bias circuit, horizontal and vertical deflection coils, and CRT. Symbols are drawn at the rate of 50 times a second on the CRT. The rapidly moving electron beam is generated by the bright-up pulse from the signal data processor. The bright-up pulse is amplified by the bright-up amplifier, and then applied to the CRT grid. The CRT, which is normally biased into cutoff, is turned on, and an electron beam is emitted. The electron beam strikes the CRT face at a position determined by the amount of current flowing in the horizontal and vertical deflection coils. The amount of light (symbol brightness) emitted from the CRT is controlled by the cathode bias circuit. A control voltage from the control panel is applied to the cathode bias circuit, which serves as a reference voltage for the autobrilliance sensor and the CRT cathode. The voltage output from the autobrilliance sensor is amplified by the autobrilliance amplifier. The output is applied to the cathode bias circuit, which changes the amount of bias (brightness) on the CRT. Thus, an optimum CRT contrast is constantly maintained under varying ambient light. DEFLECTION MODULE.— The deflection module contains the X and Y deflection amplifiers, X and Y low amplifiers, X and Y comparators, and one OR gate. The type and location of each symbol on the combiner is determined by X and Y analog voltages applied to the deflection circuits. As the need for a symbol arises, the busy signal from the HUD is sampled by the symbol generator. The busy signal is generated any time current is flowing in either deflection coil. Comparators, connected across each coil, are used to detect when the deflection coil current is equal to zero. As the current in either deflection coil reaches zero, the output from the comparator, connected across the coil, goes low. When both comparator outputs are low, the OR gate output goes low, and the busy signal is removed from the symbol generator input. At the same instant, the symbol generator applies a precise analog voltage to the X and Y deflection amplifiers. The output from the deflection amplifiers causes current to flow in the deflection coils. The current amplitude is precalculated to drive the CRT electron beam to the start point of the symbol to be drawn. A bright-up pulse is then applied to the bright-up amplifier in the video module. At the same instant, the deflection voltage is modulated at a predetermined rate and amplitude by the symbol generator. The modulation is detected by the deflection amplifiers and applied to each deflection coil. A specific part (or all) of a symbol is then drawn on the CRT face in a precise location. The bright-up pulse continues long enough to draw the prescribed line or circle that makes up the symbol. When the bright-up pulse is removed, the CRT is driven into cutoff until the next bright-up pulse is applied. This process is repeated until all symbols have been displayed on the CRT. BITE Functions Both the signal data processor and the heads-up display unit contain built-in test equipment (BITE). SIGNAL DATA PROCESSOR.— The built-in test equipment contained in the signal data processor consists of a clock pulse monitor, data test equipment, raw data test equipment, six low-voltage comparators, a signal data processor fail indicator, and a thermal overload sensor. HEADS-UP DISPLAY UNIT.— The built-in test equipment contained in the HUD unit consists of a high-voltage comparator, seven low-voltage comparators, a bright-up parity circuit, a HUD fail indicator, and a thermal overload sensor. SYMBOLOGY The symbols that are used with the various modes of operation in the F-14 are listed and described in figure 5-9. These symbols give the pilot important information, such as aircraft attitude, heading, altitude, angle-of-attack, and ground track during flight. Attack information, such as closure rate, range to target, maximum range for weapon launch, minimum range for weapon launch, and boresight reference, is available during air-to-air and air-to-ground operations. Ordnance information, such as number of rounds remaining, the type of weapon selected, and the number of weapons ready for launch, is also available. DECLUTTER The system used in the F-14 (AVA-12) has a feature known as declutter. It is used to remove preselected, unwanted symbols from the display during certain modes of operation. This feature is especially important during an air-to-air situation. When the pilot is trying to locate and engage an enemy target, the declutter feature will clear the unnecessary symbols from his/her view. 5-10