"DARPA hard" is shorthand for something considered extremely hard to achieve. The phrase comes from the US agency DARPA (Defense Advanced Research Projects Agency) who are often tasked with coming up with solutions to very difficult problems (e.g. coming up with stealth technology to prevent aircraft being shot down). The implication is that only resources of the scale available to DARPA's would be required to crack the problem.
Giving every soldier the ability to know the position of every other solidier on the battlefield - that's going to be DARPA hard.