The Cimel sunphotometer (CSPHOT) is a multi-channel, automatic sun-and-sky scanning radiometer that measures the direct solar irradiance and sky radiance at the Earth's surface. Measurements are taken at pre-determined discrete wavelengths in the visible and near-IR parts of the spectrum to determine atmospheric transmission and scattering properties. The CSPHOT collects measurements only during daylight hours (sun above horizon). For more information see the CSPHOT Instrument web page.
The CSPHOT instrument is not owned by the ARM Program, but is maintained by ARM funded Instrument Mentors. The data is collected at the site and transmitted to NASA's AERONET (AErosol RObotic NETwork) global network headquarters for processing. The data is sent to the ARM Archive via ARM External Data Center (XDC).
¶ Watch out for oddball missing values in the data files.
In addition to standard missing values, there are non-standard missing values:
:missing_value = -9999.f ;
:missing_value_explanation = "-9999 indicates data or variable was not available in raw file" ;
:missing_value1 = -100.f ;
:missing_value1_explanation = "-100 indicates a missing data point" ;
The CSPHOT produced 4 datastreams that have now been consolidated into one standard "CSPHOT" datastream. Because the data is initially processed at AERONET and then transmitted to ARM through the External Data Center, the data will be delayed a few days to a month.
The CSPHOT will only operate when the weather is clear enough for a scan. Cloudy or overcast conditions will prohibit the instrument from recording measurements. This results in a seemingly incomplete datastream. This type of operation is normal.
The primary tests performed on the data are the Minimum Threshold and Maximum Threshold tests.
The secondary test uses the TSI to try and determine if the sky is clear enough for the CSPHOT to collect data. The test is not perfect but does provide decent warning. To aid this test a plot with the TSI and CSPHOT data will be created each day, regardless if the CSPHOT datastream has data, to check if the CSPHOT should have collected data.
A third test will compare one wavelength to the other wavelengths. If a single wavelength deviates in magnitude greatly from the rest of the wavelengths, there is most likely a problem with that wavelength. If one wavelength differs in both magnitude and trend, that wavelength filter is in error. This should result in a DQPR.
A fourth test will determine if there is a potential blockage in the tube. This is still in beta test mode, so please contact someone before submitting a DQPR.
Past problems that do need to be mentioned in DQAs are mentioned below:
The most common problem with the CSPHOT will be an obstruction in the collimator tube (typically a spider's web or mud dauber at SGP). When an obstruction occurs the AOT values will follow a Cosine of the solar elevation angle curve. This plot shows the values following the dashed line, which represents a Cosine of the solar elevation angle. Low solar elevation angles may not follow the curve. The most affected values will be at higher angles.
Additional Information: Problems with the wet sensor can cause the CSPHOT to flag with a blocked channel. If the wet sensor is continuously set to 0, then the CSPHOT will operate when it is raining. This leads to moisture getting in the collimator tube and data flagging. The AOT values will follow a Cosine of the solar elevation angle curve (above image). This may also occur if the wet sensor becomes disconnected.