American stealth aircraft, specifically the B-2 bombers, are shifting from use of lighter radar reflecting materials to heavier radar absorbing materials. While the radar absorbing materials add two tons to the weight of a B-2, they require less maintenance and work better. Radar energy flows over the surface of an aircraft and is reflected only when a seam or joint causes a ripple. With radar absorbing materials, so much of the energy is absorbed that there is nothing left to reflect when it reaches a seam. Lockheed Martin insists that the B-2 is impervious to bistatic radar, such as the recent report that computer analysis of the disruptions of cell phone transmissions could track a stealth plane. The company says that its new coating design focuses these returns so sharply that the receiver would get only a single spike, not enough of a signal to zero in on the location of a bomber. Radars need two hits out of three sweeps (or three out of five) to locate an aircraft with any useful precision.--Stephen V Cole