B737 CAPT EXPERIENCED CONFUSION DURING VECTORS TO ILS AT BWI WITH REGARD TO WHAT RWY WAS IN USE.

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WHILE ON APCH TO BWI; THE PNF RECEIVED ATIS INFO WHICH STATED; 'EXPECT ILS RWY 15'S AT BWI.' WHEN WE CHKED IN WITH APCH; WE THOUGHT THEY SAID 'EXPECT VECTORS FOR THE ILS RWY 15R.' WE WERE THEN VECTORED AND TOLD 'MAINTAIN 2000 FT; INTERCEPT THE LOC; CLRED APCH.' WE INTERCEPTED THE LOC FOR RWY 15R AND WERE TOLD TO CONTACT TWR. SIMULTANEOUSLY WHILE CALLING TWR; THE LOC AND GS SIGNALS BECAME VERY ERRATIC AND UNRELIABLE; SO WE STAYED AT 2000 FT AND DID NOT START DOWN. TWR NOTICED THIS AT THE SAME TIME WE WENT AROUND AND GAVE US A VECTOR AND SENT US TO APCH CTL. WE WERE AGAIN VECTORED FOR WHAT WE ASSUMED TO BE THE ILS RWY 15R AGAIN. APCH AGAIN TOLD US TO MAINTAIN 2000 FT AND INTERCEPT 'THE LOC.' HE DID NOT SAY WHICH LOC. ONCE AGAIN; THE LOC SIGNAL BECAME VERY ERRATIC AND UNRELIABLE. (I BELIEVE IT WAS FROM DEPARTING ACFT.) WE TOLD APCH THAT THE NAVAID WAS TOTALLY UNRELIABLE WHILE SIMULTANEOUSLY REALIZING THAT HE WAS VECTORING OTHER PLANES FOR RWY 10 AND NOT RWY 15. HE THEN ASKED IF WE WOULD LIKE THE VOR APCH TO RWY 10. WE STATED THAT WE REALIZED THE PROB NOW; APOLOGIZED FOR THE CONFUSION; AND SAID WE WANTED VECTORS FOR THE ILS RWY 10 APCH. ALL WENT WELL AND WE LANDED UNEVENTFULLY. WHAT I THINK HAPPENED WAS THAT BWI WAS DOING SIMULTANEOUS APCHS TO RWY 15L AND RWY 10 WHILE DEPARTING ON RWY 15R. WHILE LISTENING TO THE ATIS; ALL THE PNF HEARD WAS ILS RWY 15 IN USE; SO WE PLANNED ACCORDINGLY FOR RWY 15R. I ALSO THINK THE FIRST CTLR DID IN FACT VECTOR US FOR RWY 15R BECAUSE WE BOTH THOUGHT WE HEAD HIM SAY THIS WOULD BE VECTORS FOR RWY 15 AND THE VECTORS LOOKED TO BE WORKING FOR RWY 15. I BELIEVE THE CTLR ON OUR SECOND ATTEMPT WAS IN FACT VECTORING US FOR THE ILS RWY 10 WHILE WE ASSUMED IT WAS AGAIN FOR RWY 15R. AGAIN; A FAILURE TO COMMUNICATE BOTH BY US AND ATC. THEY DID NOT IDENT WHICH LOC WE WERE TO INTERCEPT; NOR DID WE ASK. I BELIEVE THE ENTIRE EVENT WOULD HAVE BEEN AVOIDED IF THE CTLR WOULD HAVE STATED WHICH LOC TO INTERCEPT INSTEAD OF SAYING 'INTERCEPT THE LOC;' OR IF WE HAD SIMPLY ASKED TO VERIFY WHICH RWY WAS NOW IN USE.