TM 1-1520-238-104-734.37.3 AN/APR-39(V)1 Operation Modes.The RW setmay be operated in either the discriminator off or discrimi-nator on mode. The RW set receives 28 vdc from theemergency dc bus through the RDR WARN circuit break-er on the pilot overhead circuit breaker panel. The RW setis not monitored by FD/LS.NOTEDisplay strobe lengths indicate only relativesignal amplitude. Since many variables canaffect the atmospheric attenuation of thesignals, strobe length should not be consid-ered as a direct interpretation of the dis-tance to the emitter.a. Discriminator Off Mode.When operating in thediscriminator off mode, the DSCRM switch is placed toOFF. In this mode all high band received signals with am-plitude greater than a predetermined threshold level aredisplayed on the CRT and an audio signal, representativeof the combined amplitudes and pulse repetition frequen-cies (PRF), is present at the headset. The displays indi-cate the total radar environment in which the helicopter isoperating. Each radial strobe on the CRT is a line of bear-ing to an active emitter. When a SAM radar complex be-comes a threat to the helicopter (low band signals corre-lated with high band signals), the alarm audio issuperimposed on the PRF audio signal and the MA lightand associated strobe start flashing. Lengths of strobesand audio levels depend on the relative strength of the in-tercepted signals. A typical display when operating in thediscriminator off mode is shown in figure 4-21.NOTEIn this mode, received low band signalswhich are not correlated with a high band in-tercept will cause the MA light to flash andan alarm audio will sound.b. Discriminator On Mode.When operating in thediscriminator on mode, the DSCRM switch is set to ON. Inthis mode, signals are processed to determine their con-formance to certain threat associated criteria as follows:(1) The signal level must be greater than the mini-mum threshold level.(2) Pulse width must be less than the maximumpulse width.FILTERFILTERCONTROLM01-186Figure 4-21.Radar Warning Discriminator OffMode Display(3) PRF must be greater than the minimum pulsesper second.(4) The pulse train must exist with not less than theminimum pulse train persistence.The CRT display is divided into eight sectors. Strobes aredisplayed only in those sectors in which signals meetingall threat criteria are present. This reduces display clutterby eliminating low-level and wide pulse width signals andby selective sector display. Intercepts which meet theserequirements are displayed as described in paragraph4.37.3 a.NOTEIn this mode, uncorrelated low band signalswill not give any indications.A typical display when operating in the discriminator ONmode is shown in figure 4-22. Conditions are the same asfor figure 4-21, but it is assumed that one or more threatshave been identified in the 225 to 270 degree sector.
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