Several companies have begun design work on a prototype airship that could hover at an altitude near space where it would be able to track ground and aerial targets for up to a year at a time. Whether the program, known as Integrated Sensor is Structure (ISIS), moves beyond the design stage any time soon, however, will depend on the final version of the 2007 defense budget.

The U.S. House of Representatives funded the full $16.3 million request for the effort in its version of the 2007 Defense Appropriations Act, which passed the House in June. The Senate Appropriations Committee, however, has recommended denying the Defense Advanced Research Projects Agency’s entire $16.3 million budget request for the program in 2007. The bill is currently awaiting a vote on the Senate floor.

The House and Senate will address the issue when they meet to resolve differences between their bills later this year.

Jenny Manley, a spokeswoman for the Senate Appropriations Committee, did not respond to a request for comment on the committee’s proposed cut to the ISIS program.

Jan Walker, a spokeswoman for the Defense Advanced Research Projects Agency, said that program officials declined to comment on the ISIS effort at this time.

The Air Force Research Laboratory of Rome, N.Y., has awarded several contracts to industry to begin work on various aspects of the ISIS program. The lab awarded a two-year, $10.3 million contract to Lockheed Martin Maritime Systems and Sensors of Akron, Ohio, in June to begin work on the airship platform.

The lab awarded two contracts earlier this year to Northrop Grumman Corp. for the ISIS effort. Northrop Grumman Space Technology of Redondo Beach, Calif., will develop a transmit-and-receive module for the radar sensor that is expected to be lightweight and extremely power efficient under a $6.8 million contract awarded in April. Northrop Grumman’s Electronic Systems sector, which is based in Linthicum, Md., is developing an antenna that can handle radar as well as transmit data simultaneously under an $8 million contract. Raytheon Space and Airborne Systems of El Segundo, Calif., also is working on a design for the antenna under an $8 million contract.

If the program does go forward, the airship , will feature a radar sensor of “unprecedented proportions,” according to a Pentagon document.

The Defense Advanced Research Projects Agency’s 2007 budget justification materials, which are posted on the agency’s W eb site, describe ISIS as a sensor capable of conducting surveillance and tracking hundreds of time-critical targets in both urban and rural environments.

The sensor is being designed to track airborne targets at a range of 600 kilometers, and ground targets at a range of 300 kilometers while distributing that information to U.S. forces through hundreds of covert wideband communications links, according to the budget justification materials.

The agency wants the sensor to detect and track targets including aircraft, cruise missiles, tanks and troops, according to briefing charts posted on the agency’s Web page. The agency also wants the sensor to detect mortar and artillery fire.

Challenges involved with building the system include finding ways to keep the weight, power requirements and logistics requirements minimal, according to the budget justification materials.

The ISIS sensor would be the largest radar sensor built to date, and is much larger than the Sea Based X-Band Radar sensor built for the U.S. Missile Defense Agency, according to Michael Wechsberg, director of radio frequency programs and ISIS program director at Raytheon Space and Airborne Systems.

Much of the size of the Sea Based X-band Radar, which features a Raytheon-built sensor, is material needed to keep the radar sensor stiff, Wechsberg said in a July 19 interview. The company is able to avoid this issue on ISIS by using the structure of the airship itself to stiffen the sensor, which will be placed either inside or outside the craft, he said.

One way of handling the need for the antenna to transmit and receive data simultaneously is a technique called time multiplexing, which involves changing the function from sensing to transmitting data in less than a second, Wechsberg said.

The Missile Defense Agency is developing another large airship, known as the High Altitude Airship (HAA) that would operate at the near-space altitude, which is generally defined as the area around 20 kilometers. Lockheed Martin is under contract to build and demonstrate the HAA.

However, the HAA project is primarily focused on demonstrating the airship platform in the near term, while ISIS is more focused on the payload, Wechsberg said. The payload aboard the HAA represents about 1.7 percent of the platform’s weight, while the payload aboard ISIS accounts for roughly 30 to 40 percent, according to the Defense Advanced Research Projects Agency’s briefing charts.

Ron Browning, HAA business development director for Lockheed Martin, said during an Aug. 1 briefing for reporters that the Missile Defense Agency has not yet chosen the type of payload that will fly aboard the HAA in its first prototype flight, but said that the available room could host a communications or optical sensing system based on mature technology that would not add risk to the HAA development effort.

The small amount of dedicated payload space aboard the HAA means that that sensor will be far less capable than that envisioned for ISIS, Wechsberg said.

Dave Filicky, Lockheed Martin’s ISIS program manager, said that the company will be able to build on its experience with the HAA as it designs the ISIS platform. However, ISIS poses its own challenges with the platform, due to the size of the sensor as well as the longer duration that it is expected to hover over particular areas.

ISIS will require a lighter platform to deal with the larger radar sensor, and systems that can operate for long periods at relatively low power, Filicky said in a July 19 interview.

“If it wasn’t brand new, never been done before, [the Defense Advanced Research Projects Agency] wouldn’t have funded it,” Filicky said.

The HAA is currently expected to be demonstrated in flight for the first time around 2008; ISIS is scheduled for a reduced scale prototype flight in 2009, with plans for a full-scale demonstration to be determined afterwards, Wechsberg said.

Like ISIS, the HAA also is encountering some resistance in Congress, where members of the House and Senate have both moved to trim the $40.1 million request for the program in 2007 in order to steer funding towards other missile defense efforts. The House version of the 2007 Defense Appropriations Act cuts the request for the High Altitude Airship by $20 million, while the Senate version of the bill reduces it by $25 million.