Automatic radio compass (ARC) - an on-board direction finder designed to navigate aircraft using signals from ground-based radio stations by continuously measuring the course angle of a radio station (CSD). The heading angle of a radio station is the angle enclosed between the longitudinal axis of the aircraft and the direction to the radio station, counted clockwise.
The radio compass is called automatic because, after tuning to the carrier frequency of the radio station, it continuously measures the value of the CSD without the participation of a person (operator). CSD is displayed on a pointer or digital display, and can also be transmitted to the on-board navigation system.
Together with the exchange rate instruments, the radio compass allows the crew in any weather conditions to solve the following navigation tasks at any time of the day:
- to fly with a given CSD with continuous visual indication of the heading angle;
- determine the bearing of a radio station by the course indicator using the course system;
- operate as a backup communications radio .
Content
Principle of Operation
The action of aircraft direction finders is based on the simultaneous reception of radio station signals to two antennas - a loop antenna and non-directional. A necessary condition for the operation of such direction finders is the vertical polarization of the received radio waves , which in the used medium and long wavelength ranges is almost always fulfilled due to the specifics of the radio transmitter antennas and the prevalence of the surface radio wave propagation mechanism.
An omnidirectional antenna is a vertical pin or wire. The directivity pattern of such an antenna in the horizontal plane is a circle, that is, the level of the radio signal received by the whip antenna does not depend on the direction to the radio source.
The simplified loop antenna is one or more flat turns of wire , and the perimeter of the turn is much less than the working wavelength of the radio station. As a rule, for compactness, the coils are located on a ferrite core (magnetic loop antenna). The plane of the turns is vertical. According to the Faraday law, the radiation pattern “across the field” of such a loop antenna in the horizontal plane is described by the sin ( φ ) function (when counting the angle φ from the normal to the plane of the frame), and when it is displayed in the polar coordinate system, it resembles the figure 8 in shape (“ eight"). That is, the radiation pattern consists of two petals of the same shape and size. In particular, if the plane of the frame is parallel to the direction to the radio station (the frame is “sideways”), then the level ( amplitude ) of the signal received by the frame antenna is maximum. If the plane of the frame is perpendicular to the direction of the radio station, then the level of the received signal is minimal. Reception of radio waves of the same source on one and the other lobes of the radiation pattern of the loop antenna differs only in the phase of the radio signal: when the loop antenna is rotated around the vertical axis by 180 °, the level of the received radio signal will not change, and the phase will change by 180 ° (sign of the sin ( φ ) function changes to the opposite, the signal becomes out of phase in comparison with the initial orientation of the frame).
At the input of the radio compass receiver, the signals of the omnidirectional and loop antennas are summed with certain weighting factors. By selecting these coefficients, you can form the resulting radiation pattern in the form of a cardioid , and its maximum is directed towards one of the lobes of the radiation pattern of the loop antenna, and the minimum is directed to the other (depending on which of these directions the radio signals received by the antennas taking into account the weight coefficients turn out to be in phase, and in which they are out of phase).
In one embodiment, the radio compass loop antenna is capable of rotating about a vertical axis. The rotation is made by an electromechanical servo drive controlled by the radio compass receiver. In the direction of the maximum of the radiation pattern, with a small change in direction to the radio station, the level of the radio signal changes little (the first derivative of the cardioid with respect to the argument at the maximum point is zero, the second derivative (curvature) is small), this prevents the measurement of the bearing (direction to the radio station) by the simplest method - the method antenna pattern maximum. With a more accurate method - the minimum method, the level of the total antenna signal decreases to almost zero, and the reception of the radio signal stops. In this regard, periodic, with a frequency of several tens of hertz, switching ( inversion , 180 ° shift) of the phase of the signal of the loop antenna using a controlled phase inverter is used . The tracking electromechanical system rotates the loop antenna until the direction to the radio station falls into the minimum radiation pattern with a certain state of the phase inverter. Synchronously with the rotation of the antenna frame, the course arrow on the radio compass indicator also rotates.
In another embodiment, the outputs of the loop antennas are connected (directly or after amplification and conversion to an intermediate frequency) to a radio goniometer - a compact electromechanical device that allows instead of rotating the antennas themselves to rotate the receiving coil located in the field of two pairs of field coils whose axes are mutually perpendicular. At present, digital processing of radio signals received by all three antennas of a direction-finding antenna system is more justified in order to extract information about the bearing of a radio station.
Influence of propagation conditions on the accuracy of determining the course angle of a radio station [1]
The accuracy of determining the directional angle of a radio station with an automatic radio compass depends on the propagation conditions of the radio waves, which are affected by the terrain, ground structures (various structures, masts), structural elements of the aircraft, reflection of radio waves from the earth's surface and ionosphere.
All these factors distort the electromagnetic field due to the mechanisms of refraction in the propagation medium, diffraction by inhomogeneities and reflection from obstacles, which leads to the fact that the minimum of the cardioid deviates somewhat from the direction to the radio station, thus introducing an error in the determination of CSD. It is possible to determine direction finding errors caused by constant obstacles (relief, structures), and to take them into account only by the results of practical measurements on the ground. Some errors are seasonal in nature and depend on changes in the electrical parameters of the soil and local objects and are caused by changes in weather conditions.
Also, when determining CSD, one should take into account the phenomena characteristic of the propagation of medium-wave range radio waves: the coastal effect, the mountain effect, and the night effect.
Shore effect
Errors due to the coastal effect are associated with the refraction of surface waves during their propagation through the interface between two media: land and sea, resulting in an error Δ between the true CSD and the current count of the radio compass (ORC). Refraction will be the greater, the smaller the angle between the coastline and the direction of arrival of radio waves to the coastline. So, for example, at sharp angles between the surface wave and the coastline, the direction finding error can reach 6-8 °. The most tangible impact of the coastal effect will be observed in the immediate vicinity of the coastline. As you move away from it, the wave front gradually aligns, and the error in determining the bearing becomes insignificant.
Mountain Effect
The essence of the mountain effect is that electromagnetic waves, reflected from irregularities of the earth's surface (mountains, hills), interfere with the direct wave field of the radio station and distort it.
The magnitude of the errors of the ARC caused by the mountain effect depends on the height of the mountains and the distance to them, the wavelength of the direction-finding radio station, and the true altitude of the aircraft.
With a mountain height of 300-1500 m, the error affects a distance of 8-10 km from the mountain, with a mountain height of 1500-4000 m affects a distance of 20-40 km. For example, when flying over a mountain valley below the height of the surrounding ridges, the bearing error does not exceed 5 ° when direction finding a radio station located in the same valley at a distance of at least 10 km from the mountains. In other conditions, especially when the flight altitude is less than 300 m, the errors in determining the CSD can reach 90 °. The greatest errors are observed if the aircraft is between the radio station and the highest point of the terrain.
Behind the pass, direction finding errors are reduced, and at a distance of 30–40 km from the mountains, the error is practically absent. When flying in mountainous terrain, the speed of oscillation of the arrow of the ARC pointer at a flight altitude of 900-1200 m above the mountains is approximately 10-20 deg / s. When flying at altitudes of less than 300 m, faster fluctuations and turns of the pointer arrow by ± 90 ° are observed, therefore, when piloting, there is a danger of collision with individual mountain peaks.
To improve the accuracy of measuring the CSD in mountainous areas, radio waves with a shorter wavelength are used, and when flying at altitudes of less than 500 m, the average reading for the CSD indicator should be taken during navigation.
Night Effect
The antenna system of the radio compass during the day is usually affected only by a surface wave with vertical polarization, and the direction finding accuracy is higher. With the onset of night, the absorption of medium-wave radio waves in the lower part of the ionosphere (in layer D) decreases, the refraction (“reflection”) of the wave is more pronounced in higher layers (in layer E), as a result of which, in addition to the surface wave, a spatial wave also arrives at the receiving point . When interacting with an unstable ionosphere, the plane of polarization of a spatial wave due to the Faraday effect can rotate randomly, or, in the general case, the polarization becomes elliptical. As a result, in addition to the vertical component, a horizontal (cross-polarization) component of the electric field arises at the receiving point, which induces undesirable currents in the antenna system, leading to errors in the determination of the bearing ( polarization errors ).
Under the influence of the night effect, the direction-finding accuracy depends on the distance to the radio station, as well as on the working wavelength and power of the radio station: the closer the radio station is and the longer the wavelength, the weaker the spatial wave compared to the surface wave and the smaller the error in determining the bearing. So, at a distance of 100 km from the radio station, the night effect almost does not affect the accuracy of determining the CSD by the radio compass, since the spatial wave dominates. With greater distance, the most intensely nocturnal effect is manifested 1-2 hours before sunrise, during this period of time the error in determining the CSD can reach 30 °. At night, the bearing error is 10-15 °. The effect of a spatial wave can be especially pronounced during twilight (morning, evening), for two hours before and after sunrise (sunset), when the stability of the ionosphere is violated. The design of the antenna system uses technical solutions that weaken the influence of the cross-polarization component of the electric field.
Classification
By purpose and frequency range, the ARCs are divided into two groups - medium-wave (navigation) and emergency (search), operating in the VHF (meter waves) range .
- Medium-waved ARCs are designed to provide flights on driven (PRS) and broadcast (SRS) radio stations by continuously measuring the CSD. The operating frequency ranges of the medium-wavelength ARC are from 150 kHz to 1299.5 kHz or up to 1749.5 kHz, or up to 1799.5 kHz.
- Emergency ARCs are used to output to an emergency VHF radio station or emergency radio beacon during search and rescue operations. The frequency range of the search ARC is in the range of 100-150 MHz.
History
- July 27, 1920 - the world's first use of a radio compass for aviation navigation.
- In the spring of 1935, the radio compass created by engineer Nikolai Aleksandrovich Karbansky was tested at the Air Force Research Institute; this radio compass did not go into serial production.
- The first Soviet ARC-5 radio compasses began to be produced during the Great Patriotic War on the basis of an American design ( B-29 bomber radio compass).
Some types of radio compasses in Russia and the USSR
- ARK-5
- ARK-9
- ARK-10
- ARK-11
- ARK-15M
- ARK-19
- ARK-22
- ARK-25
- ARK-32
- ARK-35
- ARK-40
- ARK-U2 - emergency
- ARK-UD - emergency
Literature and Documentation
Literature
- Chesnokov E.V., Andreev N.N. et al. Automatic radio compasses of helicopters. The procedure for cancellation and elimination of radio-visual error. Teaching aid. - Voronezh, 2013.
- Belavin O. V., Zerova M. V. Modern means of radio navigation - M., 1965.
- Belavin OV Basics of radio navigation - M .: Sov. radio, 1977.
- Aviation and radio-electronic equipment of the An-24 aircraft - M .: Transport, 1975.
Regulatory and technical documentation
- GOST R 50860-96 airplanes and helicopters. Devices antenna-feeder communications, navigation, landing and ATC. General technical requirements, parameters, measurement methods.
Notes
- ↑ 1
Links
- P.M. Stefanovsky. “Three hundred unknowns” - Out of sight of the earth
- Automatic medium-wave radio compass ARK-9. Technical description, operating instructions, appendices 1,2.
- Automatic radio compass ARK-15M. Technical description and instruction manual. Part 1. Edition 2.
- Automatic medium-wave radio compass ARK-22. Technical operation manual.
See also
- Radio navigation
- Air navigation