A friend of mine who works at Sweden Control (but was not at work during the incident) tells me that the MSSR system (1030/1090 MHz) was compromised such that the same aircraft could be displayed up to 10 times along a 20-30 mile track. The disturbance was "medium", which is ironic; if the disturbance had been stronger, the system would have filtered out the bad echoes by itself. The "medium" strength probably explains why the event was not accompanied by increased auroral activity. We've had a few aurora nights at 64N in the recent weeks, but this clear night showed none (bummer).
My friend also stated that procedural routes have either to be purely VOR-based or defined (and approved by the authorities) as "separated", neither of which is common in Scandinavia any more.
Overflying traffic was not affected as they could easily be separated by altitude.
I have heard no report of affected GPS signals nor observed any unusual anomalies (on the ground) myself. Since GPS operate near MSSR frequencies, it would be expected that it too experienced anomalies. My speculation is that if GPS was less largely hit, the difference might be explained by the difference in signal travel direction between MSSR (effectively horizontally) and GPS (more slanted towards vertical, even at 60+ deg north).
His comment that the radar system can filter out strong disturbances intrigues me. Could this be an "automatic" effect because the return signal would appear outside some tight tolerance defined by the Mode A/C/S protocol or is it more likely it is handled in an additional software layer? Any comment from an RF/MSSR-knowledgeable person would be appreciated.
(Edit) The end result was that aircraft in the air were allowed to land but no domestic aircraft were allowed to take off. Don't know about international departures. Normal OPS were restarted after about 1.5 hours. (/Edit)
(Edit 2) After re-reading the media report of that day (
http://www.dn.se/nyheter/sverige/sol...ll-radarstopp/) I can add the following: All radar stations that were affected had the sun above the horizon. All false radar echoes appeared within an 8 degree narrow sector in the sunward direction. (/Edit 2)
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