Many communications systems now use narrow band FM = Frequency Modulation to add information to an RF signal. (This is somewhat different from “wideband”, Broadcast FM. The latter is much less efficient, requiring far more transmitter power but, at best, it offers better linearity and lower noise than AM. Broadcast FM shares some of the advantages and weaknesses of Digital Broadcast, although it is much less energy efficient than the latter). FM neatly avoids the amplitude linearity problems in amplifiers, transmitters and receivers (a problem noted for AM). One of the best things, for small rocket systems, is that tiny Integrated Circuits offer high gain (extremely nonlinear) “limiting amplifiers” which boost the received signals up to a constant voltage level before analysis. FM is not a natural for a Phase Locked system, because “Phase Lock” requires a frequency matching a stable reference. But “Frequency” is actually the “Rate of Phase Change”, and “Phase Modulation” transients will be detected by FM systems. If the modulation is contrived as a “Return to Zero” phase deviation, it will be detected in the FM system as a bipolar (+/-) frequency cycle and have no net effect on the Phase used for Doppler analysis. “Narrow Band” FM by definition involves phase deviations of less than ½ RF cycle with maximum level modulation. A fraction of that is sufficient for detection with a good signal, and the impact of this on the Doppler Phase analyzer can be predicted, and compensated. FM actually works quite well producing a fixed AVERAGE frequency with audio inputs, since these are usually symmetric AC signals, and are normally forced to have no DC frequency offset component. This is much more difficult to arrange with digital signals, but not impossible. A quick way to produce a “Net Zero” effect with digital Frequency Modulation (called “Frequency Shift Keying” = FSK) is to transmit each bit followed by its compliment (adding powerful error detection capability). A variety of techniques can encode a block of data with both a “Net 50%” sum constraint and error correction capability. This may require sending a sub-block as compliments plus adding “Parity” type compensation bits. Another technique is to use compensating “Bipolar” +/- Frequency Shift Keying so that the average frequency is fixed. Many of these techniques also improve standard FSK, by allowing “Automatic Frequency Control” to keep the receiver optimally tuned in spite of Doppler shift and transmitter drift. Most of the techniques I just mentioned send fewer real data bits with a given link bandwidth then would simple FSK, but other error detection and correction techniques also add extra bits, and reduce the real data rate. Micro-Space is working with some of these techniques to allow the addition of “Phase Locked” capability to the FSK hardware we have been using for nearly two decades. Results are promising.