Description: | The IRT output dropped to its minimum output of 223 K (0 volts). It was determined the 24
V DC converter had failed. The converter was replaced on 16 August and the instrument
began reporting normally again. The IRT data are unusable during the five day period when
the DC converter was faulty.
The GNDRAD (down-looking) IRT is used primarily for surface temperature. The PIR can be
used for this purpose by inverting the IR flux equation, Flux = sigma*temperature^4 where
sigma = 5.67x10-8 W/m^2K^4. But, unlike the IRT, the PIR measurement is strongly
effected by the air temperature in addition to the surface temperature. At night, when the air
temperature approaches the surface temperature, the PIR provides a good estimate of the
surface temperature. During the day, however, the PIR tends to underestimate the ground
temperature, particularly on sunny days. The magnitude of the underestimate can be
estimated by evaluating periods when both instruments are operating but is typically on the
order of 5-10C. |