In a post apocalyptic pandemic world a teenage girl trying to reach the safety of quarantine takes refuge in a valley, only to be thrust into a fight for her life with a man willing to kill her to get what he wants

>>it may seem a little magical.

Yeah, it certainly is a convenient prop. I like post-apocalyptic worlds where everything goes to hell in a handbasket, where characters have no recourse to technological crutches and gimmicks. They are forced to prevail the old fashioned way: by their wits.

But it's your story.

I gotta quibble on calling the antagonist "psychotic". Technically, it seems to me, he's more likely to be a psychopath than a psychotic. That is a distinction with a difference. Psychosis is a disorder so dysfunctional in terms hallucinations and an inability to engage in reality testing that is unlikely that he would be able to pursue her for any extended period. In contrast, psychopaths are fairly lucid, just mad as hell and not going to take it anymore.

This is the kind of GPS used by hikers and surveyors. It is satellite based and can be charged using a head held generator. Sounds dumb I know, but works in the story. It is a similar things to the devices that use in Africa when tracking herds.

I researched them, they would still work as long as the satellites remain up, which gives them about a decade in most situations. All data is stored on the device which means they don't need anything other than a reference point to operate.

I was worried that it may seem a little magical.

A post-apocalyptic world-- but GPS still works?

Seems to me that in a righteous post-apox world, all the infrastructure to support the power grid, Internet, GPS, POTS (plain old telephone service), etc., would have collapsed.