Frequency is measured in units of hertz, which is defined as cycles per second. Gamma rays, X-rays and visual light have very high frequencies of oscillation on the order of 1012-1017 cycles per second. These very high frequencies produce extremely short wavelengths, measured in fractions of millimeters. Radio waves, a subset of which are radar waves, much have lower frequencies, with wavelengths of propagating energy that vary from a few centimeters to at most a few tens of meters in length. The radar energy used in most GPR applications has frequencies ranging between 10 and 1500 megahertz (MHz). This energy occupies a portion of the same spectrum as television and FM radio broadcasts, cellular phones and other personal communications devices.

The electromagnetic frequency distribution within the bands used by most GPR antennas. There can be a good deal of interference of these usages with some GPR antennas, especially in the 500 to 1000 MHz frequencies that often overlap television, cellular phone and pager transmissions.

Depending on the frequency of energy transmitted into the ground and the distance between two planar interfaces (?d) reflections from the top and bottom of a layer may or may not be visible in a reflection profile. High frequency energy will generate a small enough wavelength so that the top (A) and bottom (B) will produce a reflection, and the composite reflection trace of the two (C) can define both interfaces. Medium frequency antennas with a longer wavelength will just barely have enough definition from the top and bottom (D and E) to produce a composite reflection trace (F) that exhibits both interfaces. Low frequency antennas may produce a wave that will reflect off both interfaces (G and H), but the composite reflection trace is affected by constructive and destructive interference of the two waves, and only the top of the interface is visible in the composite reflection trace (I).