## 3 Results
### Rear Plane-of-Array Irradiance (\(G_{POA,Rarr}\))
When the measured--as opposed to calculated--angular position was used in the simulation, we found that the mean bias error (MBE) improved slightly from \(-1.1\) to \(-0.7\) W\(\cdot\)m\({}^{-2}\), but the MAE, however, changed by less than 0.1 W\(\cdot\)m\({}^{-2}\). The spatial non-uniformity of irradiance on the backside of a bifacial PV array makes it difficult to identify a position for a backward-facing irradiance sensor that is representative of the entire array. Fortunately, research is being conducted on this topic by other authors, although it is still in an early stage [60]. During clear sky (i.e., low diffuse fraction) days, we observed that the bottom pyranometer can receive nearly twice as much irradiance as the top pyranometer. Therefore, the black unity line in Figure 6, which represents the average measurement from two sensors, can at times have error bars on the order of \(\pm 15\) W\(\cdot\)m\({}^{-2}\). On such clear sky days, seven of the eight software studied give \(G_{POA,Rear}\) results that are within this range (Figure 7a). In other words, when the vertical spatial non-uniformity of irradiance is considered, the reduced-order complexity 2-D VF models perform reasonably well for fixed-tilt simulations. In the HSAT scenario, we do not see the same level of model agreement (Figure 7b). We observed that the irradiance sensor closest to the sky (i.e., western sensor in the morning, eastern sensor in the afternoon) typically receives more irradiance than the pyranometer closest to the ground, which is consistent with the findings of [22]. We found that the differences between eastern and western \(G_{POA,Rear}\) measurements on the HSAT system were not as extreme as differences between the top and bottom \(G_{POA,Rear}\) measurements on the fixed-tilt system. On a clear sky day, we observed differences on the order of 10 W\(\cdot\)m\({}^{-2}\) between western and eastern pyranometers. Although five of eight software agree within 5 W\(\cdot\)m\({}^{-2}\) of each other, none of the same five tools overlap with the measurement uncertainty bars in Figure 7b. This result could likely change if alternative backside pyranometer locations were chosen. Therefore, the PV industry could benefit from a standardized best-practice protocol for mounting the rear plane of array irradiance sensors in bifacial PV monitoring systems. Figure 6: Simulated rear plane of array irradiance on the fixed-tilt and horizontal single-axis trackers (HSAT) systems as a function of being measured over a 5-week period. The solid black 45\({}^{\circ}\) line represents unity to the measurements. The measurements are the average of two pyranometers located a minimum of 12 m from the nearest array edge. The shaded areas around each regression line indicate the 95% confidence intervals. The difference between Si photodiode \(G_{POA,Rear}\) measurements and pyranometer \(G_{POA,Rear}\) measurements is most apparent at midday, when the tracker is at or near a horizontal tilt. This could be because the Si photodiodes used in this work are calibrated under the air-mass 1.5 global reference spectrum (AM1.5G), but the spectral reflectance of grass deviates strongly from the spectral distribution of AM1.5G [61]. Particularly, healthy grass has high reflectance in the near-infrared spectrum and very little reflectance in the visible spectrum where the AM1.5G spectrum peaks. It is well known that the output of silicon PV devices calibrated under the AM1.5G spectrum will increase as the observed spectrum \"red shifts\" [62; 63]. This can explain why the Si photodiode measurements are higher than the pyranometer measurements around midday when the tracker is near horizontal, and the sensor's  
Figure 7: Timeseries plots of modeled and measured rear plane of array irradiance (\(G_{POA,Rear}\)) on a cloudless day (23 March 2020). The error bars around the measured values show the maximum and minimum measurements made by two sensors. (**a**) Fixed-tilt system where rear-facing POA pyranometer measurements are made 17 m from the western array edge.