RWY TRANSGRESSION CAUSED MLG TO GAR DUE TO MISCOMS.

Narrative

ON A VISUAL APCH TO SAT RWY 12R; WE RECEIVED A CLRNC TO LAND WHILE ON A L BASE TO THE RWY. ON FINAL APCH WE NOTICED A SMALL ACFT ON THE RWY AS WE WERE APCHING THE 500 FT CALL. THE FO ASKED IF WE WERE STILL CLRED TO LAND AND THE RESPONSE WAS AFFIRMATIVE FOLLOWED IMMEDIATELY BY A COMMAND TO GAR. THE TWR WAS UNAWARE THAT THE OTHER ACFT HAD COMMENCED HIS TKOF. WE WENT AROUND AND LANDED NORMALLY. SUPPLEMENTAL INFO FROM ACN 293955: I CALLED SAT TWR AND SAID THAT I WAS READY TO DEPART RWY 12R. TWR REPLIED WITH ROGER. AND I RESPONDED WITH MY ACFT NUMBER. I HEARD THAT I WAS CLRED TO DEPART. AS I WAS DEPARTING AN MLG CALLED THE TWR AND ASKED IF THEY WERE CLRED TO LAND. THE TWR SAID THAT THEY WERE CLRED TO LAND. THEN THE MLG PLANE SAID THAT THEY WOULD EXECUTE A GAR. THE CTLR THEN REQUESTED THAT I CALL THEM WHEN I GOT TO WHERE I WAS GOING. I CALLED AND THE FAA PERSON ASKED WHAT I SHOULD HAVE DONE IN THIS SIT. IT WOULD HAVE PREVENTED THIS PREDICAMENT IF THE TWR WOULD HAVE SAID 'Y; HOLD SHORT OF RWY 12R FOR LNDG ACFT.'