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© Crown copyright 2004 Page 1
Development of a Ground Based GPS Network for the Near Real Time Measurement of Integrated Water
Vapour (IWV)
Jonathan JonesMet Office
UK
© Crown copyright 2004 Page 2
Why…?
Assimilation of IWV to model (trials have seen 4% improvement of surface temperatures)
Nowcasting applications – extreme weather events etc
Moisture and latent heat are transmitted through the water vapour phase
Validation of other remote sensing instruments (radiosonde, microwave radiometer)
Also complements satellite IWV which only available over open water
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Advantages / Disadvantages
Advantages of GPS IWVAll weather operational (vs. MWR)Continuous, high rate of measurementRelatively Low cost (Near) Real-time
DisadvantagesZTD is an integrated value (vs. radiosonde)Can’t solve directly slant delay (vs. MWR)
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Ground Based GPS Meteorology
Ionosphere
Dispersive, use L3 to remove
Troposphere + Stratosphere
Using mapping function we solve to the vertical (Zenith)
Zenith Total Delay (ZTD)= ZHD+ZWD
~90% Dry delay (Zenith Hydrostatic Delay ZHD)
~10% Wet Delay (Zenith Wet Delay ZWD)
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ZTD to IWV
ZHD mitigated using surface barometric measurements and atmospheric models
ZWD more difficult to predict, varies greatly both spatially and temporally
With further calculation (using surface temperature) we can convert ZWD to IWV
1mm IWV ≈ 3% Relative Humidity
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GPS Data Flow
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GPS Processing System
2 dual-processor PCs running Bernese v5 GPS processing software
ZTD Processing Solutions Near Real Time solution, ~25min latency 2 Quality check solutions using more accurate orbits
Quality Analysis Compare solutions (NRT vs. PP) Compare against other processing centres Compare against other obs. methods
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Equipment
Ashtech Dual Frequency GPS Receiver
Choke ring antenna, carbon fibre pole
Data collected by local PC and sent to central processing system for conversion to ZTD and IWV
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GPS IWV time series vs. RS80 Radiosonde
Camborne RS vs GPS IWV June 2003
-10
-5
0
5
10
15
20
25
30
35
40
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00
Time (Days)
IWV
(kg
/m2̂) RS IWV
GPS IWV
Difference
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Approximate Site Installation costs (EURO)
GPS Equipment ≈ 20,000PC and UPS ≈ 2,500Works Services ≈ 5,000Communication connection ≈ 500
1 Euro ≈ $1.29
~50km spatial resolution required for operational meteorology (COST716)
In UK 50km resolution ≈ 150 station network150 station network ≈ 4.2M EURO ($5.4M)
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Partnerships
Therefore….Met Office must work with other national agencies to reduce network costs
2003 to Present – UK National mapping agency (Ordnance Survey GB) ≈ 120 sites
Others (incl. offshore?) ≈ 50 sites ?
Met Office Network = 10 significant locations
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Current UK NRT GPS Network (April ‘05)
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Output of Data
COST716 format files disseminated to EU scientific community via UK Met Office
COST716 file converted to BUFR (binary format) for dissemination on the GTS
NRT animated 2D plot
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Example NRT Animated Plot
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The Future
Operational network and processing
Sub-hour processing (sub-hour RINEX)
Increasing network density
Take forward EU action through E-GVAP (EUMETNET Project)
