NavList:

Brad,

Thanks for the insightful reply.

That would leave us with the only debris-related problem being due to a collision taking place near 20,200 km.  I don't believe there's any plausible way that could occur other than as the result of hostile action.

I do understand that a partial constellation is still useful.  In fact, I used GPS receivers long enough ago that the full constellation was not yet deployed, and yes, we would get fixes of varying quality, all better than a CN fix.  Ocean navigation has an advantage here in that, unlike land-based navigation, there is not a problem with reception of some satellites being blocked by terrain, vegetation, or buildings.