What parts of Austrian airspace are not covered by primary radars?
The radar system in Austria is know by the name Goldhaube (Golden Hat). It is based on 3 fixed sites and several mobile stations. The coverage is this one:


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Another view:


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The three primary surveillance radars (PSR) are located at:


Kolomannsberg (Salzburg),
Großer Speikkogel (Koralpe)
Buschberg.


and are (or were) Selenia RAT-31 with a range of about 300 NM.


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Side notes

The difference between Austria and some incompletely covered areas is that Austria is not a large territory like China nor a water area like the Indian Ocean. It is relatively easy to get a full coverage.

In addition Austria is mountainous (famous ski resorts), so it's quite handy to use peaks for radar stations with increased range.

However, radio waves are mostly traveling in straight line. At low altitude in the valleys an aircraft is invisible. Military use this tactic under the name "ground hugging" or "nap-of-the-earth" to escape unfriendly eyes. Austrian army use mobile radars to make this possibility more complex:


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And to fuel imagination, there is this story of aircraft vanishing over Austria. Possibly a Bermuda triangle at the core of Europe.