The best modulation method, for these systems, produces the greatest energy efficiency and maintains Phase Lock with a single “quirk”. It resembles AM, but also captures all the benefits of FM. It is BPSK = Binary Phase Shift Keying. The way to split all possible Phase conditions evenly into two categories is to “bisect” the polar phase plot, and establish the reference phase states 180 degrees apart. These two states have the greatest separation, for a fixed RF voltage, and will remain reliably above the noise level with the weakest signal. Analyzed for Amplitude (using a phase sensitive detector), the states become not X and 0, for a given RF power, but X and -X. From this standpoint, the detector voltage is doubled with no increase in detector noise. Although the RF is on continuously (not off for part of the time as with CW), the voltage difference between the two states is what would be obtained with Four Times the transmitter power! This is the communication mode used for the most challenging interplanetary links. Since both the frequency and phase are stable with any modulation data, it is well suited for Phase Locked use. The “quirk” mentioned is that the inevitable, small phase errors that occur are analyzed incorrectly (backward) when the RF phase is reversed. The detected errors must be used in a control loop to maintain the “Lock”, and incorrect analysis will magnify the errors instead of reducing them. Avoiding this problem is awkward, but not particularly difficult. Correction of the small errors does not have to be done immediately, but can be implemented with a modest time delay. The error detected in a particular interval can be saved until the phase state for that bit has been determined , and then applied with a corrected polarity if necessary. The “Limiting Amplifier” in FM receiver circuits works perfectly with these signals, since the phase reversal is preserved even if the actual amplitude is hidden. The FM circuit's Frequency Detection function can even be used to guide the receiver tuning very close to the Lock condition, allowing that state to be established. These are desirable features, and worth pursuing. It is true that phase lock is harder to achieve that FM tuning, and can be very challenging with a weak signal. In addition, it is easy to lose track of which phase represents 0 data, and which represents a 1 (a brief signal interruption, producing one nanosecond of timing uncertainty, can do this). Some predictable structure must be embedded in the signal to allow recognition of the correct polarity. With the bidirectional command/telemetry loop used for Doppler navigation, this confusion can be easily detected by programming the rocket system to echo a predetermined selection of the command bits (probably with a fixed bit as well). A failure could trigger a brief reference transmission, which would reset both systems to the correct phase states. Modest complications bring profound benefits. The very high resolution tracking data mentioned earlier becomes available: the signal to noise level is enhanced: the RF link bandwidth is minimized, because of exact tuning, to further reduce the noise, and a wide range of efficient amplifiers can be used. Micro-Space is modifying our rocket Radio Systems to work in this mode. At present, we are using this mode in our “ElectroMagnetic Tractor Beam” demonstrators, which also use phase detection to determine unit positions. The BPSK data controls operation of the mobile unit, and synchronizes magnetic drive forces. We used related technology a number of years ago in a patented, optical detection and ranging system. We have also used “Phase Analysis” for precision sensing with a number of mechanical position sensors, and in automated, optical film measuring interferometers. Related to this is synchronous detection, which we have applied in many systems dating back to an old “Moon Bounce” communications effort. We have been “toying” with parts of this system for a number of years, without forcing the subsystems into workable form or shrinking them to a size consistent with our FM (FSK) production assemblies. But we are now “biting that bullet” and will soon have units in rockets ready to fly.