(CNN) US jet shoots down ‘unidentified object’ over northern Canada, Trudeau says.

After Deadhorse Alaska, this is the second balloon discovered within the polar vortex, in the vicinity of the North Warning System.

The AN/FPS-117 radar component of the  North Warning System uses pulse compression to defeat jamming. To jam this radar requires detailed knowledge of its emissions. To avoid the distortion of atmospheric dispersion, the radar pulse must be sampled close to the source.

Reliable insertion into the polar vortex requires launch from Russian territory, a courtesy that may have been provided to China.