Those of us in the service of our country overseas usually vote based by absentee ballot assigned to the state and county of one's US domestic residence, when that primary is up. A caucus is out of the question because one has to be physically present for that. So, for example, a member of the USAF here could go and send in an absentee for New Hampshire, but would have no primary election voice if he or she was from Iowa or Nevada.
Thanks for asking.