atmospheric noiseatmospheric radio waveatmospheric radio windowatmospheric refractionatomatomicatomic beam frequency standardatomic chargeatomic clockatomic constantatomic disintegrationatomic frequencyatomic frequency standardatomic impurityatomic mass unitatomic modelatomic nucleusatomic numberatomic power plantatomic shellatomic spectrumatomic structureatomic timeatomic weightatomic weight atomic massatr tubeattenuateattenuationattenuation coefficientattenuation constantattenuation distortionattenuation equalizerattenuation factorattenuation ratioattenuatorattenuator tubeattitudeattitude controlattitude gyroattitude indicatorattitude sensorattoattractattractionauaudibilityaudibility limitaudibility thresholdaudible doppler enhanceraudio