The system transfering between laser-satellites

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Introduction
The system engineering design methodology plays an important part in any systems implementation. Requirements must be clearly understood and analyzed and allocated to the functional elements, which are the building blocks of the system. Laser communication system design is not an exception to this disciplined approach.
The laser communication equation (LCE) is a basic resort of LICS’s (Laser Inter-satellite Communication System) analysis. The equation starting with the transmit source power, the designer identifies all sources of page link degradation (losses) and improvements (gains) and determines the received signal level. Based on the background and receiver noise and the type of signal modulation which is to be detected, a required signal is generated. The ratio of received signal to required signal is the system page link margin. Identifying these gains and losses requires intimate knowledge of the system design, including both the internal constraints and design choices and knowledge of the external factors, including range, data rate, and required signal criteria. These parameters are of single-way data transfer for three independent links – acquisition, tracking and data transfer (figure 1).
A crosslink, or communication between two satellites, may be needed to solve certain requirements of satellite communication architecture. Laser communications offers the user a number of unique advantages over radio frequency (RF) systems, including size, weight, power and integration ease on the spacecraft. Integration ease issues include compactness of terminals, elimination of complex frequency planning and authorization, and RF interference issues.



Acquisition link
Acquisition requires searching the uncertainty area to locate and establish the page link between satellites.
The convergence page link is not dominated by the excess communication channel shot noise, as the tracking link. The difference in required signal when the communication channel excess noise is removed is on the order of 3 dB.


Tracking link
Tracking page link model are shown in figure 5. The critical figure of merit for tracking links is the noise equivalent angle (NEA). The NEA is defined as the residual tracking error along the line-of-sight vector to the companion satellite. The NEA is the function of the received signal to noise ratio (SNR) in the tracking bandwidth, the optical spot size on the detector, and the gain of the tracking system.