THORPE PARK - SAR

In July 2026, Central Alliance (CA) was tasked by Amco Giffen, on behalf of Network Rail, to conduct airborne drone-based multiband Synthetic Aperture Radar (SAR) surveys along a key section of the York and Selby line by Thorpe Park, Leeds in West Yorkshire.

The survey was part of a subsurface study focused on identifying and delineating an area of suspected shallow mine workings adjacent to a running line.

Shallow mine workings can pose a significant, but highly site-dependent, threat to railway embankments and cuttings. The key issue is not simply whether workings exist, but whether their failure can produce ground movement within the railway earthwork or close enough to affect its stability.

For an embankment, a mine collapse can cause:

• differential settlement beneath the embankment;
• cracking or distortion of the embankment;
• localised loss of support;
• changes in drainage and pore-water pressures;
• progressive slope deformation; and, in severe cases,
• deformation of the track geometry sufficient to require speed restriction or closure

The SAR technique is an active geophysical remote sensing survey method which utilises the generation of microwaves to image the surface of the Earth, even in poor visibility or the hours of darkness. In partnership with our SAR developer Radaz, CA conducts SAR surveys all around the world with the RD350 Multiband SAR system, aboard a DJI Matrice 600 Pro Drone.

The RD350 is the only combined C, L and P band UAS radar on the market, with the only system in the UK currently being used by CA. The simultaneous use of the multiple frequency bands, C and L, allows the surveyor to define topography, and moreover investigate the near subsurface with the lower frequency P band.

As the project was taking place over rail, a fully auditable RAMS process was generated on DroneCloud and forwarded directly to the Network Rail Air Ops team to avoid scheduling conflicts and prevent distraction to track workers.

Three Multiband SAR flights were completed over the sites: three helical SAR missions spanning 120-100m AGL, 120-90m AGL and 120-80m AGL. The objective here was to provide insights into buried structure backscatter and spatial resolution with various height columns.

Using the data acquired in the field, CA processed the raw field data into GeoTIFFs showing plain and cross polarised data for the C, L and P bands; CVV, LHH, LHV, PHH, LVV and LVH.

CA have advanced its capabilities to perform refractive back projections on helical data, and create radar tiff matrices for peering into the deeper Earth, harnessing 3D tomographic techniques.

While the initial proof of concept was to examine the above ground surface roughness and below ground backscatter, the development was stretched to encompass internal capability for refractive back projection and 3D tomography. This was another successful step in providing a unique technology solution.

As a research platform, the drone-based SAR platform continues to impress and is growing continuously with methodological improvements being implemented in the field, and advances in technological capabilities in processing.