| ENVISAT - Reference Station
The principle
of differential GPS implies to set up a reference station nearby the GPS
buoys as highest precision positions require to take greatest care of the
error budget: atmospheric errors (troposphere and ionosphere) as well as
orbit uncertainties of the GPS satellites depend on the length of the baseline.
Particularly the ionospheric delays quickly degrade the measurements such
that a reliable ambiguity fixing is no longer feasible. Reliable position
estimates with centimetre accuracy can only be guaranteed up to distances
in the range of 10 to 20 kilometres between reference station and buoy.
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The
purification plant (Ciutadella Nord) where a dual-GPS reference station (CIUX)
is located.The distance to the GPS buoys is around 8 to 9 kilometres.
Video clip
here. |
The figure
above presents the scenery in which the reference station is located: Preference
was given to a small but modern purification plant situated in the north
of the town of Ciutadella. Apart from the conveniently small distance to
the GPS platforms, the power supply was the least problematic issue here
in contrast to all other available sites.
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The
two GPS antennas slightly above the apex of the purification plant's control
building. |
The
meteorological sensor station featuring a high-precision pressure, temperature
and humidity sensor. |
The
8-metres telemetry mast to establish communication links from the reference
station to the GPS buoys. |
The following
tasks of the CIUX reference station can be identified: (1)
Continuous GPS data recording at an interval of 30 seconds (IGS standard)
for static self-positioning and integrated water vapour estimation.
(2) High-frequency (1 Hz) data logging during all cross-over events
scheduled for the commissioning phase. (3) Collection of all relevant
meteorological surface quantities to aid in the data analysis. (4)
Data and position reception from the buoys using a radio transceiver system
as well as uplink capabilities in order to update the observation schedule
and to install firm- and software updates if necessary.
All
mission-critical
devices are redundant, i.e. two GPS reference
station receivers are in use and the same yields for the logging and
communication servers (see photo on the left) as well as the telemetry units
that can be either operated via a directional (Yagi) antenna or a non-directional
2-metres long antenna. Uninterruptible power supplies (UPS) operate as surge
filters and allow to bridge power outages of up to 15 minutes. All incoming
data are primarily stored on the logging host (left), additionally
stored to external discs and finally transmitted to the communication server
(middle) from where the data are sent to the analysis centre at the
Institute of Geodesy and Navigation once per day using GSM 900 communication
channels.
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