SGP/IRT/C1 - Degraded hardware

The surface temperature from the 25m IRT was ~10K less than measured from the PIR. The 
lens was replaced on 2/8/2013 but there was still a bias. The problem was corrected when the 
IRT (and that at 10m) was upgraded to a newer model.

It may be possible to determine the calibration offset, using the PIR as reference, and 
correct the surface temperature measurements.

• Surface Infra-Red Temperature, Maxima(sfc_ir_temp_max)
• Instantaneous Surface Infra-red Temperature(inst_sfc_ir_temp)
• Surface infra-red temperature(sfc_ir_temp)
• Surface Infra-Red Temperature, Minima(sfc_ir_temp_min)
• Surface Infra-Red Temperature, Standard Deviation(sfc_ir_temp_std)