Aviation Electronics Technician 1 - Historic Naval Ships Association

Precipitation StaticPrecipitation static is a type of interference thatoccurs during dust, snow or rain storms. Theprincipal cause of precipitation static is the coronadischarge of high-voltage charges from various pointson the airframe. These charges may reach severalhundred thousand volts before discharge occurs. Thecharge can be built up in two ways. First, anelectrostatic field existing between two oppositelycharged thunderclouds induces bipolar charges on thesurfaces of the aircraft as it passes through thecharged clouds. Second, a high unipolar charge onthe entire airframe occurs from frictional charging bycollision of atmospheric particles (low altitudes) orfine ice particles (high altitudes) with the aircraft’ssurface. The effects of corona discharge vary withtemperature. The effects increase as altitude andairspeed increase. Doubling airspeed increases theeffect by a factor of about 8; tripling airspeedincreases the effect by a factor of about 27.The effect of precipitation static is a loud hissingor frying noise in the audio output of a communicationreceiver and a corresponding “grass”indication on a visual output device. The radiofrequency range affected by precipitation static isnearly the same as for atmospheric static. Whenpresent, precipitation interference is severe, and oftentotally disables all receivers tuned to the low- andmedium-frequency bands.Cosmic NoiseCosmic noise is usually heard in the UHF bandand above. However, it is occasionally heard atfrequencies as low as 10 MHz. Cosmic noise iscaused by the radiation of stars. Although its effect isgenerally unnoticed, at peaks of cosmic activity,cosmic noise interference could conceivably be alimiting factor in the sensitivity of navigational andheight-finder radar receivers.MAN-MADE INTERFERENCEMan-made interference is generally categorizedaccording to the spectrum of its influence, such asbroadband and narrow band. Each type of man-madeinterference is discussed below.Broadband InterferenceBroadband interference is generated when thecurrent flowing in a circuit is interrupted or varies at arate that departs radically from a sinusoidal rate. Acurrent whose waveform is a sine wave is capable ofinterfering only at a single frequency. Any otherwaveform contains harmonics of the basic frequency.The steeper the rise or fall of current, the higher theupper harmonic frequency will be. A perfectrectangular pulse contains an infinite number of oddharmonics of the frequency represented by its pulserecurrence rate. Typical types of electricaldisturbances that generate broadband interference areelectrical impulses, electrical pulses, and randomnoise signals.For purposes of this discussion, impulse is theterm used to describe an electrical disturbance, suchas a switching transient that is an incidental product ofthe operation of an electrical or electronic device.The impulse recurrence rate may or may not beregular. Pulse is the term used to describe anintentional, timed, momentary flow of energyproduced by an electronic device. The pulserecurrence rate is usually regular.Switching transient or impulses result from themake or break of an electrical current. They areextremely sharp pulses. The duration and peak valueof these pulses depend upon the amount of currentand the characteristics of the circuit being opened orclosed. The effects are sharp clicks in the audiooutput of a receiver and sharp spikes on anoscilloscope trace. The isolated occasionaloccurrence of a switching transient has little or nosignificance. However, when repeated often enoughand with sufficient regularity, switching transients arecapable of creating intolerable interference to audioand video circuits, thus degrading receiverperformance. Typical sources of sustained switchingtransients are ignition timing systems, commutatorsof dc motors and generators, and pulsed navigationallighting.Pulse interference is normally generated bypulsed electronic equipment. This type of interferenceis characterized by a popping or buzzing inthe audio output device and by noise spikes on anoscilloscope. The interference level depends upon thepulse severity, repetition frequency, and the regularityof occurrence. Pulse interference can trigger beaconsand IFF equipment and cause false target indicationson the radar screens. In certain types of navigationalbeacons, these pulses cause complete loss ofreliability.Random noise consists of impulses that are ofirregular shape, amplitude, duration, and recurrence10-2