Aerospace Systems Radio Navigation

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  • Radio Navigation
  • Navigation is the determination of position and direction (heading) on or above the surface of the Earth
  • Radio navigation is based on the use of electromagnetic fields (radio waves) to determine a position on the Earth
  • Methods can be divided in to ‘beam based’ methods and ‘time of arrival’ methods
  • Beam based methods work by measuring changes in amplitude or phase of continuous signal. Beam based methods require extensive ground-based transmitter networks
  • Time of arrival methods work by accurately measuring the time of arrival of discrete pulsed signals, e.g. GPS
  • Time of arrival methods can use satellites as the signal source. This means that height as well as position can be obtained. Space based assets can also be used to get a fix anywhere on the planet

  • Beam based navigation - examples

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      • RDF (radio direction finder). A radio station produces a radio signal, a directional antenna on the aircraft is then moved until the strongest signal is received. First developed in the 1920s
      • Lorenz (guiding beam). Two signals of the same frequency are broadcasted in the same direction from two slightly spaced, highly directional antennas. Aircraft know that they are in the centre of the beam when the signal strength from both beams is equal. Developed in the 1930s. Used to guide German bombers, however can give away the location of the bombers
      • VOR (VHF Omnidirectional Ranging). Uses two signals of varying phase: a unidirectional master signal of constant phase, and a highly directional signal of varying phase. The antenna for the second signal spins and the phase is varied such that the angle of rotation is proportional to the phase. Thus by measuring the phase of the directional signal with the omnidirectional signal, the direction of the transmitter can be determined. Developed in the 1940s. Standard method of navigation for general and civil aviation today, but depends on the existence of ground infrastructure (not suitable for military aircraft). Not accurate for automatic landing systems - only accurate for one miles, therefore not ideal for bad weather or landing

      • Instrument landing systems

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          • An instrument landing system (ILS) is an instrument approach system which provides precise guidance to an aircraft approaching a runway. First used in basic form in the 1930s.
          • A modern day ILS consists of two independent sub-systems, one providing lateral guidance (Localizer), the other providing vertical guidance (Glideslope) to aircraft approaching runway
          • A localizer antenna array is located beyond the departure end of the runway and consists of several pairs of directional antennas. Two signals modulated at 90Hz and 150Hz are transmitted from separate but co-located antennas. Each antenna transmits a narrow…

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