Precipitation Static Precipitation static is a type of interference that occurs during dust, snow or rain storms. The principal cause of precipitation static is the corona discharge of high-voltage charges from various points on the airframe. These charges may reach several hundred thousand volts before discharge occurs. The charge can be built up in two ways. First, an electrostatic field existing between two oppositely charged thunderclouds induces bipolar charges on the surfaces of the aircraft as it passes through the charged clouds. Second, a high unipolar charge on the entire airframe occurs from frictional charging by collision of atmospheric particles (low altitudes) or fine ice particles (high altitudes) with the aircraft’s surface. The effects of corona discharge vary with temperature. The effects increase as altitude and airspeed increase. Doubling airspeed increases the effect by a factor of about 8; tripling airspeed increases the effect by a factor of about 27. The effect of precipitation static is a loud hissing or frying noise in the audio output of a com- munication receiver and a corresponding “grass” indication on a visual output device. The radio frequency range affected by precipitation static is nearly the same as for atmospheric static. When present, precipitation interference is severe, and often totally disables all receivers tuned to the low- and medium-frequency bands. Cosmic Noise Cosmic noise is usually heard in the UHF band and above. However, it is occasionally heard at frequencies as low as 10 MHz. Cosmic noise is caused by the radiation of stars. Although its effect is generally unnoticed, at peaks of cosmic activity, cosmic noise interference could conceivably be a limiting factor in the sensitivity of navigational and height-finder radar receivers. MAN-MADE INTERFERENCE Man-made interference is generally categorized according to the spectrum of its influence, such as broadband and narrow band. Each type of man-made interference is discussed below. Broadband Interference Broadband interference is generated when the current flowing in a circuit is interrupted or varies at a rate that departs radically from a sinusoidal rate. A current whose waveform is a sine wave is capable of interfering only at a single frequency. Any other waveform contains harmonics of the basic frequency. The steeper the rise or fall of current, the higher the upper harmonic frequency will be. A perfect rectangular pulse contains an infinite number of odd harmonics of the frequency represented by its pulse recurrence rate. Typical types of electrical disturbances that generate broadband interference are electrical impulses, electrical pulses, and random noise signals. For purposes of this discussion, impulse is the term used to describe an electrical disturbance, such as a switching transient that is an incidental product of the operation of an electrical or electronic device. The impulse recurrence rate may or may not be regular. Pulse is the term used to describe an intentional, timed, momentary flow of energy produced by an electronic device. The pulse recurrence rate is usually regular. Switching transient or impulses result from the make or break of an electrical current. They are extremely sharp pulses. The duration and peak value of these pulses depend upon the amount of current and the characteristics of the circuit being opened or closed. The effects are sharp clicks in the audio output of a receiver and sharp spikes on an oscilloscope trace. The isolated occasional occurrence of a switching transient has little or no significance. However, when repeated often enough and with sufficient regularity, switching transients are capable of creating intolerable interference to audio and video circuits, thus degrading receiver performance. Typical sources of sustained switching transients are ignition timing systems, commutators of dc motors and generators, and pulsed navigational lighting. Pulse interference is normally generated by pulsed electronic equipment. This type of inter- ference is characterized by a popping or buzzing in the audio output device and by noise spikes on an oscilloscope. The interference level depends upon the pulse severity, repetition frequency, and the regularity of occurrence. Pulse interference can trigger beacons and IFF equipment and cause false target indications on the radar screens. In certain types of navigational beacons, these pulses cause complete loss of reliability. Random noise consists of impulses that are of irregular shape, amplitude, duration, and recurrence 10-2