Date: 201605
Local time of day: 1201-1800
Location: ZID.ARTCC, IN
Light: Daylight
Aircraft: Military
Operator: Military
Operating under: Part 91
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZID
ATC / advisory: Center ZID
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Distraction; Confusion; Training / Qualification
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe
Detector: Person Air Traffic Control
When detected: In-flight
Contributing factors: Human Factors; Aircraft; ATC Equipment / Nav Facility / Buildings
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: ZTL sector 15 initiated a handoff to me northbound at FL350; all appeared normal. I accepted the handoff and the data block instantly began to show DATA in field E. I asked ZTL15 if they showed a target; they said yes; but I showed none; which didn't make sense. [The aircraft] checked on frequency; I asked them to ident; still showed nothing. About this time it went into COAST TRACK mode. I asked the airplane to squawk a new code and ident; I still showed nothing. Based on a position report the aircraft was now in my sector; and I again called ZTL15 to ask if they showed a target. They said they did; which didn't make sense. I tried other options; including another code change later; position reports; changing the SURV capability in the ERAM (En Route Automation Modernization) template; never saw anything. Briefly advised them 'radar contact lost' but then reestablished radar contact and tracked using primary target. The two ZID sectors north of me likewise never received a transponder reply; and I understand that ZOB never received a transponder reply. It appeared to be a transponder failure except that ZTL continued to show a target; which didn't make sense. After getting relieved; the WSIC (Watch Supervisor in Charge) and I spent some time in the SOC (Service Operations Center) while they investigated several possibilities. The best explanation they could come up with (which is the only plausible explanation I have heard) is that the airplane; a [military signals intelligence aircraft] was using Mode 4 on their transponder. When I first heard this I didn't even know what Mode 4 was; but later internet searching explained it as a special military mode. According to the SOC explanation; very few radar sites display Mode 4 data; and their initial investigation showed that the airplane was only tracked in ZTL by one sensor (radar site QRM); which for them lasted until about halfway through my sector. The initial ZID track prior to me accepting the handoff was 'piggyback data' based on the ZTL track derived from the QRM sensor; but as soon as I took the handoff we showed nothing from ZID radar sites. This would seem to explain why neither ZID nor (apparently) ZOB was never able to show anything other than just a primary target.I'm not sure if this 'Mode 4' transponder is indeed the problem; but it appears to be. Assuming that is correct; then the military pilots possibly need training to advise them about hazards associated with operating their transponders in this mode within domestic airspace. Also assuming that this is correct; controllers possibly need training about how this problem could arise when dealing with military aircraft. If the problem is something else then we seem to have a very serious radar problem.




Synopsis: Indianapolis Center Controller reported of a problem with an aircraft transponder. Aircraft did not show up on reporters' scope but showed up on another long range radar site.