Each one of these could be a potential missile used against us.
This seems to be exaggerated. Wouldn't you need a drone on American soil, already right next to the target, in order to crash it by sending falsified GPS coordinates?
Did you read the part of the article that said "The demonstration of the near-disaster, led by Professor Todd Humphreys and his team at the UTA's Radionavigation Laboratory, points to a "gaping hole" in the US's plan to open US airspace to thousands of drones," so presumably there's an existing plan to use drones in US airspace. One would imagine they would be most useful over densely populated areas.
The FAA is currently coming up with the rules to govern unmanned systems in the national airspace. They have a mandate from congress to have the rules in place by 2015. As of today, the only way to commercially operate a UAV requires case-by-case permissions.
You can't control altitude or velocity, either (unless the system uses a GPS altimeter instead of a barometric one), so "potential missile" is indeed a massive exaggeration. To be honest, it would be cheaper, easier and more dangerous for a hypothetical bad guy to buy an actual missile like the Qassam for around $800[0] than it would to try and "hijack" a real UAS.
edit: ( I think the real concern in this situation would be reverse engineering with an eye to finding vulnerabilities to be exploited in other drones, not so much reprogramming the one and only they have )
You would still need a strong signal near the target, which is more difficult than setting up a drone-trap like Iran seems to have done.
Is it possible to somehow sign GPS coordinates? (Perhaps sync an internal clock at each mission's start, and check a time-based signature?)
If not, are there practical challenges to integrating the output of a drone's engine and calculating the path travelled, instead of naively believing satellite coordinates?
Actually, wind must make that difficult. For a car, you can attach a magnet to each wheel and count the surges as it passes a sensor on each rotation, to give you an idea of where you are. Due to the external forces on a drone, you would probably want to measure forces with an accelerometer/gyroscope, not engine output.)
What are the challenges involved in such? Is internal-location tracking even feasible?
All military navigation systems are inertial, not GPS. They only accept GPS corrections within the error bars of the inertial system. As a practical matter this means that you can only make a drone deviate from its intended course by a few meters assuming you did a perfect job of spoofing the GPS.
GPS spoofing/jamming only works for systems that use GPS navigation systems; military weapons and systems have never used GPS navigation. Inertial navigation systems are spoof-proof.
