Sunday, March 05, 2006

The ionosphere influence in GPS precision

Along the path between the satellite and the receptor, GPS suffers different types of influence, either in speed propagation, polarization, signal power, direction, etc. This influence causes errors. One of the major factors that limits the positional accuracy provided by GPS, is the delay caused by the ionosphere to the passage of the signal. This is due to the elevated number of free electrons, excitated by sun light (night readings are more feasible), in this specific layer of the atmosphere.

For the GPS signal, the ionosphere is responsible for errors that can vary from 1 meter to more than 100 meters. For being a dispersive environment regarding the frequencies used in GPS, the ionosphere produces an effect to the signal, depending on the signal frequency and the refraction index. This effect is proportional to the TEC (number of electrons along the way between the satellite and the receptor).

If the value of TEC had a regular variation, the effects caused by the ionosphere would be of easy determination.

The problem is that TEC varies in time and space, by reason of the ionization solar flux, electromagnetical activity, spot solar cycle, station year, user location and direction of the satellite vector ray, making the process of ionospheric correction very difficult.

Nevertheless, the majority of the errors associated with the ionosphere can be eliminated through the observation of linear combination obtained in two different frequencies, remaining only the residual error. The problem is that most of the users work with one frequency receptors (much cheaper than double frequency receptors), taking no advantage of this fact. Also, the use of GPS in differential mode (DGPS) reduces the effects on the ionosphere, but this process is limited by the area where it can be used.

Therefore, it can be concluded that there is a need to develop models (e.g., Ionospheric Plate Model, Daily Cosine Model, Klobuchar Model) that eliminate the effect of the ionosphere in GPS readings, allowing this way, great positional accuracies.

António Araújo

The bibliography and full article is presented in this blog with the name "Modelling the Error Introduced by Ionosphere in GPS Signal in Receptors of One Frequency".

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