PDF | Introduction: Central to precision farming and natural resources Global Positioning System: The GPS consists of 24 satellites that circle the earth in six. Global Positioning. System (GPS) and its. Applications. United Nations/Croatia Workshop on the Applications of Global Navigation. Satellite Systems. Baska, Krk . This document describes the Global Positioning System (GPS) and procedures, methods and considerations to be used and observed when using GPS to.
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(NAVigation System with Time And Ranging) GPS (Global Positioning Four GPS coded satellite signals make possible the computation of position in “The Global Positioning System (GPS) is a U.S. space-based radionavigation system that provides reliable positioning, navigation, and timing services to civilian. The Global Positioning System (GPS) is a space-based navigations system that provides location and GPS is “space based satellite navigation system” that can show your exact position on or near the Earth .. edu-outreach/docs/gps-tcv. pdf.
That is, the measured satellite-to-receiver range will be longer than the actual geometric range, which means that a distance between two receivers will be longer than the actual distance. Unlike the ionospheric delay, the tropospheric delay cannot be removed by combining the L1 and the L2 observations. This is mainly because the tropospheric delay is frequency independent.
GPS Errors and Biases 39 The tropospheric delay depends on the temperature, pressure, and humidity along the signal path through the troposphere. Signals from satellites at low elevation angles travel a longer path through the troposphere than those at higher elevation angles.
Tropospheric delay results in values of about 2. Tropospheric delay may be broken into two components, dry and wet. The wet component of the tropospheric delay depends on the water vapor along the GPS signal path. Unlike the dry component, the wet component is not easy to predict. Several mathematical models use surface meteorological measurements atmospheric pressure, temperature, and partial water vapor pressure to compute the wet component.
Unfortunately, however, the wet component is weakly correlated with surface meteorological data, which limits its prediction accuracy.
However, these are not the only factors that affect the resulting GPS accuracy. The satellite geometry, which represents the geometric locations of the GPS satellites as seen by the receiver s , plays a very important role in the total positioning accuracy . The better the satellite geometry strength, the better the obtained positioning accuracy. As such, the overall positioning accuracy of GPS is measured by the combined effect of the unmodeled measurement errors and the effect of the satellite geometry.
Good satellite geometry is obtained when the satellites are spread out in the sky . In general, the more spread out the satellites are in the sky, 40 Introduction to GPS the better the satellite geometry, and vice versa. In such a case, the receiver will be located at the intersection of two arcs of circles; each has a radius equal to the receiver-satellite distance and a center at the satellite itself. Because of the measurement errors, the measured receiver-satellite distance will not be exact and an uncertainty region on both sides of the estimated distance will be present.
Combining the measurements from the two satellites, it can be seen that the receiver will in fact be located somewhere within the uncertainty area, the hatched area. As shown in Figure 3. Similarly, if the two satellites are close to each other [Figure 3.
The satellite geometry effect can be measured by a single dimensionless number called the dilution of precision DOP. The lower the value of the DOP number, the better the geometric strength, and vice versa [3, 8]. The DOP number is computed based on the relative receiver-satellite geometry at any instance, that is, it requires the availability of both the receiver and the satellite coordinates. Approximate values for the coordinates are a Figure 3. GPS Errors and Biases 41 generally sufficient though, which means that the DOP value can be determined without making any measurements.
As a result of the relative motion of the satellites and the receiver s , the value of the DOP will change over time. For example, for the general GPS positioning purposes, a user may be interested in examining the effect of the satellite geometry on the quality of the resulting three-dimensional 3-D position latitude, longitude, and height.
This could be done by examining the value of the position dilution of precision PDOP. In other words, PDOP represents the contribution of the satellite geometry to the 3-D positioning accuracy.
The former represents the satellite geometry effect on the horizontal component of the positioning accuracy, while the latter represents the satellite geometry effect on the vertical component of the positioning accuracy. As a result, the GPS height solution is expected to be less precise than the horizontal solution.
To ensure high-precision GPS positioning, it is recommended that a suitable observation time be selected to obtain the highest possible accuracy. A PDOP of five or less is usually recommended. In fact, the actual PDOP value is usually much less than five, with a typical average value in the neighborhood of two.
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