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What are the limitations for the backtracking calculation ? https://forum.pvsyst.com/viewtopic.php?f=20&t=1739 
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Author:  AndrĂ© Mermoud [ Mon Jun 30, 2014 9:05 pm ] 
Post subject:  What are the limitations for the backtracking calculation ? 
The backtracking strategy requires complex geometric calculations. In PVsyst, it is not developed for any situation. There are several limitations, for which I did not find the suitable algorithm. The backtracking is based on a given collector width and pitch between trackers. Therefore it is only defined for a set of trackers specified as one subfield in the 3D geometry. These parameters cannot be different for different subfields in a same system (or the backtracking will be established on the less favourable subfield). Single axis systems:  The backtracking works well with horizontal axis systems.  For tilted axis systems, it works well with usual configurations. But it cannot be defined if the azimuth of the axes within the subarray is not null (i.e. if the trackers are shifted with respect to each other). However it is possible if the whole subarray is not southoriented (azimuth of the whole 3D object).  It is not possible to define backtracking between trackers of different altitudes, because you cannot (in the present time) specify an eastwest slope for a tracking subfield.  For Vertical axis, the backtracking is not implemented as I couldn't find an algorithm. Twoaxis systems  For usual twoaxis systems, there are several possible backtracking strategies. The only one implemented in PVsyst in the present time is by following the sun height: the azimuth is then adjusted for avoiding the neighbour tracker's shades. This strategy alone is not very efficient, especially at lower latitudes where the eastwest sun position is preponderant. When the azimuth is big (eastwest directions), the system should adopt another strategy: following the sun azimuth and applying the backtracking to the tilt. This is not implemented yet. The backtracking from one row to the next one would require to define the Tracker's nordsouth pitch and shift (i.e. between 2 different rows), and would apply to the tracker's tilt. Moreover it would be correlated to the backtracking of the row (either in azimuth or in tilt). As for the previous case, I didn't find the algorithms for the general case, and this is not yet implemented in PVsyst. Therefore backtracking strategy is not really useable for twoaxis systems. For Tracking frames, the backtracking is implemented between trackers within the frame, but not from frame to frame. If analytical or geometrical algorithms are too difficult, I will probably try to approach the problem by successive approximations in a future version. 
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