odulation Forms tions at the time), (2) whether the satellite is receiving by means of a spot (high gain) antenna or an earth-coverage (moderate gain) antenna, and (3) the weather conditions along the surface to satellite line-of-sight communi- cation path, which might be different for the link between the communica- tor and receiver than that between the communicator and interceptor. A high elevation angle geneally means lower sidelobe radiation into the altitudes that the search aircraft fly as well as obviating main beam intecept at far range, while the use of the satellite spot antenna and clear weather condi- tions are synonymous with minimum radiated power. The remainder of this chapter deals with the issues and answers pertain- ing to the tradeoffs among the various modulation, coding, and synchro- nization techniques used by the communicator and the sophistication of the detection technique employed by the interceptor in carrying out their respective missions. In keeping with the philosophy adopted in previous chapters, we shall intentionally omit discussion of all issues related to antenna design. This is not to say by any means that such issues are neither performance nor cost effective. In fact, one of the viable options available to the interceptor to improve its detectability is to employ multiple narrow- coverage antenna/receiver combinations that accrue gain by individually covering fractional portions of the total search area. Nevertheless, we shall focus our attention only on those issues that have a direct bearing on the previously mentioned system design considerations. 1.1 SIGNAL MODULATION FORMS The various signal modulation forms that might be used by the communi- cator are no different from those already discussed in great detail in previ- ous chapters. Thus, in this chapter we shall merely review their intrinsic form with emphasis on the LPI application. The underlying modulation technique for data communication in many LPI systems reported upon thus far is MFSK. For such a modulation, the