The Department of Defense is conducting more GPS interference tests in the western U.S. through next month that will cause GPS navigation to be unreliable at times near the test centers, reported AOPA. The areas affected center on the Bonneville (BVL) Vortac in Utah, Truth or Consequences (TCS) Vortac in New Mexico and Sierra Vista Municipal Airport in Arizona. The area of unreliability can start within 160 nm of the test center at 4,000 ft agl and expand with altitude to a radius of 400 nm at 40,000 ft msl. These tests, intended to check for both accidental and deliberate interference, are fairly common as the U.S. prepares to transition to a satellite-based navigation system.