Geo Warping - Geo Warping Process

Geo Warping Process

This section explains the actual geo warping or re-projection process when applied to radar video in real time. Assume we want to display radar video on top of a satellite image. As an example we use the CIB projection which is used to display satellite data in CIB (Controlled Image Base) format.

The Figure Geo Warping Radar to CIB Projection shows dashed the maximal range circle for a range of 111 km or 60 miles using the radar projection. Such a range is typical for long range coastal surveillance radars. As stated in the last section this is a perfect circle also on the computer screen. The solid line ellipse shows the same range circle for the CIB projection.

Typically the errors occurring without Geo Warping are smallest near the radar position if at least the projection center (COP) coincides with the radar position, as realized in our example. Otherwise the error distribution depends both on the used projection and also on the projection parameters. Thus, in our case the errors are most significant near the maximum radar range. The CIB projection error corrected in east-west direction at half the radar range is 2.6 km and is 5.3 km at the full radar range of 111 km. An error of 5.3 km is quite significant compared to a typical radial radar measurement resolution of 15m.

The Figure Coordinate Re-projection explains how the radar coordinates have to be transformed to match the CIB projection coordinates. The radar world coordinates correspond to the Cartesian version of the data measured by the radar sensor. Using an inverse radar projection these coordinates are converted into geographic coordinates which represent the radar data posi-tions on the earth surface. These coordinates are then finally projected by the CIB (or any other) projection for displaying on the computer screen.

A problem which arises is that geo warping all measured radar video pixels is far too computing resource consuming as to be performed in real time. A possible solution is to use lookup tables for all points on the screen, but the lookup table re-computation after e.g. a display zoom operation still causes a noticeable delay for radar video visualization.

The Figure Geo Warping Grid depicts the solution to the problem. The circular radar coverage area is divided into a circular grid. Only the corner points of the grid are geo warped which drastically reduces the computation time. Coordinates within a grid tile are computed by a weighted bilinear interpolation of the grid corner points. As geographical projections are typically non-linear functions this introduces a certain error for the radar video display position. Keeping this error sufficiently below the radar measurement resolution makes sure that this is no restriction for the radar video display quality. The grid tile size has to be computed once for a radar position and a given projection. Thus, the grid is typically computed once for a static radar and only more often for moving radars e.g. on ships.

The OpenGL radar-scan converter does its scan conversion computations on the graphics processor (GPU) to achieve high performance and visual quality. The bi-linear coordinate interpolation mentioned above is done in dedicated hardware on the GPU and therefore causes no overhead for the scan converter.