14
better than one second, this time accuracy was relaxed to one second at the BSRN Science and
Review Workshop (Boulder, Colorado, USA, 12-16 August, 1996).
The automatic determination of time to within one second can be easily achieved on portable
computers using one of three common methods: (1) time-synchronization with the Global
Positioning System (GPS) satellites; (2) conversion of radio frequency time signals sent out by
national standards agencies; and (3) through time updates obtained via the internet.
Com puter clocks can be synchronized to within 2 milliseconds of UTC with sim ple and
inexpensive GPS systems that consist of a small antenna (< 100 mm diameter) and a decoder
box that can be plugged into the serial port. More expensive bus systems can increase the
accuracy of the synchronization to better than 1 microsecond. The need for an antenna and a
view of the sky, may reduce the applicability of such systems in built-up areas, or where the unit is
deep within a building complex. In certain regions of the world (North America, Australia, China) a
similar system has been developed, CDMA (indirect GPS) that operates through cellular networks
and is maintained by the cellular providers. CDMA, where available, is similar to GPS, but uses a
much sm aller integrated antenna and will work within office com plexes. T he tim e is kept to an
accuracy of better than 10 microseconds.
Typical Meteorological Measurement Field Specifications
Measurement Resolution Uncertainty
Air Temperature 0.1 °C ±0.3 °C
Dew Point Temperature 0.1 °C ±0.5 °C
Soil Temperature 0.1 °C ±0.3 °C
Relative Humidity 1% ±7%
Wind Speed 0.5 ms ±5% or ± 2 ms
-1 -1
Wind Direction 5° ±10°
Accumulated Precipitation 0.2 mm greater of ±0.2 mm or ± 2% of total
Precipitation Intensity 0.2 mm h greater of ±0.2 mm h or ± 2% of total
-1 -1
Snow Depth 1 mm greater of ± 10 mm or ±1% of value
Atmospheric Pressure 0.1 hPa ±0.5 hPa
Table 2.2. Recommended measurement requirements for ancillary meteorological
variables.
Many national m etrology institutes (NMI) transm it time signals, based upon the national tim e
standard, at short-wave radio frequencies. These radio time signals can be received thousands of
kilometres from the transmitter, depending on ionospheric conditions. With the proper decoding
and correction for time delays, the accuracy of the time signal can be better than 1 ms. Like the
GPS system, short-wave radio receivers require external antennas. While radio signals are as
accurate and less expensive than GPS systems, and more accurate than most internet systems,
the radio time signal is losing popularity over the ease of use of GPS and internet systems.
Computer time-synchronization has advanced rapidly since the onset of the modem
communication and the internet. The time obtained in this manner is not usually as accurate as
the GPS and radio time signals, the need to synchronize a clock to better than one second can
normally be accomplished through these methods. Many NMIs provide analog-modem dial-up
links and the associated software required to set a local computer clock to UTC. The software can
translate simple telephone codes that allow corrections to be made for the signal propagation
delay. Using this method, computer clocks can be set to within several milliseconds of UTC. New,
high speed modems that use digital processing can add a variable delay of up to 140 ms and a
20 ms jitter beyond the delay due to signal propagation. Further uncertainties associated with the
use of modems using digital technology. These telephone services are also degraded when
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