• GPS tracker-based passenger information systems



     NRL collects the GPS data along with the laser tracking data for inter comparison studies. Tracking data from the GPS Control Segment stations, USNO, the broadcast position data and DMA precise ephemerides are collected. These are continuous data over the in~rbit operation of the satellites. To utilize the GPS derived tracking data for inter comparison with the laser derived data, the local clocks at the GPS Monitor Station sites must be accounted for since they are the basis for the GPS tracking measurements. In GPS itself these clocks are accounted for by the use of GPS Time which is a common synchronization time computed at the MCS. However, the GPS ranging measurements are directly related to the local clocks whose performance must be removed if the satellite clock is to be isolated from the satellite orbital position and evaluated. The laser data is independent of this influence on ranging measurements since the local clock is used for time tagging.

    When you record a single position with a good best GPS tracking device , the position recorded will probably be within 5 to 15 meters horizontally of the true location of the antenna. When a surveyor uses good, survey-grade GPS equipment he or she can locate a point to within a centimeter of its true horizontal position. What are the factors that allow the surveyor to be 1,000 or so times more accurate than you are? This is a complicated subject. The answer includes “very good equipment,” “measuring the actual number of waves in the carrier” (as differentiated from interpreting the codes impressed on the carrier), and “spending a lot of time” at each site.1 We can cover only the basics in a book of this scope. But you will learn how to reduce errors so that you can record a fix to within half a meter to three meters of its true location.

    One primary method of gaining such accuracy is called “differential correction. Differential Correction in Summary In a nutshell, the differential correction process consists of setting a GPS receiver (called a base station) at a precisely known geographic point. Since the base station knows exactly where its antenna is, it can analyze and record errors in the GPS signals it receives–signals that try to tell it that it is somewhere else. That is, the base station knows the truth, so it can assess the lies being told to it by the GPS Tracking Device signals. These signal errors will be almost equivalent to the signal errors affecting other GPS receivers in the local area, so the accuracy of locations calculated by those other receivers may be improved, dramatically, by information supplied by the base station.



    For the logging of a given point, define “error” as the distance between what your GPS receiver records as the position of the antenna and the trueposition of the antenna. It is useful to dissect the idea of “error.” We can speak of error in a horizontal plane and differentiate it from the vertical error. This is important in 3G navigator , because the geometry of the satellites almost always dictates that no matter what we do, vertical error will almost always exceed horizontal error on or near the surface of the earth. The fact that all the satellites are necessarily above the fix being taken generally means that vertical error will be 1.5 to 2.5 as great as horizontal error. Another useful distinction is between what we might call random error and systematic error, or bias. Random errors are deviations from a “true” value that follow no predictable pattern. Systematic errors do follow a predictable pattern. An example will be illustrative. Suppose we have a machine designed to hurl tennis balls so that they land a certain distance away on a small target painted on the ground. Of course, none of the balls will hit the center of the target exactly; there will always be some error.

    Another important application of the satellite-based passenger systems is timetable planning: storing the time differences between the timetable and the real data, it is possible to create a database for timetable planning, making the work of the planning engineers easier and more accurate. Also the railway freight transportation can be supported by a GPS Vehicle Tracking System China , according to two different ways of identifying carriages. We refer to one group train, when all carriages must arrive at the same destination, hence representing a single element from a transportation point of view. In this case, tracking the position of the engine is sufficient to know the position of one of the carriages with an accuracy equal to the length of the whole train, whereas the exact positioning data of each carriage is not important for the carriers. There is a weakness in this system. To connect the carriages logically to the GPS-based engine, it is necessary to know their identification numbers; before the departure, the numbers have to be collected and transmitted by radio to the system.

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