¶ Infrared Thermometer (IRT)
¶ Background
The Infrared Thermometer (IRT) is a ground-based radiation pyrometer that provides measurements of the equivalent blackbody brightness temperature of the scene in its field of view. The downwelling version has a narrow field of view for measuring sky temperature and for detecting clouds. The upwelling version has a wide field of view for measuring the narrowband radiating temperature of the ground surface.
¶ Additional Information
The IRT data with the SKYRAD comes from the IRT instrument, but from a different port. The IRT data are serial-sampled and the SKYRAD irt values are analog sampled. If there is a problem with the infrared temperatures from one instrument and not the other, then there is likely a port/wiring issue. If the actual instrument fails, the data for both the IRT and SKYRAD datastreams should be bad.
In August 2005, an upgraded version of the IRT was installed at the SGP extended facilities (but not SGP C1). This newer model has a lower temperature range (see table below) and is located inside a positively pressurized box, with an opening for the IRT view (see image right). This will help keep dirt and other objects from collecting on the mirror between times of the bi-weekly site visits.
For more information see IRT.
¶ Expected Operations
¶ Metrics
¶ Plots
Multi-panel plot of
- Infrared Temperature (IRT) of the Sky
- Standard Deviation of the IRT
- Internal Reference IRT (Calibration)
- IRT, IRT Max, and IRT Min variables
These are also plotted up over the course of a week as well.
¶ Comparison Plots
NOTE: Some of the plots below only apply to the SGP Sites.
Extended Facility Sky IR Temperature Comparison (SGP)
This plot shows all of the extended facility IR temperatures on the same plot. This makes it easy to see any outliers. The weekly plot of this data is smoothed using a moving average of 60 points to reduce the noise.
IRT200ms Comparison
The IRT samples at a rate of 5 Hz. The data that we normally view is the minute average of this data. A plot of the minute averaged data along with the 5 Hz data is used to determine if there are any processing issues between the 2 datasets. The 5 Hz data will look a little noisier since it is an instantaneous measurement. This is also plotted up over 7 days as well.
Sky IR Temperature Comparison
Multiple instruments are plotted out in order to compare the IR temperatures. The instruments should be relatively close when there are clouds around. They will be off, sometimes by a large amount, when there is clear sky, due to the minimum detectable range of the instrument.
¶ Site-Specific Information
This instrument is located at
- ENA
- NSA
- SGP (Extended Facilities)
- TWP
- AMF's
¶ Known Issues
¶ Known Behaviors
Known issues for this instrument that MAY NOT need to be mentioned in your DQA's:
Warm Bias
An apparent warm bias of the surface temperature is exhibited by the IRT, compared to the PIR, at sites where the instrument is mounted on a tower 10 m above the ground, especially in summer. The PIR responds strongly to air temperature (and the temperature of the intervening water vapor) while the IRT responds strongly to ground temperature. The bias goes away or becomes very small at night because the ground temperature approaches equilibrium with the air. The bias is also small when it is raining or very overcast (as indicated by low solar values). The bias becomes large during the day when the sun is out. For example, on sunny days at TWP/C3, the ground temperature as seen by the IRT is about 51C while the PIR sees a temperature about 10C cooler. The actual air temperature at Darwin in the afternoon is about 32C. So it appears that the PIR is reporting a temperature approximately mid-way between the ground temperature and the air temperature. This effect is also experienced at Manus and Nauru but it is more pronounced at Darwin because the radiometers are further from the ground.
Positive "Spikes"
At tropical sites near the time of the equinoxes, positive spikes can be produced in the sky temperature measurements near local solar noon, when the sun is in the field of view of the IRT.
Positive Sky Temperature Bias
A positive bias of sky temperature is exhibited by the downwelling IRT when compared to the AERI during clear-sky conditions when the sky temperature is less than 180K. The greatest difference compared to AERI will occur when the sky is clear, dry, and cold, while the ambient temperature is quite hot. These conditions maximize the difference in temperature between the sky and the instrument, and the IRT calibration at the lower limit of 223K was not performed correctly. This bias is especially apparent at high-latitude sites, like NSA and OLI.
Internal Reference Temperature
The internal reference temperature of the downwelling and upwelling IRTs at OLI in the winter is frequently less than the minimum measurable value of -20 degrees C.
¶ Past Problems
Past problems for this instrument that DO need to be mentioned in your DQA's and possibly requiring a DQPR submittal:
Snow collecting on the gold mirror
The IRT at NSA and TWP is part of the SKYRAD system and is not using the new enclosed pressurized system. This means the mirror and camera are possibly affected by environmental conditions, like snow. This is an example of snow accumulating on the mirror and possibly on the camera lens. If the mirror is completely covered in snow the IRT will not be measuring the sky temperature, but the temperature of the snow. This particular case is not very common but does have a distinguishable trend to indicate snow contamination. If the visibility is low (less than 3 km) but IRT is able to see clear sky (flat line temperature at 213 K), the local conditions are most likely blowing snow on the surface with no cloud cover. (See image below) The IRT plot shows a ramping up in temperature as the snow starts to accumulate on the gold mirror. The maximum temperature possible will be the surface temperature, which is what happens at 08:00 GMT. Then at 09:00 GMT the snow is blown away and the gold mirror is reflecting clear sky temperatures. This process reoccurs at 12:00 GMT on the same plot. The snow is very light and would normally be blown away before substantial accumulation would be possible, except in this case the winds are constant, and out of the west. The IRT mirror faces do East, and allows the westly winds to deposit snow on the east facing mirror. Unfortunately there is not much that can be done to fix this problem, and there should NOT be a DQPR filed in this case. The Site technicians clear any snow off the gold mirror each day and log any accumulation in the log books.
IRT Removed
The IRT suffered water intrusion causing a malfunction. It was determined that a new IRT was needed. The IRT was removed from 11/16/2005 - 1/10/2006. During this time the instrument continued to produce "data". A value around 213K corresponded to the instrument outputting 0 volts. The changing values are random noise and non-perfect grounding. The first plots shows the IRT output when the instrument was not installed. The best way to tell the instrument was not connected is by comparing it to another instrument that responds to clouds. Also, note the scale of the first plot is small in comparison to how the IRT normally operates. The second plot shows the jump to correctly operating values around 02:30 GMT.
IRT showing a large negative bias and noisy signal
IRT showing a large positive bias and some maximum value flat lining
Large difference between PIR and IRT Upwelling Longwave after maintenance
Failure of IRT from power source problem
IRT25m Dirty Lens
