Radio Aids (Part One)

Radio aids are ground based navigation facilities that transmit electronic signals received by airborne units. Radio aids can be used for departure, en route navigation or arrivals, and procedures for obtaining and plotting a fix vary by category. Explanations of different types of radio aids and how to fix with them follow.

Nondirectional Radio Beacon (NDB)

This is a low, medium, or ultra high frequency (UHF) radio beacon that transmits nondirectional signals whereby the user can determine a bearing and home to the station. nondirectional radio beacons (NDBs) normally operate in the frequency band of 190 to 535 kilocycles (kHz) and transmit a continuous carrier with either 400 or 1020 cycles per second (Hz) modulation. All radio beacons, except the compass locators, transmit a continuous three-letter identification code except during voice transmissions.



Radio aids are subject to disturbances that may result in erroneous bearing information. Such disturbances result from intermittent or unpredictable signal propagation due to such factors as lightning, precipitation, static, etc. At night, radio beacons are vulnerable to interference from distant stations. Nearly all disturbances that affect the automatic direction finder (ADF) bearing also affect the facility’s identification. Noisy identification usually occurs when the ADF needle is erratic. Voice, music, or erroneous identification is usually heard when a steady false bearing is being displayed. Since ADF receivers do not have a flag to warn the user when erroneous bearing information is being displayed, the NDB’s identification should be continuously monitored.

Control Panels

There are several different types of control panels installed in specific aircraft. Refer to the aircraft technical manuals for specific guidance pertaining to equipment operation.

Plotting on a Chart

Before an ADF bearing can be plotted on a navigation chart, two things must be done. First, the bearing obtained must be converted to a TB. If a nonrotatable compass card is used, the resultant RB may be converted to TB by adding the aircraft true heading (TH) (TH + RB = TB). If a rotatable compass card is used, the TB can be found by applying the magnetic variation at the vicinity of the aircraft.

Ultra High Frequency (UHF) Direction Finders (DF)

Some aircraft are equipped with ADFs in the UHF frequency range (225.0–399.9 mHz), which utilize loop and sensing (antennas) to give bearing information. Operation of the direction finders (DF) is controlled from the UHF radio panel. It is used to obtain bearing to other aircraft and to emergency locator beacons.

VHF Omnidirectional Range (VOR)

Very high frequency (VHF) omnidirectional range (VOR) stations operate between 108.00 and 117.95 megacycles per second (mHz). VHF communications operate between 118.00 and 135.90 mHz. Station identifiers for VOR NAVAIDs are given in code or voice, or by alternating code and voice transmissions. VOR transmissions are limited by LOS and a combination of aircraft altitude and distance to the station. Accurate information may be obtained from 40 to 100 NM around the facility, although the usable range may be much greater (300 NM). VOR may be used by flying courses from one station to another as part of the high or low jet navigation airways system. It may be used as a fixing aid by taking a bearing and applying magnetic variation at the station (converting magnetic bearing (MB) to TB) and plotting an LOP. In aircraft equipped with two VORs, the bearings to two different stations may be taken simultaneously and plotted, and a fix position obtained. The aircraft is directly overflying a VOR when the bearing pointer drops rapidly below the 3 or 9 o’clock position.

Control Panel

A VOR control panel contains a power switch, frequency window, volume control, equipment self-test capability, and frequency selector controls. [Figure 5-7] To tune a VOR, turn power switch to PWR, select desired frequency and identify the station. For positive test indications, consult applicable aircraft flight or operator’s manual.

Figure 5-7. VOR navigation control panel.

Figure 5-7. VOR navigation control panel.

Indicator Panel

Several types of indicators exist that display VOR information. Examples shown here are the course indicator [Figure 5-8], the radio magnetic indicator (RMI) [Figure 5-9], and the bearing direction heading indicator (BDHI). [Figure 5-10] The course indicator has eight significant features: TO-FROM indicator, glideslope and course warning flags, course selector window, marker beacon light, glideslope indicator, heading pointer, course deviation indicator (CDI), and course set knob.

Figure 5-8. Course indicator.

Figure 5-8. Course indicator. [click image to enlarge]

Figure 5-9. Radio magnetic indicator.

Figure 5-9. Radio magnetic indicator.

Figure 5-10. Bearing distance heading indicator.

Figure 5-10. Bearing distance heading indicator.


The TO-FROM indicator shows whether the radial set in the course selector window is to or from the station, and the CDI represents this radial. If the aircraft is to the right of the radial, the CDI is displaced to the left of center on the course indicator. The glideslope indicator is similar to the CDI but represents the glideslope transmitted by an instrument landing system (ILS). If the glideslope indicator is below the center of the course indicator, the aircraft is above the glideslope. The glideslope and course warning flags inform the user that either the glideslope indicator or CDI is inoperative, or that signals received are too weak to be used. The heading pointer indicates the difference, left or right, between the aircraft MH and the radial set in the course selection window.

The marker beacon light flashes when passing over a marker beacon, such as outer marker of the ILS. The RMI is a bearing indicator, usually with two pointers and a movable compass rose. The compass rose rotates as the aircraft turns, indicating the compass heading of the aircraft under the top of the index at all times. Therefore, all bearings taken from an RMI are magnetic. Consult the specific flight or operator’s manual as to which pointer is the VOR.


Bearing Direction Heading Indicator (BDHI)

The bearing direction heading indicator (BDHI) is similar to the RMI because the needles provides MB information. Additional information concerning the BDHI is in the tactical air navigation (TACAN) section.

Tactical Air Navigation (TACAN)

The TACAN system was developed to provide information needed for precise positioning within 200 NM. As with VOR, TACAN provides an infinite number of radials radiating outwardly from the station. In addition, distance measuring equipment (DME), an integral part of TACAN, provides continuous slant-range distance information. TACAN operates in the UHF band and has 126 channels available in the X-band pulse code. Development of pulse coding has given ground equipment the capability of an additional 126 channels in the Y-band. The station identifier is transmitted at 35-second intervals in international Morse code. Airborne DME transmits on 1025–1150 mHz; associated ground-to-air frequencies are in the 962–1024 mHz and 1151–1213 mHz ranges. Channels are separated at 1 mHz intervals in these bands.

Ground Equipment

The ground equipment consists of a rotating-type antenna for transmitting bearing information and a receiver-transmitter (transponder) for transmitting distance information. Permanent ground stations are dual transmitter-equipped (one operating and one in standby) installations that automatically switch to the standby transmitter when a malfunction occurs. Each station has a ground monitor that is set to alarm at a radial shift of 1° from the alignment to magnetic north (MN). This alarm is usually located in the control tower or approach control, and sets off a light and buzzer to warn when an out-of-tolerance condition exists. It is possible to select a TACAN station and get erroneous DME and azimuth lock-on when the station is undergoing maintenance. This can be detected by an absence of signal identifier. Checks of en route or radio NAVAIDs may be made by consulting NOTAMs prior to flight or by contacting air traffic control (ATC) for advisories when airborne.