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