Modulation forces a detectable modification on the transmitted Radio Frequency (RF) energy. Usable modifications retain the primary characteristics of the electromagnetic radiation (so that the signal can be coupled through available cables, handled by practical antennas, aimed, amplified and detected without unreasonable difficulty). With such modifications, the RF signal can carry information. The simplest – and still widely used – approach is to turn the RF signal off and on to communicate using an agreed upon code. This is called CW (Continuous Wave) communication because when the RF is on, its waveform is steady and continuous. To integrate any modulation scheme into a “Doppler” (interferometric) navigation system, the phase of the RF signal must be preserved in spite of the uplink “Control” modulation or the downlink “Telemetry” modulation. This becomes iffy with the on off approach, because the phase is unknown when the signal is off. This may be tolerable for short off intervals, but still is risky if unexpected acceleration could change the velocity during such an interval. It is better if the Amplitude is changed, but not reduced to zero, so that phase detection is still practical. This brings back the still common “Amplitude Modulation” (AM) modulation mode. Since it can be implemented in a linear format, it can be used to transmit Voice, Music and Video. But these modes have lost favor for a number of reasons, the best known being their gradual degradation form “noise” when the signal is weak. Digital format modulation avoids this gradual effect (Graceful Degradation) and substitutes a VIOLENT systems failure, with weakening signal strength, including Worst Case Artifacts unless robust “error correction protocols” are incorporated in the data processing. Only the availability of such protocols make CDs, DVD and Digital TV possible! Since we are discussing Telemetry and Control which will be interfaced to digital systems, our modulation can be assumed to be digital with some level of error identification and correction included. But keep in mind: There Are No Digital Radio Systems. Everything about Electromagnetic Radiation, its generation, transmission, reception, and detection is inherently a linear process. Modifications can be induced by voltage or current step transitions, and these in turn can be controlled by computer “bit line” outputs. The magnitude of electronic voltages can be detected by a “comparator” circuit and “shaped” to become a computer “bit line” input. This process is inexact, adding noise through its uncertainty, but a decision is made about the present voltage and a subsequently “noiseless” bit of data is received by the computer. It “seems to be” digital communications. Done well, it works. Returning from that side note, AM modulation has other drawbacks: it requires linear control of the Radio Frequency amplitude, which is fairly inefficient and modestly difficult. Worse, the amplifiers which one might like to add to a transmitter, and those one must add along a very long cable connection, are not linear and significantly degrade the accuracy of the AM modulation. And worse yet, in mobile applications the extreme amplitude change resulting from signal absorbers, reflectors and “multipath” interference are difficult to separate from the more modest amplitude changes deliberately added to the signal. Motorola mastered this thorny problem with their practical “Car Radio” almost 80 years ago, and profited greatly as a result. Many of the drawbacks of AM are less important, and easier to avoid, when the information transmitted is digital – on/off bits (with a nonzero “off signal” to maintain phase analysis for our use). But it remains a bit harder to use and has not been implemented in the convenient and attractive Integrated Circuits available for FM operation. Thus AM remains a possible, but unattractive option. END 1