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Since 1909, the radiometer standard around the world has been defined by the mean of observed data from 15 types of Absolute Cavity Radiometers (Fig. 1) of the WRC in Switzerland, and determined by a comparison measurement every five years.^[11,12]Fig. 2 is the schematic diagram on the WRC’s comparison measurement of solar radiation. The International Pyrheliometer Comparison is conducted in Davos, Switzerland, every five years, the most recent being in 2010.^[13,14] In addition, the Regional Pyrheliometer Comparison (RPC) is also conducted there using the RRC’s calibrated radiometers.^[15] In this comparison, the standard radiometer of each country is calibrated through a comparison measurement, and then the participants calibrate radiometers in their countries when they return to their countries.

Fig. 1. 
Absolute cavity radiometer comparison to the WRC (Source: http://www.pmodwrc.ch)

Fig. 2. 
The schematic diagram of international pyrheliometer comparison (Source:http://www.pmodwrc.ch)

RRC of Asia was established in Tsukuba, Japan in 1964, and includes 34 countries. RRC of Asia has personnel and an environment very suitable for solar radiation measurement to the extent that it operates an observation site for Baseline Surface Radiation Network (BSRN). In addition, Japan has two Asia standard absolute cavity radiometers, and the equipment is also calibrated at the IPC. The calibrated standard absolute cavity radiometers play a role in calibrating standard absolute cavity radiometers of Asian countries along with RRC’s standard absolute cavity radiometer. Recently from January 24 to February 2, 2012, a comparison measurement was conducted on the rooftop of the Keisei Hotel (latitude: 36.13°N, longitude: 140.07°E, altitude: 560 m), which is located on Tsukuba Mountain in the City of Tsukuba, Japan. At the time, the standard absolute cavity radiometers of Korea (Fig. 3 (a) and (b)) and pyranometer (Fig. 3 (c)) were calibrated. Among them, as the pyranometer did not have a standard pyranometer, the comparison measurement was conducted by converting it into a pyrheliometer using a column as shown in Fig. 3 (c). In other words, as shown in Equation (1), global solar radiation (I_Glo) is calculated with direct solar radiation (I_Dir), diffuse solar radiation (I_Dif), and solar zenith angle (θ). Here, when converting global solar radiation to direct solar radiation, diffuse solar radiation becomes 0 W m^-2, and direct solar radiation can be calculated using global solar radiation as shown in Equation (2).

Fig. 3. 
Installment and observation of Korea’s standard instruments at RRC (Regional Radiation Center)

At this time, the inside of the column is painted black as in a black body to enable observation of only direct solar radiation, preventing diffused solar radiation from reaching the pyranometer. In addition, the sun-tracker makes the Sun and the equipment perpendicular to one another, making it precisely so only direct solar radiation can be observed.

Absolute cavity radiometers that observe direct solar radiation are precision pieces of equipment, with less than five units being manufactured per year. In Korea, two types of absolute cavity radiometers are used, including the PMO6, developed by the WRC (Fig. 3 (a)) and the AHF (Fig. 3 (b)) developed by Eppley in the United States. Absolute cavity radiometers are designed to maintain a certain level of standard heater power the entire time when opening and closing the shutter. In other words, when closing the shutter, it heats up to standard heater power by the heater as there is no absorbed direct solar radiation, but when opening the shutter, it heats up the amount subtracted by solar radiation from standard heater power, and therefore, direct solar radiation for the moment is determined as the value of heater power with the shutter closed (P_C) from which heater power with the shutter open (P_O) is set.

Here, K denotes instrument constant.

As absolute cavity radiometers have long observation intervals and are costly, they cannot be used in general observation. The radiometers used in actual observations are the equipment collecting micro-currents caused by solar energy. In other words, when solar energy reaches a radiometer, the temperature of the sensor increases, resulting in the difference in temperature between the sensor and the radiometer case. At this time, a micro-current occurs which varies across radiometers, and accordingly, the sensitivity of different radiometers are determined separately. In other words, the observed value of a radiometer (I) is represented by the simple equation shown in Equation (4).

Here, V denotes the current that occurs as solar energy reaches a radiometer (unit: mV), and c denotes the sensitivity constant (unit: W m^-2 mV^-1) of each piece of equipment.

Solar radiation is divided into direct or beam solar radiation, diffuse or sky solar radiation, and global or total solar radiation, and the devices for the observation of solar radiation are called pyrheliomter and pyranometer. Pyrheliometer is the equipment designed such that direct sunlight penetrates a small space of a cylinder and is detected by the sensor located vertically to the path of sunlight, and usually used for observation of 290-4000 mm wavelength. Pyranometer is the equipment for simultaneous measurement of direct and diffuse radiations of the sun, and used for observations of about 290-2800 mm wavelength, and the sensor must be installed horizontally in a glass dome.^[16] Diffuse radiometers are the same equipment as pyranometer, but are the equipment for measurement of only diffuse solar radiation in the atmosphere by blocking direct solar radiation using a shutter.

Fig. 4 is a time table of the comparison measurement method of absolute cavity radiometers. The observation interval is 30 minutes, with the observation being conducted for 18 minutes from the beginning, and the equipment check and preparation for the observation for the next time during the rest of the time. In addition, as absolute cavity radiometers need to observe energy when opening and closing the shutter, they make repeated observations by repeating the opening and closing of the shutter with 1 minute 30 seconds for stabilization time. In other words, the WRC’s absolute cavity radiometer comparison measurement uses only the data for the time with direct solar radiation over 800 W m^-2, and considering the characteristics of absolute cavity radiometers, data is created for three minutes (repeated observations after 1 minute 30 seconds from opening the shutter, and after 1 minute 30 seconds from closing the shutter).

Fig. 4. 
The time table for absolute cavity radiometer comparative measurement

While the WRC uses a total of 15 standard absolute cavity radiometers, the RRC usually uses five standard absolute cavity radiometers. Three of the five are standard absolute cavity radiometers of the WRC, and the remaining two are the RRC’s standard absolute cavity radiometers. The standard absolute cavity radiometer used in this study is shown in Table 1. A comparison measurement was conducted from January 24 to February 2, 2012, from 8:00 AM to 4:00 PM, with the times when observation was impossible due to clouds or precipitation having been excluded.

Table 2 shows the observation data from the standard absolute cavity radiometers for one interval among the observation data. These were observations made over 18 minutes from 10:00 AM on January 29, 2012, creating a total of 6 data points for each piece of equipment, as one observation was obtained every three minutes. Fig. 5 shows variations among observed values from Table 2, and the differences were mostly under ±0.10% (±0.001; medium dash line). S/N 0803 and 0406 overestimated compared to the mean, S/N 0401 and 32455 underestimated, and S/N 32446 showed a larger difference than the other equipment. These differences may increase with the service life of the equipment, and can be influenced by location. Although all equipment is connected to a sun-tracker and perpendicular to the Sun, differences may occur as each device has a different optical path. The factors that influence such optical paths include clouds and aerosol, and the large differences among devices at 10:18 occurred because stratus clouds were included in the optical path of the devices.

Fig. 5. 
Time series of ratio for direct solar radiations compared to mean, respectively

Fig. 6 shows time-series data from January 31, 2011 for which the largest amount of data were collected among observed days. Red dots indicate standard solar radiation as well as the mean of the solar radiation observed from RRC’s five absolute cavity radiometers, blue dots (AHF) and green dots (PMO6) indicate absolute cavity radiometers, and pink dots indicate observed values from the pyranometer (CMP22) converted to direct solar radiation. While absolute cavity radiometers did not show significant differences from the RRC mean, the pyranometers have some differences. For quantitative analysis, standard deviations are shown in Figure 7 using a scatter plot. Two absolute cavity radiometers of both the AHF and PMO6 showed standard deviations of 0.775 W m^-2 and 0.381 W m^-2, respectively, similar to the RRC values, while the CMP22 showed overestimated values compared to other observation data, with a standard deviation of 1.781 W m^-2. The data from the CMP22 used in the observation were the first observations after its production, requiring an accurate sensitivity constant based on the comparison measurement.

Fig. 6. 
Time series of solar radiation for RRC mean and Korea instruments (AHF, PMO6, CMP22) on 31 January 2012 (red point: mean of RRC standard instruments, blue point: AHF, green point: PMO6, and pink point: CMP22)

Fig. 7. 
Scatter plot of solar radiation from Fig. 6 (black circle: AHF, blue triangle: PMO6, and green square: CMP22)

A new calibration constant for domestic standard equipment (absolute cavity radiometer 2EA, pyranometer 1EA) was obtained from the RRC. Calibration constants as shown in Table 3 were reported from the WRC and RRC based on the results of a comparison measurement using five standard absolute cavity radiometers in total. All equipments overestimated compared to the observation of the RRC’s standard absolute cavity radiometer, and in particular, the first domestic standard absolute cavity radiometer, PMO6, overestimated by 0.02%, while the AHF that has been used since 2010 overestimated by 0.3%. In addition, it was found that CMP22, which will be the standard pyranometer in Korea, overestimated by 1.87%. These results are similar to the results described in section 3.1, created by the use of numerous observation data over 10 days.