ATC EQUIP PROB RADAR ASR-9. MIL X MALFUNCTIONING XPONDER CAUSED FALSE RADAR POS AND AIRSPD.

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I WAS WORKING NUMEROUS ACFT ON THE SAN ANGELO APCH POS OF MIDLAND APCH CTRL ON APRIL XX; 1994. ONE OF THE ACFT UNDER MY CTL WAS MIL X T-37 DOING A PRACTICE HI VOR DME APCH TO RWY 3 AT SAN ANGELO ARPT (SJT). I OBSERVED RAPID MOVEMENT OF X'S TARGET ON THE ASR-9 RADAR DISPLAY. X RADAR TARGET ADVANCED FROM 15 NM TO 10 NM FROM THE SJT ARPT IN UNDER 30 SECONDS. DURING THIS TIME X HAD BOTH A PRIMARY AND SECONDARY RADAR TARGET; A COMPLETE DATA BLOCK; AND THE TARGET WAS NOT IN `COAST' MODE; AND THE GND SPD READOUT INDICATED 860 KTS. THERE WAS NO INDICATION; OTHER THAN THE GND SPD; THAT THE RADAR DISPLAY WAS INACCURATE UNTIL AFTER THE EVENT. APPROX 10 TO 15 SECONDS AFTER THE TARGET RETURNED TO NORMAL SPD IT APPEARED THAT X HAD CHANGED HIS XPONDER CODE. THE PLT CONFIRMED THAT HE HAD NOT CHANGED THE CODE. X'S TARGET WAS THE ONLY ONE TO DISPLAY THIS ERRATIC PERFORMANCE. A MAINT TECH CHKED THE RADAR IMMEDIATELY AFTER THE EVENT AND COULD FIND NO CAUSE FOR THE DISPLAY ERROR. THE ATS ON DUTY REFUSED TO CALL THE RADAR OUR OF SVC AND BLAMED THE EVENT ON THE ACFT'S FAULTY XPONDER. I DO NOT BELIEVE A FAULTY XPONDER COULD CAUSE THIS ERROR. THE ONLY PROOF WE HAD THAT THE XPONDER WAS MALFUNCTIONING IS A RADAR DISPLAY THAT HAD JUST PROVEN ITS ACCURACY. TO MY KNOWLEDGE; THE ONLY INFO THE RADAR ACTUALLY RECEIVES FROM THE ACFT IS THE ALT AND BEACON CODE; ALL OTHER INFO (AIRSPD; RADAR TARGETS; ETC.) IS GENERATED BY THE RADAR AND ITS ASSOCIATED SYS. THE ASR-9 RADAR AT SJT HAS A HISTORY OF FALSE TARGETS; RING AROUND ON BEACON TARGETS; SHORT PERIODS OF RADAR OUTAGE (THE SWEEP ON THE DISPLAY SLOWING FOR A FEW SECONDS); AND ACFT TARGETS JUMPING. OUR MAINT PEOPLE ARE FINDING IT VERY DIFFICULT TO FIND THE PROBS WITH THE ASR-9 SYS. A PERFECT EXAMPLE OF THE PROBS WE ARE HAVING WITH THE SJT APCH POS HAPPENED ON THE SAME MORNING. I ARRIVED AT THE FACILITY TO FIND BOTH RADAR CHANNELS IN ALARM ON THE SJT RADAR. THE RADAR WAS CALLED OTS. WE QUICKLY LEARNED THAT THE TELEPHONE LINES WERE ONLY WORKING INTERMITTENTLY; AND THE RADIOS WERE NOT WORKING AT ALL. WE RETURNED THE APCH CTL RESPONSIBILITY TO ZFW. APPROX AN HR LATER; THE RADAR POS APPEARED TO BE OPERATING NORMALLY AND MAINT RETURNED THE RADAR TO SVC. WHILE DISCUSSING THE X INCIDENT; THE AIR TFC SUPVR TOLD ME THAT MAINT HAD NOT FOUND A PROB WITH THE RADAR. THE RADAR APPEARED TO BE WORKING NORMALLY; AND SINCE THEY COULD NOT FIND THE PROB; THEY RETURNED THE RADAR TO SVC.