How are echo tops of radar determined and displayed?

Wherever the National Weather Service (NWS) individual radar sites are providing a storm cell ID, we extract the associated echo top height for that cell.   If storm cells are not identified by the radar sites, then this won't trigger an echo top height to be extracted and displayed.  Also, if the echo tops are below 20,000 feet, you won't see one displayed.  

Image 1.  Echo top shown for a cell that is estimated to be 20,100 ft.