site needs certificate, how do I know they aren't impersonating?

When I visit secure Navy web pages like https://chart.donhr.navy.mil/ I get an error web page.

My browser gives the message:

"Secure Connection Failed

chart.donhr.navy.mil uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.

(Error code: sec_error_unknown_issuer)

* This could be a problem with the server's configuration, or it could be someone trying to impersonate the server.

* If you have connected to this server successfully in the past, the error may be temporary, and you can try again later."

I've searched the web about this message. Some Navy websites suggested downloading "DoD Root Certificates". But why would I trust this advice from the very web pages (anything navy.mil) whose authenticity is in doubt?

It seems unlikely that someone on the internet would intercept my requests for navy.mil web pages and reply with their own fake Navy pages. But it is possible, I guess.

I know that the Navy web pages I want to reach will ask for lots of my personal identity information. Identity thieves or other crooks might really like that info. That's the reason the security warning bothers me.

Is the Navy too cheap/lazy to use a trusted Certificate Authority? I'm no security expert, so I'm not sure if that is the issue.

Why would an organization as well funded as the U.S. Navy allow a problem like this to persist? Other organizations usually don't have certificate warnings block access to their secure https pages.