The U.S. Coast Guard has posted dates and times for GPS "service interruptions." The purpose is "to conduct GPS interference tests, exercises and training activities that involve jamming of GPS receivers." Affected locations are China Lake, CA, White Sands Missile Range, NM, Cape Canaveral, FL and the Nevada Test and Training Range.
Though the testing is brief (a few hours at a time), it is of multiple day duration. And don’t think it won’t affect you. Check out the map of the China Lake test area to the left. This is a massive area, defined by the red range ring (230 nautical mile radius) and blue lines (testing does not extend beyond longitude 115W and 120W). As shown, it includes all of L.A., Joshua Tree, Sequoia/Kings Canyon, Death Valley and Yosemite. I’m hoping the area really isn’t that large. Please, feel free to comment and tell me why it isn’t this extensive. There are some caveats in the test announcement; here is the full text:
"230 NM RADIUS OF THE NAWS CHINA LAKE/ARMIT AGE AIRPORT (NID) AT FL400, NOT EXTENDING BEYOND 120W AND 115W. THIS AREA DECREASES WITH ALTITUDE TO A CIRCLE OF 175 NM RADIUS FROM NID AT FL250 (BOUNDED BY 115W AND 120W), 120NM RADIUS AT FL100 (INSIDE BORDER OF
CALIFORNIA), AND 120NM RADIUS AT 4,000FT AGL."
A little research turned up the following definitions:
- FLIGHT LEVEL (FL) – A level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each is stated in three digits that represent hundreds of feet — flight level 250 represents a barometric altimeter indication of 25,000′; flight level 255 an indication of 25,500′.
- AGL – Above Ground Level, as a measurement of altitude above a specific land mass, and differentiated from MSL.
EDIT: In addition to the comments below, discussion about this topic is also taking place on Groundspeak, Geocaching.com’s message board.