Article: However, as stealth technology grows, so does anti-stealth technology. Multiple transmitter radars such as those from bistatic radars and low-frequency radars are said to have the capabilities to detect stealth aircraft. Advanced forms of thermographic cameras such as those that incorporate QWIPs would be able to optically see a Stealth aircraft regardless of the aircraft's RCS. In addition, Side looking radars, High-powered optical satellites, and sky-scanning, high-aperture, high sensitivity radars such as radio telescopes, would all be able to narrow down the location of a stealth aircraft under certain parameters. The newest SAM's have a claimed ability to be able to detect and engage stealth targets, with the most notable being the S-400, which is claimed to be able to detect a target with a 0.05 meter squared RCS from 90 km away.

Question: What can detect stealth aircraft?
Multiple transmitter radars