i have a ? or two.
first did it shut down as in off or just act dumb to where it was?
what was the distance from where it went out to when it came back on?
because of the freq's involved i doubt it was radar, if radar was the issue then many airplanes would be in trouble. but simple jammers for the gps freq's are easy and instructions can be found on the web. a simple oscillator on the right freqs putting out less than a watt will black out about a 1 mile radius for a gps.
now i doubt secret service would block it out, but maybe they where running some sort of drill at the same time in the area where they wanted to shut down gps. i am sure the SS has the equipment to do the gps and cell phone freqs. it is normal for the SS to run a low key drill in the area where the pres is, just because of all the equipment they have in place already