From the 2nd page of the article, quoting from the "Technical Working Group" (which supposedly included representatives from both LightSquared and the GPS indus

From the 2nd page of the article, quoting from the "Technical Working Group" (which supposedly included representatives from both LightSquared and the GPS industry): "the interfering signal at a distance of 500 feet from the [cell] tower [was] up to 800 billion times more powerful than the distant GPS signals being received from space," and a mile away power levels were "400 million times those of the GPS signals." That's a difference of ~87 to 120 dB, which seems consistent with the 90 dB difference you cite. (edited: s/LightSpeed/LightSquared/)