Even better are distant quasi-stellar radio sources (e.g., [0]). The detectable pulsars and neutron stars are close enough (inside our galaxy) that they will no

Even better are distant quasi-stellar radio sources (e.g., [0]). The detectable pulsars and neutron stars are close enough (inside our galaxy) that they will not form a fixed reference frame, due to slow motion across the sky. Though, I suppose they're probably good enough over short time intervals, depending on the needed accuracy. There's also the issue of X-rays not penetrating the Earth's atmosphere. One could try triangulating based on time of arrival for pulsar radio pulses, but that requires a reasonable amount of collecting area with a radio telescope and substantial processing power. [0] https://en.wikipedia.org/wiki/International_Celestial_Reference_Frame https://en.wikipedia.org/wiki/International_Celestial_Refere... Edit: slight clarification