Inertial Locks
An Inertial Lock is a planetary facility capable of instantly negating the Transnewtonian pseudovelocity imparted by small TN-era ship drives within a suborbital radius of a planet, most notably the drive of any practical missile design. Larger drives, even those of fighters, are immune to the effect.
Any missile salvo that intercepts a population or ground force on a body protected by an Inertial Lock would have an additional flight time added to it, similar to ICBM flight time mechanics. Speed would be the sole determinant of this flight time. Even a moderately capable point defense array would thus be capable of destroying a far disproportionate # of incoming missiles. Although such missiles speed would have dropped to essentially that of a sitting duck, the inertial lock's interference would make missile ECM far more capable than normal.
Inertial locks would be permanent installations owing to the immense complexity of the fine tuning process adapting it to a local geomagnetic field. The primary cost would be in Boronide for the massive power requirements. They could be destroyed as normal as a side effect of ground combat, but the easiest way of defeating them would be attaining total ground superiority. Alternatively, they could be PDC-mounted
It would be relatively easy to slot Inertial Locks into existing gameplay, but interaction with NPRs is a little more problematic. Since NPRs do not build PDCs, they would not benefit much from Inertial Locks once any orbiting space stations are destroyed. They would work well with any conflict between two player races, but to work for NPRs it requires either PDCs or some other means for ground forces to contest a beam-armed opponent in orbit.