Most will agree that Tour of Duty is very unbalanced in terms of how stars are earned.
Many challenges take days worth of gameplay to unlock. Daily challenges are too few and unreliable to get stars.
Stars towards the tour ranks should be awarded each time you level up. Re-Ups should award even more stars (possibly an entire rank).
I feel that this would not only make the tour of duty more balanced, but it would make the player levels more rewarding.
As of now… there is virtually no reward or point in leveling or re-uping. I was very disappointed to see the lack of anything really upon getting my first re-up.