Defense

Airborne multifunction radar of the next generation

FHR research airplane »Delphin« with PAMIR-Ka wing unit.
© Fraunhofer FHR

FHR research airplane »Delphin« with PAMIR-Ka wing unit.

Block diagram of the Pamir-Ka SAR demonstrator.
© Fraunhofer FHR

Block diagram of the Pamir-Ka SAR demonstrator.

A point target response measured at a reference target to verify the resolution.
© Fraunhofer FHR

A point target response measured at a reference target to verify the resolution.

Reliable airborne surveillance and reconnaissance with radar requires sensors with highly flexible functionality and the best possible image quality. The radar reconnaissance systems that are currently available for utilization on flight platforms already offer a variety of operating modes and resolutions in the decimeter range. But does this mean that the feasibility limits for pulsed multifunction systems have already been reached?

Work within the framework of the project PAMIR-Ka (Phased Array Multifunctional Imaging Radar in Ka-band) reveals perspectives for novel methods and technologies for airborne, wide-area surveillance and reconnaissance in the Ka-band frequency domain.

In the final development stage, PAMIR-Ka will be a pulsed multi-channel and multi-functional radar system with synthetic aperture for ultra-high imaging and moving target indication in the Ka-band. The system is being developed as the successor of PAMIR-X system, which operated in the X-band, and will open up new technical possibilities for the frequency band between 30 and 38 GHz. The development of airborne radar systems using Ka-band technology offers many advantages compared to traditional X-band systems. More compact antennas and high frequency modules can be used in the sensor systems. Moreover, the higher bandwidth also significantly enhances the resolution of the processed image products in range direction.  Combining these properties with a high-precision pulse operation in conjunction with an active electronically steerable 2D array antenna for transmit and receive, the range can be increased by distributed transmission power generation. Further enhancement of the range resolution is possible by antenna beam tracking in the direction of flight. In conjunction with multi-channel design, enhanced radar modes can be integrated, e.g. for the better detection and location of moving targets.

2D array technology for Ka-band radar operation with synthetic aperture

Two separate antennas for transmitting and receiving are envisaged in the final stage of development. Each equipped with 48 x 8 antenna elements, the fully populated array antennas will allow very  agile electronic 2D beam scanning. Combining  this with a high instantaneous bandwidth of up to 8 GHz, SAR operating modes with range resolutions of less than 2 cm and resolutions in the direction of flight of 1 to 2 cm could be acieved. By appropriate  thermal design, the transmit antenna array will have a high pulsed transmit power of approximately 800 watt to achieve the best possible range and image dynamic in spite of the unfavorable path attenuation in Ka-band compared to X-band.

First demonstration phase of the high-resolution SAR mode

The realization of a multi-functionalradar in Ka-band requires step-by-step methodological and technological development in demonstrator phases. In the first stage of development, PAMIR-Ka is equipped with a transmitting and receiving channel in a quasi-monostatic antenna arrangement. Prior to the first flight tests, the system was installed on board the FHR airplane »Delphin«. The tests aimed to verify the functionality of the system under flying conditions and investigate the relevant aspects of pulsed SAR imaging in the Ka-band.

The SAR demonstrator consists of two parts, a wing unit and a cockpit unit. The wing unit accommodates the system's front end and is installed in an aerodynamic pod. The front end comprises the antennas, signal generation, the frequency converter modules, pulse distribution and an analog-digital converter with a sampling rate of up to 20 GS/s. The pod is mounted under the wing and the antennas, which are inclined at an adjustable angle, illuminate the ground scenes. The cockpit unit consists of an inertial navigation system, a GNSS receiver, a power converter, data storage and the radar process control.

Numerous flight tests were carried out with PAMIR-Ka in 2017. The images in the stripmap mode were taken at heights of up to 800 meters above the ground with an antenna depression angle of 35°. To achieve the best possible range resolution, the complete simultaneous bandwidth of 8 GHz was used, which theoretically allows a range resolution of 19 mm. The relatively small opening of the horn antenna allows a long synthetic aperture and a correspondingly fine resolution in the direction of flight. The evaluation of the two-dimensional point spread function in the processed image shows a resolution of 9 x 20 mm on the ground (azimuth x range).

In future development stages, the system will be extended to create a multi-channel, multi-functional phased array system with Significantly increased output power and an active electronically scanned array (AESA).