Light Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 14
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic
Procedure
Working the arrival portion of east combined with DRE and SATE. Aircraft Y checked on the RNAV arrival descending via and Aircraft X checked on about 7-8 miles behind also descending via. As Aircraft Y began a slow descent they slowed while Aircraft X did not. As the distance rapidly decreased I stopped Aircraft X at 11;000 to keep them from hitting Aircraft Y. The Conflict Alert (CA) went off (way too late like it always does now that we have been forced to change to fusion and the STARS software wasn't updated). Aircraft X continued at 11;000 and actually passed above Aircraft Y well over 100 knots faster. Aircraft X ended up first in the sequence due to their speed which may have been their plan all along. There is nothing in the RNAV arrivals that made this illegal. However this shows it is not a safe procedure. Almost every pilot flies it different and there is no automatic safety net provided where all the aircraft should be at the same speed descending via a profile to the same altitude. They can come into our airspace at flight levels going 400 plus knots or come in over the boundary already at 100 with 70 miles left to fly doing 210 knots and both are perfectly legal. As you can see by this example this can really burn you. If you are distracted with other duties it can sneak up on you in a heartbeat. I recommend that we add speeds and altitudes on the arrival tying the pilots down to more of a 'tunnel' instead of leaving it so wide open.
Discussion with reporter was about the Fusion Radar system and STARS. Reporter thinks that STARS may not have the capability for such a fast update from the Fusion radar; which causes the CA to possibly be delayed. Fusion updates according to the reporter every half second; where before using STARS and a single radar it updated about every 10 seconds.
Controller reports of incident where the trailing aircraft doesn't slow and overtakes preceding aircraft which are separated by 1;000 feet. Reporter states that Conflict Alert goes off but not in a timely manner which may be due to use of fusion radar and a software problem.