I'm not sure that it needs to be passive, a sufficiently advanced adversary nation-state (Russia, China) is assumed to have their own equivalent of this, and to be aware of the orbits of US satellites:

https://en.wikipedia.org/wiki/United_States_Space_Surveillan...

Much as the US is aware of the orbits of their satellites.

If you already assume that it's hard to 'hide' the existence and orbits of your satellites from an advanced observer with good telescopes and ground radar and ways of correlating the data, the fact that your ocean surveillance satellites emit radar might not be a problem. No need to be passive about it.