[Varcht]

Pulsar seemed like a good idea but seems it has some difficulties.

Due to their small size, pulsars are relatively weak radio sources. Therefore, the largest radio telescopes in the world are usually needed to observe them. As we have seen, pulsars emit their largest intensity at low radio frequencies around 400 MHz. In particular at such frequencies, however, the pulses suffer from propagation effects when they travel to Earth through the interstellar medium. [0]

[0http://www.jb.man.ac.uk/distance/frontiers/pulsars/section4....