Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP)
Other / Unknown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
ATC Equipment / Nav Facility / Buildings
On approach and heading vectors at 5;000 FT nearing MVY Approach pointed out traffic at our assigned altitude at 12 o'clock. There was no contact visually or TCAS indication. Approach was partially blocked as it sounded as if he was giving an immediate 45 degree turn. We quickly jumped in to have him repeat for our aircraft and he again stated turn immediate to hug 180 degrees. I was ready with TCAS switch to override automation if need be and clicked off automation with a positive turn. In the turn; plane dropped less than 200 FT to 4;850 FT and I quickly corrected. Nothing from ATC was mentioned and we shortly given another heading once clear toward the airport. [Flight was] uneventful from there on.
On arrival; a GLF4 was descending to 5;000; on a vector for a visual approach to Runway 6 at MVY. I observed an unassociated beacon splat ahead of the GLF4 indicating an altitude of 5;000. At times on our RADAR system we see 'false targets' that often last only a sweep or two; and my initial reaction was that the unassociated beacon splat was one of those. When the unassociated beacon splat lasted more than a few sweeps; I decided to treat it as an actual aircraft. The GLF4 was at the same altitude heading right towards the splat; therefore I called traffic and subsequently issued a safety alert for the GLF4 to turn due south. I issued this control instruction while the GLF4 was still in PVD Approach's airspace; a violation of LOA. Reviewing NOP data; the unassociated splat was on a beacon code which mirrored the beacon code of an Air Taxi; who was at 5;000 FT on the FMH 357R/31NM (splat was on the FMH 240R/31NM). The unassociated splat lasted a total duration of 1 minute and 30 seconds. Our FMH ASR-8 RADAR system often detects 'false targets' and it is becoming increasingly difficult to differentiate the false targets from real ones; as they are often lasting for many sweeps in duration. The RADAR needs further troubleshooting to remedy this problem.
An IFR arrival into MVY reported an apparent traffic conflict observed on TCAS and issued by ATC; the ATC report indicated that the conflicting target was likely a 'false target' sometimes experieced with the RADAR system.