Before detailing other uses of “Phase Locked Doppler” for navigation and control of orbital, cislunar and interplanetary spacecraft, I want to discuss other uses of “Phase Detection” for tracking and navigation.

These techniques actually have a long history. One of the most important is used in the VOR or OMNI (VHF Omnidirectional Radio) navigation beacons long used by aircraft pilots. These compare the Phase of an Amplitude Modulated reference signal transmitted with the radio beam, with a direction dependent Frequency Modulation. The frequency modulation is actually produced by Doppler Shift: electronically “wobbling” the transmitting antennas in a small circular path.

These two types of modulation (AM and FM) – which have little interaction in good receivers – are separated from the RF signal and compared with a phase detector or phase comparator. The standard aircraft form uses a dial to set the desired aircraft path from the VOR transmitter as a “Radial” compass direction, and a single needle shows if the plane is to the right or left of that path. (A TO or FROM flag indicates whether the received signals have the relative phase for outbound flight in that direction, or if they have the reversed – 180 degree shifted – phase relationship indicating inbound flight along the same compass track from the opposite side of the transmitter.)

Other application have also been in widespread use. But for our, customized use, these techniques also have great value. Closely related to the VOR application, a phase comparison of the RF signals from two antennas can give extreme accuracy in identifying the direction to the transmitter. Two antennas, separated by a one wavelength distance (66 centimeter at 500 MHz) will see a 180 degree phase shift for a 30 degree change in the line of sight direction to the transmitter. If these two signals are electrically combined in a single “antenna array”, a 30 degree FWHP “Beam Width” results. But if a good phase detector is used to compare the signals, the resolution becomes 1/6 degree = 10 minutes of arc. If the array of antenna elements is increased to a 20 meter width, the beam width can be reduced to about one degree. But two antennas spaced 20 meters apart will increase the Phase Detector based resolution to 1/180 degree = 20 seconds or arc! It is true that RF signal reflections can seriously distort this analysis, but additional antenna units can detect and correct for some of these errors, and antennas to monitor a rocket nearly overhead can have high immunity to such reflected signals.

Thus, by applying Phase Detection techniques to the output from a small group of antennas, the “Line of Sight” direction to a small rocket can be determined to very high resolution (even with a low power transmitter), and when combined with information from the “Coherent” (Phase Locked) Doppler system, the distance to the rocket along that path is obtained to even higher accuracy. Simple tools thus yield extreme navigational precision in three dimensions.

For vertical flight into “Space” (at 100 km altitude) using the described system, a one milliwatt transmitter would be sufficient to provide second by second tracking of the flight with 1 to 5 meter lateral resolution (usable for guidance corrections) and better than 1 cm altitude resolution.

(Careful design, construction and calibration are necessary to consistently achieve the best performance mentioned.)

Next: Space Tracking for Lunar Transfer, Lunar Descent and Orbital Rendezvous for Sample Return.