A B737 flight crew received a TCASII TA and RA on short final at lax.

Attributes
ACN	441276
Time
Date	199906
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lax.airport
State Reference	CA
Altitude	msl single value : 1200
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : lax.tower
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 25l other
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	411276
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	conflict : airborne less severe
Independent Detector	aircraft equipment : tcas
Resolutory Action	flight crew : took precautionary avoidance action
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Ambiguous

Narrative:

XA06Z, on short final to 25L at lax (3-4 mi) at approximately 1200 ft, a TA was observed and a target on our navigation display indicated slightly left and below our aircraft by 300 ft. The TA was immediately followed by an RA and guidance to 'pull up' and 'monitor vertical speed.' the target at that time was displayed at 200 ft immediately below our aircraft. Visual identify of the target aircraft was not made at any time. Confign was gear down and 30 degrees flaps. A climb was initiated immediately upon receiving the RA and guidance. The response to an RA was reported. A return to the normal glide path was executed at about 400-500 ft and a normal landing accomplished. I called the lax tower chief upon arrival at the gate, and was informed that at no time was a primary target reported on radar and that an anomaly known as 'split signal' was suspected. (A condition that overlapping radar returns can actually build a false target and generate a TA and/or RA. I reported the incident to dispatch as a near miss and requested follow-up action.

Original NASA ASRS Text

Title: A B737 FLC RECEIVED A TCASII TA AND RA ON SHORT FINAL AT LAX.

Narrative: XA06Z, ON SHORT FINAL TO 25L AT LAX (3-4 MI) AT APPROX 1200 FT, A TA WAS OBSERVED AND A TARGET ON OUR NAV DISPLAY INDICATED SLIGHTLY L AND BELOW OUR ACFT BY 300 FT. THE TA WAS IMMEDIATELY FOLLOWED BY AN RA AND GUIDANCE TO 'PULL UP' AND 'MONITOR VERT SPD.' THE TARGET AT THAT TIME WAS DISPLAYED AT 200 FT IMMEDIATELY BELOW OUR ACFT. VISUAL IDENT OF THE TARGET ACFT WAS NOT MADE AT ANY TIME. CONFIGN WAS GEAR DOWN AND 30 DEGS FLAPS. A CLB WAS INITIATED IMMEDIATELY UPON RECEIVING THE RA AND GUIDANCE. THE RESPONSE TO AN RA WAS RPTED. A RETURN TO THE NORMAL GLIDE PATH WAS EXECUTED AT ABOUT 400-500 FT AND A NORMAL LNDG ACCOMPLISHED. I CALLED THE LAX TWR CHIEF UPON ARR AT THE GATE, AND WAS INFORMED THAT AT NO TIME WAS A PRIMARY TARGET RPTED ON RADAR AND THAT AN ANOMALY KNOWN AS 'SPLIT SIGNAL' WAS SUSPECTED. (A CONDITION THAT OVERLAPPING RADAR RETURNS CAN ACTUALLY BUILD A FALSE TARGET AND GENERATE A TA AND/OR RA. I RPTED THE INCIDENT TO DISPATCH AS A NEAR MISS AND REQUESTED FOLLOW-UP ACTION.

Data retrieved from NASA's ASRS site as of July 2007 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.