In the Fengyun-4 microwave detection satellite mission, a frequency scanning interferometry (FSI) LiDAR was used to conduct real-time, high-precision measurements of the satellite’s microwave antenna surface profile. This enabled on-orbit adaptive antenna adjustment, ensuring normal operation of the satellite’s payloads. The LiDAR system includes an FSI laser ranging component and a two-dimensional scanning mirror. The precision of the laser ranging system is crucial as it directly influences the accuracy of the antenna surface profile measurements, which in turn impacts the satellite’s overall performance.

The laser ranging system uses a distributed feedback (DFB) laser, which modulates the drive current to produce a laser frequency scanning output. However, the output characteristics of DFB lasers tend to degrade with increasing operational duration, particularly under the harsh conditions of geostationary orbit. This degradation can cause a drift in the laser’s frequency scanning characteristics curve, leading to anomalies in the FSI laser ranging system. Consequently, the nonlinearity of the laser frequency scanning should be periodically calibrated and corrected to ensure optimal function of the FSI laser ranging system. Currently, these nonlinearity issues are addressed by iteratively changing the current and using optoelectronic phase-locked loops for active correction. Given the constraints imposed by the harsh space environment, which limits the use of high-performance processors and complex circuits, there is a pressing need for a calibration and correction method that consumes minimal computational resources.

Optical fibers, which are primarily used in the FSI laser ranging system to construct internal optical paths, introduce length errors during the fabrication process. Additionally, factors such as temperature changes and laser frequency variations can alter the equivalent optical path length of the fibers, introducing measurement errors. Therefore, periodic calibration of the fiber length or system parameters is necessary. In ground-based conditions, methods such as optical cavities or gas absorption cells are used for fiber length calibration, and precision ranging instruments for auxiliary calibration of the system parameters. However, when deployed as satellite payloads, the use of these optical devices or precision instruments becomes impractical, limiting the applicability of these calibration methods. Thus, there is a need for a calibration process that does not require additional devices or instruments and can rapidly calibrate and correct the parameters of the FSI laser ranging system.

This study first establishes a mathematical and physical model for the FSI laser ranging system, based on the principles of light interference. It explores the use of an equidistant optical frequency sampling method to derive the fundamentals of an FSI laser ranging system, and investigates the relationship between the measured distance and peak frequency of the beat frequency signal spectrum, standardizing and simplifying the parameter description method. Subsequently, the analysis addresses system errors induced by laser output frequency drift and changes in fiber length. A method is proposed to measure the system’s zero-point by altering the scanning mirror’s pointing angle [Fig. 2(a)], measuring the distance and angle at the targets at both ends of the baseline ruler [Fig. 2(b)], and rapidly calibrating system parameters based on spatial geometric relationships (Eq. 16). Following this, the study delves into the nonlinear error in laser frequency scanning, suggesting a method for calibrating the laser scanning nonlinearity characteristics by linearly modulating the drive current, performing time-frequency analysis on the interferometric beat frequency signals generated by the internal optical path, and constructing a modulation current function to correct for frequency scanning nonlinearity (Eq. 29), accompanied by simulation analysis (Figs. 3 and 4). An experimental setup is then constructed (Fig. 5) using the proposed methods to calibrate the system parameters and correct the laser frequency scanning nonlinearity, with subsequent analysis and discussion of the experimental results.

An experimental setup was constructed to verify the calibration method for the FSI laser ranging system as outlined in this study. The FSI laser ranging system and a baseline ruler were positioned on a large optical platform (Figs. 5 and 6). Following the procedures described in Section 2.3, the scanning mirror was initially controlled to rotate to a position perpendicular to the ranging laser to measure and record the peak position in the spectrum, establishing the system zero-point relationship. Subsequently, the scanning mirror was manipulated to take measurements of the targets at both ends of the baseline ruler, also recording the scanning mirror’s pointing angle and the spectrum peak positions to establish the system linearity (Fig. 7). Experimental data were categorized into four sets, corresponding to different angles of the measuring optical path, distances between the reference scale targets, and center positions of the spectrum peaks. System parameters were calculated using Eq. (16) (Table 1), and an average of the parameters estimated from the four sets was computed to obtain relatively accurate calibration results. Following the protocol in Section 2.4, first, a linear current function was used to modulate the laser, sample the reference beat frequency signal, and conduct time-frequency analyses. Then, a polynomial fitting method was employed to determine the laser frequency scanning nonlinearity function relationship. Finally, a correction current function was formulated using Eq. (29), effectively correcting the laser frequency scanning nonlinearity (Fig. 8).

In response to the measurement errors induced by laser frequency drift in FSI laser ranging systems, this study establishes mathematical and physical models, and conducts formula derivations to determine the expressions for system parameters influenced by the characteristics of lasers and optical fibers. The issue is thus reframed as a calibration problem for the coefficients of linear relationships. The proposed method, which involves altering the scanning mirror angle in conjunction with using a reference scale, measures the system’s zero-point and linear relationships. Subsequently, system parameters are calibrated through geometric relationships. Experiments conducted within a constructed experimental environment validate the accuracy of this method.

Addressing the issue of frequency scanning nonlinearity caused by changes in DFB laser characteristics, this paper develops a mathematical and physical model of laser frequency scanning and conducts formula derivations to explore the principles of correcting frequency scanning nonlinearity. A method is proposed for calibrating the laser frequency scanning characteristics by driving the laser with a linear current function, and another method has been proposed for correcting laser frequency scanning nonlinearity by constructing a current function. The validity of these methods is confirmed through simulation analysis and experimental verification.

The calibration method for the FSI laser ranging system introduced in this article facilitates automated calibration without the need for manual intervention. This method eliminates the need to augment the existing system with additional optical components such as optical cavities or gas absorption cells. Furthermore, precision instruments, such as high-accuracy displacement stages, laser interferometers, or wavelength meters, are not required for calibration purposes. It is suitable for application in on-orbit deployment and ground testing of the system, demonstrating considerable value for engineering applications.