AN ACR MLG MADE A GAR UPON FINDING ITSELF LOW OVER THE OM.

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WE WERE EXECUTING AN ILS RWY 17L CAT III TO DFW. WE TURNED TO FINAL AT 15 DME AND CROSSED PENNY AT 4000 FT. CLRNC WAS TO MAINTAIN 4000 FT UNTIL ESTABLISHED; CLRED FOR APCH. AUTOPLT CAPTURED GS FROM ABOVE (ABOUT ONLY 1/3 DOT). LOC AND SLOPE APPEARED NORMAL; BUT ACFT CROSSED JIFFY AT 1400 FT AGL VERSUS 1713 FT; DSNDING AT 1500 FPM. I TOLD THE FO 'LET'S GAR.' HE EXECUTED GAR AT ABOUT 1200 FT; FOLLOWED BY RADAR VECTORS TO UNEVENTFUL ILS CAT 1 TO RWY 17L. AS WE WERE CLBING; WE HEARD TWR CALL THE ACFT BEHIND US WELL BELOW GS; AND THEN A FEW SECONDS LATER REPEATED THE CALL AND TOLD THEM TO CLB IMMEDIATELY. IT IS LIKELY THE 2 OCCURRENCES WERE RELATED. POSSIBLE REASONS FOR THE PROBS MIGHT HAVE BEEN A BLANKING OF THE ILS SIGNAL; OR GETTING WRONG SENSING BECAUSE OF THE SHARP TURN AT 15 DME; 4000 FT JUST AS THE AUTOPLT IS CAPTURING THE GS INSTEAD OF BEING ESTABLISHED ON FINAL AND CAPTURING FROM BELOW. SUPPLEMENTAL INFO FROM ACN 236400: TAXIING FROM E AIR FREIGHT RAMP. WE WERE TOLD TO HOLD SHORT OF RWY 13L. AFTER ABOUT 20 MINS WE WERE TOLD BY DFW GND TO TAXI ACROSS RWY 13L; TURN L AND THEN HOLD SHORT OF RWY 17L ON TXWY 19 AND TO EXPECT RWY 17L FOR TKOF. AFTER ABOUT 5 MINS IN HOLDING SHORT POS; WE HEARD ON TWR FREQ; WHICH WE HAD BEEN INSTRUCTED TO MONITOR; THAT AT LEAST 2 AIRPLANES FOUND THEMSELVES LOW OVER THE MARKER AND WERE EXECUTING MISSED APCHS. TWR THEN INSTRUCTED US TO CROSS RWY 17L AND TOLD US WE HAD OBSTRUCTED THE ILS GS ANTENNA. COMMUTER ACFT WERE TAXIING TO RWY 13L ON TXWY 18 AND ONE OF THEM NOTIFIED TWR THAT THE ILS HOLD SHORT MARKERS WERE REMOVED DUE TO CONSTRUCTION ON A NEW TXWY PARALLEL TO RWY 17L. DEP ATIS DID NOT MENTION WHICH ILS'S WERE IN PROGRESS; BUT WE SAW ACFT LNDG ON RWY 17L WHEN WE TAXIED ON TO TXWY 19; AND A CLOSER LOOK AT THE TAXI CHART WOULD HAVE TOLD US THAT OUR GND CLRNC MIGHT HAVE BEEN QUESTIONABLE. SINCE OUR CLRNC ONTO TXWY 19 MADE US CROSS THE ILS HOLD LINE (BASED ON TAXI CHART) THE CAPT WAS UNDER THE IMPRESSION THAT WE WOULD GET A QUICK TKOF CLRNC SINCE GND CLRED US ONTO TXWY 19. WE WERE IN A WDB FGT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ANALYST CALL TO ARPT OPS REVEALED THAT ALTHOUGH THERE WAS SOME CONSTRUCTION WORK BEING ACCOMPLISHED IN THE AREA; THE RWY AND ILS HOLD LINES WERE IN PLACE AS REQUIRED.