Narrative:

I was working tustin radar; the arrival sector for sna in the morning. The weather was IFR with strong gusty winds from the west. Aircraft X was on a downwind heading at 3000 ft. For the ILS approach to sna. Shortly after issuing the base leg heading of 100 I observed the target take a hard turn due south and a rapid descent from 3000 to 1600 ft. In a 3000 ft. MVA. The turn was so sharp and the descent so rapid that I thought it was a bad return from the stars (standard terminal automation replacement system) tracker. Instead of issuing low altitude alert instructions I asked the pilot to verify that they were level at 3000. The pilot reported back that they were climbing back up and the altitude read out went to XXX. Once the aircraft climbed back up I issued a heading of 090 for the ILS. After clearing aircraft X for the ILS the aircraft again dove down and dramatically turned to a heading approximately 30 degrees west bound away from the approach and into another aircraft on the downwind at 3000. If the pilot had not already done this I would have also thought it was a stars issue. The two aircraft lost separation. Approximate separation was 1.9 miles and 100 ft. I turned aircraft X back towards the final and instructed the aircraft to maintain 3000 as the MVA in the area is 3000. I issued traffic but both aircraft were IMC. I then canceled the approach clearance for aircraft X and climbed the aircraft to 4000. When aircraft X reached 4000 he advised that he was canceling IFR and wanted to proceed VFR to rnm. This event has been reported to the highest level of management at sct.the week prior to this incident I was working an aircraft into toa. The aircraft was established on the GPS approach to toa when the stars tracker showed the aircraft jump 1/2 mile north of the final towards traffic. When the aircraft's target turned northbound toward traffic I asked the pilot if they had turned northbound. The pilot reported that they were still pointed at the airport. The aircraft's speed increased to 360 kts and then jumped 1/2 mile to the south of the final into a 2700 ft. MVA. This incident was reported as a stars issue.the controllers at sct have lost confidence in stars. We do not know if the data we observe on the scope is accurate information. We hesitate to issue instructions as we wait to see if what we are seeing is a stars issue or is an accurate representation of what is actually occurring. We hesitate to issue immediate instructions because if the data is wrong we risk creating other issues. If I would have issued a traffic alert to the aircraft landing toa and issued instructions to climb and turn left immediately to avoid traffic I would have interrupted the pilot in a critical phase of flight; turning the aircraft into an MVA based on erroneous data provided by stars. If both incidents I described occurred with carts (common automated radar terminal system) I would have known that what I was seeing was true and I would have issued instructions immediately. But instead; we have become so numb to the erroneous information from stars that we hesitate and second guess. This hesitation and second guessing may cause fatalities as the instructions we give will be too late to save the situation.I recommend that sct falls back to carts or fatalities may occur.

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Original NASA ASRS Text

Title: SCT Controller reported of a target making a hard turn and descending into a lover MVA. Controller couldn't tell if the aircraft was in fact acting erratic or an issue with STARS. Controller stated he and others at SCT would like to see a return to CARTS.

Narrative: I was working Tustin radar; the arrival sector for SNA in the morning. The weather was IFR with strong gusty winds from the west. Aircraft X was on a downwind heading at 3000 ft. for the ILS approach to SNA. Shortly after issuing the base leg heading of 100 I observed the target take a hard turn due south and a rapid descent from 3000 to 1600 ft. in a 3000 ft. MVA. The turn was so sharp and the descent so rapid that I thought it was a bad return from the STARS (Standard Terminal Automation Replacement System) tracker. Instead of issuing low altitude alert instructions I asked the pilot to verify that they were level at 3000. The pilot reported back that they were climbing back up and the altitude read out went to XXX. Once the aircraft climbed back up I issued a heading of 090 for the ILS. After clearing Aircraft X for the ILS the aircraft again dove down and dramatically turned to a heading approximately 30 degrees west bound away from the approach and into another aircraft on the downwind at 3000. If the pilot had not already done this I would have also thought it was a STARS issue. The two aircraft lost separation. Approximate separation was 1.9 miles and 100 ft. I turned Aircraft X back towards the final and instructed the aircraft to maintain 3000 as the MVA in the area is 3000. I issued traffic but both aircraft were IMC. I then canceled the approach clearance for Aircraft X and climbed the aircraft to 4000. When Aircraft X reached 4000 he advised that he was canceling IFR and wanted to proceed VFR to RNM. This event has been reported to the highest level of management at SCT.The week prior to this incident I was working an aircraft into TOA. The aircraft was established on the GPS approach to TOA when the STARS tracker showed the aircraft jump 1/2 mile north of the final towards traffic. When the aircraft's target turned northbound toward traffic I asked the pilot if they had turned northbound. The pilot reported that they were still pointed at the airport. The aircraft's speed increased to 360 kts and then jumped 1/2 mile to the south of the final into a 2700 ft. MVA. This incident was reported as a STARS issue.The controllers at SCT have lost confidence in STARS. We do not know if the data we observe on the scope is accurate information. We hesitate to issue instructions as we wait to see if what we are seeing is a STARS issue or is an accurate representation of what is actually occurring. We hesitate to issue immediate instructions because if the data is wrong we risk creating other issues. If I would have issued a traffic alert to the aircraft landing TOA and issued instructions to climb and turn left immediately to avoid traffic I would have interrupted the pilot in a critical phase of flight; turning the aircraft into an MVA based on erroneous data provided by STARS. If both incidents I described occurred with CARTS (Common Automated Radar Terminal System) I would have known that what I was seeing was true and I would have issued instructions immediately. But instead; we have become so numb to the erroneous information from STARS that we hesitate and second guess. This hesitation and second guessing may cause fatalities as the instructions we give will be too late to save the situation.I recommend that SCT falls back to CARTS or fatalities may occur.

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.