The B-21 Raider is, by design, a low-observable long range strike platform built to operate in highly contested environments. The public unveiling in December 2022 confirmed what many working pilots, controllers, and airspace managers already suspected: this program prioritizes reduced radar and signature exposure so the aircraft can penetrate dense air defenses and operate with tactical surprise. That design intent has clear implications for how civilian surveillance and air traffic management see and manage airspace when low-observable military traffic is present.

Start with the basics every pilot and ATC professional lives with today. U.S. terminal primary radars such as the ASR family operate in the S band, approximately 2.7 to 2.9 GHz, while cooperative surveillance via transponders and SSR operates in the 1030/1090 MHz L band. The FAA has also moved much of the NAS toward ADS-B for position reporting and traffic situational awareness, with ADS-B Out mandated for most controlled airspace since 2020. That combination means civilian traffic picture depends heavily on cooperative avionics plus S-band primary returns as a fallback. Low-observable aircraft are designed to be hard for S-band, centimeter-wave radars to detect when noncooperative. This is an important mismatch between the way modern civilian surveillance is provisioned and the physics that govern stealth.

How do countermeasures to stealth change the arithmetic? Several approaches under discussion and field trial existed before mid‑2023. Long wavelength radars in VHF/UHF bands and multistatic or passive coherent location networks exploit physics and geometry that reduce the effectiveness of shaping and some RAM treatments. Passive systems that listen to third‑party transmitters or fuse many low‑quality bearings can, when networked and processed with modern compute and algorithms, reveal tracks that would be marginal or invisible to a single S‑band primary radar. Industry demonstrations and NATO measurement campaigns have shown passive systems can contribute a usable air picture in coastal and constrained environments. Raytheon and others have also publicly documented long‑wavelength approaches aimed at improving detection of very low observable targets. None of this is magic; each method brings tradeoffs in resolution, altitude coverage, false alarms, and integration complexity.

From an operational standpoint that matters to civilian pilots and controllers the takeaway is simple and practical. If a noncooperative low‑observable military aircraft is operating in or transiting civilian airspace with its transponder and ADS-B off, it may not appear on the ATC traffic display the way a conventional aircraft would. That creates two risks. First, separation assurance is degraded because controllers lose the cooperative data stream they rely on for identity and altitude. Second, controllers and pilots get less time to detect, identify, and mitigate a convergence threat when primary returns are weak or intermittent. These are not speculative scenarios; they follow directly from known radar band physics and how civilian surveillance systems are deployed.

What can be done within the civil domain to reduce risk and keep skies safe without compromising legitimate military requirements? Practical mitigations that respect both safety and security include:

  • Preserve cooperative equipage and procedures for non‑exempt military flights when flying through civil airspace. If mission constraints permit, military aircraft should use transponders or ADS-B with an agreed protocol so controllers can see and identify them. When operational security prevents full cooperation, limited coordinated modes and flight notification can still reduce risk.

  • Use airspace management tools like NOTAMs, special use airspace activation, or temporary flight restrictions to segregate known test or training activity where noncooperative operations are planned. Those procedural measures are often the fastest way to restore a predictable traffic picture for commercial and GA operators.

  • Improve civil‑military data sharing and sensor fusion. Sharing the outputs of passive sensors, low‑frequency radars, airborne EO/IR, and military surveillance through secure feeds into ATC fusion systems creates a composite picture controllers can act on even if individual sensors provide partial or noisy data. NATO and industry trials have shown this fusion model is feasible; turning demonstrations into operational feeds is a governance and engineering problem worth solving.

  • Invest in fallback surveillance for safety, not force posture. Civil authorities should consider limited deployments of complementary sensors—passive radars or low‑frequency receivers—in high‑density or high‑risk corridors to give controllers alternate cues. These systems are not replacements for primary S‑band radars but can provide extra situational awareness when a return is intermittent or absent. Any such deployments must be evaluated for performance in altitude coverage, clutter tolerance, and false alarm rates before relying on them for separation decisions.

  • Train for noncooperative traffic scenarios. Controllers and flight crews should rehearse the procedures for handling unknown or intermittent targets: increased separation buffers, altitude or route vectors that reduce convergences, prearranged communications protocols, and immediate coordination channels with military authorities. Flight crews benefit from ADS-B In traffic displays and simple contingency phraseology that reduces cognitive load during surprise encounters.

A final, practical note for operators: low observables reduce detectability, but they do not confer invisibility in every environment. Long wavelengths, bistatic geometries, emissions from the aircraft, IR/optical signatures, and the simple existence of chase aircraft and support platforms mean detection is a multidimensional problem. The civilian system of record will continue to be ADS‑B and S‑band primary radar in the near term. That reality argues for policy and procedural solutions that assume cooperation when safe and coordinated segregation when necessary, while building technical bridges where possible between civil and military sensing communities.

If you work in the cockpit or in a TRACON, keep two practical habits. First, assume the traffic display can be incomplete and fly defensively when operating near known military ranges or when NOTAMs indicate test activity. Second, push for clear preflight coordination between military range control and civilian ATC when any noncooperative test profile is planned. Those habits cost almost nothing and pay off when the technology at the edge of physics creates uncertainty in the traffic picture.

The B‑21 is a reminder that the airspace we share will only get more complicated as advanced military platforms and new civil entrants arrive. Civil aviation does not need to match stealth technology to remain safe. It needs better integration, smarter procedures, and a realistic appreciation for what its radars can and cannot see. That combination keeps pilots and passengers safe while letting legitimate defense activity occur under sensible, coordinated controls.