Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 27)

Three locations where further information on time synchronization can be found are:

(1) NIST: http://www.boulder.nist.gov/timefreq/service/its.htm

(2) PTB: http://www.ptb.de/en/org/q/q4/q42/ntp/_ntp_main.htm and

(3) Time Synchronization Server: http://www.eecis.udel.edu/~ntp/index.html.

communications are made via satellite links (long-distance services), but again, correction can be

applied. Obtaining a true time via the internet is more difficult than with modems because of the

increased variability in response times of the service. To overcome some of the variability

associated with these delays, the Network Time Protocol (NTP) was developed. The advantage of

NTP is its ease of use and its ability to be used on m ost com puter platform s. Both the NTP service

and the software required to use it is freely available on the worldwide web, along with details of

its operation . It must be noted that the NTP service will not operate correctly if the computer

being updated is behind a firewall, unless the firewall is set up to allow NTP packets through. NTP

is a very effective means of standardising computers on a local area network. For remote

locations, a high quality GPS system associated with the network server and NTP can

synchronize the LAN to better than 1 ms of the GPS time base.

These signals can either be incorporated directly into more advanced data acquisition systems or

set on a daily basis for less advanced systems or externally controlled data acquisition system.

The software operating many PC Card Data Acquisition Systems (DAS) uses the computer clock

for tim e information. Therefore, m aintaining the computer clock tim e as accurately as possible is

essential. For a PC with a tim e gain of 10 seconds per day, the clock would require autom atic

updating approximately once each hour. Communication with many new external DAS can be

accomplished through local area network protocols, especially Internet Protocol (IP) addressing

that provides an easy m eans of ensuring that all system s, computers and dataloggers, m aintain

time precisely.

For data acquisition systems with internal time keeping, the same clock correction must be

maintained for the relative sam pling rate, while the absolute tim e can be corrected during data

processing.

2.3.2 Data Acquisition System Accuracy

The specification for data acquisition system requirem ents for BSRN radiation measurem ents is

set forth in the report of the WCRP BSRN Implementation Workshop, Davos, Switzerland, 6 - 9

August 1991. Uncertainty of the complete system (digital voltmeter (DVM), scanner (multiplexer)

and cabling) was set as ±0.01% of the reading or ±1 :V, whichever is greater. If the overall

accuracy of the data acquisition system is greater than 10% of the accuracy required for the

observation (e.g., 1 W m for an instantaneous uncertainty of 10 W m ), then a high quality

-2 -2

preamplifier should be used. If such an amplifier is required, it should be placed as close to the

signal transducer as possible so that line noise is not also amplified, as it would be if the amplifier

were associated with the DAS. Care must be taken to ensure the temperature stability of the

amplifier, either through amplifier selection or temperature control, so that temperature influences

do not increase significantly the uncertainty in the measurement.

Each instrument should be scanned at least once per second with the analog signals integrated to

provide one-second values. On those systems where integration time is programmable, the

shortest period to be used for sampling of radiation signals is one power line cycle (PLC).

Sample averaging and special filtering techniques are required when employing many new high-

speed DAS systems, especially those that are directly connected to a computer bus, to reduce

uncertainties associated with electromagnetic noise.

While the primary aim of the BSRN is to obtain accurate radiation fluxes, the accuracy of the data

acquisition system used in the collection of ancillary data should be commensurate with the

general aims of the program. Therefore, ancillary measurements should be sampled and recorded

following the same principles as applied to the radiation observations.

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