  The large constellations of spacecraft planned for use in cislunar space (on
the Lunar surface, in Lunar orbit, and in the vicinity of the Lunar Gateway)
require new solutions for positioning, navigation and timing (PNT). Here, I
describe COMPASS (Combined Observational Methods for Positional Awareness in
the Solar System), a spacecraft navigation system to provide cost-effective
techniques for the positioning of large numbers of spacecraft in cislunar
space. COMPASS will use beacons that emit coherent ultra-wideband signals
designed to be interoperable with existing and future Very Long Baseline
Interferometry (VLBI) networks. Using differential VLBI, COMPASS will provide
rapid determination of the interferometric phase delay with picosecond level
accuracy during routine VLBI observing sessions. Multi-baseline
phase-referenced COMPASS-VLBI observations with simultaneous calibrator
observations should thus enable sub-meter accuracy transverse positioning and
meter level lunar orbit determination using with small femtospacecraft beacons
and a few seconds of observation per position determination.
