The National PNT Advisory Board invited Dr. Humphreys to speak at their "GPS toughening" working group meeting on June 10 and then to present before the full Advisory Board on June 11.  His presentation concerned GPS navigation message authentication as a means of "toughening" GPS receivers against unintentional and intentional GPS spoofing.  As part of the presentation, Humphreys offered a categorization and an ordering of spoofing attacks and defenses that will be a good starting point for a proper civil GPS threat assessment.

See the meeting agenda, the posted slides (pdf) for "Toughening Techniques for GPS Receivers: Navigation Message Authentication," and the full presentation (pptx).