The operation in Abbottabad involved another U.S. aircraft with stealth features, a Black Hawk helicopter equipped with special cladding to dampen noise and evade detection during the 90-minute flight from a base in Afghanistan. The helicopter was intentionally destroyed by U.S. forces — leaving only a tail section intact — after a crash landing at the outset of the raid.
‘A difficult challenge’
The assault and the months of surveillance leading up to it involved venturing into some of Pakistan’s most sensitive terrain. Because of the compound’s location — near military and nuclear facilities — it was surrounded by Pakistani radar and other systems that could have detected encroachment by Predators or other non-stealth surveillance planes, according to U.S. officials.
“It’s a difficult challenge trying to secure information about any area or object of interest that is in a location where access is denied,” said retired Air Force Lt. Gen. David Deptula, who served as head of intelligence and surveillance for that service. The challenge is multiplied, he said, when the surveillance needs to be continuous, which “makes non-stealthy slow-speed aircraft easier to detect.”
Satellites can typically provide snapshots of fixed locations every 90 minutes. “Geosynchronous” satellites can keep pace with the Earth’s rotation and train their lenses on a fixed site, but they orbit at 22,500 miles up. By contrast, drones fly at altitudes between 15,000 and 50,000 feet.
In a fact sheet released by the Air Force, the RQ-170 is described as a “low observable unmanned aircraft system,” meaning that it was designed to hide the signatures that make ordinary aircraft detectable by radar and other means. The sheet provides no other technical details.
Stealth aircraft typically use a range of radar-defeating technologies. Their undersides are covered with materials designed to absorb sound waves rather than bouncing them back at sensors on the ground. Their engines are shielded and their exhaust diverted upward to avoid heat trails visible to infrared sensors.
Unlike the Predator — a cigar-shaped aircraft with distinct wings and a tail — the RQ-170 looks like more like a boomerang, with few sharp angles or protruding pieces to spot.
The Air Force has not explained why the RQ-170 was deployed to Afghanistan, where U.S. forces are battling insurgents with no air defenses. Air Force officials declined to comment for this story.
Strikes along the border
Over the past two years, the U.S. military has provided many of its Afghanistan-based Predators and Reapers to the CIA for operations in Pakistan’s tribal region, where insurgent groups are based. The stealth drones followed a similar path across the Pakistan border, officials said, but then diverged and continued toward the compound in Abbottabad.
U.S. officials said the drones wouldn’t have needed to be directly over the target to capture high-resolution video, because they are equipped with cameras that can gaze at steep angles in all directions. “It’s all geometry and slant ranges,” said a former senior defense intelligence official.
Still, the missions were regarded as particularly risky because, if detected, they might have called Pakistani attention to U.S. interest in the bin Laden compound.
“Bin Laden was in the heart of Pakistan and very near several of the nuclear weapons production sites,” including two prominent complexes southeast of Islamabad, said David Albright, a nuclear weapons proliferation expert at the Institute for Science and International Security.
To protect such sites, Pakistan’s military has invested heavily in sophisticated radar and other aircraft-detection systems. “They have traditionally worried most about penetration from India, but also the United States,” Albright said.
Largely because of those concerns, Pakistan has placed strict limits on the number and range of CIA-operated Predators patrolling the country’s tribal areas. U.S. officials refer to the restricted zones as “flight boxes” that encompass North and South Waziristan.
Staff writers Craig Whitlock and Greg Jaffe and staff researcher Julie Tate contributed to this report.