The lowest detectable layer, the D region, extends from about 50 km to 90 km. Here are some general ideas on the subject. Neither the boundaries between these regions, nor the upper layer of the ionosphere, can be defined strictly. It can be divided into the mesosphere and thermosphere, et cetera that are themselves composed of layers: D, E, and F. Source: GPS for Land Surveyors Ionospheric Stratificationįor ease of reference, the ionosphere can be said to extend from 50 kilometers to 1,000 kilometers above the earth's surface. The effect of the ionosphere on the GPS signal usually reaches its peak in March, about the time of the vernal equinox. It is also nearly four times greater in November, when the earth is nearing its perihelion, its closest approach to the sun, than it is in July near the earth’s aphelion, its farthest point from the sun. During the daylight hours in the midlatitudes, the ionospheric delay may grow to be as much as five times greater than it was at night, but the rate of that growth is seldom more than 8 cm per minute. ![]() It is usually least between midnight and early morning, and most around local noon or a little after. The ionospheric delay changes slowly through a daily cycle. The higher the electron density, the larger the delay of the signal, but the delay is by no means constant. This density is often described as total electron content or TEC, a measure of the number of free electrons in a column through the ionosphere with a cross-sectional area of 1 square meter: 10 16 is one TEC unit. As their number and dispersion varies, so does the electron density in the ionosphere. When gas molecules are ionized by the sun’s ultraviolet radiation, free electrons are released. Source: GPS for Land Surveyors Ionosphere and the Sun ![]() Also, the daytime ionosphere is rather different from the ionosphere at night. The sun plays a key role in the creation and variation of these aspects. The magnitude of these delays is determined by the state of the ionosphere at the moment the signal passes through, so it's important to note that its density and stratification varies. It is the first part of the atmosphere that the signal encounters as it leaves the satellite. The ionosphere is ionized plasma comprised of negatively charged electrons which remain free for long periods before being captured by positive ions. This causes an apparent delay in the signal's transit from the satellite to the receiver. Through both refraction and diffraction, the atmosphere alters the apparent speed and, to a lesser extent, the direction of the signal. The long, relatively unhindered travel of the GPS signal through the virtual vacuum of space changes as it passes through the earth’s atmosphere. One of the largest errors in GPS positioning is attributable to the atmosphere. ![]() Source: Cathryn Mitchell, University of Bath
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