Data Processing Methods

Source of Information: Review of C code taken from the program "tu" (the unpacker). The program, written by personnel at ASRC, Atmospheric Solar Radiation Group at SUNY-Albany NY, uses methodology described in Harrison et al. (1994) Instrument cosine responses used in this procedure were determined either by Yankee Environmental Systems, Inc. (YES) or by NOAA's Central UV Calibration Facility (CUCF). Older determinations were typically made by YES while more recent ones are from CUCF. These cosine response files can be found under Access to Data and Products, Angular Cosine Corrections.

The instrument's most recent cosine responses, determined in the laboratory of the testing agency (YES or CUCF) prior to the date of the data being corrected, are retrieved from the database.

For each of the 7 channels, there are 2 sets of responses: one from the south to north scan (labelled SN), and one from the west to east scan (WE). Each set is comprised of 181 responses; each response corresponds to 1integer degree. The responses of each set are organized into a list in the following order:
M90, M89, M88 ... M3, M2, M1, P0, P90, P89, P88 ... P3, P2, P1

The responses represent the deviations from ideal instrument responses and are unitless. The character M indicates minus; P indicates plus; and the integer indicates the angle of the beam with 0 being the point at which the beam is normal to the top of the diffuser.

For simplicity, the remainder of this section describes the procedure as it applies to a single channel; all channels are treated in the same manner.

The azimuth and solar elevation (both in degrees) at the time the UV-MFRSR voltages were measured in the field are used to determine the angular correction to be applied. Both values must be within certain limits. The degrees of azimuth must be in the range 0 - 359. If the azimuth is less than zero, 360 is added to it; if it is greater than 359, 360 is subtracted from it. If the elevation is outside the range 0.001 - 89.5, the rest of the procedure is skipped and no correction is made.

The quadrant corresponding to the location of the sun determines the starting locations of pointers into the sets of angular responses. The quadrant is defined as the integer portion of:
azimuth degrees/90.0
The degree of elevation (el_deg) is defined as the integer portion of the degrees of solar elevation at the time of the field measurement. The pointers are set as shown in the table below, depending upon the quadrant. These are the starting points for Step 5.

Weighting factors are determined for each direction and for both the degree of elevation (el_deg) and the next highest degree of elevation.
Azimuthal Weighting Factors:
awt2 = weight of the second direction = the fractional portion of azimuth degrees/90.0
awt1 = weight of the first direction = 1.0 - awt2

Elevational Weighting Factors:
ewt2 = weight of next highest degree = the fractional portion of the degrees of solar elevation
ewt1 = weight of this degree = 1.0 - ewt2

Four values are needed from the lists of angular responses: 2 from SN and 2 from WE. Advance the pointers 1 value for each degree of el_deg. For example, if el_deg is 25 and the quadrant index is 0, advance pointer #1 from SN P90 to SN P65 and pointer #2 from WE P90 to WE P65. After saving the angular responses at those locations (r1deg, r2deg), advance each pointer to the next location in each list to get the responses for the next highest degree (r1next, r2next).

The angular correction is then calculated as follows:
c1 (for 1st direction) = (r1deg x ewt1) + (r1next x ewt2)
c2 (for 2nd direction) = (r2deg x ewt1) + (r2next x ewt2)
final correction factor = (c1 x awt1) + ( c2 x awt2)