GPS Solution - Differential GPS and real-time kinematic
Most, not all, GPS surveying relies on the idea of differential positioning. The modeof a base or reference receiver at a known location logging data at the same time as a receiver at an unknown location together provide the fundamental info rmation for the determination of accurate coordinates. While this basic approach remains today, the majority of electronic tracking device surveying is not done in the static postprocessed mode just described. Postprocessing is most often applied to control work. Now, the most commonly used methods utilize receivers on reference stations that provide correction signals to the end user via a data link sometimes over the Internet, radio signal, or cell phone and often in real-time.
In this category of GPS surveying work there is sometimes a distinction made between code-based, DGPS, and carrier based, RTK, solutions. In fact, most systems use a combination of code and carrier measurements so the distinction is more a matter of emphasis rather than an absolute difference.
The General idea
Errors in satellite clocks, imperfect orbits, the trip through the layers of the atmosphere, and many other sources contribute inaccuracies to GPS signals by the time they reach a receiver. These errors are variable, so the best to way to correct them is to monitor them as they happen. A good way to do this is to set up a electronic tracker on a station whose position is known exactly, a base station. This base station receiver’s computer can calculate its position from satellite data, compare that position with its actual known position, and find the difference and presto, error corrections. It works well, but the errors are constantly changing so a base station has to monitor them all the time, at least all the time the rover receiver or receivers are working. While this is happening the rovers move from place to place collecting the points whose positions you want to know relative to the base station, which is the real objective after all. Then all you have to do is get those base station corrections and the rover’s data together somehow. That combination can be done over a data link in real-time, or applied later in postprocessing.
DGPS
The term DGPS is most often used to refer to differential GPS that is based on pseudoranges, aka code phase (Figure 6.8). Even though the accuracy of code phase applications was given a boost with the elimination of Selective Availability (SA) in May 2000 consistent accuracy better than 5 meters or so still requires reduction of the effect of correlated ephemeris and atmospheric errors by differential corrections. Though the corrections could be applied in postmission processing services that supply these corrections, most often operate in real-time.
Real-Time
Usually, pseudorange corrections, rather than coordinate corrections, are broadcast from the base to the rover or rovers for each satellite in the visible constellation. Rovers with an appropriate input/output (I/O) port that can receive the correction signal and calculate coordinates. The real-time signal comes to the receiver over a data link. It can originate at a project specific base station or it can come to the user through a service of which there are various categories. Some are open to all users and some are by subscription only. Coverage depends on the spacing of the beacons, aka transmitting base stations, their power, interference, and so forth.
More GPS Tracking Solution at http://www.jimilab.com/ .
In this category of GPS surveying work there is sometimes a distinction made between code-based, DGPS, and carrier based, RTK, solutions. In fact, most systems use a combination of code and carrier measurements so the distinction is more a matter of emphasis rather than an absolute difference.
The General idea
Errors in satellite clocks, imperfect orbits, the trip through the layers of the atmosphere, and many other sources contribute inaccuracies to GPS signals by the time they reach a receiver. These errors are variable, so the best to way to correct them is to monitor them as they happen. A good way to do this is to set up a electronic tracker on a station whose position is known exactly, a base station. This base station receiver’s computer can calculate its position from satellite data, compare that position with its actual known position, and find the difference and presto, error corrections. It works well, but the errors are constantly changing so a base station has to monitor them all the time, at least all the time the rover receiver or receivers are working. While this is happening the rovers move from place to place collecting the points whose positions you want to know relative to the base station, which is the real objective after all. Then all you have to do is get those base station corrections and the rover’s data together somehow. That combination can be done over a data link in real-time, or applied later in postprocessing.
DGPS
The term DGPS is most often used to refer to differential GPS that is based on pseudoranges, aka code phase (Figure 6.8). Even though the accuracy of code phase applications was given a boost with the elimination of Selective Availability (SA) in May 2000 consistent accuracy better than 5 meters or so still requires reduction of the effect of correlated ephemeris and atmospheric errors by differential corrections. Though the corrections could be applied in postmission processing services that supply these corrections, most often operate in real-time.
Real-Time
Usually, pseudorange corrections, rather than coordinate corrections, are broadcast from the base to the rover or rovers for each satellite in the visible constellation. Rovers with an appropriate input/output (I/O) port that can receive the correction signal and calculate coordinates. The real-time signal comes to the receiver over a data link. It can originate at a project specific base station or it can come to the user through a service of which there are various categories. Some are open to all users and some are by subscription only. Coverage depends on the spacing of the beacons, aka transmitting base stations, their power, interference, and so forth.
More GPS Tracking Solution at http://www.jimilab.com/ .
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