Are there Rydberg antenna SDRs for radio astronomy? What sensitivity (?) is necessary to navigate by the EMF of stars? FWIU this is called astronometry? There

Are there Rydberg antenna SDRs for radio astronomy? What sensitivity (?) is necessary to navigate by the EMF of stars? FWIU this is called astronometry? There's probably a better word than "astral"?: From https://news.ycombinator.com/item?id=44054783 https://news.ycombinator.com/item?id=44054783 : > How many astral signals does a receiver need to fix to determine lat/long/altitude given the current time? > How many astral signals does a receiver need to fix to determine to infer the current time, given geometrically-impossible triangulation and trilateration solutions given the known geometry of the cosmos and the spherical shape of the earth?