Signal propagation through the ionosphere
|The Indian ionosphere is characterized by large horizontal gradients, intense irregularities, large dayto- day variations and equatorial anomaly conditions, there is clear necessity to thoroughly understand the ionospheric time delay effects on the GPS signals. The ionospheric delay error is a function of Total electron content (TEC) which is one of the dominant errors.
The ionosphere is the upper part of the earth’s atmosphere between approximately 60 to 70 and 900 – 1000 km above the earth. The signal propagation is mainly affected by free charged particles. As the GPS signals travel through the medium, it is slowed down in a proportion that varies according to time of the day, solar activity etc. When light travels through the ionosphere it slows down at a rate inversely proportional to its frequency squared. The generation of ions and electrons in ionosphere is proportional to the radiation intensity of the sun, and to the gas density. Chapman profile is an indicative of the number of ions produced as a function of height. The exact shape of the pro?le and the related numerical numbers depend on several parameters. The spatial distribution of electrons and ions is mainly controlled by insolation of the sun and consequent motion of ionized layers. These processes create different layers of ionized gas in different heights. The main layers are known as the D, E, F1 and F2 layers . In particular, the F1 – layer, located directly below the F2 – layers, shows large variations that correlate with the respective sun spot number. The four principle layers are designated with height domains corresponding to approximately to 60- 90 km (D layer), 90-140 km (E layer) 140- 200 km (F1 layer) 200 -1000 km (F2 layer). Also, geomagnetic infiuences play an important role.
Hence, signal propagation is affected by solar activity, near the geomagnetic equator, and in high latitudes. Near the geomagnetic equator, the earth’s magnetic field is horizontal along with the orthogonal dynamo electric field at E region heights. The electric field is eastward during the day and westward during the night. As a consequence, the dynamic eastward field ionospheric plasma from equatorial F region moves upward and then diffuses downward along the
sloping magnetic field lines to low altitudes on both sides of the equator. The electron concentration is thus depleted on the magnetic equator and
enhanced in two regions, one on each side. The phenomenon is known as equatorial ionization anomaly. Due to the variable insolation of the sun the spatial distribution of the layers varies during the day. The D-layer is only generated at the daylight side the earth. The distribution of ionospheric plasma is also affected by solar and magnetic disturbances like occurrence of solar flares etc. There is a short term i.e. 27 days and long term i.e. 11 years periodicity in solar activity. A radio signal when penetrates the ionosphere is modified by the medium due to the presence of electrons in the earth’s magnetic field. The impact of the state of the ionosphere on the propagation of waves is characterized by the electrons content . The electron density is quantified by counting the number of electron in a vertical column with a cross sectional area of one square meter called TEC. The TEC is a function of amount of solar radiation. On the night side of the earth, the free electrons have a tendency to recombine with the ions, there by reducing the TEC. As a consequence, the TEC above a particular observation station on the earth has a strong diurnal variation.
The ionosphere is a disperse medium for radio waves. Dispersion or differential time delay due to the ionosphere, causes pulse distortion and produces a difference in pulse arrival time across a band width Δf. If Ne is electron density then approximate correction “a” for the delay in signal propagation with reference to phase velocity component can be computed from
a = 1 – 40.3 Ne / f2.
The correction for group velocity component “b” is
b = 1 + 40.3 Ne / f2.
The effect of ionosphere on the phase and group velocity is equal in magnitude but has a different sign. The relationships for a and b indicate that the index of refraction, and thus the time delay of signal propagation, is proportional to the inverse of the squared frequency. Consequently, one part of the ionospheric delay can be modeled when two frequencies are used.