Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 27)

T hree locations where further inform ation on tim e synchronization can be found are:
9
(1) N IS T : http://www.boulder.nist.gov/tim efreq/service/its.htm
(2) PT B: http://www.ptb.de/en/org/q/q4/q42/ntp/_ntp_main.htm and
(3) T ime S ynchronization Server: http://www.eecis.udel.edu/~ntp/index.htm l.
15
com m unications are m ade via satellite links (long-distance services), but again, correction can be
applied. Obtaining a true tim e via the internet is m ore difficult than with m odem s because of the
increased variability in response tim es of the service. To overcom e som e of the variability
associated with these delays, the Network Tim e Protocol (NTP) was developed. The advantage of
N T P 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 m ust be noted that the NTP service will not operate correctly if the com puter
9
being updated is behind a firewall, unless the firewall is set up to allow N T P packets through. NTP
is a very effective m eans of standardising com puters on a local area network. For rem ote
locations, a high quality GPS system associated with the network server and N T P can
synchronize the LAN to better than 1 m s of the GPS tim e base.
T hese signals can either be incorporated directly into m ore advanced data acquisition system s or
set on a daily basis for less advanced system s or externally controlled data acquisition system .
The software operating m any PC Card Data Acquisition System s (DAS) uses the com puter clock
for tim e inform ation. Therefore, m aintaining the com puter 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 approxim ately once each hour. Com m unication with m any new external DAS can be
accom plished through local area network protocols, especially Internet Protocol (IP) addressing
that provides an easy m eans of ensuring that all system s, com puters and dataloggers, m aintain
tim e precisely.
For data acquisition system s with internal tim e keeping, the sam e clock correction m ust be
m aintained 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 m easurem ents is
set forth in the report of the W CRP BSRN Im plem entation W orkshop, Davos, Switzerland, 6 - 9
August 1991. Uncertainty of the com plete system (digital voltm eter (DVM), scanner (m ultiplexer)
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
pream plifier should be used. If such an am plifier is required, it should be placed as close to the
signal transducer as possible so that line noise is not also am plified, as it would be if the am plifier
were associated with the DAS. C are m ust be taken to ensure the tem perature stability of the
am plifier, either through am plifier selection or tem perature control, so that tem perature influences
do not increase significantly the uncertainty in the m easurem ent.
Each instrum ent should be scanned at least once per second with the analog signals integrated to
provide one-second values. On those system s where integration tim e is program m able, the
shortest period to be used for sam pling of radiation signals is one power line cycle (PLC).
Sam ple averaging and special filtering techniques are required when em ploying m any new high-
speed DAS system s, especially those that are directly connected to a com puter bus, to reduce
uncertainties associated with electrom agnetic noise.
W hile the prim ary aim of the BSR N is to obtain accurate radiation fluxes, the accuracy of the data
acquisition system used in the collection of ancillary data should be com m ensurate with the
general aim s of the program . Therefore, ancillary m easurem ents should be sam pled and recorded
following the sam e principles as applied to the radiation observations.