# What are the limitations for the backtracking calculation ?

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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 sub-field in the 3D geometry.

These parameters cannot be different for different sub-fields in a same system (or the backtracking will be established on the less favourable sub-field).

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 sub-array is not null (i.e. if the trackers are shifted with respect to each other). However it is possible if the whole sub-array is not south-oriented (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 east-west slope for a tracking sub-field.

- For Vertical axis, the backtracking is not implemented as I couldn't find an algorithm.

Two-axis systems

- For usual two-axis 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 east-west sun position is preponderant.

When the azimuth is big (east-west 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 nord-south 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 two-axis 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.