VHF/UHF low-frequency radar — China's JY-26 "Skywatch" and Russia's Nebo-M are specifically engineered to detect stealth aircraft by exploiting the fact that radar-absorbing materials lose effectiveness when the radar wavelength approaches the aircraft's physical dimensions. A flying wing like the RQ-180 has a large chord length — VHF-band radar can produce enough of a return to cue a targeting radar. The problem for adversaries is converting that soft "bloom" into an actual weapons-grade fire control solution before the aircraft maneuvers or the contact fades. It's detectable in theory; it's not shootable from that data alone.

Space-based infrared — Russia's Tundra constellation and China's equivalent look for IR signatures. The RQ-180's turbofan engine produces heat. At 65,000 ft the signature is faint and the target is small, but a dedicated space-based IR sensor can potentially cue further investigation.

Bistatic radar networks — China has invested heavily in widely-distributed, geographically separated transmitters and receivers (the radar transmitter is separate from the receiver), which defeat many stealth coatings that are tuned to defeat monostatic radar. This is China's single most credible counter to the RQ-180's stealth coating specifically.

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SIGINT detection of its own emissions — If the RQ-180 activates its AESA in active radar mode or transmits data, that emission is detectable. Russia and China have world-class ELINT capabilities that could triangulate its position if it transmits at the wrong moment. Passive-only operation mitigates this but limits its usefulness as a networking node.

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Kinetic engagement range problem — Even if China detected an RQ-180 at 70,000 ft with a VHF cue, their best interceptors (J-20, Su-57) max out around 65,000 ft, and high-altitude SAMs like the S-500 have a theoretical ceiling of ~180 km altitude but real-world intercept probability against a low-RCS maneuvering target is largely untested. Against Iran: irrelevant. Against China or Russia: genuinely uncertain.

(http://www.autoadmit.com/thread.php?thread_id=5840265&forum_id=2Ã#49707363)