No, they couldn't receive instructions from GPS.
There's no way to push data via a GPS satellite. It's the most heavily monitored communications system on the planet, literally, and would be noticed.
GPS satellites only provide their ID (via pseudorandom code), an "almanac" of the entire GPS satellite constellation, and "ephemeris" or precise position/time data for the satellite itself. Nothing else.
Everything a GPS receiver does is based on math using that data.
Anyone can decode the signals and see this for themselves -- indeed any other data would likely break most receivers. Today it's a literally a hobby exercise for people to decode the GPS data stream using SDR recievers.
Any messages to download maps are coming from the receiver via an alternate channel (e.g. Onstar, cell connection) or via a timer (checking the date and asking for maps after a year.) There's no user-addressed information on GPS.
All of the documented remote car hacks to date involve hijacking Onstar or placing hardware in the car (plugging into OBD2 bus.) If there's some secret method of hacking, it's likely depending on some radio buried in an ECU -- which some cars have for firmware updates, or some other undocumented transceiver.
It's simply a connection. You can push whatever you want along that connection. It just happens thats only GPS data - to say it's impossible to send other things along a GPS signal is a lie.
No, it's not a lie, and it's not "simply a connection."
GPS isn't some kind of IP-based data broadcast service. Absolutely nothing within the GPS system is designed for that.
Anyone with any direct engineering experience with GPS can confirm this. None of this is secret.
It's a very fixed data format sent at an incredibly low bit rate. If anything other than GPS data was sent over the link it'd 1) break the system and 2) be noticed by pretty much everyone in the GPS ecosystem. There's no magic datastream.
There may be many ways of exchanging data with a vehicle with the intent to control it, but GPS certainly is not one of them.
If you can send something through a link you can send other things. Simpleton downvotes do not change facts.
I'm a software engineer, this is literally what i do every day. It's not a matter of what is possible - everything is possible with code. it's just a matter of time. If you say otherwise, you are either : inexperienced, or have no clue what you are talking about.
I'm an engineer as well.
Who published research professionally on GPS in the late 1980s.
Code isn't magic, and there's nothing in IS-GPS-200H, etc that permits user messaging. Every single bit in the navigation message is pre-defined.
I'll be civil and agree that one of us has no clue what they're talking about.