Narrative:

Air carrier X, an medium large transport, was descending from 14000 ft to 10000 ft on a course from bands intersection direct to kayoh intersection, en route to lgb. This route passes approximately 2 mi north of hdf on a 245 degree bearing. As air carrier X was approaching 11000 ft, he advised ATC he was climbing for a TCASII RA. As I looked at the radar for traffic, I observed the mode C of a 1200 (VFR) beacon code change from 2700 ft MSL to 10700 ft MSL in one sweep of the radar. This occurred when the targets were about 2 mi, or less, apart. The aircraft on the 1200 beacon code was a USAF mlt Y in the pattern at riv (march AFB). As soon as the targets passed each other, the mlt's mode C indicated 2500 ft MSL. Air carrier X stated he never saw any traffic. About 9 months ago this phenomena was a rather common event. Frequently, when working an IFR aircraft, I would observe that the mode C of a 1200 beacon code aircraft would suddenly jump from a low altitude to a higher altitude nearly approximating the mode C of the IFR aircraft. This would only occur when the 2 aircraft would reach a point where the targets were about to merge. Once the targets passed each other, the mode C of the 1200 beacon code aircraft would return to normal. It appears that this problem may have returned. Callback conversation with reporter revealed the following information: reporter states that the mode C jump occurs maybe once a day, or possibly once a week. It only happens when there is another aircraft approaching on a discrete beacon code. It is not restr to only military aircraft, but also involves other types. No conflicts have been reported, but reporter feels that one may if the controller or pilot react to the false track jump. Facility has an ASR-9 system that has been in service for about 1 month. The mode C jump problem has been ongoing for about 1 yr. Doesn't think that a ucr has been filed on this, and not sure if facility manager is aware of it.

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Original NASA ASRS Text

Title: MODE C JUMP OCCURS WHENEVER A DISCRETE BEACON CODE ACFT GETS CLOSE.

Narrative: ACR X, AN MLG, WAS DSNDING FROM 14000 FT TO 10000 FT ON A COURSE FROM BANDS INTXN DIRECT TO KAYOH INTXN, ENRTE TO LGB. THIS RTE PASSES APPROX 2 MI N OF HDF ON A 245 DEG BEARING. AS ACR X WAS APCHING 11000 FT, HE ADVISED ATC HE WAS CLBING FOR A TCASII RA. AS I LOOKED AT THE RADAR FOR TFC, I OBSERVED THE MODE C OF A 1200 (VFR) BEACON CODE CHANGE FROM 2700 FT MSL TO 10700 FT MSL IN ONE SWEEP OF THE RADAR. THIS OCCURRED WHEN THE TARGETS WERE ABOUT 2 MI, OR LESS, APART. THE ACFT ON THE 1200 BEACON CODE WAS A USAF MLT Y IN THE PATTERN AT RIV (MARCH AFB). AS SOON AS THE TARGETS PASSED EACH OTHER, THE MLT'S MODE C INDICATED 2500 FT MSL. ACR X STATED HE NEVER SAW ANY TFC. ABOUT 9 MONTHS AGO THIS PHENOMENA WAS A RATHER COMMON EVENT. FREQUENTLY, WHEN WORKING AN IFR ACFT, I WOULD OBSERVE THAT THE MODE C OF A 1200 BEACON CODE ACFT WOULD SUDDENLY JUMP FROM A LOW ALT TO A HIGHER ALT NEARLY APPROXIMATING THE MODE C OF THE IFR ACFT. THIS WOULD ONLY OCCUR WHEN THE 2 ACFT WOULD REACH A POINT WHERE THE TARGETS WERE ABOUT TO MERGE. ONCE THE TARGETS PASSED EACH OTHER, THE MODE C OF THE 1200 BEACON CODE ACFT WOULD RETURN TO NORMAL. IT APPEARS THAT THIS PROB MAY HAVE RETURNED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE MODE C JUMP OCCURS MAYBE ONCE A DAY, OR POSSIBLY ONCE A WK. IT ONLY HAPPENS WHEN THERE IS ANOTHER ACFT APCHING ON A DISCRETE BEACON CODE. IT IS NOT RESTR TO ONLY MIL ACFT, BUT ALSO INVOLVES OTHER TYPES. NO CONFLICTS HAVE BEEN RPTED, BUT RPTR FEELS THAT ONE MAY IF THE CTLR OR PLT REACT TO THE FALSE TRACK JUMP. FACILITY HAS AN ASR-9 SYS THAT HAS BEEN IN SVC FOR ABOUT 1 MONTH. THE MODE C JUMP PROB HAS BEEN ONGOING FOR ABOUT 1 YR. DOESN'T THINK THAT A UCR HAS BEEN FILED ON THIS, AND NOT SURE IF FACILITY MGR IS AWARE OF IT.

Data retrieved from NASA's ASRS site as of July 2007 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.