Originally Posted by
055166k
At London ACC Swanwick your mode C height readout disappears completely from the SSR label during high rates of descent. I might not see the rapid descent rate, and I would be left with a callsign label and no visual cue to any problem. I think a "fix" is under way after years of asking.
NATS’ radars are optimised to record changes of height of up to 1,000ft/scan. Any ROC/D above this rate will be marked as invalid and will not be displayed by any RDP system. Scan rates vary between 10 seconds per scan for a 6 RPM radar and 4 seconds per scan for a 15 RPM radar. This equates to an ability to track a climb/descent of between 6,000 and 15,000 fpm.
NATS’ radars turn at different rates so in order to prevent the application of different rates in different parts of UK airspace, an average scan rate has been adopted. The average scan rate of 7.5 secs/scan therefore equates to a ROC/D of 8,000ft/min.
The design parameters of STCA include the ability to track ROC/D up to 11,000ft/min. However STCA is dependant on the data rates received from the sensors and so the 8,000ft/min average becomes the limiting factor in STCA performance.
As a consequence of these limitations until such time as changes are made to both ground RDP systems and ground/airborne safety nets,
all aircraft operating within UK controlled airspace will be restricted to a maximum ROC/D of 8,000ft/min. The restriction is effective AIRAC 7/2007 on 5 July 2007. It will be promulgated by AIC, with a subsequent AIP amendment in ENR 1-1-3-1 Para 2.2.1.
BD