We present a method of characterizing the horizontal and vertical electron density roughness of the D-region ionosphere using Nationwide Differential GPS (NDGPS) transmitters as Low Frequency (LF; 30-300 kHz) and Medium Frequency (MF; 300-3000 kHz) signals of opportunity. The horizontal roughness is characterized using an amplitude cross-correlation method, which yields the correlation length scale metric. The vertical roughness is characterized using a differential phase height, which is needed to mitigate the effects of transmitter phase instability. The ranges and typical values of roughness metrics are investigated using data from several field campaign measurements. Finally, the roughness metrics for an NDGPS transmitter and VLF transmitter are compared. It is found that the roughness detected by the VLF transmitter is significantly smoother and demonstrates the utility of this method to complement traditional VLF measurements.