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2.0 Sampling Frequency and Accuracy Requirements for BSRN Stations

2.1 Sampling Frequency

2.1.1 Sampling Frequency of Radiation Measurements

The BSRN requires that all radiation variables be sampled at 1 HZ with an averaging time of one

minute. The final output for each variable should consist of the one-minute mean, minimum,

maximum and standard deviation. This specification is based upon the typical 1/e response time

of first class pyranometers and pyrheliometers being approximately one second. Although some

instruments require the measurem ent of m ore than one signal for the calculation of a specific

radiation element, the archived data will consist only of the mean, minimum, maximum and

standard deviation of the radiation element.

When an element requires more than one signal to be measured or the conversion from the

signal to the final value of the element is non-linear, difficulties arise in providing a single true

sample standard deviation for the one-minute mean value. This can be accomplished if the

measurements are stored each second and the calculations done later or the data acquisition

system is capable of calculating the parameter each second.

The m ost com m on radiation observation made in the BSRN that requires multiple signals is

infrared irradiance, where between 2 and 5 measurements are made each second, depending on

instrument type. There are two methods of data handling that provide the exact standard

deviation for the flux and two methods that provide an estimate of the standard deviation if the

standard deviation cannot be calculated.

(1) Observations can be made of each of the required signals once per second and stored.

Using this data, the one minute average can be calculated by applying the appropriate

instrument responsivity to each voltage measurement and the appropriate effects of the

case and dome temperatures. The standard deviation can then be calculated from the

individually calculated flux values. The primary drawback of this method of signal

processing is storage requirement associated with collecting one-second data.

(2) With the increasing computation power of data acquisition equipment, the determination

of the infrared flux can be made following the measurement of the appropriate signals.

This would require the conversion of the instrument thermopile signal into a flux, and

between 1 and 4 thermistor resistance measurements into temperatures and then the

equivalent blackbody fluxes. This m ethod requires the em bedding of the thermopile

responsivity into the data acquisition system. Many scientists are unwilling to include

such information in the acquisition stage of an observation because of the risk of error

and the difficulty of correcting the problem when discovered. To reduce the potential of

this type of error, while maintaining the capability of calculating the standard deviation,

the mean, minimum, maximum and standard deviation of each of the raw signals can be

stored along with calculated infrared irradiance.

Alternatively, given the difficulty associated with observing, storing and calculating the exact

standard deviation of the infrared flux, the standard deviation of the flux can be reasonably

estimated based by the standard deviation of the thermopile signal. This estimate assumes that

over one-minute the temperatures of the case and dome remain nearly constant and therefore do

not affect greatly the overall standard deviation of the flux. This assumption is substantiated by

reference to Figure 8.1 that illustrates that a 5% change in thermistor resistance alters the overall

flux by less than ±1.6 % over an extended temperature range.

The observation of temperature using thermistor technology is illustrative of a non-linear

conversion from resistance to temperature using the Steinhart and Hart equation (see Sec. 9.2.3).

In cases where the one-second data is not stored or the conversion of resistance to temperature

is not accomplished within the data acquisition system each second, the standard deviation of the

temperature should be estimated based on the positive standard deviation

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Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador Página 20

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