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Old 9th Mar 2014, 14:46
  #875 (permalink)  
andrasz
 
Join Date: Sep 2008
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Nevertheless, I'm confused - why can they not say...
Because they don't know. The issue of radar coverage is rather complex:

Civilian ATC is used to monitor and control civilian airspace. It uses two types of radars - Primary Surveillance Radar (PSR) and Secondary Surveillance Radar (SSR). The first is a physical 'echo' from any object within range (as already discussed, that is typically 60NM), the second is essentially a two-way radio communication between two computers using an interrogation - response protocol. Most of the world (except the deep ocean regions and some very remote land areas) are covered by SSR, but PSR is typically based at major airports to cover the surrounding airspace. The information from both sources are displayed on ATC radar screens, and in an area with dual coverage an aircraft with no transponder signal will show up as an unidentified blip on the screen. However in areas with only SSR, aircraft with no transponder signal will not show up.

When MH370 'disappeared from radar', the transponder signal was lost. But that does not immediately equate to anything happening with the aircraft, it just means that there is no transponder response. This is why Malaysian ATC tried for another hour to contact the aircraft, and only when it should have been visible to Vietnamese PSR did they raise the alarm that something was amiss.

Military radar in normal peacetime works the opposite way. Rather than controlling civilian airspace, it just monitors it, with a real-time data link to civilian ATC. On military monitors the known and identified targets are blanked out, so observers can focus on any unidentified targets. Thus military radar would not have monitored MH370, and if in the few seconds between loss of transponder signal and descent below observable height an unidentified blip would have appeared, that would have likely gone unnoticed. In all such cases it is a lengthy reconstruction process to retrieve the primary military radar data (which may first need to be 'weeded' to remove traces of any hush-hush activity), then match all targets with known and identified aircraft before anything may be said with any certainty.

I'm sure this is being done as we speculate in vain. In such cases the radar manufacturer may also be involved to attempt to amplify any possible weak signals that would have been filtered out as background noise by the processing software, but would still be recorded within the primary raw data. Naturally this takes time measurable in days or weeks, and is usually done as a part of an investigation process if no other sources are available to reconstruct the last moments of an aircraft.

Last edited by andrasz; 9th Mar 2014 at 15:03.
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