SFO LCL CTLR RECEIVES AN AMASS (ARPT MOVEMENT AREA SAFETY SYS) ALERT; AND ISSUES A GAR TO A TWIN ON SHORT APCH TO RWY 28R WITH A B737 ON SHORT FINAL TO RWY 28L; BEING UNABLE TO DETERMINE THE NATURE OF THE ALERT.

Narrative

AMASS OPERATIONAL ON CONFIGURED FOR LNDG RWYS 28; DEPARTING RWY 1. SFO WAS LNDG RWYS 28; DEPARTING RWY 1. IT WAS NIGHT; NO MOON; BUSY SIDE-BY-SIDE ARRS TO RWY 28 WITH VERY FEW DEPS FROM RWYS 1. ARRS WERE MAKING SIMULTANEOUS QUIET BRIDGE AND TIPP-TOE VISUAL APCHS. ACFT X A LIGHT TWIN ACFT (KING AIR OR TWIN CESSNA OR SIMILAR) MADE QUIET BRIDGE ARR FOR RWY 28R ALONGSIDE ACFT Y ON TIPP-TOE FOR RWY 28L. THE 2 ACFT WERE SIDE-BY-SIDE FOR THE LAST 3-4 MI OF THE APCH AND WERE SIDE-BY-SIDE OVER THE LIGHTS. I SCANNED THE RWYS 28 VISUALLY AND ON THE ASDE AND BOTH RWYS 28 APPEARED CLR. JUST AFTER BOTH ACFT PASSED THE NUMBERS; THE AMASS ALERTED. I LOOKED AT THE ASDE AND SAW THE TARGET FOR ACFT X (CIRCLED) AND THE TARGET FOR ACFT Y (CIRCLED) BUT I COULD NOT DETERMINE THE REASON FOR THE ALERT. WE ARE REQUIRED TO SEND AN ACFT AROUND IF WE GET AN AMASS GAR ALERT ON IT. SINCE BOTH ACFT X AND ACFT Y WERE ALERTING; AND I COULD NOT SEE ANY CONFLICTING TFC FOR EITHER ONE; I DID NOT KNOW WHAT TO DO. THEN THE AMASS VOICE SAID 'WARNING; GAR RWY 28R' AND THE SAME MESSAGE DISPLAYED ON THE ASDE. I IMMEDIATELY TOLD ACFT X TO GO AROUND. IT WAS SO DARK I COULD NOT SEE IF ACFT X WAS STILL FLYING. HE COULD HAVE ALREADY TOUCHED DOWN OR PERHAPS HE WAS JUST IN THE FLARE. THE PLT ANSWERED THAT HE WAS GOING AROUND. I COULD NOT GIVE HIM A REASON FOR THE GAR BECAUSE I DID NOT REALLY KNOW WHAT IT WAS; EXCEPT FOR AN AMASS ALERT. ACFT Y LANDED WITHOUT INCIDENT. THE NATURE OF AMASS IS THAT WHEN AN ALERT IS RECEIVED; ACTION MUST BE TAKEN IMMEDIATELY. FALSE ALERTS LESSEN THE CTLR'S CONFIDENCE IN THE SYS AND THUS INCREASE REACTION TIME TO THE ALERT. SUGGESTION: AMASS REFRESHER TRAINING FOCUSING ON THE BASIC CAPABILITIES OF THE SYS AND ITS USE AS WELL AS A REVIEW AND BRIEFING OF FALSE ALERTS TO THE CTLRS WHO USE IT.