US$10 million skin for DARPA's remarkable ISIS stratospheric airship

“To put the ISIS challenge into perspective, consider that tracking large numbers of low flying targets at hundreds of kilometers requires a radar with the capability of the largest ballistic missile defense radars These are ground based, the size of large buildings, and need their own power plants to function. ISIS must get similar radar capability while constrained by the limited lift capability of a stratospheric airship.

“The best way to reduce the system weight and power requirements is to exploit the airship’s size, roughly fifty meters in diameter. ISIS will have a phased-array antenna that is nearly as large as the airship itself. Since radar performance depends on the power-aperture product, the extremely large antenna aperture size allows us to significantly reduce the transmit power. This greatly simplifies the onboard power and cooling systems allowing us to replace the conventional, heavy, high power antenna with a larger but lighter lowpower- density antenna.

“However, this solution faces several technology challenges.

“First, weight is still a critical problem. The lightest space-based X-band active radar antennas weigh approximately twenty kilograms per square meter. Even if next generation space-based technology weighs in at only three kilograms per square meter, it will still be too heavy to realize the full ISIS potential. The ISIS program must develop phased-array antennas lighter than any spacebased technology. To do this, ISIS is exploiting several inherent advantages the airship platform provides. Unlike a space-based antenna, the ISIS antenna does not need to be stiffened to survive launch or stowed to fit in a small cargo bay and then deployed. Nor does it require radiation shielding. In fact, a stratospheric airship environment should enable the use lightest antenna technology ever developed.

“The second challenge introduced by an extremely large and lightweight antenna is electrical calibration to compensate for the inevitable flexing and distortion of the huge aperture. The phased-array will consist of millions of elements. The position of each element must be known to within a millimeter. We absolutely need ideas on how to dynamically measure, model, and calibrate such a large system.

“The third challenge faced by the large phased-array antenna is beamformer complexity. The huge antenna will be constantly reconfiguring itself to adapt to changing battlefield conditions. The aperture will be made up of thousand of small subarrays. All subarrays will be combined together with the proper time delay to form one large array to track an airborne target at the horizon. A millisecond later, the subarrays will be regrouped into multiple horizontal strips to simultaneously track many ground targets at closer range. New light weight and low power consuming beamformer concepts are needed, perhaps digital or photonic to allow for this flexibility. If you have other ideas, we’d love to hear them.

“The fourth challenge faced by the huge antenna is a new level of integration complexity. A single ISIS system will be both sensor and airship. The antenna will be nearly as large as the airship itself. Therefore, we cannot separately develop a platform and a payload. Innovative functional integration across subsystems can allow each bit of mass to serve multiple sub-system functions. Here lies another huge development opportunity. How do we most efficiently integrate the sensor, structure and power sub-systems into a multi-functional airship?

“Once we solve the antenna weight, size, and integration challenges, ISIS must contend with the fifth challenge, annual wind storms. These can last for several days and exceed 80 knots in some locations. This will challenge the airship’s ability to stay on station. The propulsion power scales as the cube of the wind velocity, so we must find a way to store large amounts of energy in reserve for use during peak winds. We will need technologies with ten times the specific energy density of today’s lightest batteries.

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