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 low power-density antenna.
The system faces several technology challenges.
1) weight is 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. 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. (2 kgs per square meter or less)
2) Electrical calibration of the antenna to compensate for the inevitable flexing and distortion of the huge aperture is needed. The phased-array will consist of millions of elements. The position of each element must be known to within a millimeter.
3) Beamformer complexity is a problem. The huge antenna will be constantly reconfiguring itself (in milliseconds) to adapt to changing battlefield conditions.
4) The level of integration complexity is a problem. A single ISIS system will be both sensor and airship.
5) Once the antenna weight, size, and integration challenges are solved, 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. They will need technologies with ten times the specific energy density of today’s lightest batteries.
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