Narrative:

I was working a radar-side sector. I had two aircraft; aircraft X at 37000 ft and aircraft Y at 33000 ft. Since I had aircraft X already faster and slightly behind aircraft Y I decided to speed up the aircraft X and make him first. I was watching the speed and had around 40 knots in front. When they started to get closer to a fix we have to have them cross at 29000 ft I realized I needed to get aircraft X down to 35000 ft for crossing traffic at 36000 ft. At this time aircraft Y was barely outside of the 5 mile ring I had up so I descended aircraft X to 35000 ft now then cross a fix at 29000 ft. At that time conflict alert went off between aircraft X and aircraft Y. After observing the alert I descended aircraft Y to have him cross a fix at 29000 ft. I was watching it very closely; the speed; the altitude and the distance between them. The closest I saw them get on my conflict alert box was 5.03 miles and then I started to gain distance from there. Now fast forward to the next week when I come in from my days off. I was immediately pulled behind the sectors and told that I had a loss of separation between these two aircraft. I was asked if I remembered what happened and I told my supervisor that I never went below 5 miles and she told me I did; I had 4.98 miles. Then she told me that her boss told her she needs to write me a performance write up for running aircraft too close. I told her again that I watched the conflict alert box as they got close and the closest they got was 5.03. She told me that falcon radar replay showed something different. Then an it guy came and said that he had the replay of my scope loaded and that I could watch it if I wanted. I said absolutely and went back with him to watch it. After watching it again the closest it showed was 5.03 miles like I had already said. I told my supervisor that and told her I wanted to see the falcon replay. I asked two times throughout the night if I could and both times she told me later so it never happened.when I came into work the next day I told them I needed time to watch the falcon replay and file my report. The supervisor then asked why I didn't do it yesterday and I told her she never gave me time to do so. I went back and watched the falcon and this is what I saw. I had 5.03 and 800 ft; [a few seconds later] I had 4.98 and 600 ft; [and then a few seconds after that] I had 5.05 and 500 ft. I feel like they are getting that 4.98 miles from the aircraft Y updating on the radar and the aircraft X not yet. I have watched the replay of what my scope showed and it never went below 5.03 miles. So I feel like I didn't have a loss of separation based on watching both. I don't know what I recommend but I would like to know how my radar scopes' conflict alert says 5.03 miles and they are saying 4.98 and then that means I had a deal. I feel like I have to have full confidence in what my display is showing me. If I'm being judged on some other superior equipment then why doesn't my scope use that same equipment?

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Original NASA ASRS Text

Title: ZLC Center Controller reported that a Falcon replay indicated a loss of separation between two aircraft while the actual radar scope indicated nominal separation.

Narrative: I was working a Radar-side sector. I had two aircraft; Aircraft X at 37000 ft and Aircraft Y at 33000 ft. Since I had Aircraft X already faster and slightly behind Aircraft Y I decided to speed up the Aircraft X and make him first. I was watching the speed and had around 40 knots in front. When they started to get closer to a fix we have to have them cross at 29000 ft I realized I needed to get Aircraft X down to 35000 ft for crossing traffic at 36000 ft. At this time Aircraft Y was barely outside of the 5 mile ring I had up so I descended Aircraft X to 35000 ft now then cross a fix at 29000 ft. At that time conflict alert went off between Aircraft X and Aircraft Y. After observing the alert I descended Aircraft Y to have him cross a fix at 29000 ft. I was watching it very closely; the speed; the altitude and the distance between them. The closest I saw them get on my Conflict Alert box was 5.03 miles and then I started to gain distance from there. Now fast forward to the next week when I come in from my days off. I was immediately pulled behind the sectors and told that I had a loss of separation between these two aircraft. I was asked if I remembered what happened and I told my supervisor that I never went below 5 miles and she told me I did; I had 4.98 miles. Then she told me that her boss told her she needs to write me a Performance write up for running aircraft too close. I told her again that I watched the Conflict Alert box as they got close and the closest they got was 5.03. She told me that Falcon Radar replay showed something different. Then an IT guy came and said that he had the replay of my scope loaded and that I could watch it if I wanted. I said absolutely and went back with him to watch it. After watching it again the closest it showed was 5.03 miles like I had already said. I told my supervisor that and told her I wanted to see the Falcon replay. I asked two times throughout the night if I could and both times she told me later so it never happened.When I came into work the next day I told them I needed time to watch the Falcon replay and file my report. The Supervisor then asked why I didn't do it yesterday and I told her she never gave me time to do so. I went back and watched the Falcon and this is what I saw. I had 5.03 and 800 ft; [a few seconds later] I had 4.98 and 600 ft; [and then a few seconds after that] I had 5.05 and 500 ft. I feel like they are getting that 4.98 miles from the Aircraft Y updating on the radar and the Aircraft X not yet. I have watched the replay of what my scope showed and it never went below 5.03 miles. So I feel like I didn't have a loss of separation based on watching both. I don't know what I recommend but I would like to know how my radar scopes' conflict alert says 5.03 miles and they are saying 4.98 and then that means I had a deal. I feel like I have to have full confidence in what my display is showing me. If I'm being judged on some other superior equipment then why doesn't my scope use that same equipment?

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.