[Hannah LS (OP)]

Paul asks:

Is there any research going into new Radar systems or something similar that can detect stealth aircraft?

Do you have any ideas?

[syhprum]

During the Balkan war in 1999 Yugoslav forces were able to detect a F-117A and bring it down with the aid of three missles


https://en.wikipedia.org/wiki/1999_F-117A_shootdown

[evan_au]

Stealth aircraft that have angular faces work by reflecting the bulk of the radar signal away from the direction of the incoming radar beam.

One way to defeat this is to have multiple passive radar receivers dotted around the countryside, increasing the chance that a reflection will reach one receiver. It is helpful if these receivers are very mobile, so the incoming aircraft doesn't know where they are. It also helps if they use "flat panel" antennas (rather than the traditional rotating parabola); these can pick up reflections from multiple directions at once, with sufficient processing.

Stealth planes are often designed to hide from ground-based radar, but may be more susceptible to detection from higher altitudes.

It is also possible to conduct observations from space. Due to the risk of debris in orbit, most countries would avoid destroying a satellite (after China made a big orbital mess by destroying one of their own satellites).

[Kryptid]

I've heard that low-frequency (L-band) radars are better at picking up low-observable aircraft because the larger wavelength can't be reflected away as effectively by features on the aircraft that are smaller than the wavelength itself.

[syhprum]

I understand that the latest stealth aircraft  have complex and very expensive coatings that must be replaced  each time the aircraft is flown

[alancalverd]

All fast jet aircraft emit hot gas. I'm fairly certain than an infrared-imaging satellite could track the CO2 plume, particularly as they all start from known land bases or very large ships.

[evan_au]

Being stealthy means minimal openings for cooling air or heat exchangers.
I understand that the electronics on one stealthy aircraft tends to overheat, so they have to regularly open air vents to keep things cool.
With the airvents open, it shows up better on radar 

[alancalverd]

A few hundred watts of radar kit may light up the fuselage a bit, but a megawatt of burning kerosene leaves quite a trail of hot gas behind it.

[Bored chemist]

I read somewhere that the smoke trail from some  shoulder-launched very expensive automatically targeted missile turned it from "fire + forget" to "fire + run away" because the enemy could see exactly where it was launched from.