Avionics II. Radio technical Navigation instruments (4)


Aircraft navigation is the science about the means and tools of controlling aircrafts while flying from one point on earth’s surface to another according to the trajectory selected in time and space. Radio navigation is area of aircraft navigation that deals with the theory and its practical use of radio devices in navigation. (Continue, Begining at No.20 )

 Simpler radio navigation tools that identify only one navigation element are called radio navigation devices (radio altimeters, radio locators, etc.).Radio navigation system is called set of radio navigation equipment and devices that are used to determine the coordinates of the aircraft. It’s short and long range radio navigation systems, landing systems, collision avoidance systems, and so on. Radio navigation system together with magnetic, inertia and satellite systems and different types of devices makes complex navigation or flight management systems.
Radio navigation opportunities, especially in range, depends on the electromagnetic wave propagation properties. When the range is about a thousand kilometers or less, this is called short distance radio navigation. Long distance radio navigation instruments range to several thousand kilometers.

Radio navigation systems measure navigation elements and determine coordinates of the aircraft or its location in relation to terrestrial equipment. This is done by measuring angles and distances or differences between multiple ground installations. According to the method of measuring systems are divided into measuring angles, distances, distance differences and cruise speed.
Angle measuring systems (terrestrial radio beacons and aircraft receivers) measure angle between the baseline and the direction of the line to ground radio beacon in the horizontal plane. Beacons emit electromagnetic waves that propagate to the aircraft. These waves are accepted by the aircraft antenna and receiver, measured parameters depend on the angular coordinates of the aircraft in relation to the lighthouse. Aircraft determines the coordinates of the transmitter of electromagnetic waves by measuring the direction of the waves.
Radio navigation systems which measures distances determines the distance between the aircraft and the electromagnetic wave transmitter (also can be located in the Earth’s moon), or between aircraft and the ground surface. This is short distance radio navigation, radio altimeters and satellite navigation systems.
Non-autonomous radio navigation systems that measure angles and distances can not independently determine aircraft position. They just lay down the line, where navigation parameter has the same values. The line is called the position line. To find the location of the aircraft, it is necessary to use multiple radio navigation instruments together.
Differences in distance measuring radio navigation tools can identify an aircraft by taking signals from several electromagnetic wave transmitters, which locations are known. This is a long-distance radio navigation system.
Speed is measured by Doppler speedometers.

2.1. Short range of radio navigation

These radio navigation instruments determine the direction of the ground beacon and the distance to it. Knowing the distances and directions to several ground-based beacons, which place is known, it is possible to determine the location of the aircraft and to land it based on readings of devices.

Range of short range radio navigation systems depend on the wavelength and their propagation conditions. Used are medium, ultrashort and decimeter waves , thus working range is only a few hundred kilometers, but often it is quite sufficient. Short range radio navigation system consists of ground beacons and aircraft equipment. These systems are used for quite a long time and are one of the most important in the aircraft navigation, although do not have wide variety in accordance with the principle of operation. These are radio compasses, distance measurers, VHF radio navigation system VOR / ILS , microwave landing systems MLS, radio pelengators, emergency radio ranger and others. Operation of these devices is based on radio waves emitting from the beacon in a straight line. Then, one can determine their direction, and knowing their travel time, can measure distances. Waves reflected from the ground and the ionosphere is not suitable or even hinders navigation.

Aircraft radio compasses set the direction to the surface wave radio beacon, which location is known. It is the line running from the aircraft through the beacon and away from it. The angle between the longitudinal axis of the aircraft and the direction of radio waves source is called the beacon relative bearing, RB. Using one radio compass, aircraft location can not be determined, as the distance to the beacon must be known.

Location of the aircraft and can be determined after finding several directions to different beacons. Aircraft location will be the position of the intersection of lines. Rescue aircrafts have emergency VHF radio compasses that operate similarly to ADF. They determine the direction of the emergency beacon signals ELT (Emergency Location Transmitter). Flying in this direction, it is possible to find an aircraft crash site.
VOR radio-navigation systems of the aircraft determine direction (azimuth) in relation to the beacon.
This direction is a line from the beacon over the aircraft and extending away from it. The aircraft azimuth is measured in relation to beacon.
The aircraft azimuth in relation to beacon is angle between the magnetic meridian of the beacon and the direction aircraft-beacon. Knowing only one VOR beacon azimuth, one can fly to or from the beacon, but the location of the aircraft can not be established, as the distance from the beacon to the aircraft is not known. An aircraft location can also be found by measuring the direction of the aircraft to several different beacon locations; it will be in the intersection line.

Position line determined by radio compasses ADF and VOR is the straight line connecting the aircraft to the beacon. The most often radio compasses are used flying to the beacon or from it, when the beacon relative azimuth or bearing RB is zero or 180 degrees. Moving away from the beacon, aircraft’s location accuracy set by VOR radio compasses deteriorate as the same angle corresponds to greater linear distance.
DME distance measuring system measures the distance to the ground radio station. According to the rate of change of distance some distance measuring equipment determines the relative speed of the aircraft in respect of station: when the aircraft flies directly to or from station, the speed is equal cruising speed. Distance measuring system determines the distance to the station, regardless of which side of the aircraft it is. For example, when the aircraft is to the south of the station in 200 kilometers, it will show 200 km. The same distance it will display when the aircraft is 200 km to the east of station, or west or north. Alternately measuring the distances to two different stations, the two locations of an aircraft will be found, but only one of them will be right (the two circles intersect at two points). To find out which one of these places is right, it is necessary to measure distance to the third station or to recognize which point is not correct, where the aircraft at the time not to be.
ADF, VOR and DME are the main short range navigation aids to be used to identify the location of the aircraft during the flight. Using them alone, the aircraft position can be determined only by means of receiving signals from several ground beacons and after quite complex calculations. Using distance meter VOR with DME, the aircraft location is at the intersection of the circle and the line. Complicated calculations are not required. Found the aircraft position will be more accurate, the closer is VOR beacon to distance measuring terrestrial radio station.

Landing is the most responsible flight moment. The plane must touch runway at its beginning. If landing happens in the middle of runway, it may not be enough distance for stopping the plane. Although the runways are quite broad, it is better to try to land the plane in the middle, because the suspension can start bringing the plane into the side. It is important to get close in the correct angle. If the plane when preparation for the landing, begin to descend too fast, its vertical speed might be too high the plane will be more difficult to handle. In case of early descent plane may use too much fuel and it may be “pulled“giving less time for the pilot to manage situation… Some airports have hills or tall buildings nearby. The line, which the plane is moving when landing, should be not very inclined or very steep and should always coincide with the middle of the runway. This line must make approximately 2.5 ° to 3 ° angle to the horizon.
The smaller the deviations from this line, the easier and safer landing is. The pilot, even seeing the runway, without devices can accurately follow this landing line. Not seeing runaway it is not possible to land without landing equipment signals. Terrestrial radio landing system forms a precise landing line. Aircraft ground equipment accepts signals and shows pilot deviation from landing line, or even automatically controls the plane that it do not deviate from it . The most widely used are precision ILS landing system operating in the VHF wave band. They consist of ground equipment (beacons), and aircraft equipment (receivers). The aircraft precision landing system ILS stops working close to the runway, so it is necessary to complete the touchdown visually monitoring the path and its light signaling systems. The minimum height to which the ILS works is called landing category. It depends on the airport and in the aircraft landing equipment. ILS landing systems are of varying complexity: the better system, the closer to the runway it ends operation. Close to the surface of the radio signals can not be used, as they induce currents in the ground that interfere with the bacon signal. Given the complexity of the ground landing system, aerodromes have landing categories of I, II, IIa, IIIb, IIIc. For example, ground-based ILS Category II beacon shape landing line just above 30 m to the ground and within 300 m from the start of the runway. Aircraft ILS equipment measures the deviation from the runway to the sides, but close to the ground there are no ILS signals. For example, the category II airport does not have landing signals when aircraft altitude is less than 30 m and closer 300 meters to runway. Then pilot must land visually.

In the cockpit deviations from the landing line are shown by aviahorizon or similar display of other device in vertical and horizontal scales, where tolerance is shown by points, for example, every 0.5°. Sliding scale index shows deviations from the landing line. For example , if the vertical scale of the index moved up two points , which means that the aircraft is 1 ° below landing line ( the pilot appears to fly above rather than below).

2.2. Other short range radio navigation equipment

Radijo altimeters

Radio altimeters send radio signals to the ground and receive reflected signal. Since radio wave propagation speed is constant, aircraft altitude is proportional to measured time difference between sent and received signals. Radio altimeters measure the actual (absolute) height from the ground to the aircraft. Passenger aircrafts use radio altimeters that measure low altitudes (up to 2,500 feet). Radio signal with gradually changing frequency is sent to the ground. The absolute altitude is proportional to the difference of emitted and received signal frequency.

Flying with maintaining a constant altitude by radio altimeter is uncomfortable, especially in hilly terrain. Aircraft trajectory must then comply with the ground profile (hills and gullies). Pilots do not fly this way. Radio altimeters are the most necessary during airplane landing on a runway, as the altitude is measured with ± 0.5 m accuracy. Helicopters can hang with the precision only using the radio altimeter.

Larger airplanes and helicopters have radio altimeters.

Doppler meters

Speed is measured by Doppler speedometers. Their operating principle is based on radiolocation: speed is measured by the difference between the frequency of signal, sent from the aircraft to the ground and same signal, reflected from the surface. Speed is proportional to frequency change. The speed also can be found by the processing of signals received from the satellites based on distance change. Relative speed of the aircraft and land-based beacon can also be determined by change of the distance between them.

Meteorological locators

Aircraft flight safety may be affected by the storm clouds along the way. Contact with storm can cause distracting from the course and aircraft icing, and then one can fly only by instruments. Air turbulence can damage aircraft control surfaces, it is uncomfortable for passengers. Occurring in the aircraft way dangerous meteorological conditions can be detected meteorological locator and shown on cockpit device. Then the pilot can decide whether or not to fly through them, or change the route bypassing dangerous area. This is the main purpose of the locator. The locator can also look to the ground and see large objects in front of the airplane- larger lakes and rivers, the coastline, larger cities. Some locators also can measure drift.
Aircraft radio locator antenna, which is in front of the aircraft, emits radio waves ahead of the plane. Antenna directional diagram is very narrow. If on the way of radio waves are rain clouds, a part of the pulse energy is reflected back of the antenna. The distance to the cloud can be found by measuring the time between sent and returned pulse. In uniform pulse propagation speed, the distance to the cloud is always proportional to half this time. Measuring the position of the antenna, where it receive reflected signal, direction of the cloud to the aircraft longitudinal axis is set.

Landing radio signals

ILS is a complex and expensive landing system; they are installed in larger aerodromes. In a runway approach, few kilometers away, one or several ground-based radio beacons may be installed. Their arrangement coincides with the runway center line. All beacons emit 75 MHz radio waves with narrow directional chart. An aircraft has beacons signals receiver. The cockpit has no complex controls, just few lights of different colors. Audio signals are audible on the headphone. When aircraft flies over the beacon, lights flashes and beacon signal is heard in headphones. The distance from the runway to the airfield beacon is known by the airport documents. Then pilot know what is the distance to the runway, and looking at the altimeter indicate is aircraft deviated from landing line. If the beacon signal at the expected is not heard, it means that the aircraft is highly deviated from the runway, further descending is dangerous and it is necessary to make a circle and try once more.

2.3. Remote radio navigation

Long-distance radio navigation instruments determine position of the aircraft by measuring the distance from a few terrestrial transmitters in the aircraft. There is only one long-distance radio navigation tool – Loran C”. Aircraft location initially is determined only approximately by measuring the travel time of radio pulses from several transmitters to the aircraft. Then radio signal phases are compared and distance is adjusted (see Fig. 9). Approximate measurement method by radio pulse time allows avoiding measurement ambiguity. Such systems are called phase-impulse.
Transmitters of radio navigation system „Loran –C“ are in different parts of the globe. Several ground-based transmitters operate coherently with each other, they are called chain. One transmitter in the chain is the main (master), and the other – slaves. In the chain there are usually two or three transmitters. All the coordinated chain transmitters operate at a 100 kHz frequency.
The distance between the main and slave beacons is large, about 960-1300 km. The straight line connecting the two land-based beacons is base line, and the distance between the beacons is called base.
Beacons operation is synchronized by the atomic clock. The system is based on well-known phenomenon that the radio wave propagation speed is 300 000 kilometers per second. By measuring the time between the signals one send from the main and slaves beacons, the aircraft calculates distances to the beacons and fixes location of the aircraft. Successive measurements help determine the direction of flight , speed, azimuth and other aircraft navigation data , especially if the „Loran –C“ operates in conjunction with the aircraft’s navigation system or satellite navigation aids .