In orbit proximity intelligence

The attacker, mainly a military organization, can use satellites positioned in proximity to the victim satellite to gather information, visual or radio, on the satellite's capability or on its work. ^[1]

Proximity intelligence can be visual, with cameras or other optical sensors, to gain information about satellite's hardware, or electromagnetic, using antennas to intercept communications or to measure other EM emissions to attempt a side-channel attack.

Standards/references: ^{[2] [3] [4][5][6]}