The Brookhaven National Laboratory (BNL) has developed a ship-board shortwave radiation instrument platform called the Portable Radiation Package (PRP). The PRP is composed of a broadband Precision Spectral Pyranometer (PSP) made by Eppley Laboratory, Inc., a fast-rotating shadowband (FRS) spectral radiometer, which is the subject of this paper, attitude sensors for the measurement of platform pitch, roll, and azimuth, and a special microprocessor for data collection and instrument control. microprocessor for data collection and instrument control.
The FRS spectral radiometer is designed to provide quality estimates of direct and diffuse shortwave irradiance from a moving platform without need of special gyro-stabilized tables. It is small, environmentally robust, low power, and is well suited for research ships, for ice floes, or for larger buoys in all but the higher Beaufort conditions.
The arm rotational speed is 7.0 sec per revolution, meaning the arm moves across the upper hemisphere, relative to the sensor, in 3.5 sec. The hemispherical shape of the arm ensures that the sensor will see a shadow, regardless of its azimuth heading. The arm moves across the face of the sun in a few tenths of a second and the head is in full shadow for about one tenth of a second.
Spectral irradiance measurements are made with a modified spectral radiometer head (Yankee Environmental Systems, Inc.). The head has seven detectors (channels), a broadband channel and six 10 nm bandpass measurements at 415, 500, 610, 655, 860, and 960 nm. It can be accurately calibrated and has an excellent zenith angle response. The head construction, adeptly described by Harrison et al.(1994), is environmentally sound and the package is weatherproof, robust, and suitable for use in a marine environment. <\p>
|The figure shows a typical solar spectrum at the top of the atmosphere and at the Earth's surface. Superimposed on the graph are the pass bands of the silicon cell photo diode and the six FRS spectral radiometer narrowband channels. Also shown are the SeaWiFS channels.|
The FRS spectral radiometer data logger is custom-made and packaged in a waterproof housing that resides in close proximity to the PRP . The low-power microprocessor computer, a Tattletale Model 8 by Onset Computer Company, has the power to accommodate a sophisticated program in a remote package. Using a Motorola 68332 microprocessor, it has a 500 Kbps RS-232 interface and a tunable system clock from 160 KHz to 16 MHz. It programs in C and has a PIC 16C64 microcontroller which operates as a super-controllable clock, and provides improved external bus expansion, a precision real-time clock, and increased memory as required. It weighs only one ounce and can draw less than 250 microamps in low power mode. The data logger incorporates an RS232/422 interference and surge protection, operates over a -40 to 65\degC\ temperature range, and is waterproof and immune to shock and vibration.