Time Domain Transmissometry (TDT) is similar to TDR, however it measures the transmission, rather than reflection, of a pulse along a looped, or closed circuit, rod. TDT measures the time taken for an electromagnetic wave to propagate (travel) along a given length of a transmission line in the soil. With TDT, a step pulse with a fast rise time is transmitted into a transmission line. The step pulse travels down the transmission line and a voltage threshold is detected at the other end of the transmission line. There is no complex waveform analysis as with TDR. The shape of the transmitted waveform is not relevant, but a measurement of the pulse travel time through the transmission line will allow an estimate of the dielectric constant of the medium.
Similar to TDR, a pulse measured via TDT will be slower in wetter soils than drier soils. The velocity of the pulse is related to the dielectric constant.
TDT sensors, being a more refined version of TDR, generally provide greater accuracy and lower power consumption than TDR sensors. However, they are usually more expensive.
Examples of this type of sensor include the GroPoint sensor family and the Acclima TDT Soil Moisture Sensor.