Radio and Navigation Systems

The ionosphere affects radio signals, ie various communication and navigation systems. The disturbance is dependent on signal frequency. Signals of a frequency below approximately 30 MHz, so-called short wave, reflected in the various jonosfärsskikten, allowing radio communication over long distances. It is the so called plasma frequency, related to the electron density, which sets an upper limit for the reflection of radio waves. Signals containing frequencies above 30 MHz, however passes through the ionosphere, which is used for communication with satellites and other spacecraft.

Occasionally increases ionization greatly from solar flares, SPE and geomagnetic storms. Frequencies between about 2 to 30 MHz affected by increased absorption. Signals with higher frequency can be affected by interference. As an example, TV and FM radio (VHF) rarely affected by solar activity and thus to changes in the ionosphere. Short wave (HF) with aircraft, ships, amateur radio, etc. however, is affected much more often. Kortvågskommunikationen depend on the parameters Maximum Usable Frequency (MUF) and the lowest usable frequency (LUF). MUF is calculated based on the so-called critical frequency and angle of the outgoing radio signal. The critical frequency is the highest frequency that a radio wave may have to be reflected in the ionosphere if it is sent straight up (about 108 Hz). This is in turn dependent upon the electron density in the F layer. Flux is determined by the proportion of radio waves absorbed in the lower D and E layers.

In conjunction with a solar flare X-ray emission increases sharply, leading to an increased ionization in the lower layers of the ionosphere on the sunlit part of the Earth. Both short-wave and long wave affected. Depending on the strength of the outbreak may be interference from minutes to hours. Also produces a wide range of radio waves that interfere directly out the signal produced. The plasma frequency increases so that signals with higher frequency reflected.

Ionization also occurs from high-energy particles. During a so-called polar cap absorption, or PCA, increases ionization in the polar region, and may last several days to weeks depending on strålningsutbrottets strength. During such an event, it is sometimes impossible to communicate via short-wave in the polar regions. Coronal mass ejections, and the fast solar wind from coronal, often leads to interference in radio signals, especially around the auroral oval.

A geomagnetic storms affect HF communication mainly higher lattituder, unlike a solar flares which may affect the communication at all lattituder. There are also areas of the ionosphere, eg polar regions, and shortly after sunset, when the ionosphere is more turbulent. This small-scale turbulence leads to random scattering of radio waves and thus to fluctuations, so called Ionospheric Scintillation in the signal.

Mobile phone communications can be disrupted by the so-called Solar radio bursts. The degree of interference depends on the direction of the antenna. Because these are horizontally directed, they can intercept the radio waves, especially during the morning or evening hours when the sun is low in the sky. If the output power from mobile phones decreases in the future, the interference may increase.

The requirements for better navigation systems has increased with technological advances, more transport and more people traveling. General navigation is such Loran-C, Omega, and GPS. Navigation systems can be divided into land-based and space-based navigation systems. For land-based systems include, among others Loran-C and Omega. They use radio signals around 100 kHz and 10 kHz, which goes along the ground and is reflected in the ionosphere. Both of these systems are affected by conditions in the D and E layer. Today is still used both Loran-C and GPS, but not Omega.

As the flight, and the latter space, have evolved, it also became important with a three-dimensional positioning system. The advantage of a space-based system is that the user can get a detailed position in real time; latitude information, longitude and altitude, and unlike terrestrial systems, satellites provide better coverage. GPS determines the position by three measurements: the distance between a known satellite and terrestrial receiver, time for the signal to travel from the satellite to the receiver, the signal speed. Scintillations causes a fade in or fade out of the signal so that the receiver loses lock on the signal, or the GPS signal travels slightly slower, due to the increased electron density in the ionosphere, so that posistionen errors. Improvements in GPS has been implemented through the introduction of a two signals GPS, DGPS (Differential Global Positioning System), which has a much better accuracy. The GPS system consists of a total of 24 satellites. Unlike land-based system uses GPS (GHz) radio signals passing ionosphere. GPS is not affected as much by changes in the ionosphere due to solar flares, but instead of the total electron density, TEC (total electron content), the ionosphere along the signal path during a geomagnetic storm. During the period from October 29 to 30, 2003, was the interference of the so-called WAAS so great that height error exceeded 50 meters.

To save time and money, some airlines started flying over the poles, such as the route London – Hong Kong or New York – Singapore. At the beginning of the route usually used VHF (30-300 MHz), but later, in the polar region, switch to HF (3-30 MHz). Satellite communications (SATCOM) are used, but mostly as a backup. Additionally, SATCOM only be used up to 82 degrees latitude. A solar storm could cause pilots get radio interference, higher radiation for people and electronics, and interference with the navigation system. Because of these effects, airlines have to sometimes choose a more southerly route, which in turn results in increased costs. During the period October 26 to November 5, 2003 experienced flight control, every day, interference from “minor” to “severe.” Several flights had to be diverted to more southerly routes to SATCOM could be used. Another problem for aviation, satellite navigation. It plans to use DGPS to reduce separation between aircraft and landing. The United States has an improved system, WAAS, which can correct the calculation errors for GPS positions in aircraft. In Europe there is a similar system called EGNOS.