Narrative:

I was on the sector and conflict alert activated between an IFR aircraft; believe it was as 29;000 feet but do not remember the call sign; and a mode C intruder (data tag beacon code 1200) at 29;300 feet. I gave a traffic call to the IFR aircraft; at this point he was about 25-28 miles north and his course kept him 4-6 miles west of the target. I told the aircraft that there was a target; with a position from his location; with mode C altitude reading 29;300 feet appearing to be maneuvering on a beacon code of 1200. I also included that we believed it was an aircraft with bad mode c; but we were trying to verify that; he requested to remain updated. After about a minute I updated him again; and let him know that it appeared he was still maneuvering and would be 4-6 miles west of the IFR aircraft course; he acknowledged with no request made. Shortly thereafter he passed abeam the 1200 mode c intruder; no closer than 4.5 miles. That first aircraft did report seeing a single engine aircraft passing 1;000 feet above as the mode c intruder was indicating FL293 and 4.5 miles west of the IFR aircraft. I advised the IFR aircraft that there was an E170 that just passed over at 34;000 feet and he acknowledged with no other comments made in regard to the mode c intruder.for the next hour and a half the mode c intruder remained in the same general area maneuvering. During that time there were multiple aircraft that either elected to take a vector around; or climb above the mode c target; do not have an exact number but it would easily be 20 or more. This does not include all traffic calls for other aircraft passing in close proximity.all aircraft; outside of the first aircraft that I believe saw an E170 crossing above him; reported seeing the target on TCAS; but never saw anything outside.the mode c intruder went away and was not seen again. I recall seeing a few hits of a non mode c target being displayed in the vicinity of airport ZZZ shortly after that; but that is all.

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Original NASA ASRS Text

Title: A Controller observed a Beacon Target squawking 1200 with a Mode C indicating 29;300 feet. Numerous aircraft were vectored away from this target. No aircraft reported seeing any traffic at the position of the beacon target.

Narrative: I was on the sector and conflict alert activated between an IFR aircraft; believe it was as 29;000 feet but do not remember the call sign; and a mode C intruder (data tag beacon code 1200) at 29;300 feet. I gave a traffic call to the IFR aircraft; at this point he was about 25-28 miles north and his course kept him 4-6 miles west of the target. I told the aircraft that there was a target; with a position from his location; with mode C altitude reading 29;300 feet appearing to be maneuvering on a beacon code of 1200. I also included that we believed it was an aircraft with bad mode c; but we were trying to verify that; he requested to remain updated. After about a minute I updated him again; and let him know that it appeared he was still maneuvering and would be 4-6 miles west of the IFR aircraft course; he acknowledged with no request made. Shortly thereafter he passed abeam the 1200 mode c intruder; no closer than 4.5 miles. That first aircraft did report seeing a single engine aircraft passing 1;000 feet above as the mode c intruder was indicating FL293 and 4.5 miles west of the IFR aircraft. I advised the IFR aircraft that there was an E170 that just passed over at 34;000 feet and he acknowledged with no other comments made in regard to the mode c intruder.For the next hour and a half the mode c intruder remained in the same general area maneuvering. During that time there were multiple aircraft that either elected to take a vector around; or climb above the mode c target; do not have an exact number but it would easily be 20 or more. This does not include all traffic calls for other aircraft passing in close proximity.All aircraft; outside of the first aircraft that I believe saw an E170 crossing above him; reported seeing the target on TCAS; but never saw anything outside.The mode c intruder went away and was not seen again. I recall seeing a few hits of a non mode c target being displayed in the vicinity of Airport ZZZ shortly after that; but that is all.

Data retrieved from NASA's ASRS site 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.