Power reflection coefficients at normal incidence in excess of 10^-14 or -140db attenuation are thought possible on occasion (Atlas 1959), so given a peak power in the order of 100 kW then for a spillover lobe 30db down on the main lobe this gives about 10^-17 of 100 kW or 10^-12W, which is about the power commonly reckoned to be in one Just Detectable Echo or a faint blip at the limit of detectability on a typical PPI. Evidently much higher efficiency than one JDE would be indicated in this case, given expert testimony of an echo significantly stronger than that from a very large jet at close range. Since the ratio of normal signal intensities on a PPI might easily range up to 10^4 (a single aircraft at constant range could vary by as much as 10^3 depending on aspect) we should probably assume an echo of at least 10^4 or 10^5 JDE.