Nonlinear Compensation Method Based on Data for Increasing Absolute Measurement Precision of FID Signal

To increase proton magnetometer (PM) absolute precision, a compensation method is presented to compensate the nonlinear part of the free induction decay (FID) signal. The nonlinear model of frequency measurement is modeled according to the exponential attenuation features of the FID signal first, and then the hardware setup is proposed and realized to capture the raw data of frequency measurement. Thereafter, the approximate period and the parameter k in the nonlinear model of frequency measurement are estimated by the least square (LS) linear regression method with the captured raw data. The data, which is used to revise the nonlinear part of raw data, are calculated by estimating the approximate period and parameter k. Finally, the frequency of the FID signal is estimated by the LS linear regression method with the revised data. Simulation and experimental results show that the absolute precision of frequency measurement for the FID signal is increased obviously if the proposed method is adopted. The outdoor experimental results of geomagnetic-field measurement show that the absolute precision of PM is increased by about 0.05 nT when the captured raw data are revised by the proposed method.