Introduction to: Radio Navigational Aids

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1 Introduction to: Radio Navigational Aids 1

2 Lecture Topics Basic Principles Radio Directional Finding (RDF) Radio Beacons Distance Measuring Equipment (DME) Instrument Landing System (ILS) Microwave Landing System (MLS) Hyperbolic Navigational System 2

3 Defining Navigation Navigation is the determination of the Position and Velocity of a Moving Vehicle on land, at sea, in the air, or space. 3

4 What are Radio Navigational Aids? Specialised Electronic and Communication Systems used for navigational purposes. Long-range systems employ LF and HF bands. Short-range systems employ VHF and UHF bands. Microwave frequencies are used for long-range and short-range systems. 4

5 Basic Principles of Radio Navigational Aides Measuring the distance to a known location by measuring the round trip time of the Electromagnetic Waves. Utilizing the directional radiation pattern of antennas to find the direction to a known location. Antenna radiation patterns are of a prime interest in the field of radio navigation aids. 5

6 Triangulation Systems X B A A and B are landmarks of known coordinates 6

7 Line of Position (LOP) 90 o 90 o 135 o 45 o 45 o 135 o 180 o A B 180 o 225 o 315 o 315 o 225 o 270 o 270 o 7

8 Bearings Measurement North A X B A B Heading B base-line A 8

9 Radio Direction Finding RDF equipment are used on board of aircraft, ships, and other moving platforms. RDF systems employ a radio receiver with a rotatable Loop antenna. Loop antennas have a special radiation pattern which is suited to the RDF function. 9

10 Loop Antenna one or more loops of wire connection terminals base The loop can be circular or square. 10

11 Loop Antenna Radiation Pattern Loop Top View of Loop Antenna Radiation Pattern 11

12 Loop Antenna Orientation Tx Tx vertically polarised radio field Direction of Propagation Top View of Loop Antenna 12

13 Maximum Induced Voltage vertically polarised radio field Direction of Propagation Tx Tx Top View of Loop Antenna Magnetic component 13

14 Minimum Induced Voltage vertically polarised radio field Direction of Propagation Tx Tx Top View of Loop Antenna Magnetic component 14

15 Simple Direction Finder Bearing is found by searching for the minimum signal. Why? Rx Rotating Loop Antenna Field Strength meter 15

16 Ambiguity of Simple RDF Tx? Tx? Top View of Loop Antenna Radiation Pattern 16

17 Addition of Sense Antenna Loop Antenna Omnidirectional Antenna (Sense Antenna) Voltage sum of Loop and Sense antennas 17

18 Cardioid Antenna Pattern Loop Antenna v is proportional to 1 +cos 18

19 Automatic Direction Finder (ADF) Loop Sense Antenna Synchro Gen. Amp /2 Rx Motor IF Amp Detector Indicator IF Amp Detector 19

20 Radio Beacons A Radio Beacon (RB) is a transmitter used with RDF (300 khz - 3 MHz). Technically, RDF can be used to take bearing of any transmitter operating at the appropriate radio frequency. Non-directional Beacons (NDB) are used specifically with RDF. NDB transmit CW interrupted at regular interval by identification Morse code. 20

21 Simple Radio Range System Tx /2 21

22 Radio Range Antenna Pattern Morse Code: A : N : NW CW A North CW NE CW = A + N West N N East CW A CW SW South SE 22

23 VHF Omnidirectional Range (VOR) VHF Omnidirectional Range (VOR) provides Azimuth reading at the aircraft. No direction finding antenna required at the aircraft position. VOR Beacon, transmits continuous signals on one of 20 assigned channels in the MHz band. Limited range to line-of-sight, about 100 km. Several VORs are needed to cover long distance over land. 23

24 Principle of Operation Antenna with Cardioid pattern, rotating electrically at the rate of 30 Hz (Goniometer). Bearing information is transmitted in the phase of the signal. A separate 30 Hz reference signal is transmitted by an Omnidirectional antenna. Bearings are calculated from the phase information received at the aircraft. 24

25 Cardioid Antenna Pattern 315 o 0 o 360 o 45 o 270 o 90 o 225 o 180 o 135 o 25

26 VOR Antenna Pattern 30 revolutions per second North X 26

27 The signal at x is proportional to 1 + cos The signal at X is effectively AM modulated by 30 Hz North X 30 revolutions per second 27

28 Bearing Calculation The amplitude of the signal at x is proportional to 1 + cos where is the bearing of x relative to North. The Cardioid pattern is rotated anticlockwise at the rate of 30 revolution s per second. The amplitude of the signal at x is then proportional to: 1 + cos (2 30 t + The phase difference = cos (2 30 t + cos (2 30 t 28

29 Distance Measuring Equipment 29

30 Introduction Distance Measuring Equipment (DME) is used to measure the distance between an aircraft and a fixed point. Generally the fixed point is a VOR or a destination point as an airport. The useful range of DME systems is about 180 km, corresponding to a roundtrip delay of 1200 m sec. 30

31 System Architecture Airborne equipment (Interrogator): Transmitter Receiver Ground equipment (Transponder) located at the fixed point. 31

32 Basic Principle MHz (126 channels) Tx Rx Interrogation Reply Transponder MHz or MHz 32

33 Operation The transmitter sends pseudo-random coded interrogation pulses The transponder responds by sending reply pulses using the same pseudo-random pattern. The onboard receiver intercepts the reply pulses that conforms to its pseudo-random pattern. The time difference between sending the pulses, transponder processing delay, and receiving the pulses is used in calculating the distance. 33

34 Transponder Access Method The system employs a random access method. Up to 50 aircrafts may use the same transponder. The interrogation pulses are pseudo-random coded. The receiver only accepts replies that conform to the same pseudo-random pattern. 34

35 Instrument Landing System 35

36 Introduction Instrument Landing Systems (ILS) are designed to guide an aircraft in its final approach and landing. Three distinct subsystems are used: Localiser, Glide Slope, and Markers. 36

37 Localiser Plan Runway 5 o 90 Hz 150 Hz (top view) Localiser Tx ( MHz) DIRECTION OF APPROACH 37

38 Localiser Aids the pilot in lining up his/her aircraft in the proper azimuth approach to the runway. Consists of a group of transmitters and antennas positioned at the far end of the runway. The antenna radiation pattern has a 5 o beawidth, centred along the runway. The VHF frequencies used for the localise are in the range to MHz. The useful range of the system is about 40 km. 38

39 Glide Slope Plan Runway 5 o 90 Hz Tx Localiser Tx 150 Hz (top view) ( MHz) 1 o 90 Hz 150 Hz 2.5 o o ( MHz) Tx DIRECTION OF APPROACH (side view) 39

40 Glide Slope Aids the pilot in making his/her approach at the proper elevation angle to the runway. Consists of a group of transmitters and antennas positioned beside the runway. The antenna radiation pattern has a 1 o beawidth, and elevated about 2.5 o to 2.75 o in the direction of approach. The VHF frequencies used for the glide slope are in the range to MHz. The useful range of the system is about 40 km. 40

41 Frequency Arrangement 90 Hz 150 Hz 90 Hz 150 Hz 5 o 1 o 41

42 Markers Plan 75 MHz narrow vertical radio beam Marker Tx Marker Tx Runway 400 Hz Carrier 75 MHz 1300 Hz (top view) 1 km 6-10 km DIRECTION OF APPROACH 42

43 Markers Markers are transmitters that radiate continuous narrow vertical radio beams. The carrier frequency is 75 MHz modulated by special tones. The first transmitter is modulated by a 400 Hz tone, located at 6-10 km from the end of the runway. The second transmitter is modulated by a 1300 Hz tone, located at 1 km from the end of the runway. 43

44 Microwave Landing System 44

45 Introduction Microwave Landing System (MLS) was designed to handle the increase in air traffic volume and to satisfy the demand for all-weather landing facilities. Employs microwave frequencies ( GHz band) rather than VHF. MLS provides better accuracy, ease of application, and automation. enables landing down to zero ground visibility. ICAO-approved replacement for the ILS system. 45

46 Architecture MLS consists of three subsystems: Localiser, Glide Slope, and Flare. The Localiser and Glide Slope subsystems serve the same purpose as in ILS. The Flare provides information on the actual height of the aircraft above the plane of the runway. 46

47 Aircraft Height Measurement h = d tan h Flare Tx Runway (end view) d 47

48 Height Calculation The angle q made by the aircraft and the runway at the point where the flare transmitter is situated is measured. The distance d between the flare transmitter and the runway is known. The height is calculated using the equation: h = d tan 48

49 Time Reference Scanning Beam A Runway Localiser Tx (top view) B DIRECTION OF APPROACH 49

50 Operation The system is based on Time-Reference Scanning Beams (TRSB), referenced to the runway, that enables an aircraft to determine precise azimuth and elevation angles. The angular position is made by measurement of the time intervals between the TO and FRO azimuth antenna beam scan (typically +/- 60o) and UP and DOWN scan (typically 0o - 30o) of the elevation antenna pattern. 50

51 Calculation The angles (or are calculated using the equation: 1 t T t is the time difference between pulses and T is the scanning period from A to B and back to A 51

52 Hyperbolic Navigation Systems 52

53 Introduction Hyperbolic systems are based on measuring the difference in distance between a craft and two known sites. Fixed distance difference between the craft and the two sites lie on a hyperbolic locus that passes between the sites. Definition of terms: base-line base-line extension line of position (LOP) centre-line (CL) 53

54 Basic Concept CL X base-line extension A base-line B LOP 54

55 Hyperbolic Navigation D A B 55

56 Decca Concept Decca was the first of several hyperbolic electronic navigation systems developed during and after World War II. It uses radio waves as a means of measuring the difference in distance between sites. Master/slave radio transmitters arrangement. Calculating the difference in distance based on phase difference measurement. The range difference is given by: d= 360 o 56

57 Decca Lines 360 o 0 o or 0 o 720 o or 0 o A B 57

58 A Master and two Slaves A B1 B2 58

59 Decca System To overcome ambiguities and increase accuracy. Three bas-lines system. A set of four transmitters, a master and three slaves, referred to as a chain. The base lines identified as Red, Green, and Purple. The system reading is displayed on Red, Green, and Purple Decometers respectively. 59

60 Decca Station Topology B 3 purple B 1 A red green B 2 60

61 Basic System In phase Master Tx 340 khz Tx 340 khz Slave Red 340 khz Q 340 khz Onboard receiving equipment Red Decometer 61

62 Possible Working System In phase Master Slave Tx 340 khz X 1/ X 1/3 Red Tx 340 khz Q X 4 X Onboard receiving equipment Red Decometer 62

63 Practical 85 khz Scheme Master Tx 85 khz 85 In phase Slave X 1/ X Q X 4 X Onboard receiving equipment Red Decometer 63

64 Decca Communication Receiver Multipliers In Out Purple Slave 5f 5f 30f Decometers 30f 30f Purple Master 6f 6f 18f 24f 18f Green Green Slave 9f 9f 18f Red Slave 8f 8f 24f 24f Red 64

65 Thank you for your attention 65

EE Chapter 14 Communication and Navigation Systems

EE Chapter 14 Communication and Navigation Systems EE 2145230 Chapter 14 Communication and Navigation Systems Two way radio communication with air traffic controllers and tower operators is necessary. Aviation electronics or avionics: Avionic systems cover