Narrative:

Aircraft X was climbing to his requested altitude. Aircraft Y was underneath and in front of aircraft X. Due to westbound traffic; and not wanting to delay aircraft Y at FL270 for 60 miles; I decided to stop the climb of aircraft X at FL270; and climb aircraft Y through aircraft X. Aircraft Y was out climbing aircraft X 4:1 at one point. There was a 60-ish knot overtake the entire time; with aircraft X behind aircraft Y; however it was a controlled and calculated overtake and climb. I calculated the closure rate; the climb rate of aircraft Y; put a 'J-ring' on aircraft Y; and decided there was no risk involved with this maneuver. Apparently; my calculation was 8 seconds off. The loss of separation occurred. This is the first operational error of my career. There was no traffic complexity or volume in the sector. I had nothing to watch except this situation; and that's what I was watching. Not once did I observe the target of aircraft X go inside of the 'J-ring' on aircraft Y. Had I thought my calculations were incorrect and this was going to be a loss of separation; it was incredibly easy to fix; using turns; stopping an aircraft's climb; etc. It was well calculated and was not going to be an issue. The fact that I was apparently 8 seconds off (2/3 of a single radar sweep) in my calculation could be attributed to any number of factors; including a late report by the pilot; right down to the acceptable margin of error for the radar mosaic resolution; especially considering multiple radar sort boxes were in play. The lateral separation was reported to me as 4.96 NM...or less than 50 feet horizontally. If someone can actually prove to me that the aircraft were 49 feet off of those 5 miles; I'll eat my shoe. Like I said; I monitored the entire situation almost exclusively and not once did I observe the target inside the 'bubble'. A move to ads-B/nexgen radar so we can actually get accurate position resolution.

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Original NASA ASRS Text

Title: Enroute Controller experienced a loss of separation detected by automation; the reported estimating the separation loss at less than fifty feet.

Narrative: Aircraft X was climbing to his requested altitude. Aircraft Y was underneath and in front of Aircraft X. Due to westbound traffic; and not wanting to delay Aircraft Y at FL270 for 60 miles; I decided to stop the climb of Aircraft X at FL270; and climb Aircraft Y through Aircraft X. Aircraft Y was out climbing Aircraft X 4:1 at one point. There was a 60-ish knot overtake the entire time; with Aircraft X behind Aircraft Y; however it was a controlled and calculated overtake and climb. I calculated the closure rate; the climb rate of Aircraft Y; put a 'J-Ring' on Aircraft Y; and decided there was no risk involved with this maneuver. Apparently; my calculation was 8 seconds off. The loss of separation occurred. This is the first operational error of my career. There was no traffic complexity or volume in the sector. I had nothing to watch except this situation; and that's what I was watching. Not once did I observe the target of Aircraft X go inside of the 'J-Ring' on Aircraft Y. Had I thought my calculations were incorrect and this was going to be a loss of separation; it was incredibly easy to fix; using turns; stopping an aircraft's climb; etc. It was well calculated and was not going to be an issue. The fact that I was apparently 8 seconds off (2/3 of a single RADAR sweep) in my calculation could be attributed to any number of factors; including a late report by the pilot; right down to the acceptable margin of error for the RADAR mosaic resolution; especially considering multiple RADAR Sort Boxes were in play. The lateral separation was reported to me as 4.96 NM...or less than 50 feet horizontally. If someone can actually prove to me that the aircraft were 49 feet off of those 5 miles; I'll eat my shoe. Like I said; I monitored the entire situation almost exclusively and not once did I observe the target inside the 'bubble'. A move to ADS-B/NEXGEN RADAR so we can actually get accurate position resolution.

Data retrieved from NASA's ASRS site as of July 2013 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.