Narrative:

I was working south radar (sr) and observed 2 VFR targets (1200 squawks) to the southeast that were rdr (primary targets). They would FLIP flop from beacon targets with altitude to rdr targets. One in particular was probably in the rdr state for 2/3 of the time. This is noticed when observing these targets utilizing cmh modes south. South satellite (ss) was not open; so I had its display set to utilize our long-range radar in london; oh (qwo). On that display; every time I compared; both of those 1200 targets were showing as solid beacon targets. One of those aircraft; a C172; called me for VFR TA's when approximately 40 mi ese of uyf (madison co airport; located next to the town of london; oh). It wasn't busy; so I asked the pilot what make and model of transponder he was utilizing. The pilot responded that it was a narco AT150. I didn't inquire as to whether it had been modified per the airworthiness directive (docket no. 2002-northeast-32-ad; amendment 39-13586; ad 2004-08-16/rin 2120-AA640). That airworthiness directive fixes it so it'll not be invisible to mode south interrogations (as well as TCAS interrogations). I have been noticing for the past couple months that more and more GA low-end type aircraft appear to have intermittent xponders when viewed with our cmh mode south radar; yet appear as good beacon targets when utilizing our nearby qwo air route surveillance radar. I recently came across the following statement. 'Turning off mode-south: a maintenance alert was issued instructing bi-6 sites to stop interrogating mode-south.' from what I can find; qwo is an atcbi-6 site. 'If' qwo is utilizing classical atcrbs interrogations; and our ASR-9 is intermode (full mode south...without the interrogation patch that included classical style atcrbs interrogations...ie; no P4 pulse); that 'may' explain why I observe this difference.

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Original NASA ASRS Text

Title: CMH CONTROLLER DESCRIBED TRANSPONDER RECEPTION PROBLEM WHEN UTILIZING CERTAIN RADAR EQUIPMENT ALONG WITH MODE-S; ALLEGING SOME EQUIPMENT FAILS TO DISPLAY TRANSPONDER.

Narrative: I WAS WORKING SOUTH RADAR (SR) AND OBSERVED 2 VFR TARGETS (1200 SQUAWKS) TO THE SE THAT WERE RDR (PRIMARY TARGETS). THEY WOULD FLIP FLOP FROM BEACON TARGETS WITH ALT TO RDR TARGETS. ONE IN PARTICULAR WAS PROBABLY IN THE RDR STATE FOR 2/3 OF THE TIME. THIS IS NOTICED WHEN OBSERVING THESE TARGETS UTILIZING CMH MODES S. S SATELLITE (SS) WAS NOT OPEN; SO I HAD ITS DISPLAY SET TO UTILIZE OUR LONG-RANGE RADAR IN LONDON; OH (QWO). ON THAT DISPLAY; EVERY TIME I COMPARED; BOTH OF THOSE 1200 TARGETS WERE SHOWING AS SOLID BEACON TARGETS. ONE OF THOSE ACFT; A C172; CALLED ME FOR VFR TA'S WHEN APPROX 40 MI ESE OF UYF (MADISON CO ARPT; LOCATED NEXT TO THE TOWN OF LONDON; OH). IT WASN'T BUSY; SO I ASKED THE PLT WHAT MAKE AND MODEL OF XPONDER HE WAS UTILIZING. THE PLT RESPONDED THAT IT WAS A NARCO AT150. I DIDN'T INQUIRE AS TO WHETHER IT HAD BEEN MODIFIED PER THE AIRWORTHINESS DIRECTIVE (DOCKET NO. 2002-NE-32-AD; AMENDMENT 39-13586; AD 2004-08-16/RIN 2120-AA640). THAT AIRWORTHINESS DIRECTIVE FIXES IT SO IT'LL NOT BE INVISIBLE TO MODE S INTERROGATIONS (AS WELL AS TCAS INTERROGATIONS). I HAVE BEEN NOTICING FOR THE PAST COUPLE MONTHS THAT MORE AND MORE GA LOW-END TYPE ACFT APPEAR TO HAVE INTERMITTENT XPONDERS WHEN VIEWED WITH OUR CMH MODE S RADAR; YET APPEAR AS GOOD BEACON TARGETS WHEN UTILIZING OUR NEARBY QWO AIR RTE SURVEILLANCE RADAR. I RECENTLY CAME ACROSS THE FOLLOWING STATEMENT. 'TURNING OFF MODE-S: A MAINT ALERT WAS ISSUED INSTRUCTING BI-6 SITES TO STOP INTERROGATING MODE-S.' FROM WHAT I CAN FIND; QWO IS AN ATCBI-6 SITE. 'IF' QWO IS UTILIZING CLASSICAL ATCRBS INTERROGATIONS; AND OUR ASR-9 IS INTERMODE (FULL MODE S...WITHOUT THE INTERROGATION PATCH THAT INCLUDED CLASSICAL STYLE ATCRBS INTERROGATIONS...IE; NO P4 PULSE); THAT 'MAY' EXPLAIN WHY I OBSERVE THIS DIFFERENCE.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.