Proximate orbits are defined as two circular orbits whose difference in altitude is smaller than 500 m and difference in orbital plane angle is smaller than 1 So.
ITU-R SA.1263-1997 history
1997ITU-R SA.1263-1997 Interference Criteria for Meteorological Aids Operated in the 400.15-406 MHz and 1 668.4-1 700 MHz Bands