[trod1234]

More than likely that conjectured capability is the yesteryear Predator.

Seems tech may have received some upgrades:

https://www.technologyreview.com/2019/06/27/238884/the-penta...

There was also a hot-air balloon program that would persist at stationary at high altitude and track all moving targets over a metropolitan area for up to a month without maintenance.

The news article which said the project was called "Basilisk Stare", seems to have vanished now though from all search results. Edit: Maybe I'm misremembering and its actually Gorgon Stare.

https://en.wikipedia.org/wiki/Gorgon_Stare

[potato3732842]

>There was also a hot-air balloon program that would persist at stationary at high altitude and track all moving targets over a metropolitan area for up to a month without maintenance.

They could do this with a single (bajillion dollar) military radar slung from the bottom of a plane over Iraq 15yr ago.

The current tech is probably cheaper and better.

[sawjet]

DHS has these and is already using them. There is an aerostat that flies in South Padre whenever SpaceX is launching starship doing exactly what you suggest.