Planetary Defenses (GURPS Traveller)

Picked up a copy of GURPS Traveller yesterday.

Planetary Defense Fire (p. 172)

"TL8+ planets with large populations and/or good starports often have
planetary defenses.  Ground-based lasers and particle beams cannot
penetrate a planetary atmosphere and still carry enough energy to hit hogh
orbital targets, so static planetary defenses generally take one of two
forms: ground-based missile launchers, or a number of armed 'orbital
platforms' (built as ships with no drives), or meson guns buried deep
underground. Such "deep meson guns" are immune to most attacks except other
meson guns, sabotage or destruction or their orbital or surface sensor."