# Method to Measure Light Spot Position in Beacon-Less Inter-Satellite Communication System
## 1 Introduction  
Inter-satellite optical wireless communication (Is-OWC) is one of the remarkable technologies that can be used to communicate between satellites. Its major subsystem is the acquisition, tracking and targeting (ATP) system, which guarantees high-quality communication link by detecting the incident angle of the laser beam and controlling the rotation of optical antenna to align the incident light. Traditional ATP system needs an independent laser beam as beacon light, and also needs an independent optical path. In recent years, a kind of beacon-less Is-OWC system is proposed [1; 2; 3], which offer the advantage of lower size, weight and power cost. In this system, spatial ATP subsystem is performed using the collimated communication beam instead of an additional highly divergent beacon laser. Furthermore, the transmitted and received lights adopt the same wavelength and use the same optical path. A part of transmitted light is reflected and mixed within the received light. Thus, a part of reflected light will be leaked into the detector of ATP system. Therefore, it is a problem to measure the spot position of weaker received light in the interference of the stronger reflected light.  
In this article, quadrant detector (QD) is used to measurement light spot position because of its advantages, including high resolution, low natural noise and fast response speed [4; 5]. Many methods have been presented to improve the detection accuracy for a single spot on the QD. [PERSON] et al. [6] proposed that the detection accuracy of QD was related to spot radius, spot position and output SNR. [PERSON] et al. [7] analyzed the nonlinear characteristic of QD and proposed a linear correction methodbased on Boltzmann function. For multiple light spots position detection, [PERSON] et al. [8] proposed to use pulses to modulate the projected light beams at different frequencies and demodulating the combined output signals of the position sensitive detector. [PERSON] et al. [9] used a sensor array to detect the position of multiple spots. [PERSON] et al. [10] proposed a method for detecting beam deviation angle by using four light spots in the multi-aperture system on a single QD. However, when the intensity of each light differs too much, the above methods cannot work.  
Therefore, we propose a method based on intensity modulation and cyclic cross-correlation (CCR). First, two cosine signals with different frequencies are used to modulate the intensity of transmitted and received laser beams respectively, so that the photocurrent signals output from QD contain two cosine signals whose amplitudes correspond to the energy of the two laser beams irradiated on QD's target plane. Then, a reference cosine signal which has the same frequency as the modulation signal of the received light is used to CCR process with the QD output signals. Finally, the amplitude of the weaker beacon signal can be extracted from the result of CCR, and the spot position of received light is calculated. This improved method has good performance even when the two light is different in intensity by a factor of 91.7.