Synthetic Aperture Radar (SAR) is installed on drones, aircraft, and even satellites. They create high-resolution images which are similar to photographic images to some extent. Here, SAR devices scan the surface of certain areas via electromagnetic waves and display the terrain in two dimensions. This type of SAR image is used, for example, by military and intelligence organisations for image intelligence of ground activities. The advantage of SAR radar devices is that they can be used to obtain valuable reconnaissance images both by day and by night as well as in cloudy conditions. This is because radar waves do not require sunlight and can penetrate through clouds without any problems. 

Detecting stationary and flying objects

However, in future, these type of imaging reconnaissance devices will also be able to detect and differentiate flying objects such as aircraft and helicopters. This is very special, as SAR systems are primarily designed for imaging the ground and not for detecting flying objects. This new operating mode, known as the Air Moving Target Indication AMTI mode, will therefore require notable adjustments to the radar algorithms. To this end, armasuisse S+T, together with partners, collected initial valuable measured data for assessing AMTI in a measurement campaign in Switzerland at the end of August 2022.

Initial measurements in Switzerland

For the measurement campaign, the SAR research sensor known as MIRANDA-35 from the Fraunhofer Institute for High-Frequency Physics and Radar Technology was used on the light aircraft Delphin. In order to image flying objects with MIRANDA-35 for the first time, a Eurocopter EC635 of the Swiss Armed Forces flew various flight paths under the light aircraft. These flight paths were characterised by different speeds and flight directions. The aim is to obtain well-focused, high-resolution images, known as re-focused inverse SAR images, of the flying helicopter. Should this be successful, aircraft, larger drones, and helicopters in low-level airspace could not only be detected but even differentiated based on the image. This would provide an important contribution and supplement to the local recognized air picture. The recognized air picture provides the users with a list of the flying objects in a particular airspace.The data collected is currently being processed by researchers at the University of Zurich using special algorithms. It will be of particular interest to assess the limits of the image focusing, for example, up to which flight speed an interpretable image can still be created.

This elaborate flight experiment was led by the two specialist areas Flight Testing, armasuisse Aeronautical Systems, and Sensor Technology, armasuisse S+T. The measurement campaigns took place as part of the activities of the armasuisse S+T research programme Reconnaissance and Surveillance. These and other trials are used to assess the development potential of SAR systems.