# TRC-Based High-Precision Spot Position Detection in Inter-Satellite Laser Communication
quality, the ATP system will not separate too much beam energy from the signal beam, so the energy of the beacon light entering the ATP system is greatly reduced [7; 8; 9]. How to perform high-precision spot position detection under extremely low signal-to-noise ratio (SNR) is a problem that must be faced. To detect a weak light spot position, this article uses a four-quadrant detector (QD) which has a fast response speed and outputs four continuous photocurrents. However, QD is susceptible to thermal noise and background radiation noise. To suppress these two kinds of noise, [PERSON] [10] and [PERSON] [11] used pulse signals to modulate signal light at the transmitting end, and then used narrow-band filters and sampling pulses to select receive signals at the receiving end. However, when the center frequency of the narrowband filter and the frequency of the sampling pulse are the same, the SNR of the received signal is not improved. In the field of distance measurement, [PERSON] [12] used the modulated laser as the transmission signal, and performed the correlation operation between the modulation signal and the reflected signal. In the ATP system based on position sensitive detector (PSD), to suppress background light and dark current noise, [PERSON] [13] used a square wave to modulate the beacon light, and used the same frequency square wave signal at the receiving end to perform correlation calculation with the photocurrent signal output by PSD, which improves the detection accuracy. However, this method requires high synchronization between the two signals. [PERSON] [14] used the Kalman filter to process the photocurrent signal output by the QD to improve the accuracy of spot position detection. [PERSON] [15] established the beam detection range model of the laser guidance system based on QD, calibrated the influence of temperature parameters on the beam detection range, and performed temperature compensation for this, which greatly improves the beam detection performance of QD. In the moving target tracking system based on QD, [PERSON] [16] used low-order least squares fitting and Kalman filter to improve the detection accuracy of the spot position under strong noise environment. However, the methods discussed in the literatures [14; 16] cannot provide sufficient detection accuracy when the SNR is extremely low (such as \(-17\) dB). To solve this problem, this paper attempts to provide sufficient SNR gain from the perspective of line spectrum signal enhancement. Therefore, we proposed a new method based on the time reversal convolution (TRC) line spectrum enhancement algorithm. This method uses a cosine signal to modulate the intensity of light beam, and converts the photocurrent signal output by the QD into a line spectrum signal. Then, the TRC algorithm with window function is used to suppress the noise and provide higher SNR gain to accurately detect the amplitude of the photocurrent. In this way, the detection accuracy of the spot position can be improved. When the output SNR of the QD is as low as \(-17.86\) dB, the proposed method can still detect the spot position and the absolute error is limited within \(0.0238\) mrad.