A CRJ100 ON ARR TO FSD DSNDED BELOW CLRED ALT BECAUSE THEY PICKED UP A CLRNC FOR ANOTHER ACFT.

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DSNDING INTO FSD; WE WERE CLRED TO 12000 FT THEN 8000 FT; THEN DIRECT NEISH (FAF FOR VOR 33) DOWN TO 4500 FT. ALL OF THIS WAS DONE MORE THAN 30 MI FROM THE ARPT. WE HAD BRIEFED THE VISUAL FOR RWY 33 USING THE VOR 33 PLATE AND SET THE FMS UP FOR THE VISUAL TO RWY 33. WE BRIEFED THE MSA AS 3300 FT AND I SET THE TERRAIN MODE ON MY SIDE. AT SOME POINT; DSNDING THROUGH ABOUT 10000 FT I MAY HAVE ACCEPTED A CLRNC MEANT FOR ANOTHER PLANE TO DSND TO 3000 FT AND SET THE ALT IN THE ALERTER. WE THOUGHT THIS WAS A LITTLE LOW; BUT I HAD THE LARGE TV TWR IN SIGHT SEVERAL MI N OF OUR COURSE; AND IT WAS PLOTTED ON MY TERRAIN MAP ON THE MFD. WE DISCUSSED THE LOW ALT AND SINCE WE WERE VISUAL WITH THE RWY AND THE TWR IN SIGHT WE DSNDED TO 3000 FT. A FEW MINS LATER; FSD APCH ASKED US TO VERIFY THAT WE WERE AT 4500 FT. I SAID WE WERE AT 3000 FT AND HE TOLD US TO CLB IMMEDIATELY TO 4500 FT. WE CLBED TO 4500 FT AND WERE THEN CLRED FOR A VISUAL APCH FOR RWY 33 AND TO CONTACT TWR. WE CONTINUED FOR A NORMAL APCH AND LNDG ON RWY 33. AT THE GATE I PHONED ATC AND GAVE THEM MY INFO. ATC CLAIMED THAT THEY REVIEWED THE TAPES AND I WAS CLRED TO 4500 FT AND NOT 3000 FT; SO IT MUST BE A PLTDEV. PROB: SOMEHOW SET 3000 FT AND DSNDED WHEN ONLY CLRED TO 4500 FT. DISCOVERED: ATC ASKED US TO VERIFY WE WERE AT 4500 FT WHEN WE HAD BEEN AT 3000 FT FOR A FEW MINS. CORRECTIVE ACTION: CLBED IMMEDIATELY TO 4500 FT AND THEN CLRED FOR A VISUAL APCH AND LNDG. PERCEPTION: HOW DID 3000 FT GET SET? WE BOTH VERIFIED AND DISCUSSED IT WHEN IT WAS SET. I MUST HAVE ACCEPTED SOMEONE ELSE'S CLRNC. DECISION: WE DSNDED THINKING IT WAS A LOW ALT; BUT WE HAD THE OBSTACLES AND THE FIELD VISUALLY. SUPPLEMENTAL INFO FROM ACN 707888: DURING DSCNT FOR APCH INTO FSD WE WERE TOLD TO EXPECT THE VISUAL 33 AND CLRED DIRECT NEISH; MAINTAIN 3000 FT. I WAS THE PF AND BRIEFED THE APCH DURING THE DSCNT; NOTING THAT MSA WAS 4500 FT. WE DISCUSSED THIS AS A CREW; TURNED ON TERRAIN AND NOTED OBSTRUCTION ON MFD. WE THOUGHT IT WAS ODD THAT WE WERE CLRED TO 3000 FT WHEN THE OBSTRUCTION WAS A 3444 FT TWR APPROX 15 NM TO THE NE OF OUR POS. WE WERE VMC AND HAD VISUALLY ACQUIRED THE TWR (AND MATCHED IT POS WITH THE MFD) PRIOR TO DSNDING BELOW MSA. AT THIS POINT WE FELT COMFORTABLE DSNDING BELOW MSA SINCE WE HAD VISUAL SEPARATION FROM THE OBSTRUCTION. THE VOR DME RWY 33 APCH DEPICTS 3100 FT WHEN XING NEISH; SO WE ASSUMED THAT SINCE WE WERE ON VECTORS WITH ATC THAT 3000 FT WAS ABOVE MVA. FROM OUR POS WE FELT COMFORTABLE EXECUTING A VISUAL APCH AND WE WERE JUST ABOUT TO CALL THE FIELD IN SIGHT WHEN APCH CALLED TO VERIFY OUR ALT. WE RESPONDED; '3000 FT.' APCH NOTIFIED US THAT OUR ASSIGNED ALT WAS 4500 FT. WE IMMEDIATELY EXECUTED A CLB TO 4500 FT. UPON REACHING 4500 FT (APPROX 20 NM SE OF SFD) WE WERE CLRED FOR THE VISUAL APCH. AN UNEVENTFUL APCH AND LNDG ENSUED. ONCE SAFELY AT THE GATE AFTER PAX DISEMBARKED; THE CAPT CALLED FSD APCH ON THE TELEPHONE. APCH INFORMED US OF A POSSIBLE PLTDEV.