Time domain reflectometer (TDR) probes are used in the Long Term Pavement Performance (LTPP) Seasonal Monitoring Program to obtain the moisture content in unbound base and subgrade materials. The TDR technique is based on the measurement of the travel time by an electromagnetic wave induced into a waveguide, in this application, a moisture probe. The apparent length is the length between the beginning and end points on the waveform that
correspond to the beginning and end of the metal tube portion of the moisture probe. This apparent length of the probe can be used to calculate the dielectric constant of the material surrounding the probe. The dielectric constant is an input to the calculation of moisture content.