The sensor head is pointed by direct drive step by step azimuth and zenith motors with an accuracy 
of 0.05 degrees. A microprocessor computes the position of the sun based on time and input locational 
coordinates and directs the sensor head to approximately one degree of the sun after which a four quadrant 
detector tracks the sun precisely prior to a programmed measurement. After the routine measurement is 
completed the instrument returns to the "park" position awaiting the next measurement sequence. A "wet 
sensor" exposed to precipitation will cancel any measurement sequence in progress. 
The data are downloaded under program control to a Data Collection Platform (DCP) typically used 
in the geostationary satellite telemetry system. See section 3. 
2.2. Measurement Concept 
The radiometer has sufficient S/N that observations can be made from 300 to 1030 nm as currently 
configured. The basic sun measurement is three spectral observations (triplet) taken 30 seconds apart. 
Triplet observations are made during morning and afternoon langley calibration sequences and at 15 minute 
intervals in between. The time variation of clouds is typically greater than that of aerosols and would 
manifest itself in an observable variation in the raw digital data. Additionally the 15 minute interval allows 
a longer temporal frequency check for clouds. 
Two basic sky measurement sequences are the almucantar and principle plane. The almucantar is a 
series of measurements taken at a constant view zenith angle and specified azimuth angles relative to the 
position of the sun. During a measurement sequence for one of four channels the instrument makes a sun 
measurement, begins the aureole measurement sweep through the solar disc and continues through 360 
degrees of azimuth in about 40 seconds. The almucantar sequence is typically made at an optical airmass of 
2 or less to maintain large scattering angles. 
The principle plane sequence operates in much the same manner but in the principle plane of the 
sun. Because the scattering angles are constant with solar zenith these observations are made for optical 
airmasses between 2 and 1 when the almucantar measurements are less effective. 
2.3. Instrument Stability and Calibration Accuracy 
We define the stability of the instrument as its ability to accurately reproduce results from multiple 
measurements under constant conditions using standardized techniques. This is best measured by examining 
the instrument output under conditions required for instrument calibration. A two meter integrating sphere 
at Goddard Space Flight Center is used to calibrate the sky radiance channels and mountain top langley plots 
are used for the sun channels. 
Our field calibration instrument has been in almost continuous operation since March 1993. Dark 
current values show a variation of ±1 count compared to typical values of 2000 to 15000 counts depending 
on optical depth and airmass. With respect to the long term stability of calibration, two measurements 
taken eight months apart have been made to date at NASA Goddards integrating sphere facility. Assuming 
the absolute calibration of the sphere is known exactly, the calibration of the instrument has decreased by 
about 1 to 5% depending on channel. This is less than or equal to the absolute accuracy (±5% )of the 
integrating sphere. 
Langley calibrations were made on Mt. Lemmon near Tucson Arizona in May 1993 and December 
1993. The change in zero airmass voltage between the two dates was 2.3% at 1019 nm and 0.5 to 1.1% for 
all other channels. These results are typical for interference filters 
Measurements of the spectral temperature sensitivity of the instrument showed the results agreed 
with the published temperature sensitivity of the detectors and that only the 1019 nm channels showed 
significant temperature variation (0.25%/°C) warranting a correction to a reference temperature in the 
processing. 
3 - DATA TRANSMISSION 
Data are transmitted from the memory of the sun photometer via the Data Collection Systems of either of 
three geosynchronus satellites GOES, METEOSAT or GMS to the appropriate ground receiving station. 
The data can be retrieved for processing either by modem or internet resulting in near real-time acquisition 
from almost any site on the globe. The DCS is a governmental system operated for the purpose of