With an authenticated GPS signal, where you can't change the satellite position data in the signal, you can jam the signals and send a delayed copy to the GPS receiver, tinkering with the relative timing of each satellite's signal so that the receiver solves for the position you want. It's tough to do, but apparently not tough enough.
I would think you could _*calculate*_ what the signal including the delay as percieved by the GPS receiver should look like at any given time, in any given location. In principle, all the information is public available.
The Radionavigation Lab result is noteworthy as it was an entirely civilian effort by one professor and fewer than half-a-dozen students.
Just demonstrates my point: GPS is an open protocol, free for all to read and understand. Anyone with the proper skills in mathematics and engineering (and a little ingenuity) can get the information they need to develop spoofers and jammers.
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