"For the iPhone, there are four different radio interfaces that could potentially be used for malicious purposes: the cellular modem, Wi-Fi, GPS, and NFC"
Well, GPS only receives... so how is it going to detect that?
Possibly the idea is to prevent malware already on the phone from logging location data while the radios are dark, and then uploading the historical location data when the phone reconnects?
That said, it's obviously a very limited form of protection. I guess it speaks to the sad state of mobile security that this is the best Snowden and Bunnie can come up with - the only sane choice for a potential target is to assume the mobile device is untrusted and try to reduce the scope for it to snitch on the user.
In the the linked article, it states they intend to attach signal probes to test points on the iPhone mainboard to read when certain interfaces are active.
I assume similar to the way radar detector-detectors work. Superheterodyne receivers mix incoming RF with an internal signal to make a much lower intermediate signal they can tune to. The device will leak some of the internal signal and you can detect it.
Well, GPS only receives... so how is it going to detect that?