# High-Precision Light Spot Position Detection in Low SNR Condition Based on Quadrant Detector  
[PERSON]  
1 Institute of Optics and Electronics, Chinese Academy of Sciences, No. 1 Guangdian Road, Chengdu 610209, China; [EMAIL_ADDRESS] (J.Y. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.X.) [PERSON]  
1 Institute of Optics and Electronics, Chinese Academy of Sciences, No. 1 Guangdian Road, Chengdu 610209, China; [EMAIL_ADDRESS] (J.Y. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.X.) [PERSON]  
1 Institute of Optics and Electronics, Chinese Academy of Sciences, No. 1 Guangdian Road, Chengdu 610209, China; [EMAIL_ADDRESS] (J.Y. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.X.) [PERSON]  
1 Institute of Optics and Electronics, Chinese Academy of Sciences, No. 1 Guangdian Road, Chengdu 610209, China; [EMAIL_ADDRESS] (J.Y. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.X.) [PERSON]  
1 Institute of Optics and Electronics, Chinese Academy of Sciences, No. 1 Guangdian Road, Chengdu 610209, China; [EMAIL_ADDRESS] (J.Y. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.X.) [PERSON]  
1 Institute of Optics and Electronics, Chinese Academy of Sciences, No. 1 Guangdian Road, Chengdu 610209, China; [EMAIL_ADDRESS] (J.Y. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (H.L. ); [EMAIL_ADDRESS] (Q.W. ); [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.X.) [EMAIL_ADDRESS] (G.Z. ); [EMAIL_ADDRESS] (D.H.); hitxos@163. A laser beam of 1550 nm wavelength is widely applied to laser communication systems because it is weakly affected by atmosphere turbulence compared with other laser wavelengths [3]. We usually use a charged coupled device (CCD) as the position feedback device, but the low quantum efficiency of CCD at 1550 nm limits its use [4; 5]. This is especially true when the distance between satellites and the ground increases; considering the effect of atmosphere channel, the energy of the beacon light decreases considerably. Here, we apply a quadrant detector (QD) as our detector because of its high resolution, which can help us acquire the signals whose energy are at the nanowatt (nW) level. What is more, its natural noise is low and response speed fast, all of which indicating that it can play the role as the detector of satellites to ground communication well. [PERSON] and colleagues analyzed the nonlinear characteristics of QD and proposed a linear correction method based on the Boltzmann function [4]. When they received the position of a light spot, it could be calculated by the Boltzmann function, whose parameters were known by experiment. [PERSON] and colleagues analyzed the characteristics of the detector output signal [6]. The estimated spot position values and the real spot position values were calibrated by [PERSON] and colleagues, the calibrated values then fitted to the polynomial [7]. This method is similar to the Boltzmann function method, the main difference being the greater complexity of the function employed in the method proposed by [PERSON] and colleagues. In a word, the scientists above adopted the direct detection method, which directly detects the amplitude of the output photocurrents of QD to correct the non-linearity. The methods they proposed are used in the situations when the QD output signal is greater than the noise, or almost at the same. That is to say, when the SNR is at a low level, the methods do not work well.