(CNN) Kyiv denies involvement in alleged Kremlin drone attack.

TERCOM, terrain contour matching cruise missile guidance, became widely known during the 1991 Gulf War. In conjunction with separate terminal guidance, near zero CEP was feasible, while rendering cruise missiles immune to air defense systems. This remains almost true today. In the April 2018 missile strikes against Syria, non-stealth cruise missiles advantaged mountainous terrain to avoid detection by S400 installations, with complete avoidance of interception.

The terrain between  Ukraine and Moscow, part of the East European Plain, is not advantageous to this technique.  But small plastic drones are much stealthier than all but the newest cruise missiles. This admits two possibilities:

• Ukraine has developed terrain mapping and air defense knowledge of Moscow routes to an extremely detailed degree, flying at near-zero altitude.
• The drones were launched from inside Russia.

A false flag operation is discounted in this case. The Russians responded with surprise and confusion.

If Ukraine is responsible, which is not established, this was the equivalent of the  Doolittle Raid. Putin will likely employ his double on May 9, which will not go unnoticed by his inner circle.