1. A method performed by a computing device for suppressing effects of extraneous signals in a current return signal acquired by a receiver from a signal emitted by a transmitter, the current return signal collected at a current cross-track location of a current down-track location, the method comprising: calculating an estimated background signal for the current return signal based on a combination of an estimated background signal for a prior return signal for a prior down-track location that has a similar air gap to the current return signal and an estimated gain and bias adjusted return signal for the prior return signal; calculating a gain and bias adjusted return signal for the current return signal; and calculating an estimated foreground signal for the current return signal based on a difference between the gain and bias adjusted return signal for the current return signal and the estimated background signal for the current return signal.