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.