QinetiQ has used a suite of modelling tools to predict the millimetric plume signatures from a range of ballistic missile types, based on the accepted theory that Bremsstrahlung emission, generated by the collision of free electrons with neutral species in a rocket motor plume, is the dominant signature mechanism. Plume signatures in terms of radiation temperatures varied from a few hundred Kelvin to over one thousand Kelvin, and were predicted to be dependent on emission frequency, propellant type and missile thrust. Two types of platform were considered for the passive mmw imager launch detection system; a High Altitude Platform Station (HAPS) and a satellite based platform in low, mid and geosynchronous earth orbits. It was concluded that the optimum operating frequency for a HAPS based imager would be 35GHz with a 4.5m aperture and a sensitivity of 20mK providing visibility through 500 vertical feet of cloud. For a satellite based platform with a nadir view, the optimum frequency is 220 GHz. With such a system, in a low earth orbit at an altitude of 320km, with a sensitivity of 20mK, a 29cm aperture would be desirable.