GPS Systems: Technology, Operation, and Applications by Levitan Ben & Lawrence Harte

Author:Levitan Ben & Lawrence Harte [Ben, Levitan]
Language: eng
Format: azw3
Publisher: UNKNOWN
Published: 2017-03-12T05:00:00+00:00

Figure 1.15, GPS Data Formats

GPS Errors GPS errors are unwanted variations in position measurements that come from a variety of sources. Without corrections to the errors, the location accuracy of the GPS system is within approximately 50 to 100 meters. GPS errors include propagation errors that can result from changes in atmospheric conditions, inaccurate satellite position data (ephemeris), reference timing errors (clock drift), multipath transmission and selective availability.

Atmospheric Conditions The atmosphere is composed of materials or elements that surround the earth. The earth’s atmosphere can be divided into layers that have different effects on the propagation of radio signals.

Atmospheric conditions define the characteristics of the atmospheric regions. Atmospheric layers are regions separated by height above the surface of the earth, which contain gases and materials with common characteristics. Atmospheric layers include the troposphere (directly above the earth to 12 km), the mesosphere (12 km to 50 km), the stratosphere (50 km to 90 km), and the ionosphere (90 km to 400 km).

Atmospheric data is information that identifies the characteristics of the atmosphere. Atmospheric data may be used to define how objects (such as satellites and radio signals) will interact with and be changed by the earth’s atmosphere.

Radio propagation is the process of a radio signal (electromagnetic signal) traveling from one point to one or more alternate points. Radio propagation may involve a direct wave (space wave) or a wave that travels along the surface (a surface wave). Radio propagation characteristics typically vary based on the medium of transmission (air) and the frequency of radio transmission.

Troposphere The troposphere is a layer of the earth’s atmosphere, between the surface and the stratosphere, in which about 80 percent of the total mass of atmospheric air is concentrated and in which the temperature normally decreases with altitude.

The ionosphere and troposphere both refract GPS signals. This causes the speed of a GPS signal in the ionosphere and troposphere to be different from the speed of a GPS signal in space. Therefore, the distance calculated from the formula: signal speed x time, will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

Stratosphere The stratosphere is the atmospheric layer that lies between the troposphere and the mesosphere, which is approximately 50 km to 90 km above the surface of the earth.

Mesosphere The mesosphere is the atmospheric layer that is located between the stratosphere and the ionosphere, and is approximately located between 12km to 50 km above the surface of the earth.

Figure 1.16 shows the different layers of atmosphere above the earth. The troposphere is the first layer that extends up to approximately 12 km. It is followed by the mesosphere layer that ranges from 12 km to 50 km. The stratosphere layer is located from 50 km to 90 km and the ionosphere ranges from 90 km to over 400 km.

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