## 1 Introduction  
Over past decade, concentrating solar power technology (CSP) has presented great advances, and the linear focus parabolic trough solar system is one of the most widespread and mature solar power technologies [1]. It mainly consists of a parabolic trough reflecting mirror, tracking system and vacuum pipe receiver. The tracking system plays an important role in the operation of the parabolic solar collector which could drive the collector to track the sun, so that the reflector can focus solar rays projected on the mirror onto the receiver, which can convert the collected solar energy into heat energy and transfer it to the heat-transfer medium. The CSP technologies require high-accuracy solar tracking systems in order to achieve better performance. The algorithms used in the active solar system could be classified according to the solar tracking control strategies: open-loop, closed-loop and hybrid-loop. The open-loop strategy has no control feedback from a solar sensor, based on solar coordinates calculated from SPA to estimate the sun's apparent position with respect to a geographical location on Earth. The closed-loop strategy has control feedback from a solar sensor and can achieve high solar tracking accuracy without an SPA [2]. For the open-loop strategy, in order to compute the sun's position, SPA need to input parameters such as the geographic location and time. The sun's apparent position generated by the SPA represent the set point of the control algorithm, which will finally be transformed into angular position for the actuators of the active solar tracking system in the control unit. In order to keep down the price of the tracking system, the calculation of sun position and control other tracking system functions in controller is based on a low-cost microprocessor. This imposes significant restrictions on the SPA to be used in the tracking system [3]. The solar trackers can be classified as single-axis tracking (SAT) mechanism and double-axis tracking (DAT) mechanism [4; 5]. DAT mechanism requires the direction of incident solar ray and optical axis to be consistent. The single-axis tracking (SAT) mechanism requires the incidence ray to be located in the plane containing the main optical axis and focal line. SAT tracking method can be further sub-divided according to its focal line, such as EW axis system tracking from south to north and NS axis system tracking from east to west. In addition, the spin axis of the polar axis tracking system points directly to the North Pole and tracks from east to west. Therefore, there are four common tracking methods for parabolic trough solar system: (1) north-south axis tracking method; (2) east-west axis tracking method; (3) polar axis tracking method; (4) two-axis tracking method [6]. For the SAT system, there is an included angle between the incident ray and the optical axis, which reduces the intensity of the incident sunlight. Among the four tracking methods, the DAT system has the best performance. It has two rotating axes which are perpendicular to each other, and the reflector rotates around the two axes at the same time so that the optical axis of the mirror is consistent with the direction of solar rays [6]. However, the complexity of DAT system is higher and it has greater manufacturing and maintenance costs than the one-axis tracking system. There are several algorithms for computing the sun position with different levels of accuracy and complexity which have been published for solar radiation applications. These calculation methods can be classified into two groups. The first group is relatively simple methods with low accuracy that uses empirical formulae to directly calculate equations of time and solar declination. The second consists of more complex algorithms considering nutation, aberration and other factors. For relatively simple algorithms, basic solar parameters such as solar declination and equation of time can be calculated using the time of year.