Everyone’s talking about how the F-35 Lightning II has a RCS the size of a Golfball but I don’t know what it means or how significant it is.
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Radar is a type of electromagnetic radiation, like light. Radar cross section (RCS) is a method of measuring what fraction of the radar energy that hits an object will be reflected back to the radar.
When performing scientific measurements and comparisons of RCS, values are measured in decibel-metres or dBsm, where 0dBsm = 1m^(2) and every increase or decrease of 3dBsm roughly either doubles or halves the RCS in m^(2), so -3dBsm is about 0.5m^(2) and +3dBsm is roughly 2m^(2).
For layman comparisons however, it’s common for RCS values to be expressed as spherical objects like golf balls, marbles, beach balls, etc. The reason for this is that a conductive metal sphere just so happens to have an RCS in m^(2) that’s equal to it’s physical cross-section, and with a sphere’s RCS being independent of aspect angle (whether you view it from above, below, one side or another).
This isn’t meant to suggest that a jet’s RCS is the same from all sides, but rather it means engineers, etc don’t have to get super specific about how someone should imagine a cube, or flat plate, or cylinder, etc being orientated to give a rough idea of the RCS of an object.
As for its significance, while early on the F-35 was expected to have an RCS roughly the size of a golfball, it exceeded expectations and these days an F-35 is claimed by USAF and Lockheed leadership to have an RCS smaller than that of an F-22, which puts it somewhere in the ballpark of a frontal RCS of -40dBsm (0.0001m^(2) – the physical cross-section of metal sphere around 1cm in diameter)
By comparison, a typical fighter jet like an F-16, F-15, F/A-18, etc will have an RCS somewhere around the -10dBsm to +10dBsm (0.1 to 10m^(2)) range, with that value varying with things like what the jet’s carrying externally. As a rule of thumb, the range that a radar can detect an object roughly halves with every 10dBsm decrease in RCS. That means that if a radar could detect a 0dBsm fighter at 100 nautical miles (115mi / 185km) then it would only be able to detect a -40dBsm fighter at about 6.25 nautical miles (100 divided by 2^(4)) / 7.2mi / 11.6km.
Being stealthy doesn’t necessarily mean you’re outright undetectable / invisible, it just means you can get significantly closer before being detected. That said, it can be pretty comparable to outright invisibility when you can launch your weapons and destroy adversaries without ever entering the detection range of their radars.
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