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The three platforms on which the instruments and support equipment are mounted are aligned north to south. Upward looking radiometers rest on a long rectangular platform (~0.3 m by 3 m) that is made of an off-white fiberglass grating. It is elevated about 1.5 to 2 m off of the ground by two steel posts that are set in concrete (below the frost line). The data logging, power distribution, and communication equipment are fastened to these posts. A smaller platform about 3-4 m south of the main platform supports an automatic sun tracker that holds the pyrheliometer, shaded pyrgeometer and shaded pyranometer. About 15 m north of the main platform is a 10-m tower with downward looking radiometers and meteorological instruments. A crossarm supporting the downward looking radiometers is also aligned north to south. This station layout ensures that the tower, and the instruments mounted on it, will never shade the upward looking radiometers on the main platform. Electrical power for the station is conditioned by a 1.15 KVA uninterruptable power supply that has the capability to supply power through battery backup for a short period (~2 hours) if the line power fails.

Radiation measurements at SURFRAD stations cover the range of the electro- magnetic spectrum that affects the earth/atmosphere system. Global solar and its components are measured separately. Total downwelling (global) solar radiation is measured on the main platform by an upward looking broadband pyranometer. The direct component is monitored with a normal incidence pyrheliometer (or NIP) mounted on an automatic sun tracker, and the diffuse component is measured by a shaded pyranometer that rides on the solar tracker. Diffuse solar was not in the original suite of SURFRAD measurements. The shaded pyranometer was added in 1996 when a support platform with a shade arm mechanism was fitted to the trackers. A third pyranometer is mounted facing downward on a crossarm near the top of the 10-meter tower to measure solar radiation reflected from the surface. An upward looking pyrgeometer on the main platform measures long wave (thermal infrared) radiation emitted downward by clouds and other atmospheric constituents. Another pyrgeometer, mounted facing downward on the crossarm atop the tower, senses upwelling long wave radiation. These measurements of upwelling and downwelling in the solar and infrared wavebands constitute the complete surface radiation budget.

Besides the surface radiation budget, other ancillary parameters are monitored. There are two instruments on the main platform that monitor wavebands of special interest. A UVB radiometer measures the degree of harmful ultraviolet radiation (290-320 nm) that evades the ozone layer and reaches the surface. Another instrument monitors the intensity of the waveband active in photosynthesis (400 to 700 nm). The final radiometer in the SURFRAD suite is the Multi-Filter Rotating Shadowband Radiometer (MFRSR) which measures both global and diffuse solar radiation in one broadband and six narrow bands of the solar spectrum. These are useful for obtaining aerosol loading information. Instruments for measuring wind speed and direction, air temperature, and relative humidity also reside at the top of the 10-meter tower. A barometer in the data logger box measures station pressure.

The redundancy of three component solar measurements (global, direct and diffuse) provides a useful tool for quality control of the SURFRAD data. In addition, the sum of the diffuse and direct is actually a better measure of total solar than the global measurement alone because when the sun is near the horizon, a change in the cosine response of the global pyranometer's sensor introduces errors in the global measurement.

Future enhancements to SURFRAD stations depend on funding, but may include Global Positioning Satellite (GPS) receivers and associated hardware and software which would allow for the retrieval of total-column water vapor. An inexpensive unattended cloud imager, recently developed at SRRB, will be deployed at all SURFRAD stations. The addition of GPS and cloud imagers will make SURFRAD stations more beneficial as satellite validation sites and will provide information crucial to radiative transfer calculations.

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