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Generally, space waves are “line of sight” receivable, but those of lower frequencies “bend” somewhat over the horizon. The VOR signal at 108 to 118 MHz is a lower frequency than distance measuring equipment (DME) at 962 to 1213 MHz. Therefore, when an aircraft is flown “over the horizon” from a VOR/DME station, the DME is normally the first to stop functioning. 

## Disturbances to Radio Wave Reception 

Static distorts the radio wave and interferes with normal reception of communications and navigation signals. Low- frequency airborne equipment, such as automatic direction finder (ADF) and LORAN (LOng RAnge Navigation) ,are particularly subject to static disturbance. Using very high frequency (VHF) and ultra-high frequency (UHF) frequencies avoids many of the discharge noise effects. Static noise heard on navigation or communication radio frequencies may be a warning of interference with navigation instrument displays. Some of the problems caused by precipitation static (P-static) are: 

• Complete loss of VHF communications. 

• Erroneous magnetic compass readings. 

• Aircraft flying with one wing low while using the autopilot. 

• High-pitched squeal on audio. 

• Motorboat sound on audio. 

• Loss of all avionics. 

• Inoperative very-low frequency (VLF) navigation system. 

• Erratic instrument readouts. 

• Weak transmissions and poor radio reception. 

• St. Elmo’s Fire. 

## $^{ }$Traditional Navigation Systems 

## Nondirectional Radio Beacon (NDB) 

The nondirectional radio beacon (NDB) is a ground-based radio transmitter that transmits radio energy in all directions. The ADF, when used with an NDB, determines the bearing from the aircraft to the transmitting station. The indicator may be mounted in a separate instrument in the aircraft panel. [Figure 9-2] The ADF needle points to the NDB ground station to determine the relative bearing (RB) to the transmitting station. It is the number of degrees measured clockwise between the aircraft’s heading and the direction from which the bearing is taken. The aircraft’s magnetic heading (MH) is the direction the aircraft is pointed with respect to magnetic north. The magnetic bearing (MB) is the direction to or from a radio transmitting station measured relative to magnetic north. 

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## NDB Components 

The ground equipment, the NDB, transmits in the frequency range of 190 to 535 kHz. Most ADFs also tune the AM broadcast band frequencies above the NDB band (550 to 1650 kHz). However, these frequencies are not approved for navigation because stations do not continuously identify themselves, and they are much more susceptible to sky wave propagation especially from dusk to dawn. NDB stations are capable of voice transmission and are often used for transmitting the Automated Weather Observing System (AWOS). The aircraft must be in operational range of the NDB. Coverage depends on the strength of the transmitting station. Before relying on ADF indications, identify the station by listening to the Morse code identifier. NDB stations are usually two letters or an alpha-numeric combination. 

## ADF Components 

The airborne equipment includes two antennas: a receiver and the indicator instrument. The “sense” antenna (non- directional) receives signals with nearly equal efficiency from all directions. The “loop” antenna receives signals better from two directions (bidirectional). When the loop and sense antenna inputs are processed together in the ADF radio, the result is the ability to receive a radio signal well in all directions but one, thus resolving all directional ambiguity. The indicator instrument can be one of four kinds: fixed- card ADF, rotatable compass-card ADF, or radio magnetic 

