Instrumentation

Most of the ground stations in the SAURAN network use Kipp & Zonen CMP11 pyranometers for global and diffuse irradiance measurements, SOLYS trackers and CHP1 pyrheliometers for direct normal irradiance measurements. Other instrument types are also in use at certain sites. All radiometric data are time-averaged over 1-minute, hourly and daily periods. Instruments are subject to regular maintenance and all glass domes and windows are cleaned multiple times each week. Detailed instrument specifications can be obtained from Kipp & Zonen and The Eppley Laboratory. 

A summary of the acronyms and units that apply to SAURAN measurements is given below. A list of the instruments in use at a specific site can be downloaded from the supporting documents link on each station page. When downloading data, the column headings indicate the type of measurement (eg. global horizontal irradiance, or GHI) followed by the instrument model (eg. CMP11). 

We recommend restricting the date range to a maximum of 1 month when downloading 1-minute values, so as to limit the file size and download time.

When opening files in spreadsheet software the comma-delimited option may be selected. SAURAN does not allow for the selection of specific instruments when downloading data - all measurements at a station are included in the file. All averages are calculated for the period immediately preceding the time-stamped value; for example, the daily average values time-stamped as 2014-04-05 00:00 are generated over the course of 2014-04-04. 

Please note that despite our best efforts gaps in the data record are inevitable over time. Although these are kept to a minimum SAURAN users are advised to check all files for missing measurements and to run all downloaded data through their own quality control algorithms. We are currently adding a flag-based quality function to the site for certain stations.

Measurement uncertainty quantifies the extent to which a radiometric measurement differs from the 'true' value of irradiance at a given place and time. Please note that many factors affect uncertainty including the quality of an instrument, the time period since it was last calibrated, the extent to which sensor components are properly cleaned, the weather conditions under which measurements are taken and the period over which the data are averaged. It is therefore not possible to use the manufacturers' specifications as definitive measures of uncertainty under all circumstances. Users of SAURAN data are referred to the literature for a more comprehensive discussion of this topic, including the following texts:

1. Myers, D.R., 2013. Solar Radiation: Practical Modeling for Renewable Energy Applications, Taylor and Francis.

2. Stoffel, T., Renné, D., Myers, D., Wilcox, S., Sengupta, M., George, R., Turchi, C., 2010. Best practices handbook for the  collection and use of solar resource data. National Renewable Energy Laboratory, NREL/TP-550-47465.

3. Joint Committee for Guides in Metrology, 2008. Evaluation of measurement data - Guide to the expression of uncertainty in measurement JCGM 100. GUM 1995 with minor corrections.

  

Acronyms and units:  

Radiometry:

GHI: Global horizontal irradiance [W/m^2]

DNI: Direct normal irradiance [W/m^2]

DHI: Diffuse horizontal irradiance [W/m^2]

UV:  Ultraviolet irradiance [W/m^2]

PB: Perforated shadow band [W/m^2]

Meteorology:

Air_Temp: Air temperature [degC]

BP: Barometric pressure [mbar]

Rain_Tot: Total rainfall [mm]

RH: Relative humidity [%]

WD: Wind direction [deg]

WD_SD: Wind direction standard deviation [deg]

WS: Wind speed [m/s]

General:

RN: Record Number [ ] (Note: Record numbers may not be continuous.)

TS: Time stamp [ ] (All times are South African Standard Time (GMT+2))