Radar geo-warping
These echoes are typically presented on a computer screen with a color-coding scheme depicting the reflection strength.
In older systems the polar oriented picture has been displayed in so called plan position indicators (PPI).
These echoes have to be displayed together with other real world data like object positions, vector maps and satellite images in a consistent way.
Building a link from real world earth positions to display pixels is commonly called geographical referencing or in short geo-referencing.
This article describes in further detail the Geo Warping of radar video images in real time.
The radar supplies its measured data in polar coordinates (ρ,θ) directly from the rotating antenna.
The RSC process is influenced by the current zoom, shift and rotation settings defining which part of the 'world' shall be visible in the display image.
As detailed later the RSC process also takes the currently used geographical projection into account when the radar video images are Geo Warped.
The OpenGL RSC is implemented using a reverse scan conversion approach which calculates for every image pixel the most appropriate radar amplitude value in the polar store.
By applying bi-linear filtering between adjacent pixels in the polar store during the conversion process the OpenGL RSC finally achieves a very high visual quality radar display image for every zoom level, creating smooth images of the radar echoes.
This section illustrates how radar video data are geo referenced and displayed on a computer screen.
The position of the real world object is typically given in geographical coordinates (latitude, longitude and height above the earth surface).
This section explains the actual geo warping or re-projection process when applied to radar video in real time.
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.
As geographical projections are typically non-linear functions this introduces a certain error for the radar video display position.
The OpenGL radar-scan converter does its scan conversion computations on the graphics processing unit 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.
