Function singleaxis 

pub fn singleaxis(
    solar_zenith: f64,
    solar_azimuth: f64,
    axis_tilt: f64,
    axis_azimuth: f64,
    max_angle: f64,
    backtrack: bool,
    gcr: f64,
    cross_axis_tilt: f64,
) -> (f64, f64, f64)

Calculate single-axis tracker positions with backtracking support.

Determines the rotation angle of a single-axis tracker using the projected solar zenith angle approach from Anderson & Mikofski (2020). Backtracking uses the slope-aware method (Eq. 14 from the same reference).

Arguments

• solar_zenith - Apparent solar zenith angle in degrees.
• solar_azimuth - Apparent solar azimuth angle in degrees.
• axis_tilt - Tilt of the tracker axis from horizontal in degrees.
• axis_azimuth - Azimuth of the tracker axis in degrees (North=0, East=90).
• max_angle - Maximum rotation angle of the tracker from horizontal (e.g. 45.0 or 60.0 degrees).
• backtrack - Enable backtracking (True).
• gcr - Ground Coverage Ratio (module width / row pitch).
• cross_axis_tilt - Cross-axis tilt angle in degrees (typically from calc_cross_axis_tilt). Use 0.0 for flat terrain.

Returns

A tuple containing (surface_tilt, surface_azimuth, aoi). All in degrees.

References

Anderson, K., and Mikofski, M., 2020, “Slope-Aware Backtracking for Single-Axis Trackers”, Technical Report NREL/TP-5K00-76626.