Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 26)

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better than one second, this tim e accuracy was relaxed to one second at the BSRN Science and
Review W orkshop (Boulder, Colorado, USA, 12-16 August, 1996).
The autom atic determ ination of tim e to within one second can be easily achieved on portable
com puters using one of three com m on m ethods: (1) tim e-synchronization with the G lobal
Positioning System (GPS) satellites; (2) conversion of radio frequency tim e signals sent out by
national standards agencies; and (3) through tim e updates obtained via the internet.
C om puter clocks can be synchronized to within 2 m illiseconds of UTC with sim ple and
inexpensive GPS system s that consist of a sm all antenna (< 100 m m diam eter) and a decoder
box that can be plugged into the serial port. More expensive bus system s can increase the
accuracy of the synchronization to better than 1 m icrosecond. The need for an antenna and a
view of the sky, m ay reduce the applicability of such system s in built-up areas, or where the unit is
deep within a building com plex. In certain regions of the world (North Am erica, Australia, China) a
sim ilar system has been developed, CDMA (indirect GPS) that operates through cellular networks
and is m aintained by the cellular providers. CDMA, where available, is sim ilar to GPS, but uses a
m uch sm aller integrated antenna and will work within office com plexes. T he tim e is kept to an
accuracy of better than 10 m icroseconds.
Typical Meteorological Measurement Field Specifications
Measurem ent Resolution Uncertainty
Air Tem perature 0.1 °C ±0.3 °C
Dew Point Tem perature 0.1 °C ±0.5 °C
Soil Tem perature 0.1 °C ±0.3 °C
Relative Hum idity 1% ±7%
W ind Speed 0.5 m s ±5% or ± 2 m s -1 -1
W ind Direction 5° ±10°
Accum ulated Precipitation 0.2 m m greater of ±0.2 m m or ± 2% of total
Precipitation Intensity 0.2 m m h greater of ±0.2 m m h or ± 2% of total -1 -1
Snow Depth 1 m m greater of ± 10 m m or ±1% of value
Atm ospheric Pressure 0.1 hPa ±0.5 hPa
Table 2.2. Recom m ended m easurem ent requirem ents for ancillary m eteorological
variables.
Many national m etrology institutes (NMI) transm it tim e signals, based upon the national tim e
standard, at short-wave radio frequencies. These radio tim e signals can be received thousands of
kilom etres from the transm itter, depending on ionospheric conditions. W ith the proper decoding
and correction for tim e delays, the accuracy of the tim e signal can be better than 1 m s. Like the
GPS system , short-wave radio receivers require external antennas. W hile radio signals are as
accurate and less expensive than GPS system s, and m ore accurate than m ost internet system s,
the radio tim e signal is losing popularity over the ease of use of GPS and internet system s.
C om puter tim e-synchronization has advanced rapidly since the onset of the m odem
com m unication and the internet. The tim e obtained in this m anner is not usually as accurate as
the GPS and radio tim e signals, the need to synchronize a clock to better than one second can
norm ally be accom plished through these m ethods. Many NMIs provide analog-m odem dial-up
links and the associated software required to set a local com puter clock to UTC. The software can
translate sim ple telephone codes that allow corrections to be m ade for the signal propagation
delay. Using this m ethod, com puter clocks can be set to within several m illiseconds of UTC. New,
high speed m odem s that use digital processing can add a variable delay of up to 140 m s and a
20 m s jitter beyond the delay due to signal propagation. Further uncertainties associated with the
use of m odem s using digital technology. These telephone services are also degraded when