Guided wave radar or time domain reflectometry (TDR) works very similarly to pulse-generated free space radar. The main difference is the addition of a cable or rod from the radar unit to the process surface to guide and focus the radar signal (figure). Guided radar also operates on a lower frequency of approximately 1.2 GHz.
Figure. The signal from a guided wave radar transmitter follows a cable or rod to the process surface for improved performance.
The advantage of guided wave radar technology is the signal is very concentrated on the cable or rod. When it encounters foam, the radar signal does a better job of going through the foam to reflect off the liquid surface. Neither condensation nor dust has any effect on TDR.
Another major advantage in solids level measurement is that the angle of repose can be accounted for with careful placement of the rod or cable. The angle of repose results from the way solids pile up in a vessel, creating an angle on the side of the pile. When using guided wave radar, the point at which the rod or cable contacts the product will determine the signal reflecting back to the transmitter.
Installation considerations include material compatibility, possible excessive pull force on the cable in solids applications when installed in tall silos, and avoidance of interferences between the cable or rod and items such as agitators
