Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRFuture-proof space situational awareness with radar: Fraunhofer FHR has further developed the signal processing systems of the target-tracking radar of the large radar facility TIRA, significantly expanding the possibilities for radar-based space observation. For initial experiments with the new instrument, a special object was chosen: the Moon. The result is a high-resolution radar image of the entire Moon surface visible from Earth.
Fraunhofer FHR succeeded with the upgraded target-tracking radar of the large radar facility TIRA (Tracking and Imaging Radar) in producing a high-resolution, continuous radar image of the entire Earth-visible Moon surface—from a distance of 385,000 kilometers! The demonstration underscores the potential of TIRA for demanding observation and analysis tasks of objects in space.
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.
Software-defined radar for a future-proof space situational awareness
The continuous improvement of the capabilities of the large radar facility TIRA is a central and important aspect of the research at Fraunhofer FHR. In the further development of signal processing systems, researchers consistently rely on the concept of software-defined radar to create a flexible and future-proof research instrument for space situational awareness with radar.
Signal sampling and signal generation occur directly in the microwave L-band at 22 centimeters wavelength, close to the analog front end of the antenna. Further processing of the digitized signals takes place in real time on graphics processors using software-defined procedures. This enables flexible implementation of innovative methods and their scientific use.
Various successful experiments demonstrate the suitability of the overall system concept developed at Fraunhofer FHR and the realized subsystems.
Surveying the Moon as the first experiment
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, the self-rotation of the Moon and Earth was used to virtually create a much larger antenna—the synthetic aperture—over a period of about 30 minutes. In Figure 1, this synthetic aperture (green) is shown together with the relative motion of TIRA (red) relative to the Moon’s surface.
Combined 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. To separate these two overlapping images, the target-tracking radar’s monopulse system was used for post-forming 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.
These generated SAR images are shown in Figures 2a and 2b. The horizontal axis represents the synthetic aperture, while the vertical axis shows the distance to the radar system from top to bottom. Figure 3 shows a zoomed-in section of the image featuring the iconic Tycho crater on the Moon’s southern hemisphere.
