Physics dictate that a tactical fighter-sized stealth aircraft must be optimized to defeat higher-frequency bands such the C, X and Ku bands, which are used by fire control radars to produce a high-resolution track. Industry, Air Force and Navy officials all agree that there is a “step change” in an LO aircraft’s signature once the frequency wavelength exceeds a certain threshold and causes a resonant effect—which generally occurs at the top part of the S-band.Russian air defenses may appear formidable as part of Moscow’s increasingly sophisticated anti-access/area denial (A2/AD) capability, but areas protected by these systems are far from impenetrable bubbles or 'Iron Domes’ as some analysts have called them.While it is true that a layered and integrated air defense may effectively render large swaths of airspace too costly—in terms of men and materiel—to attack using conventional fourth generation warplanes such as the Boeing F/A-18E/F Super Hornet or Lockheed Martin F-16 Fighting Falcon, these systems have an Achilles’ Heel. Russian air defenses will still struggle to effectively engage fifth-generation stealth aircraft such as the Lockheed Martin F-22 Raptor or F-35 Joint Strike Fighter.(This first appeared in August 2016.)“In terms of establishing viable air defenses against opponents with fifth generation aircraft, it's quite clear how Russia is trying to tackle the problem of stealth," said Mike Kofman, a research scientist specializing in Russian military affairs at CNA Corporation during an interview with The National Interest. “Russia's advanced radar, variety of capable missiles and systems that try to integrate large amounts of data for a more potent air defense will increasingly segregate Western air forces into two benches. In a future where these systems have proliferated to China, Iran and other regional powers there will be those that can penetrate and survive against advanced air defenses in a high end fight, and those whose job it is to bomb ISIL or its successor.”Kofman notes that advanced Russian-built air defenses like the S-300, S-400 and forthcoming S-500 family come with systems designed to detect and track the presence of low observable (LO) aircraft such as the F-22 and F-35. That’s just a function of physics, as I have noted before. The problem for Moscow is that while Russian early warning and acquisitions radars operating in the VHF, UHF, L and S bands can detect and even track a tactical fighter-sized stealth aircraft, those systems don’t deliver a weapons quality track. “Russia has invested in low-band early warning radars, with some great variants out there, but can it use these to put a good picture together, and process it to develop a track against low-observation aircraft?” Kofman asked rhetorically.