The plan is reminiscent of a strategy used during the Gulf War, when U.S. military planners rushed a sensor aircraft into service to track fleeing Iraqi ground forces. The Joint Surveillance Target Attack Radar System helped turn Iraq's withdrawal from Kuwait into a rout.

While program officials are circumspect about discussing details of the sensors being considered this time around, they did not rule out accelerated deployment of Global Hawk, which is still in its engineering and development phase. Currently scheduled for deployment in 2005, Global Hawk has been singled out by Department of Defense planners as a "transformation" technology that could significantly upgrade U.S. fighting capabilities. Accelerated production could speed delivery by two years.

Global Hawk's base is Beale Air Force Base, Calif., home of the 9th Reconnaissance Wing, which also operates U-2 spy planes. The Air Force plan prior to the air war in Afghanistan was to have Global Hawk replace the U-2 by 2007.

Global Hawk sensors now include synthetic aperture radar and electro-optical/infrared sensor payloads. The Defense Advanced Research Projects Agency awarded a research contract to Lockheed Martin Corp. to develop a foliage-penetrating radar, also known as a "tank under trees" sensor. The Army has conducted extensive research on ground-penetrating radar based on ultrawideband technology.

Although the Air Force wouldn't comment on the Global Hawk sensor program, a recent Pentagon announcement soliciting industry ideas for combating terrorism included sensor projects for detecting, mapping and destroying underground targets. Bin Laden's al Qaeda network and his Taliban supporters are thought to operate from mountain bunkers around Afghanistan.

According to the Oct. 25 DOD notice, "Applicable methods may include ground-penetrating radar, acoustics, and unconventional surveillance and reconnaissance."

Global Hawk's builder, Northrop Grumman Corp.'s Ryan Aeronautical Center (San Diego), has delivered five aircraft thus far to the Air Force and for now expects to build at least two aircraft a year. Alfredo Ramirez, the program's deputy chief engineer, said heightened security restrictions imposed after Sept. 11 prevented him from discussing details of the sensor program. But he acknowledged the consideration of "additional payload systems for Global Hawk."

A congressional source said foliage-penetrating radar development is part of the Pentagon's "payload investment strategy." So far, there is no funding for a Global Hawk earth-penetrating radar. "Are people working on this? Sure. But the big issue is the tempo and maturity of the sensor program," the aide said.

Lawmakers have also encouraged DOD efforts to accelerate Global Hawk development and deployment. Congressional panels overseeing military spending have backed "additional development of enhanced target detection algorithms and improved target detection hardware," said a House report on the fiscal 2002 budget.

Senate appropriators are pushing a signals intelligence payload that would give Global Hawk sensor capabilities similar to those of the aging U-2.

Asked about sensor payloads for Global Hawk, a senior DOD official said during an Oct. 31 briefing on unmanned aircraft technologies that Pentagon officials were considering adding an operational requirement for a future signals intelligence payload.

As for plans to speed deployment of the craft, the official added: "Global Hawk is definitely one of the considerations that's on the table. . . . We're certainly looking at a lot of options. And Global Hawk, I will say, is a very attractive one."

PowerPC enlisted

A big difference between Global Hawk and previous unmanned aircraft is its ability to perform on-board processing of sensor imagery. "This is a big departure," said Northrop Grumman's Ramirez, who said designers are migrating to PowerPC processing architectures. One design implication is a heavier payload that also includes the transmitters, exciters and communications gear needed to transmit imagery to ground controllers.

Communications include two pairs of narrow- and wideband line-of-sight and satellite links, Ramirez said.

Global Hawk is a high-altitude unmanned reconnaissance aircraft capable of flying 13,500 nautical miles and initially carrying a 2,000-pound payload. It is designed to loiter over targets for extended periods.

"It can operate in excess of 35 hours," the DOD official said. "It's equivalent to flying 1,200 nautical miles, hanging out for over 24 hours and then flying back 1,200 nautical miles."

The Air Force hopes to raise Global Hawk's payload to 3,000 pounds by the time it reaches the production phase. The current plan is for Global Hawk to carry high-resolution synthetic-aperture radar (SAR) with a ground moving-target indicator mode and an electro-optical/IR sensor. The sensors are being built by Raytheon E-Systems.

So far, the craft's sensors generate only text data on target location and speed. SAR and EO/IR imagery are transmitted as individual frames that can be combined in a mosaic before being forwarded to analysts and targeters, program officials said.

Ramirez said that wideband communications links might eventually permit transmission of video imagery.