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Avionics and Aircraft Electrical Systems

Chapter 3 Navigation Systems. Avionics and Aircraft Electrical Systems. Navigation Systems. Learning Objectives. Understand the different Radio Navigation Aids and t heir principles of operation. Understand the principle of operation of Inertial Navigation System (INS).

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Avionics and Aircraft Electrical Systems

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  1. Chapter 3 Navigation Systems • Avionics and Aircraft • Electrical Systems

  2. Navigation Systems Learning Objectives Understand the different Radio Navigation Aids and their principles of operation. Understand the principle of operation of Inertial Navigation System (INS). Understand the principle of operation of the Global Positioning System (GPS). Understand aircraft radar systems and their principles of operation.

  3. Navigation Systems Since the early days of flight navigation has improved immensely. The first flights were done using road maps, but now we have progressed to GPS and INS. To start we will look at the various radio navigation aids. Automatic Direction Finding – ADF(NDB). Omnidirectional Beacon –VOR. Tactical Air Navigation System –TACAN. Instrument Landing System –ILS.

  4. Navigation Systems ADF(NDB) A non-directional (radio) beacon (NDB) is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies the signal transmitted does not include  inherent directional information, The signal is in Morse Code incorporating the station's identifier which is used to confirm the station and its operational status. NDBs operate on a frequency between190 kHz and 1750 kHz The aircraft equipment that receives the radio signal is an ADF.

  5. Navigation Systems An automatic direction finder (ADF) is an aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the aircraft to the transmitter. ADF receivers are normally tuned to aviation NDBs operating in the LW band between 190 – 535 kHz. Most ADF receivers can also receive medium wave (AM) broadcast stations, although these are less reliable for navigational purposes.

  6. Navigation Systems The operator tunes the ADF receiver to the correct frequency and verifies the identity of the beacon by listening to the  morsecode signal transmitted by the NDB

  7. Navigation Systems ADFs contain a small array of fixed aerials which use electronic sensors to deduce the direction using the strength and phase  of the signals from each aerial. In flight, the ADF's RMI(Radio Magnetic Indicator) or direction indicator will always point to the broadcast station regardless of aircraft heading. However a banked attitude can have a slight effect on the reading. The needle will still generally indicate towards the beacon, however it suffers from DIP error where the needle dips down in the direction of the turn.

  8. Navigation Systems The red needle is the ADF pointer and will always point directly to the station in relation to the nose of the aircraft. The compass card is driven by the compass system and the bearing that the station is from the aircraft can be read off the pointer i.e. 021º. A compass card is not essential as the aircraft can be flown in the direction of the needle. Depending on the power of the transmitter, the signal can be received up to 300nm for navigational NDBs or for an airfield NDB it could be as little as 35nm.

  9. Navigation Systems Aerials The NDB aerial is vertically polarised and has the flat array on top to improve it’s radiating efficiency.

  10. Navigation Systems VOR A VOR is a type of short-range radio navigation system for  aircraft, enabling aircraft to determine their position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beaconsusing a receiver unit. It operates in the VHFband from 108 to 117.95 MHz. A VOR ground station sends out a master signal, and a highly directional second signal that varies in phase 30 times a second compared to the master. By comparing the phase of the secondary signal to the master, the angle (bearing) to the station can be determined. The VOR has a range of about 200 nm.

  11. Navigation Systems A VOR Ground Station Each ground station sends an identifying signal in Morse Code. The VOR signal is displayed on an instrument on the aircraft which requires a compass input to display the correct data. Red needle No 1 VOR (Bearing 031º) Green needle No 2 VOR (Bearing 012º) The frequency is selected on a control unit.

  12. 1 1 0 Navigation Systems The VOR information can also be displayed on the HSI to show lateral displacement. 335 N MILES COURSE Required bearing to VOR Required course pointer GS Course deviation bar. To/From arrow pointing to the VOR. Shows aircraft is left of required course.

  13. Navigation Systems TACAN Is a navigation system used by military aircraft. It provides the user with bearing and distance (slant-range) to a ground or ship-borne station (aircraft carrier). The bearing unit of TACAN works on the same principle as the VOR but is more accurate since it makes use of a two frequency principle, with 15 Hz and 135 Hz components. It operates in the frequency band 960-1215 MHz. The distance system is a transponder-based radio navigation technology that measures slant range distance by timing the  propagation delay of VHF radio signals. The aircraft sends and receives a pulsed pairs signal– two pulses of fixed duration and separation. It’s range can be up to 240nm.

  14. Navigation Systems A radio signal takes approximately 12.36 microseconds to travel 1 nautical mile to the target and back. The time difference between interrogation and reply is measured by the interrogator's timing circuitry and converted to a distance measurement (slant range), in nautical miles, then displayed in the cockpit. TACANantenna

  15. Navigation Systems The frequency is selected on a control box and the distance can be displayed on a separate digital display or on the HSI. The bearing is displayed on needles on a compass card repeater. To display correctly the TACAN needs a compass feed.

  16. Navigation Systems VOR/DME This is the civilian version of the TACAN. The distance part of a TACAN is co-located with a VOR and works on a paired frequency, so when the VOR is selected the DME is automatically selected. A VOR/DME ground station.

  17. ILS is a runway approach aid: Navigation Systems Instrument Landing System (ILS) ILS is used to guide the aircraft to the runway threshold in poor visibility. Fixed transmitters on the ground send out a special pattern of radio signals These define a radio beam that is like a pathway in the sky The pathway then leads to the touch-down point on the runway

  18. Navigation Systems ILS Glidepath ILS transmits 2 frequencies 90Hz signal is strongest, so aircraft is above the Glidepath 90Hz Glidepath Transmitter Localizer Transmitter Glidepath 150Hz Runway

  19. Navigation Systems ILS Glidepath ILS transmits 2 frequencies 150Hz signal is strongest, so aircraft is below the Glidepath 90Hz Glidepath Transmitter Localizer Transmitter Glidepath 150Hz Runway

  20. Navigation Systems ILS Glidepath ILS transmits 2 frequencies Both signals are equal, so aircraft is on the Glidepath 90Hz Glidepath Transmitter Localizer Transmitter Glidepath 150Hz Runway

  21. Navigation Systems ILS Localizer ILS transmits 2 frequencies 90Hz signal is strongest, so aircraft is right of the Localizer Localizer Transmitter 90Hz Runway Runway Centre Line 150Hz Glideslope Transmitter

  22. Navigation Systems ILS Localizer ILS transmits 2 frequencies 150Hz signal is strongest, so aircraft is left of the Localizer Localizer Transmitter 90Hz Runway Runway Centre Line 150Hz Glideslope Transmitter

  23. Navigation Systems ILS Localizer ILS transmits 2 frequencies Both signals are equal, so aircraft is on the Localizer Localizer Transmitter 90Hz Runway Runway Centre Line 150Hz Glideslope Transmitter

  24. Navigation Systems ILS Localizer Transmitter 90Hz Glidepath Transmitter Localizer Transmitter 90Hz Glidepath Runway Runway Centre Line 150Hz 150Hz Runway Glideslope Transmitter

  25. Navigation Systems INS INS is based on measuring acceleration in all planes and calculating distance and speed from the acceleration. To initiate the INS system the unit must be powered up from a completely stationary position. It also needs a very accurate present position entered. This information can be found on parking area TAPs or from an en-route supplement. The accelerometers are mounted on three axis, Vertical, North facing and East facing. When the unit is powered up it runs through a set up procedure which aligns these axis, by measuring the earths rate of rotation.

  26. Navigation Systems INS When the unit is aligned only the East axis will measure any movement as the earth rotates. It normally takes 8 mins to align the unit but the longer it is left aligning the more accurate it becomes. Once aligned it is selected into Navigation Mode and the aircraft can now move. From this point onwards the unit works out how far it has moved in any direction and plots a new position in three dimensions. To be able to give full Navigation information it also needs a TAS feed to work out ETAs and wind.

  27. Navigation Systems INS Due to unit drift error rates of 3nm/hr are normal. The replacement of mechanical accelerometers by Ring Laser Systems has reduced this to below 1nm/hr. This system measures the bend of a laser beam and equates the amount of bend to an acceleration.

  28. Navigation Systems GPS The GPS system is based on radio signals from satellites in space which then give a 3 dimensional fix to the receiver unit. The unit can be initiated at any time, even while flying, and requires 4 satellite signals to operate fully and accurately. It can operate with 3 satellites but accuracy is degraded. To initiate quickly it requires an approximate position entered but can initiate by itself. This will take a lot longer time. There are 24 satellites positioned in orbit around the earth and they are positioned to ensure that at any place on earth 4 or more satellites can be received. Each satellite can be identified by it’s individual radio signal which is coded in time pulses.

  29. Satellite 2 Satellite 1 Satellite 3 3 1 2 2 1 3 Navigation Systems Each satellite has an atomic clock which is synchronised with each other to ensure best accuracy for the system.

  30. Navigation Systems GPS A stand alone GPS is a present position system only and to calculate all navigational information the GPS requires a TAS and compass feed. GPS accuracy is now down to 1 or 2 metres and can be used for precision approaches to approved airfields. The system was initially administered by the US Air Force, but in the mid 2000s it was handed to civilian administration. When run by the military accuracy was kept to a lesser level for non-military users and only friendly allies given the ‘P’ codes to allow the GPS to operate to full accuracy. This practice was stopped when it became civilian.

  31. Navigation Systems Flight Management System (FMS) In modern nav systems a GPS/INS mixed together is used and gains the best parts of each system to provide a very accurate navigational system. Control and Display Unit The FMS can be tied into the navigational displays and coupled to the auto-pilot, so that the whole flight can be programmed while on the ground before engine start. Most FMS systems can be programmed on a computer in Flight Planning and taken to the aircraft in a loading unit and loaded in seconds on arrival at the aircraft.

  32. Navigation Systems Weather Radar This is an aircraft mounted radar system which can detect cloud and thunderstorms and can also be used for ground mapping. The returns come from the water droplets in the cloud. The more water content the larger the return. Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Weather radars send directional pulses of microwave  radiation, in the order of a microsecond long, using a  cavity magnetron or klystron tube connected by a  waveguide to a parabolic antenna.

  33. Navigation Systems Weather Radar The transmitted radar beam is like a torch beam. The antenna sweeps from side to side over a 160º arc. This produces a continuously updating picture of the weather. The elapse rate of the display can be adjusted to give either a constant picture or a rapidly fading picture.

  34. Navigation Systems Weather Radar How the scan works

  35. Navigation Systems Weather Radar The beam scan is normally automatically stabilized to ensure the beam scans horizontally to the earth’s surface. This is done by feeding signals from an INS or Vertical Gyro to keep the antenna level no matter the attitude of the aircraft. The antenna has a tilt mechanism which allows the operator to move the beam up or down, usually to a maximum of 15º. This can be useful when determining the top or bottom of a cloud structure. If the tilt is increased or decreased the return will disappear from the screen. If the range at which this happens is multiplied by the tilt angle and then by 100, the result is the height above or below your present altitude. i.e. 40nm x 2º x100 = 8,000ft

  36. Navigation Systems Weather Radar Weather returns show as colours starting at light green for light rain, working through yellow and red to purple for thunderstorms. The weather picture can be displayed on a purpose designed scope or under-layed on the navigation display of a flat screen flight instrument system.

  37. Navigation Systems Weather Radar Because the water droplets in a thunderstorm absorb and scatter some of the radar pulse the area behind a storm does not always appear on the screen. This shows as a dark shadow behind the bright thunderstorm.

  38. Navigation Systems Weather Radar In the ground mapping mode the radar beam is turned through 90º. This gives a better ground mapping picture.

  39. Navigation Systems Secondary Surveillance Radar (IFF/SSR) SSR is a radar system used in air traffic control, that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude The ground station interrogates the aircraft and the aircraft transponder replies. The transponder can reply in 4 different modes which give various information to the ground station.

  40. Navigation Systems Mode 1 – Identifies the role of the aircraft. Mode 2 – Is only used on operations and identifies a specific airframe. This code would normally change every 6 hrs. Mode 3 – Identifies the specific flight and the code is allocated by ATC. Mode C – Identifies the aircraft altitude to the nearest 100ft.

  41. Navigation Systems Successor IFF (SIFF) SIFF is an upgraded IFF system which incorporates modes ‘S’ and 4. With the introduction of Digital Air Data Units more information is available to be used electronically. All aircraft will provide Mode S basic DAPs. The following parameters are downlinked to the interrogating system: DATA Input • (a) Flight Identification - flight crew. • (b) Transponder Capability - automatic. • (c) Altitude – Digital Altitude Unit (DAU). • (d) Flight Status - automatic.

  42. Navigation Systems Successor IFF (SIFF) Mode S Level 2 Enhanced Surveillance Platforms that require frequent access to international civil Air Traffic Management (ATM) systems are required to conform to the requirements for enhanced surveillance. In these cases, the following parameters are down-linked to the interrogating system in addition to the basic parameters: • (a) Magnetic heading. • (f) True track angle. • (g) Track angle rate. • (b) True airspeed. • (h) Ground speed. • (c) Indicated airspeed. (i) Vertical rate. • (d) Mach number. • (e) Roll angle.

  43. Navigation Systems Successor IFF (SIFF) Mode Selectors

  44. Navigation Systems Successor IFF (SIFF) Mode 4 is a military only system which automatically identifies an aircraft as part of an operation. The identification is encrypted and automatically changes the code to a pre-loaded pattern. This should ensure that no friendly fire incidents occur. Also the Mode 4 data can be programmed into Ground Defence Missile Systems (Patriot) which will disable lock on to a target squawking Mode 4. Mode 5 is pre-positioned to facilitate a planned upgrade.

  45. Navigation Systems Traffic Collision and Avoidance System (TCAS) This system can only be used if SIFF has been installed. It uses the information in Mode S to anticipate possible collision courses between aircraft and gives commands to keep them apart. The information can be displayed on an electronic VSI or over-layed on a flat screen display. Each aircrafts transponder interrogates the others to gain the required information. The corrective command is programmed to ensure the aircraft carry out different manoeuvres to keep them apart.

  46. 15NM TCAS II 40NM + 10,000 ft TCAS II - 10,000 ft TCAS II Navigation Systems TCAS • TCAS communicates with transponder equipped aircraft • that are within its Surveillance Volume. 46

  47. 40NM 25NM 25NM 15NM Navigation Systems TCAS RNG 1 Surveillance Volume 47

  48. Navigation Systems TCAS The TCAS system uses two types of display to present information to the aircrew. Traffic Advisory display (TA) This is given when an aircraft is within certain parameters of your aircraft but is not deemed a threat yet. There are various levels of Traffic Advisory which increase as the intruder gets closer. Resolution Advisorydisplay (RA) This is given when an aircraft is deemed a threat to your aircraft. The time to closest point of approach with the intruder is now between 15 and 35 seconds.

  49. +13 BRT Navigation Systems TCAS Non Threat Traffic: Range beyond 6 NM and altitude is greater than +/- 1200 feet from your aircraft. It is not yet considered a threat. 2 1 RNG 10 Traffic is +1300ft climbing. 4 UP .5 0 6 DN 4 .5 2 1 49

  50. -05 BRT Navigation Systems TCAS Proximity Traffic: Range within 6 NM and altitude is within +/- 1200 feet of your aircraft. Still not yet considered a threat. 2 1 RNG 10 Traffic is – 500ft descending. 4 UP .5 0 6 DN 4 .5 2 1 50

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