I would love to know this too. Incidentally there has been testing of air breathing ion engines to allow long lived very low orbit satellites on Earth. I'm wondering if this is applicable to Mars.Using the upper atmosphere as fuel can mean lower mass and make it easier to have a largeish onstellation with good coverage.Multifunction satellites (GPS included) which would give direct, accurate and real time data on the upper atmosphere.
You're definitely going to need realtime global data.