GAN/SKYRAD/M1 - Degraded IRT mirror

The gold mirror was worn and needed to be replaced. Degradation of the mirror occurs 
gradually and it is difficult to determine when it begins to affect the measurement. To 
estimate the beginning of the problem, I used the time when the difference in IR sky 
temperature from the MWR3C was consistantly greater that 2.4K (2 times the resolution of the IRT 
measurement).

The problem was corrected when the mirror was replaced.

• Sky Infra-Red Temperature Minima(sky_ir_temp_min)
• Sky infrared temperature, standard deviation(sky_ir_temp_std)
• Sky Infra Red Temperature Maxima(sky_ir_temp_max)
• Sky Infra-Red Temperature(sky_ir_temp)