A TVS (Tornado Vortex Signature) is a Doppler RADAR alarm resulting from at least 45-knot outbound adjacent to at least 45-knot inbound radial velocity. The supercell must be reasonably close to the RADAR in order for the RADAR to trigger the TVS. If it is above the RADAR, the tornadic circulation is in the RADAR's cone of silence. If it is too far from the RADAR, the beam will overshoot the circulation. The optimum distance occurs where the beam can sample the mesocyclone (and possibly the actual tornadic circulation) with several tilt angles.
The NWS will often issue a tornado warning from RADAR evidence alone if the mesocyclone associated with the TVS is deep and persistent. Most of the strong and violent tornadoes have deep and persistent mesocyclones. If the tornadic circulation is in a favorable location to the RADAR (not too close or too far away), volume scans (through using various tilt angles) along with examining the output every 6 minutes can be used to assess the depth and persistence of the mesocyclone.
A TVS on RADAR does not necessarily mean a tornado is occurring. To be a tornado the circulation must be on the ground. The RADAR is not able to tell if the "tornado signature" is on the ground. Less than 30% of even strong mesocyclones produce a tornado. There is another "volume of silence" the RADAR has and that is the volume underneath the lowest tilt angle. If the lowest tilt angle is 0.5 degrees, the RADAR will be unable to detect anything that could be detected under 0.5 degrees (a RADAR is only able to sample a planar area with each tilt angle). RADAR is sometimes taken out into the field during storm chases to collect data on the circulation that is near the ground for research purposes.