I was reading in 'How could one navigate on Mars' about communications satellites, and it got me thinking about radio relay and navigation infrastructure. Pure nitrogen is an inert lifting gas in the carbon dioxide atmosphere there. Carbon monoxide is too and could be refined from the air. Hydrogen could be gotten from the water in the soil but would slowly leak from the balloon, though it is so much lighter and plentiful it would maybe still be a better choice. With the very thin atmosphere it would take very little to fill a balloon - but it would also mean a balloon would have to be extremely large to have much lifting capacity. Still, if all it is is an antenna for a radio transceiver it is tethered to, and a navigational aid, maybe it could be a good way to cheaply set up reliable radio contact on Mars and an array of landmarks for triangulation visible over great distances. Wind loads are a lot lower on Mars, there aren't any thunderstorms - though maybe static discharges would be a problem.
Could this be useful? How high would they need to be to get above most of the dust in a storm and thus work despite the static interference during such storms? What kind of calculations do you need to do to analyze such a system?