I take it the show doesn't have any military experts? Not that there's anything seriously amiss about how military is presented in the show, but the rank insignia on the uniforms does bother. Or in the case of the UN Admiral, lack thereof. But what really gets me is the use of chevrons for the officers in the Martian Navy. It makes them look like enlisted personnel. Hell, at a glance one would think Donnager's captain was a sergeant and Lopez a corporal.
Why do you think they must use the same insignia used today? They are 2 centuries in the future and are from a different planet.