NAV - DME & VOR Flashcards
What is DME? (5)
- Accurate, slant range indicator that requires dedicated equipment both on the ground and in the aircraft
- 2 frequency operation - avoids interrogating each other
- Interrogator recognizes own original pulse train and calculates time taken from transmission to reception
- Increased working range due to two transmitters
- Operates in UHF waveband
DME diagram
How accurate are DME? (3)
- Within 1nm
- Can accept up to 100 A/C simultaneously
- Subject to Slant error - Closer the beacon, less accurate the readings
What uses do DME include? (5)
- Co-located VOR/DME stations provide range and bearing
- Provides positive ranges for aircraft flying same track and interrogating same DME (ATC separation)
- Co-location with precision approach aid (Accurate range) from touchdown
- Enhances the accuracy of holding patterns Suitable computer + DME (Accurate area navigation)
- Military use for air-to-air refuelling
What is VOR? (6)
VOR - VHF Omni-directional Radio range
- Automatically and continuously provides an aircraft with an accurate magnetic bearing from a beacon
- Exploits radio wave characteristics
- VHF band: 108 - 117.95 Mhz
- Identifies with 3 letter morse code / speech
- “line of sight”: operating range depends on aircraft height
What 2 signals do VOR use? (2)
- Reference Signal - transmitted Omni-directionally
- Variable Signal - transmitted by a circular array of aerials around the Reference Signal antenna, simulating a revolving antenna
How are the 2 signals transmitted on a VOR? (1)
Transmitted simultaneously but the simulated rotation causes a phase difference which varies with change of bearing
How does a VOR not transmit? (2)
- Does not transmit a usable signal vertically upwards
- The period during which the aircraft receives no signal increases with height
What are the characteristics of different VOR antenna systems? (5)
- Circle of spherical aerials, surrounded by larger circle of horizontal reflectors (25m diameter)
- Radial information transmitted by FM
- Fed to antenna aerial that phase angle changes radially through 360°
- Permits installation in difficult terrain but is expensive to equip and install
- Radials always extend outwards from the VOR
How many spot frequencies does a VHF receiver operate? (1)
180
How is the VOR displayed in a cockpit? (2)
- Magnetic Bearing (QDM) by an arrow at the end of a VOR pointer
- QDR shown at opposite end
What is OBS? (1)
OBS - Omni-bearing Selector
What are the 2 Forms of OBS? (2)
- Earlier Form
- Later Form
What does an Earlier Form OBS display? (3)
- Displays only VOR information
- Shows deflection left or right of pre-selected QDM
- Shows whether the aircraft is generally heading to or from the beacon
What does an Later Form OBS display? (2)
- Shows all the same information as earlier form, but for the purposes of the Instrument Landing System as well as VOR
- Includes a Fail Flag when no signal is being received
OBS diagram
OBS on a QDM 270° example (3)
- Required QDM is 270° (which gives QDR of 090°)
- Needle is showing “fly left” to acquire that QDM
- Each dot on displacement scale indicates 2°
What type of guidance does a VOR provide? (1)
Lateral guidance
How is To / From indicated on an OBS? (2)
- The TO/FROM indicator tells the aircraft position in relation to the beacon
- It does not tell whether the aircraft is flying towards or away from the beacon
