Most GPR systems use a high-fidelity receiver, such as a 16-bit Analog to Digital Converter (ADC), to measure return reflections. These devices cannot measure the entire return signal in one operation, so instead combine many incremental measurements.
Impulse GPR systems do this by sampling the magnitude of the return for brief instant(s) after each broadband pulse. A varying delay is applied between pulse generation and sampling, and responses from many pulses are then combined to form the equivalent of one complete return (A-scan). Although simple, this approach is also highly inefficient because the ADC is only “listening” to a very small fraction of each return signal.
Step frequency GPR systems, on the other hand, emit a series of narrowband signals at discrete frequency steps. The amplitude and phase of the return signal is measured continuously at each step. An Inverse Fourier Transform (IFT) is then used to combine measurements from the complete set of frequency steps and produce the A-scan. While this is a far more efficient use of receiver hardware, it is also complex and difficult to scale. Furthermore, the narrowband signals are problematic in terms of regulatory compliance.