High-precision ranging (optimetric) instruments are highly desired for both space navigation and communications as well as gravitational-based science missions. Traditionally, these measurements are made with instruments solely dedicated to ranging (i.e. Doppler shift of a microwave carrier providing spacecraft-Earth relative velocity along line of sight); however, this approach is limited by clock stability, frequency, and plasma-induced group delay variations, resulting in lower range and range rate accuracies.
NASA Goddard Space Flight Center has developed a directed modulated ranging technique to improve free space optical communications. Through utilizing coherent optical communication to combine optimetric measurements over an optical carrier, one can accurately measure Doppler and absolute ranging. This process works through a looping and synchronizing iteration, measuring frame, bit, and phase change values using a phase detector and clock data recovery apparatus. A dual-mixer time difference (DMTD) approach is also employed, making the system more phase sensitive and easier to calibrate.