Radionavigation

The radionavigation is a technique of Navigation using radioelectric signals to determine its position or a place of position. The points obtained are independent of the conditions of visibility. One distinguishes the passive systems of positioning, without transmitter on board mobiles, active systems with transmission between mobile and bases, often called radiolocation. The embarked radar can be also regarded as a technique of radionavigation.

History

See also: History of navigation

The use of the radio waves as helps with navigation simultaneous with is discovered directional loop antennae and associated Radiogoniométrie. The Radiophare S started to help the ships in North Atlantic before their use in aviation.

By measuring the direction of two radio beacons or more with a radio compass, the determined navigator a probable point by triangulation. These " radiocompas" improved until the current models still largely used in aeronautics. The system CONSOL is an improvement developed during the second world war, not requiring an embarked directing antenna, the layer of the transmitter is determined by the pulse repetition frequency heard, gràce with a system of sending antennas complex.

A major change was obtained with the first systems " hyperboliques" , Loran and Decca, about 1940-1945. The hyperbolic systems determine the position by measuring the difference in travel time between two transmitters (at least), the place of the points with equal difference is a hyperbole on the chart. Three transmitters are necessary for a point (intersection of hyperboles). To avoid vague or ambiguous geometries, three transmitters or more necessary, are synchronized in a " chaîne". The first hyperbolic system the LORAN-A functioned with 1800KHz, the stations emit impulses of a few milliseconds with precisely synchronized phase and beginning. Measurement was carried out initially with an oscilloscope aboard aircraft, then progress of electronics allowed the direct posting of the differences in time (" TD"), then today of the geographical point. DECCA initially used continuous waves on four frequencies between 70 with 130KHz, the mobile having to follow the 3 relative phases between the frequency Master and to count the " tours" on " deccamètres". It was sophisticated in the years 1970 by an automatic identification of the " tours" grace a system of pseudo-impulse. the point was obtained as for Loran on special charts of hyberboles, then calculated in the receiver dice the appearance of the first microprocessors. The Decca chains are stopped today. Other local systems hyberbolic were developed, as in France the RANA, and TORAN, in order to mitigate the absence of cover DECCA in the Bay of Biscay

The OMEGA was also a hyperbolic system, of total cover. It was developed for the need for the American navy, and comprised 8 transmitters of very strong power in the waveband 10-14KHz. Waves VLF have the property to be propagated on all the ground by " guide onde" between the ground and the ionosphere. a mobile could thus receive everywhere 4 or 5 stations, and calculate its position with a precision of some miles. The system, not very precise, heavy and expensive on the ground, was abandoned in the years 1990. A similar system was developed by the ex-USSR.

The first operational system by satellite is the TRANSIT. It used the phenomenon of the " doppler" , which varies the frequency of reception of a satellite (for example) according to the angle its speed. Knowing the position of the satellite (its " éphémérides") the measurement of Doppler during its passage makes it possible to make a calculation of position. The frequency used was of 400MHz, ten satellites allowed a point approximately every hour, of precision similar to the point astro.

Current systems

Systems of positioning per satellite

They make it possible a mobile receiver to position in a geodetic Système, using a constellation of satellites in orbit. One can quote GPS (American), GLONASS (Russian), and in the future GALILEO (European) and Beidou (Chinese)

Systems of localization per satellite

They allow the follow-up of a ship from a fixed telephone. One distinguishes the systems public ARGOS, COSPAS/SARSAT, EUTELTRACS and GEOSTAR, and private.

Systems at terrestrial base

These systems use fixed beacons (Radiophare S, Radiobalise S) to make it possible a mobile receiver to position.

coastal systems: SYLEDIS in Western Europe and the Radio beacon S (the latter giving only one place of position);
deep-sea systems: LORAN C (northern, Peaceful Atantique northern, northern Indian Ocean), the LORAN has (China, Japan) and Chaïka (the Peaceful North-West);
aeronautical systems: Radio beacon S, VOR, THEY and DME;

The systems of positioning at terrestrial base function according to two modes:

circular mode: the measurement of the run time of a signal (generally way return ticket) between the mobile and a transmitter located at ground made it possible to calculate the distance D transmitter-ship; the ship was thus on a circle of radius D; using two (or better three) transmitting, one determined the position of the mobile thus, with the intersection of the circles. The SYLEDIS functions either in circular mode, or in hyperbolic mode
the hyperbolic mode: the measurement by the mobile of the difference of times of arrival of signals resulting from synchronized transmitters gave a place of position (hyperbole whose transmitters were with the hearths); the combination of the signals resulting from several beacons gave the position of the mobile (ship or aircraft), with the intersection of the hyperboles; hyperbolic examples of systems: LORAN

Basic principles in aviation

Radials and QDR

The radials are radioelectric axes which are located by their angular measurement starting from the Magnetic north . They are generated by a radioelectric beacon.

QDR: Magnetic raising of the Aircraft by a station (QD' R' = R adial: center towards the outside of the circle, along the ray).

QDM: Magnetic raising of the station by the Aircraft (QD' Me = towards the M oyeu)

A radioelectric beacon makes it possible to define 360 radials 0° in 360° of one in a degree.

One notices that the radial one pointed towards North is the 0 or 360, towards the East the 090, the South the 180, and the West the 270.

Let us note that each radial is a half-line and that one should not confuse, for example radial the 200 with radial the 020

Note: QDM = QDR-180° (if ΔR = ΔDm or if DM varies little and station-aircraft outdistances it is weak)

Maritime use

Evolutions in progress

The GPS answer the majority of the needs for sea transport, but the principle of a second back-up system is maintained, ensured today by the LORAN, and the radio beacons, in the event of breakdown or of degradation of the GPS.

In aeronautics, the problem is more complex because of the stakes of safety. The improvements of the precision and the integrity of signal GPS are today assured by the complementary systems (WAAS EGNOS), but a second independent system is under development with Galileo.

This remains insufficient to give up the other terrestrial systems local or regional, and pleasing with the discussions in progress on a hyperbolic system of replacement based on the LORAN, called E-LORAN, of precision and sufficient cover in navigation with broad or approach, maritime or aeronautical.

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