VHF/HF Communication Systems PDF

Chapter 1 VHF/HF COMMUNICATION SYSTEMS Learning Outcomes  Airborne Very High Frequency Communication  Airborne High Frequency Communication  Concept of Selective Calling System (SELCAL)  Operation of Emergency Locator Transmitter (ELT) AVIONIC SYSTEMS VHF/HF COMMUNICATION SYSTEMS...

Chapter 1 VHF/HF COMMUNICATION SYSTEMS Learning Outcomes  Airborne Very High Frequency Communication  Airborne High Frequency Communication  Concept of Selective Calling System (SELCAL)  Operation of Emergency Locator Transmitter (ELT) AVIONIC SYSTEMS VHF/HF COMMUNICATION SYSTEMS 1.1 Introduction Communication in aviation is currently mainly achieved by voice modulation of radio waves. The future seems to lie in data transfer, which can be achieved without using the human voice. Nonetheless, voice communication is still important for safe movement of air traffic, and will remain so in many parts of the world for some time. 1.2 Very High Frequency (VHF) Communication Communication by VHF is essentially line of sight by direct wave; it provides short range air-to-air and air-to-ground voice communications (Figure 1-1). The primary use is for air traffic control in aeronautical communications band (118-136 MHz). The range available can be approximated by R = 1.25  Hr  Ht  Where R is in nautical miles, Hr and Ht are receiver and transmitter antenna heights in feet above Mean Sea Level (MSL). The factors affecting the range of VHF transmission are:  Transmission power both at aircraft and ground station  Height of the transmitter  Height of the receiver  Obstacles at the transmission site will block the signals A VHF communications system shown in Figure 1-2, consists of  Frequency control panel  Transceiver  Antenna ET0171/Chaganti Page 1-1 AVIONIC SYSTEMS VHF VHF ATC Figure 1-1 Very High Frequency Communications ET0171/Chaganti Page 1-2 AVIONIC SYSTEMS ANTENNA RF-COAX AUDIO TO FLIGHT INTERPHONE MIC INPUTS VHF TRANCEIVER AUDIO TO SELCAL FREQUENCY 136.00 FLIGHT RECORDER SYSTEM CONTROL PANEL INPUTS Figure 1-2 VHF System Communication ET0171 /Chaganti Page 1-3 AVIONIC SYSTEMS Frequency selection and status data will be provided by control panel, Flight interphone system provides Microphone (MIC) audio input for voice transmission. The received and transmitted radio frequency signals travel between transceiver and antenna on a RF-Coax line. De-modulated audio is guided to the flight interphone system for voice communications and to SELCAL for signalling purpose. Flight recorder system receives analog pulse to signify that the transmission has acknowledged. In certain circumstances the aircraft may receive both direct and reflected waves which may cause fading or even short-term loss of voice communication. Example 1-1: At what maximum range would a VHF signal transmitted from an aircraft flying at 12000 ft be received by another aircraft flying at 9000 ft?   Range 1.25 9000  12000  255 nm 1.3 High Frequency (HF) Communication Communication by HF is essentially by ground wave for short range and sky wave for long range, it provides long range air-to-air and air-to-ground voice communications (Figure 1-3). The primary use is for air traffic control in aeronautical communications band (2-22 MHz). For long range it depends on ionospheric reflection, which may take single or multiple bounces to cover the required distance. The distance between any two successive ground reflections is known as skip zone, useful for selecting critical operating frequency. ET0171/Chaganti Page 1-4 AVIONIC SYSTEMS The factors affecting the range of HF transmission are:  Transmission power both at aircraft and ground station  Time of day  Season of the year  Any disturbance in the ionosphere  Geographical location  Frequency in use A HF communications system shown in Figure 1-4, consists of  Frequency and Power control panel  Transceiver  Coupler (matching network for antenna) Frequency selection and power setting will be provided by control panel, Flight interphone system provides Microphone (MIC) audio input for voice transmission. The received and transmitted high frequency signals travel between transceiver and antenna with a proper matching (coupler) to save power. De-modulated audio is guided to the flight interphone system for voice communications and to SELCAL for signalling purpose. Flight recorder system receives analog pulse to signify that the transmission has been acknowledged. In certain circumstances the aircraft or ATC loss voice communication, reason being changes in ionosphere conditions. Sky wave communication is not reliable and needs to adjust frequency from time to time. AM radio system is most suitable for high frequency communication. ET0171 /Chaganti Page 1-5 AVIONIC SYSTEMS IONOSPHERE HF/SKY WAVE HF/GROUND WAVE SKIP ZONE HF/GROUND WAVE ATC ATC Figure 1-3 High Frequency Communications ET0171/Chaganti Page 1-6 AVIONIC SYSTEMS ANTENNA RF COUPLER AUDIO TO FLIGHT INTERPHONE MIC INPUTS HF TRANCEIVER AUDIO TO SELCAL POWER & FREQENCY 12.000 FLIGHT RECORDER SYSTEM CONTROL PANEL INPUTS Figure 1-4 HF System Communication ET0171 /Chaganti Page 1-7 AVIONIC SYSTEMS 1.4 Selective Calling System (SELCAL) Pilots on long haul flights used to have to listen to the radio all the time, waiting for their own call sign to alert them to a message for them. This was tiring, especially on HF frequencies with a lot of static as well as receiver noise. The SELCAL system (Figure 1-5) allows pilots to mute the receiver until ATC transmits a group of two pulses, each of 1±0.25 seconds separated by a period of 0.2±0.1 seconds. These pulses are designated ‘RED x’, where x is a letter corresponding to the audio frequency of the pulses transmitted as a modulation on the carrier frequency (Table 1-1). Each code is allocated to a specific aircraft listening on the frequency. Table 1-1: SELCAL Tone Allocation Designation Frequency (Hz) RED “A” 312.6 RED “B” 346.7 RED “C” 384.6 RED “D” 426.6 RED “E” 473.2 RED “F” 524.8 RED “G” 582.1 RED “H” 645.7 RED “J” 716.1 RED “K” 794.3 RED “L” 881.0 RED “M” 977.2 RED “P” 1083.9 RED “Q” 1202.3 RED “R” 1333.5 RED “S” 1479.1 ET0171/Chaganti Page 1-8 AVIONIC SYSTEMS IONOSPHERE HF VHF ATC ATC Figure 1-5 Selective Calling ET0171/Chaganti Page 1-9 AVIONIC SYSTEMS SELCAL communications system shown in Figure 1-6, consists of  Decoder  Tone Selector  Selcal Lamps/Switches This decoder consists of five channels, each VHF/HF transceiver used for voice communications delivers received audio to the decoder. When the relevant code is received, it activates an alarm in the cockpit, either a light or a bell or both, telling the crew to de-select the mute function and use normal communications. 1.5 Emergency Locator Transmitter (ELT) Emergency locator transmitters are radio devices which can be carried on to the aircraft. In case of emergency ELT’s are activated, its radio transmission aids in locating crash sites and survivals by rescue people. As an example ELT-RESCU 406 from Honeywell Aerospace is shown in Figure 1-7. RESCU 406 This Programmable compact transmitter consists of  Battery workable in water  Long erecting Antenna  Lanyard card  Plastic water bag This beacon transmits two kinds of rescue signals Civil Rescue 121.5 MHz Military Rescue 243.0 MHz ET0171/Chaganti Page 1-10 AVIONIC SYSTEMS ALERT AUDIO INPUTS TONE SIGNALS BURSTS SELCAL LAMP /SWITCHES RESET SELCAL DECODER PILOT CALL PANEL TRANCEIVER KEYING SELCAL FOUR_BIT ASSIGNED LETTER ALERT SIGNAL WARNING CODE CODES Figure 1-6 SELCAL System ET0171/Chaganti Page 1-11 AVIONIC SYSTEMS Examine the standard operational procedure of the beacon in water and on land Water Operation (Figure 1-8a) At the top of beacon, unroll lanyard card and tie it to a raft Put the beacon into water This beacon automatically operates and starts sending distress frequencies Land Operation (Figure 1-8b) Erect and position the antenna in right direction without any obstacles Pour liquid into container to activate water activated switch Liquid in the container can be fresh water or salt water Battery casing end of the beacon is inserted into liquid container Beacon is held properly, so that it transmits effectively Normal emission duration of ELT is 48 hours Example 1-2: An ELT is operating for 24 hours on Nickel-Cadmium battery of 240 mAh rating, calculate the current consumption to power the transmitter? ET0171/Chaganti Page 1-12 AVIONIC SYSTEMS LANYARD CORD TRANSMITTER PART ANTENNA BATTERY PART PLASTIC BAG Figure 1-7 ELT-RESCU 406 (Source: Honeywell Aerospace) ET0171/Chaganti Page 1-13 AVIONIC SYSTEMS FLIGHTLEVEL IN FEET 40000 ANTENNA 20000 ELT MAXIMUM LIQUID 5000 243 MHz (Military) MINIMUM LIQUID DISTANCE IN NM 250 175 100 LIQUID CONTAINER 121.5 MHz (Civil) Figure 1-8a ELT-Water Operation Figure 1-8b ELT-Land Operation ET0171 /Chaganti Page 1-14 AVIONIC SYSTEMS Notes ET0171/Chaganti Page 1-15