1. A method for measuring the amount of radial platform motion of a handheld portable Moving Target Indicator (MTI) radar sensor, comprising: transmitting a series of radar pulses at frequencies less than about 5 GHz; processing radar return signals resulting from said series of radar pulses to develop frequency domain data in a plurality of range bins, including frequency domain phase data in a range bin corresponding to a large stationary object in a scene under surveillance; computing a radial correction factor based on the frequency domain phase data for the range bin corresponding to the large stationary object, said radial correction factor indicative of radial motion of the sensor caused by postural sway or respiration of a user; and applying the radial correction factor to time domain samples of the radar return signals prior to processing by Doppler filters used to measure motion in the scene, to compensate for the radial motion of the radar sensor caused by the user.