Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRFirst Tests Successfully Completed
High-Resolution Radar Imaging of the Moon’s Surface with the New Target Tracking Radar of the Large Radar Facility TIRA
Researchers at Fraunhofer FHR have successfully conducted initial experiments with the new target tracking radar of the TIRA facility: They produced a high-resolution radar image of the entire Moon’s surface visible from Earth - from a distance of 385,000 kilometers! By utilizing the motion of Earth and the Moon, a significantly larger, virtual aperture was created using the 34-meter antenna of the TIRA facility, thereby achieving high resolution imaging. This method of generating a synthetic antenna aperture (SAR) enables coherent imaging of the entire visible Moon’s surface.
A New Target Tracking Radar for the TIRA Facility
The continuous improvement of the capabilities of the large radar facility TIRA (Tracking and Imaging Radar) is a central and important part of the research at Fraunhofer FHR. In the further development of the signal processing systems of the target tracking radar, the concept of software-defined radar is being consistently pursued in order to create a flexible and future-proof research instrument for space situational awareness with radar.
The signals are directly sampled and generated in the microwave domain in L-band at a wavelength of 22 centimeters, near the analog frontend of the antenna. Further processing of the digitized signals takes place in real-time on graphics processors using software-defined methods. This allows for the flexible implementation of innovative methods and their scientific utilization.
In recent weeks, an important milestone in development was reached with the successful completion of various experiments, demonstrating the suitability of the overall system concept developed by Fraunhofer FHR.
First Light for the New System
In the first experiment with this new instrument, referred to as First Light, a measurement of the Moon was conducted to investigate the system’s stability. The Moon was illuminated by the powerful transmitter of TIRA with the antenna beam of the Cassegrain antenna, and the echoes reflected from the Moon’s surface were received after approximately 2.6 seconds. The width of the antenna beam roughly corresponds to the apparent diameter of the Moon.
Since the Moon appears as a single point to the antenna beam and is not resolved by it, the rotation of the Moon and Earth was utilized over a period of about 30 minutes to virtually span a much larger antenna, the so-called synthetic aperture. Figure 1 shows this synthetic aperture (green) with the relative motion of TIRA (red) to the Moon’s surface. Along with the radar system’s distance measurement, this results in an image of the Moon’s surface with a resolution in the range of 20 meters. The reflections from the northern and southern hemispheres of the Moon overlap as they map to the same distances to the radar respectively. These two overlapping images were separated using the monopulse system of the target tracking radar, which allows post-shaping of the antenna beam. The monopulse system comprises an array of feed antennas in the focal plane of the Cassegrain reflector system, used for precise direction measurement during target tracking.
The generated SAR images of the Moon’s hemispheres are shown in Figures 2a and 2b. The synthetic aperture is given along the horizontal and the vertical coordinate gives the distance to the radar system. Figure 3 shows an excerpt of the image with the iconic Tycho crater on the southern hemisphere of the Moon.
