Area Navigation (RNAV)
Area navigation (RNAV) is a general term used to describe the navigation from point A to point B without direct overflight of navigational aids, such as VOR stations or ADF non-directional beacons. It includes VORTAC- and VOR/DME-based systems, as well as systems of RNAV based around LORAN, GPS, INS, and the FMS of transport category aircraft.
However, until recently, the term RNAV was most commonly used to describe the area navigation, or the process of direct flight from point A to point B using VORTAC- and VOR/DME-based references which are discussed in this section.
All RNAV systems make use of waypoints. A waypoint is a designated geographical location or point used for route definition or progress-reporting purposes.
It can be defined or described by using latitude/longitude grid coordinates or, in the case of VOR-based RNAV, described as a point on a VOR radial followed by the point’s distance from the VOR station (i.e., 200/25 means a point 25 nautical miles from the VOR station on the 200° radial).
Figure 1 illustrates an RNAV route of flight from airport A to airport B.
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| Figure 1. The pilot uses the aircraft’s course deviation indicator to fly to and from RNAV phantom waypoints created by computer. This allows direct routes to be created and flown rather than flying from VOR to VOR |
The VOR/DME and VORTAC stations shown are used to create phantom waypoints that are overflown instead of the actual stations. This allows a more direct route. The phantom waypoints are entered into the RNAV course-line computer (CLC) as a radial and distance number pair.
The computer creates the waypoints and causes the aircraft’s CDI to operate as though they are actual VOR stations. A mode switch allows the choice between standard VOR navigation and RNAV.
VOR-based RNAV uses the VOR receiver, antenna, and VOR display equipment, such as the CDI. The computer in the RNAV unit uses basic geometry and trigonometry calculations to produce heading, ground speed, and time readouts for each waypoint. VOR stations need to be within line-of-sight and operational range from the aircraft for RNAV use. [Figure 2]
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| Figure 2. RNAV unit from a general aviation aircraft |
RNAV has increased in flexibility with the development of GPS. Integration of GPS data into a planned VOR-based RNAV flight plan is possible as is GPS route planning without the use of any VOR stations.
Radio Altimeter
A radio altimeter, or radar altimeter, is used to measure the distance from the aircraft to the terrain directly beneath it. It is used primarily during instrument approach and low-level or night flight below 2,500 feet. The radio altimeter supplies the primary altitude information used to determine landing decision height.
It incorporates an adjustable altitude bug that creates a visual or aural warning to the pilot when the aircraft reaches that altitude. Typically, the pilot will abort a landing if the decision height is reached and the runway is not visible.
A radio altimeter uses a transceiver and a directional antenna to broadcast a frequency-modulated continuous wave (FMCW) signal at a center frequency of 4.3 GHz directly toward the ground. This signal travels at the speed of light, strikes surface features, and reflects back to a separate receiving antenna.
Because the transmitter continuously sweeps its frequency, typically across a 50 MHz band, the frequency of the return signal slightly lags behind the frequency currently being transmitted. The transceiver measures this frequency difference, which is directly proportional to the elapsed travel time of the signal and the aircraft’s height above the terrain. The resulting data is displayed as altitude above-ground level (AGL). [Figure 3]
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| Figure 3. A digital display radio altimeter (top), and the two antennas and transceiver for a radio/radar altimeter (bottom) |
A radar altimeter is more accurate and responsive than an air pressure altimeter for AGL information at low altitudes. The transceiver is usually located remotely from the indicator. Multifunctional and glass flight deck displays typically integrate decision height awareness from the radar altimeter as a digital number displayed on the screen with a bug, light, or color change used to indicate when that altitude is reached.
Large aircraft may incorporate radio altimeter information into a ground proximity warning system (GPWS) which provides aural alerts to the crew of potentially dangerous proximity to the terrain below the aircraft. A decision height (DH) window displays the radar altitude on the EADI in Figure 4.
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| Figure 4. The decision height, DH200, in the lower right corner of this EADI display uses the radar altimeter as the source of altitude information |
What is a "phantom waypoint" in VOR-based RNAV?
How does a radio altimeter calculate height above ground (AGL)?
Why is a radio altimeter used instead of a barometric altimeter for landing?
What happens when an aircraft reaches its "Decision Height"?
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