5.1 WAVEFORM CRITERIA

The primary unique criterion for stealth waveform design is reasonably flat, total operating frequency band coverage. This objective isn't always compatible with best data link or radar mode performance. Some obvious criteria are stated in this section. These criteria are then applied to various spread spectrum strategies such as frequency diversity, discrete phase codes, linear FM, and hybrid waveforms. LPI requirements, in addition to SNR considerations, dictate the use of high duty cycle waveforms. This result has two implications: (1) That the transmitted pulse period must be incrementally variable and (2) that large expansion/compression ratios usually are involved.

Another result based on stealth requirements is that the instantaneous (not just the average) bandwidth of the transmitted signal be as large and as uniform as possible. For each geometry, the power must be managed to the lowest level consistent with acceptable performance or bit error rate (BER) as mentioned in Section 2.1.4. It obviously is desirable to keep the preprocessing and the bandwidth to a minimum; therefore, the waveforms chosen should result in the lowest possible data rate prior to compression or decompression. Lastly, LPI time and frequency constraints described in Sections 2.2.4 and 2.2.5 require noncontinuous or burst transmission for both data links and radars. Some systems naturally operate in a burst mode such as the JTIDS, which uses a TDMA format. As mentioned in Section 2.2.4, the stealth platform, whether aircraft, ship, or vehicle, must move between transmissions to create an uncertainty volume.

These...