The baseband still has access to your battery when your phone is off.
For the example of a protest march, if you're a participant or even a bystander walking home wanting to avoid being caught up in a surveillance dragnet, turning your phone off should be sufficient. You'd probably want to take stronger measures if you were involved in organising the protest, though.
I would not trust that any modern smartphone is fully off without yanking the battery.
Are you sure they don't have a backup battery or similar that can continue broadcasting your location after you pull the battery. That would be an ideal countermeasure as pulling the battery would be a great indication you may be about to do something "naughty" and you'd likely do such things shortly after too ...
That is not enough. Use a faraday cage.
Modern phones have two batteries [0].
Burner phones bought in cash from one of the little independent shops we have all over (here in the UK anyway) with pre-paid sims - not a new idea I saw it cropping up in fiction 20 or more years ago but still effective, used for a day or two and then dumped.
Isn't it tragic that the way you have to protect yourself from the state when organising a legal protest is pretty much the same way you would do it if you where a drug dealer or a terrorist (at least a reasonably smart one).
Citation needed?
Because the definition of off, for most phones, is exactly that.
http://blog.erratasec.com/2013/07/no-nsa-cant-track-phones-e...
There is NSA method called 'implant', but it requires installation of software (possibly in the firmware). It's not so easy and it can't be used in large scale.
Snowden is right when he says that every phone is vulnerable for targeted surveillance. For people like him, every phone is dangerous even when they are closed.
Mass surveillance is different. NSA can't just pick random person and remotely track their phone when it's turned off.
>"The FBI can access cell phones and modify them remotely without ever having to physically handle them," James Atkinson, a counterintelligence security consultant, told ABC News. "Any recently manufactured cell phone has a built-in tracking device, which can allow eavesdroppers to pinpoint someone's location to within just a few feet," he added.
Source: http://blogs.abcnews.com/theblotter/2006/12/can_you_hear_me....
I believe (but cannot cite) that this capability became mainstream when E911 & GPS was mandated.
This stuff is a big reason why BlackBerry was so critical to regulated industries and public company execs. PIN messaging on legacy BlackBerry was secure between devices on the same BES, and there wasn't a direct linkage to the user's identity, which made it easy to lower the value of metadata collection.
The fact that BlackBerry designed around this early on leads me to suspect (as an outside observer with an imagination) that mass metadata collection was known by folks in industry back in the 90s.
Wouldn't a simpler explanation be that it was just over-designed for 90s-era threats in a way that hardened it to some more recent threats?
Consider however that none of their US based competitors managed to do address those threats in a meaningful way.
https://github.com/SecUpwN/Android-IMSI-Catcher-Detector/iss...
'Dreamy Smurf handles power management, which according to The Guardian includes "an ability to stealthily activate a phone that is apparently turned off"'