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Aeronautical Nondirectional Radiobeacons (NDBs)
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According to the U.S. Federal Radionavigation Plan (2010):
NDB serve as nonprecision approach aids at some airports; as compass locators, generally collocated with the outer marker of an ILS to assist pilots in getting on the ILS course in a non-radar environment; and as en route navigation aids.
The NAS includes more than 1,300 NDB. Fewer than 300 are owned by the Federal Government; the rest are non-Federal facilities owned predominately by state, municipal, and airport authorities.
FAA has begun decommissioning stand-alone NDB as users equip with GPS. NDB used as compass locators, or as other required fixes for ILS approaches (e.g., initial approach fix, missed approach holding), where no equivalent ground-based means are available, may need to be maintained until the underlying ILS is phased out. Most NDB that define low- frequency airways in Alaska or serve international gateways and certain offshore areas like the Gulf of Mexico will be retained.
Except in Alaskan airspace, no future civil aeronautical uses are envisioned for these bands after the aeronautical NDB system has been decommissioned throughout the rest of the NAS. Marine radiobeacons have been phased out.
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Further information from the Radionavigation Plan:
Radiobeacons are nondirectional radio transmitting stations that operate in the low- and medium-frequency bands to provide ground wave signals to a receiver. Aeronautical nondirectional beacons are used to supplement VOR-DME for transition from en route to airport precision approach facilities and as a nonprecision approach aid at many airports. An automatic direction finder (ADF) is used to measure the bearing of the transmitter with respect to an aircraft or vessel. Marine radiobeacons have been phased out.
A. Signal Characteristics
Aeronautical NDB operate in the 190 to 415 kHz and 510 to 535 kHz ARNS bands. (Note: NDB in the 285-325 kHz band are secondary to maritime radiobeacons.) Their transmissions include a coded continuous- wave (CCW) or modulated continuous-wave (MCW) signal to identify the station. The CCW signal is generated by modulating a single carrier with either a 400 Hz or a 1,020 Hz tone for Morse code identification. The MCW signal is generated by spacing two carriers either 400 Hz or 1,020 Hz apart and keying the upper carrier to give the Morse code identification.
B. Accuracy
Positional accuracy derived from the bearing information is a function of geometry of the Lines of Position (LOP), the accuracy of compass heading, measurement accuracy, distance from the transmitter, stability of the signal, time of day, nature of the terrain between beacon and craft, and noise. In practice, bearing accuracy is on the order of +/-3 to +/-10 deg. Achievement of +/-3 deg accuracy requires that the ADF be calibrated before it is used for navigation by comparing radio bearings to accurate bearings obtained visually on the transmitting antenna. Since most direction finder receivers will tune to a number of radio frequency bands, transmissions from sources of known location, such as amplitude modulation (AM) broadcast stations, are also used to obtain bearings, generally with less accuracy than obtained from radiobeacon stations. For FAA flight inspection, NDB system accuracy is stated in terms of permissible needle swing: ±5 deg on approaches and +/-10 deg in the en route area.
C. Availability
Availability of Aeronautical NDB is in excess of 99%.
D. Coverage
Extensive NDB coverage is provided by 1,575 ground stations, of which FAA operates 728.
E. Reliability
Reliability is in excess of 99%.
F. Fix Interval
The beacon provides continuous bearing information.
G. Fix Dimensions
In general, one LOP is available from a single radiobeacon. If within range of two or more beacons, a two-dimensional fix may be obtained.
H. System Capacity
An unlimited number of receivers may be used simultaneously.
I. Ambiguity
The only ambiguity that exists in the radiobeacon system is one of reciprocal bearing provided by some receiving equipment that does not employ a sense antenna to resolve direction.
J. Integrity
A radiobeacon is an omnidirectional navigation aid. For aviation radiobeacons, out-of-tolerance conditions are limited to output power reduction below operating minimums and loss of the transmitted station identifying tone. The radiobeacons used for nonprecision approaches are monitored and will shut down within 15 s of an out-of-tolerance condition.
K. Spectrum
Aeronautical NDB operate in the 190-435 and 510-535 kHz frequency bands, portions of which it shares with maritime NDB.
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Frequency Bands |
| Band | Use | Service11 | Table |
| 190 - 285 kHz | Aeronautical NDBs | Radionavigation | - |
| 285 - 325 kHz | Aeronautical NDBs (secondary to maritime radiobeacons) | Radionavigation | - |
| 325 - 415 kHz | Aeronautical NDBs | Radionavigation | - |
| 510 - 535 kHz | Aeronautical NDBs | Radionavigation | - |
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