4
Holben et al.
(AOT). An almucantar is a series of measurements taken
at the elevation angle of the Sun for specified azimuth
angles relative to the position of the Sun. The range of
scattering angles decrease as the solar zenith angle de-
creases; thus almucantar sequences made at an optical
airmass of 2 or more achieve scattering angles of 120⬚
or larger. Scattering angles of 120⬚ are typical of many
sunsynchronous viewing satellites; thus a measure of the
satellite path radiance is approximated from the ground
station. During an almucantar measurement, observations
from a single channel are made in a sweep at a constant
elevation angle across the solar disk and continues through
360⬚ of azimuth in about 40 s (Table 2). This is repeated
for each channel to complete an almucantar sequence.
More than four almucantar sequences are made daily at
an optical airmass of 4, 3, 2, and 1.7 both morning and
afternoon and, an almucantar is made hourly between 9
a.m. and 3 p.m. local solar time for the standard instru-
ment and skipping only the noon almucantar for the po-
larization instrument. A direct Sun observation is made
during each spectral almucantar sequence.
The standard principle plane sequence measures in
much the same manner as the almucantar but in the
principal plane of the Sun where all angular distances
from the Sun are scattering angles regardless of solar ze-
nith angle. This measurement sequence begins with a
sun observation, moves 6⬚ below the solar disk, and then
sweeps through the sun taking about 30 s for each of the
four spectral bands (Table 2). Principal plane observa-
tions are made hourly when the optical airmass is less
than 2 to minimize the variations in radiance due to the
change in optical airmass.
Polarization measurements of the sky at 870 nm are
an option with this instrument. The sequence is made in
the principal plane at 5⬚ increments between zenith
angles of ⫺85⬚ and ⫹85⬚. The configuration of the filter
wheel requires that a near-IR polarization sheet is
attached to the filter wheel. Three spectrally matched
870 nm filters are positioned in the filter wheel exactly
120⬚ apart. Each angular observation is a measurement
of the three polarization filter positions. An observation
takes approximately 5 s and the entire sequence about 3
min. This sequence occurs immediately after the stan-
dard principle plane measurement sequence.
Instrument Precision
We define the precision of the instrument as its ability
to accurately reproduce results from multiple measure-
ments under constant conditions using standardized
techniques. Three methods will be used to assess the ra-
diometric precision: 1) the variability of the digital num-
bers (DN) from the spectral response acquired from the
2-m-diameter integrating sphere at Goddard Space Flight
Center, which is used to determine the gain and offset
Table 1. Measurement Sequences of the CIMEL Sun/Sky Scanning Spectral Radiometer
Spectral
Range (nm) Target No. Obs. Obs. Interval Application
Basic direct Sun 340–1020 Sun 1 each k ⵑ8 s for 8 k AOT, Pw, a
Triplet observation 340–1020 Sun Three direct sun 3 at 30 s apart, 1 min total AOT, Pw, a and cloud screening
Standard measurement sequence 340–1020 Sun Variable: depends on day length Ea. 15 min, m⫽2 a.m. to m⫽2 p.m. AOT, Pw, a
Langley 340–1020 Sun 16, a.m. and p.m. between m 7 and 2 m⫽7–5, incr. of 5 m; m⫽5–2, incr. of 25 m Langley, Cal, AOT, Pw, a
Basic sky 440–1020 Sky 1 each k None Sky radiance
Langley sky 440–1020 Sky 16 between m 7 and 2 m⫽7–5, 0.5; m⫽5–2, 0.25 Stability of Langley plot
Almucantar 440–1020 Sky 72 (Table 2) ⬎8/day: m⫽4, 3, 2, 1.7 hrly 9 a.m. to 3 p.m. Size dist. and P (h ), AOT, a
Polarization 870 Sky 42 (Table 2) Hourly; 9 a.m. to 3 p.m. Size dist. and P (h )
Principal plane 440–1020 Sky 42 (Table 2) Hourly; 9 a.m. to 3 p.m. Size dist. and P (h ), AOT, a
calibrations of the sky radiance channels, 2) examination