AN MD83 FLC MISINTERPRETS THEIR CLRNC AND DSND TO 2000 FT VERSUS 3000 FT 10 MI N OF SAT; TX.

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WE STARTED A 13 HR DUTY DAY OFF A SHORT 10 HR LAYOVER IN MSP. ON THE 4TH LEG; STL-SAT; WE WERE DELAYED 25 MINS FOR DIFFICULTY WITH THE START VALVES OPENING; WHICH CUT INTO OUR PLANNED SHORT 11 HR LAYOVER. ON APCH TO SAT ILS RWY 12R; IT SEEMED TO US THAT THERE WAS LITTLE TFC AND WERE PLANNING ON ATC VECTORING US CLOSE IN ON FINAL. I; THE FO WAS FLYING. I FOUND MY WORKLOAD HIGH TRYING TO DSND RAPIDLY ON VECTORS; FATIGUED AFTER THE LONG DAY. THE CAPT HEARD AND READ BACK A DSCNT TO 2000 FT; WHICH I COMPLIED WITH. LEVEL AT 2000 FT ON BASE I DID NOT NOTICE WE SEEMED LOW; BUT FIGURED IT WAS SINCE WE WERE FAIRLY CLOSE IN. TEXAS BEING FLAT AND HAVING WHAT WE BELIEVED TO BE A CLRNC TO 2000 FT GAVE ME LITTLE CAUSE FOR ALARM. ATC THEN CALLED AND SAID THEY WANTED US TO MAINTAIN 3000 FT. I IMMEDIATELY ADDED PWR AND CLBED TO 3000 FT. WE COULD NOT BE SURE IF IT WAS US WHO MISTAKENLY HEARD AND READ BACK 2000 FT OR IF THE CTLR HAD ISSUED US A DSCNT TO 2000 FT THEN DISCOVERED THE MISTAKE. AT OUR LOWEST; WE WERE 1200 FT AGL AND CLRED ALL OBSTACLES BY AT LEAST 500 FT. THE APCH AND LNDG WERE UNEVENTFULLY COMPLETED.