On the decrease of the radar cross section of the Apollo Command Module due to reentry plasma effects

SUMMARY A knowledge of the radar cross section of the Apollo Command Module is important in connection with reentry acquisition and skin-tracking. It is known from actual reentry tests, however, that effects associated with the re- entry plasma sheath can cause large decreases from the free space cross section of reentering objects. A number of mechanisms have been proposed to explain the decreases that have been observed. Among these are, first, a diverging lens type of ef- fect where a photo-ionized cloud preceding the sheath acts to divert the in- cident radar signal around the object. Second, the plasma sheath, at certain combinations of altitude and speed, may act as an absorbant coating. Third, the inhomogeneous plasma sheath may present to the incident radar wave a reflecting surface having, because of its shape, a lower cross section than the original body surface. With the reentry object, radar frequency, and radar look-angle fixed, the regions where these mechanisms are likely to have effect depends primarily on the altitude and speed of the object. In this report, the regions of decrease due to the last two of the mechanisms listed above