Narrative:

Aircraft X was descending via the RNAV arrival. Aircraft Y was from the south and also descending via the WITTI2 arrival. The aircraft were initially tied. Aircraft Y started down first and slowed anywhere from 10-80 knots slower than aircraft X. Spacing was building steadily between the two aircraft. When the two aircraft were just about in trail; there was 6.5 miles with aircraft Y 10-30 knots slower. Less than a minute later there was 6 miles with aircraft Y and still 10-30 knots slower. Passing it off as a bad hit; I knew the spacing was fine with the speeds shown. I reduced aircraft Y to 250 knots to insure the separation remained. Maybe a minute later; around the time to handoff the aircraft to approach; I saw that space had decreased again to 4.84 miles with aircraft Y still showing slower! By this time; the aircraft were entering TRACON airspace. Knowing that they only needed 4 miles between the aircraft; I just shipped them over.I am completely baffled by this scenario. How could I lose separation with 6.5 miles and faster in front? We replayed the data from TRACON's radar feed and it showed an overtake situation. We are waiting to pull the data from our facility to see what ours showed. I do not recall an overtake the entire time I was watching this. Talking to a veteran controller that has been in this facility for over 25 years; he said he has seen this happen too. He said he saw separation decreasing with faster in front as they were approaching the gate at the TRACON boundary. We do have a radar outage at one site but we still have coverage from other radar sires. I don't know that this would affect anything. I don't have any recommendations yet. There shouldn't be a disparity between what the targets are doing and what the speeds say.

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Original NASA ASRS Text

Title: A Center controller reported an aircraft was overtaking a leading aircraft even though the ground speed readouts indicated the trailing aircraft was slower.

Narrative: Aircraft X was descending via the RNAV arrival. Aircraft Y was from the south and also descending via the WITTI2 arrival. The aircraft were initially tied. Aircraft Y started down first and slowed anywhere from 10-80 knots slower than Aircraft X. Spacing was building steadily between the two aircraft. When the two aircraft were just about in trail; there was 6.5 miles with Aircraft Y 10-30 knots slower. Less than a minute later there was 6 miles with Aircraft Y and still 10-30 knots slower. Passing it off as a bad hit; I knew the spacing was fine with the speeds shown. I reduced Aircraft Y to 250 Knots to insure the separation remained. Maybe a minute later; around the time to handoff the aircraft to Approach; I saw that space had decreased again to 4.84 miles with Aircraft Y still showing slower! By this time; the aircraft were entering TRACON Airspace. Knowing that they only needed 4 miles between the aircraft; I just shipped them over.I am completely baffled by this scenario. How could I lose separation with 6.5 miles and faster in front? We replayed the data from TRACON's radar feed and it showed an overtake situation. We are waiting to pull the data from our facility to see what ours showed. I do not recall an overtake the entire time I was watching this. Talking to a veteran controller that has been in this facility for over 25 years; he said he has seen this happen too. He said he saw separation decreasing with faster in front as they were approaching the gate at the TRACON boundary. We do have a radar outage at one site but we still have coverage from other radar sires. I don't know that this would affect anything. I don't have any recommendations yet. There shouldn't be a disparity between what the targets are doing and what the speeds say.

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.