Spectral Solar Radiation Data Base Documentation, Vol. I

Table of Contents

5.0 Measurement Uncertainty


The measured data, field notes, quality-control information, and measurement uncertainty are stored on magnetic tape in 80-character lines with the filenames given in Table 6-1. The filenaming convention used for the measured data was

	Filename			Content

	FLA8705.DAT			FSEC data  1987  May
	PGE8801.DAT			PG&E data  1988  January
	SER8711.DAT			SERI data  1987  November
Data were not acquired in all months, so the months may not be contiguous.

Quality-control information has the same filenaming convention, with ".QC" extension (for example, FLA8705.QC). Daily notes are contained in the files FSEC.NOTES, PGE.NOTES, and SERI.NOTES, respectively. The measurement uncertainty is in the ".UNCERT" files.

The ".DAT" files contain two types of information: "D" for data, and "C" for instrument configuration. The first line of a data or configuration segment begins with a "D" or a "C", respectively, in the first column, followed by the site, year, day, and time stamp; the number of lines in the segment is in columns 76-80 so that the user can easily skip the desired number of lines. An example of the information in the configuration, or "C," segment is shown in Figure 6-1. It begins with a "C" followed by the site "FSEC," year "1987," day "122," and time "0830," latitude, longitude, and elevation. There are19 channels for measurements, zero spectra in the "C" segment, and 40 lines in the segment. The configuration information appears whenever the instrument set-up or calibration was changed, or the data acquisition system was restarted. If the configuration was not changed at the beginning of the month, the configuration segment will be in the previous month's ".DAT" file.

An example of a "D" segment is shown in Figure 6-2. This segment contains the measured meteorological data, broadband and spectral solar radiation data, and ancillary data. The data contained in each line and column, and its format, are as follows:


Line/Column	Contents				Format

							a = character
							i = integer
							f = floating point


1:1		D ( for data )				(a1)
3:15		Site, Year, Day Number, Time Stamp	(a13)
		3:6    FSEC, PG&E, or SERI		(a4)
		7:8    Year				(a2 or i2)
		9:11   Day Number			(a3 or i3)
		12:15  Standard Time			(a4 or i4)
16:23		Latitude (°)				(f8.4)
24:24		N (north)				(a1)
25:33		Longitude (°)				(f9.4)
34:34		W (west)				(a1)
35:39		Elevation (m)				(i5)
40:40		M (m)					(a1)
41:52		Reference to Configuration Segment	(a12)
54:69		Starting year, day, time for the 
		spectral scan(s), if any, and number 
		of attempts to acquire the scan 
		(which indicates possible time delay 
		from the starting time)
54:62		Year, Day Number, Time			(a9 or i9)
63:63		"+" for plus number of attempts to 
		acquire a spectrum			(a1)
67:67		# of attempts to acquire a spectrum	(i1)
70:73		Possible # of Measurements (19)		(i4)
74:75		Number of Spectra in this Segment 
		( 0, 1, or 2 )				(i2)
76:80		Number of Lines in Segment		(i5)

LINE 2:		Pointer to the quality-control, 
		daily field notes, and measurement 
		uncertainty information.  Quality-
		control information is in the ".QC" 
		file of the same month, with the 
		same site, year, day, and time 
		stamp.  Daily field notes are in 
		the ".NOTES" file for the same site, 
		with the same site, year, and day 
		stamp.  Measurement uncertainty is 
		in the files ".UNCERT" under the 
		instrument serial number identified 
		by PRS-xxx, and the columns in the 
		uncertainty files for "+" and "-" 
		uncertainty limits.			(a80)

LINE 3:		Broadband measurements before the 
		spectral scan.
 1:3		Channel #1				(i3)
 4:10		Direct Normal (thermopile) (W/m2)	(f7.2)
11:13		Channel #5				(i3)
14:20		Direct Normal (silicon) (W/m2)		(f7.2)
21:23		Channel #2				(i3)
24:30		Global Normal (W/m2)			(f7.2)
31:40		Channel not used	
41:43		Channel #3				(i3)
44:50		Global Horiz. (thermopile) (W/m2)	(f7.2)
51:53		Channel #19				(i3)
54:60		Global Horiz. (silicon) (W/m2)		(f7.2)
61:63		Channel #4				(i3)
64:70		Global Tilt (W/m2)			(f7.2)
71:80		Channel not used
LINE 4:		Broadband measurements after the 
		spectral scan; same format as 
		before-scan (LINE 3).
LINE 5:	
 1:6		Tilt of the fixed-tilt instruments 
		from the horizontal (°)			(f6.1)
 7:12		Azimuth of the fixed-tilt 
		instruments (°) (180° = south)		(f6.1)
13:80		Description of any special 
		instrument (for example, diffuse-
		horizontal measurements at SERI)	(a88)

LINE 6:	
 1:3		Channel #10				(i3)
 4:10		Albedo (W/m2)				(f7.1)
11:13		Channel #15				(i3)
14:20		Cloud Cover (tenths)			(f7.1)
21:23		Channel #7				(i3)
24:30		Surface Pressure (mb) Measured at 
		FSEC and PG&E; calculated from 
		elevation at SERI [11, p. 100]		(f7.1)
31:33		Channel #6				(i3)	
34:40		Ambient Temperature (° C)		(f7.1)
41:43		Channel #8				(i3)
44:50		Relative Humidity (%)			(f7.1)
51:53		Channel #9				(i3)
54:60		Wind Speed (m/s)			(f7.1)
61:63		Additional channel described in 
		line 5					(i3)
64:70		Value					(variable)
71:73		Additional channel described in 
		line 5					(i3)
74:80		Value					(variable)

LINE 7:		Sun Photometer readings (only 
		recorded for SERI data; not used 
		except for relative comparisons).
LINE 8:	
 1:5		Earth-Sun distance correction (% 
		deviation from 1.0)			(f5.2)
 6:12		Extraterrestrial Radiation (W/m2)	(f7.1)
13:18		Zenith Angle (°)			(f6.2)
19:24		Kt (ratio calc'd from thermopile 
		instruments)				(f6.1)
25:30		Kn (ratio calc'd from thermopile 
		instruments)				(f6.1)
31:36		D/GH (ratio calc'd from thermopile 
		instruments)				(f6.1)
37:42		Albedo (%)				(f6.1)
43:48		Air Mass				(f6.2)
49:68		Reserved for Turbidity, if calc'd
69:72		Precipitable Water Vapor, if calc'd 
		from sun photometer
73:76		Precipitable Water Vapor from the 
		National Weather Service		(f4.1)
77:80		Precipitable Water Vapor Calc'd 
		from Relative Humidity			(f4.1)

LINE 9:	
 1:2		Channel #17				(i2)
 3:6		Number of Wavelengths Recorded by 
		Spectroradiometer #1			(i4)
 7:11		Beginning Wavelength (nm)		(i5)
12:16		Ending Wavelength (nm)			(i5)
17:20		Wavelength Step Size (nm)		(f4.1)
21:25		Spectroradiometer #1 Tilt from the 
		Horizontal (°)				(f5.1)
26:31		Spectroradiometer #1 Azimuth (°)	(f6.1)
32:36		Angle of Incidence of the Direct 
		Beam on Spectroradiometer #1 (°)	(f5.1)
38:39		Measurement Type (DN, GN, GH, 
		GT, DF)					(a2)
40:40		Attachment (Teflon Dome D, 
		Integrating Sphere S, View-limiting 
		Tube T)					(a1)
41:42		Channel #18				(i2)
43:46		Number of Wavelengths Recorded by 
		Spectroradiometer #2			(i4)
47:51		Beginning Wavelength (nm)		(i5)
52:56		Ending Wavelength (nm)			(i5)
57:60		Wavelength Step Size (nm)		(f4.1)
61:65		Spectroradiometer #1 Tilt from the 
		Horizontal (°)				(f5.1)
66:71		Spectroradiometer #2 Azimuth (°)	(f6.1)
72:76		Angle of Incidence of the Direct 
		Beam on Spectroradiometer #2 (°)	(f5.1)
78:79		Measurement Type (DN, GN, GH, 
		GT, DF)					(a2)
80:80		Attachment (Teflon Dome D, 
		Integrating Sphere S, View-limiting 
		Tube T)					(a1)

LINE 10:	Saved for extra spectroradiometers 
		if needed.
If no spectral data were recorded, the number of lines in the "D" segment is 10, the number of spectra is 0, and there is no corresponding quality-control information for this date/time in the ".QC" files. No quality control was performed because the atmosphere was unstable. If the number of spectra is 1, there are 60 lines in the "D" segment; if there are two spectra, the number of lines is 100.

If the number of spectra is 1 or 2, the spectral data follow after the first 10 lines of the "D" segment. There are 10 spectral measurements per line, with the number in the left-most column indicating the first or second spectroradiometer (1 or 2), followed by the starting wavelength (in nanometers) and the 10 spectral measurements (in W/m2-nm). The 10 measurements are at wavelength increments of 2 nm. There are 401 spectral measurements from 300 to 1100 nm in 2-nm steps, with 10 measurements per line for 40 lines, and one measurement in the 41st line. The last number in the 41st line is the spectral integral. If a second spectroradiometer was used, the next 41 lines follow the same format. Blank lines follow the spectral data to fill the "D" segment to 60 or 100 lines.

An example of a ".QC" file is shown in Figure 6-3. The first line of each segment begins with a "Q" followed by the site, year, day number, and time stamp, with the number of lines in the segment given in the last columns of the line. Following the identifier are messages about variables that were flagged by the automatic quality-control processing (see appendix), noted by an operator at the measurement site, or noted during inspection at SERI. The last line of the segment shows the QC code for variables such as DN (direct normal, thermopile detector), SN (direct normal, silicon detector), AL (albedo), and SP (spectrum), as described in the appendix. Three quality-control codes are used: 1 = good; 2 = suspect, see messages and daily notes; and 3 = poor or missing, see messages and daily notes. The broadband codes (DN, SN, GN, GH, SH, and GT) have two values; the first is the before-scan measurement and the second is the after-scan measurement.

Figure 6-4 shows an example of a segment of the daily notes from the FSEC.NOTES file. Each segment of the file begins with an "N" followed by the site, year, and day number stamp, with the number of lines in the segment given in the last columns of the line. The next line contains the site operator log sheet number, month, day, year, day number, spectroradiometer serial number(s), and measurement mode(s). The PG&E and SERI notes do not include the daily log sheet number. The first two lines are followed by field notes. Notes that were added after inspection at SERI are separated from the operator notes with "-------". The existence of slides or photographs is indicated as well as their quality, i.e., good (G), marginal (M), or poor (P). In the example in this figure, there were two log sheets for this day (number 116 and 117) because the measurement mode changed from global horizontal (GH) to global tilt (GT).

An example of a segment of the measurement uncertainty file "UNCERT.CAL" is shown in Figure 6-5. The first line begins with a "U" followed by the serial number of the spectroradiometer. The upper and lower measurement uncertainty limits are given in the columns following wavelength, with the appropriate measurement mode, dates, and description at the top of the column. The second line in each segment of the ".DAT" files is a pointer to the correct uncertainty information in "UNCERT.CAL". The broadband solar radiation and meteorological instrument measurement uncertainty follows each spectral measurement uncertainty segment in "UNCERT.CAL".

The format of this data base was established at SERI to accommodate expanded, multiyear, long-term data collection. Users can reformat the data to suit their particular needs. We recommend the following steps for accessing and documenting particular data sets:

  1. Select and extract the desired spectral data from the ".DAT" files using information such as measurement mode, day number, time, air mass, or atmospheric conditions, depending on the desired attributes.

  2. Extract and review the corresponding segments of the quality-control information from the ".QC" files.

  3. Extract and review the corresponding field notes from the ".NOTES" files.

  4. Extract the measurement uncertainty information from the ".UNCERT" files and quote measurement uncertainty for all data used.

  5. Examine the daily broadband and spectral solar radiation data graphs in the catalog (Volume II of this report [18]), if available. These graphs provide information on atmospheric conditions during the day. For example, Figure 4-3 shows daily broadband and spectral plots for partly cloudy conditions when solar radiation is changing rapidly throughout the day. The sun is obscured when the direct-normal values drop to zero and the global values approach each other. Figure 4-1 shows a clear day.

No data should be used without consulting the quality-control information, daily notes, and measurement uncertainty.


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