The Effective Shielding Effectiveness of a Space Vehicle

Abstract

A mathematical formula has been developed to predict the shielding effectiveness, from inside to outside, of a space vehicle. Such a formula shows that the shielding effectiveness of such a space vehicle varies as the point of observation on the outside of the space vehicle is varied. A possible method of measuring the shielding effectiveness of a space vehicle is presented. The electromagnetic compatibility of equipments inside a space vehicle depends upon the shielding effectiveness of the space vehicle. Antennas mounted externally receives desired signals in the presence of interfering signals generated by equipments within the space vehicle. Such interfering signals penetrate the structure and are received by the external antennas. The amplitude of such interfering signals may be reduced by cable shielding, equipment case shielding and vehicle structural shielding. The shielding effectiveness of the structure, from inside to outside, depends upon the location of the source inside the space vehicle, the apertures and shims on the vehicle structure, and the method of cable penetrations through the vehicle structure. A typical example of a space vehicle is the GPS satellite. It consists of four antenna systems: i) an L-band antenna system consisting of a helex antenna array. (2) telemetry, tracking and command antenna system consisting of conical spirals. (3) an UHF dipole and (4) a W-Sensor antenna as shown in figure 1. A typical method of measuring the shielding effectiveness of a space vehicle is shown in figure 2. In this method, the space vehicle structure is rotated about its axis and the electric field intensity, at various points on the structure, may be measured with the source located inside the space vehicle. The location of the source inside the space vehicle needs to be varied to take into account the effect of source location on the shielding effectiveness of the ; ructure. The measured minimum shielding effectiveness of the structure at a given frequency should be accepted as the true shielding effectiveness of the structure. If such a true shielding effectiveness value is unacceptable, various techniques may be implemented on the structure to increase its shielding effectiveness. CH2294-7/86/000-0062 $01.00©1986 IEEE 62