Narrative:

While working harbor radar; lgb tower called and requested diverging headings on 2 'general aviation' departures; which I approved. While working other traffic; a C152 departed lgb on a left turn heading 160. I identified the aircraft and climbed him to 4;000 ft. The C152 was about 2 NM south of lgb. Shortly there-after; a C182 checked in on frequency at the same time lgb tower was calling to advise that the C182's transponder wasn't showing a squawk code. After hearing lgb tower over the speaker; I immediately issued the correct code to the C182; and then answered lgb tower acknowledging their call. I observed a target near lgb's departure corridor at 400 ft 1 NM west of the airport. I assumed that was the targets I was waiting to 'tag up'. At the same time; I issued a heading of 200 degrees to the C152 for climb above arriving traffic to toa and also to sequence behind other en route traffic he was following. While issuing that heading; I placed a 'bat' on the target I thought to be the C182 and saw the 'bat' pointing northeast bound. Not a heading any departure would be on. Immediately I turned the C152 back to a heading of 160 for divergence. The pilot of the C182 then replied that they got their transponder working; and they tagged up on their code about 2 NM northwest of the C152. Prior to this; there was a short time where there was no indicated primary or secondary return in that area. This is why I turned the C152 in that direction. [We need] better radar coverage for low level aircraft in the vicinity of lgb harbor area.

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Original NASA ASRS Text

Title: SCT Controller experienced a loss of separation event between two LGB IFR departure aircraft when questionable RADAR tracking failed to properly display the target aircraft; the reporter suggesting improve low level RADAR coverage in the subject area.

Narrative: While working Harbor RADAR; LGB Tower called and requested diverging headings on 2 'General Aviation' departures; which I approved. While working other traffic; a C152 departed LGB on a left turn heading 160. I identified the aircraft and climbed him to 4;000 FT. The C152 was about 2 NM south of LGB. Shortly there-after; a C182 checked in on frequency at the same time LGB Tower was calling to advise that the C182's transponder wasn't showing a squawk code. After hearing LGB Tower over the speaker; I immediately issued the correct code to the C182; and then answered LGB Tower acknowledging their call. I observed a target near LGB's departure corridor at 400 FT 1 NM west of the airport. I assumed that was the targets I was waiting to 'Tag Up'. At the same time; I issued a heading of 200 degrees to the C152 for climb above arriving traffic to TOA and also to sequence behind other en route traffic he was following. While issuing that heading; I placed a 'bat' on the target I thought to be the C182 and saw the 'bat' pointing northeast bound. Not a heading any departure would be on. Immediately I turned the C152 back to a heading of 160 for divergence. The pilot of the C182 then replied that they got their transponder working; and they tagged up on their code about 2 NM northwest of the C152. Prior to this; there was a short time where there was no indicated primary or secondary return in that area. This is why I turned the C152 in that direction. [We need] better RADAR coverage for low level aircraft in the vicinity of LGB harbor area.

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.