An Experiment of Near Real-time Preciptable Water Vapor Retrieval

Using Ground-based GPS Stations in South East Asia Yoshinori SHOJI

Meteorological Research Institute / JMA

Motivation Contribute severe weather forecast in SE Asia with temporally high resolution water vapor information under all weather from ground-based GPS network.

▲：Radio sonde site●：IGS GPS site(hourly)●:IGS GPS site(daily)

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● Nation wide permanent GPS observation network with more than 1,200 stations.

● Operated by Geographical Survey Institute.● Can be used as water vapor sensors.

GEONET- Dense GPS network in JAPAN -

JMA AWSAMeDas・：4-elements・：Rain gauge

Data assimilation experiment of GPS PWVusing JMA Meso 4D-Var

mm

9hour forecastw/o GPS

3 hour precipitation amount2004/6/30/06jst

9 hour forecastwith GPS

φij

= ρij

+ Nij

+ Tij

- jil

+ c×δi−δj

+ εi

Satellite j

Receiver i

：　Distance between J and I

：　ambiguity bias

：　Tropospheric delay

：　Ionospheric Delay

：　Clock error

：　Thermal noise, multi-path, antenna phase center variation

parameter estimation

L1 and L

2

Basic concept of GPS meteorology

Observed phase of GPS radio wave

・GPS satellites transmit stable signal using cesium and/or rubidium atomic clock.・GPS receiver observes number of radio waves. ・Distance between GPS satellite and receiver can be analyzed by time variation of signal phase.・Signal delay caused by neutral atmosphere can also be estimated simultaneously.・Accumulated water vapor along GPS ray path can be calculated from atmospheric delay.

ZWDZHDZTD +=

Zenith Total Delay(ZTD) is a sum of Zenith Hydrostatic Delay and Zenith Wet Delay

∫∫=

= −

dzz

dzzgzg

PgmRkZHD

m

sfcmd

)(

)()(

10 16

ρ

ρ

ZHD is proportionate to surface pressure ∫

∫=

+−

=Π

×Π= −

dzTP

dzTP

T

Tk

mmkkR

PWVZWD

v

v

m

md

vv

2

312

5

1

,10

sfcm TT 72.02.70 +≈

PWV is proportional to ZWD. Coefficient can be estimated by using surface temperature.

Relation between the Atmospheric delay and water vapor

IGS productsGPS Satellite Orbit and Clocks

Data Latency

Ultra rapidRealtime

4 hours < 5cm

Rapid 17 hours < 5cm

Final < 5cm

AccuracyOrbit

AccuracyClock

Predicted half 〜10cm

〜 5ns(150cm)

Analyzed half

〜 0.2ns(6cm)0.1 ns(3cm)

〜13days < 0.1ns(< 3cm)

Key point of Near Real Time analysisis

to get accurate GPS satellite clock offset in NRT

Accuracy of GPS satellite clock in IGS Ultra-rapid orbit (IGU) is insufficient.

IGSInternational GNSS Serivice

・〜Jan. 1, 1994 part of International Association of Geodesy

・Mission provide high accuracy data and products for GNSS in support of Earth science research, multidisciplinary applications, and educations.

Feb. 2008 336 stations are active

Several IGS Tracking network sites use H-maser atomic clock.

“USUD” IGS station (1993〜)uses H-maser atomic clock

・for LEO tracking・operated by JPL・Only one GPS station using H-Maser atomic clock in Japan.

Clock offset of PG01-：IGU-：IGS

Clock offset of “USUD”-：Analyzed with JPL rapid orbit-：Linear extrapolation-：Analyzed with JPL final orbit

Differences of Satellite Clock offset（IGUーIGS）

Error of USUD clock offset-：Rapid orbit - Final orbit(JPL）-：Extrapolation - Final Orbit(JPL）

Clock offsets of “USUD” is much stable compared to those of GPS satellites

USUD

Horizon

How to correct GPS satellite clock under horizon ?

By forming data link using both GPS satellite and receivers

Improvement of PWV accuracy(MRI)

Red circle: using IGS final orbitGray Square with dashed line: using IGUBlack Solid Line: using IGU ( clock modified)Blue Triangle: Radio Sonde Red: Jul. 2007

Green: Jan. 2007

Expansion of analysis area (Japan → Global)using IGS’s Global GPS network

Green: hourly providing stationBlue: Daily providing stationRed: Used in satellite clock offset analysis

USUD

IGS station “ntus” and Radio Sonde station “48698” in Singapore

NTUS: Nanyang Technological University48698: Changi

30km

PWV time series at “NTUS”Jul. 27-28, 2007

●: using IGS final orbit and clock (IGS)××××： using IGS ultra rapid orbit and clock (IGU)ーーーー: using IGS ultra rapid orbit and modified clock(IGUm)□ : radio sonde (48698)

PWV time series at “NTUS”Jul. 22-31, 2007

●: using IGS final orbit and clock (IGS)××××： using IGS ultra rapid orbit and clock (IGU)ーーーー: using IGS ultra rapid orbit and modified clock(IGUm)□ : radio sonde (48698)

Radio-sonde vs GPSJul. 22-31, 2007

●： using final orbit and clock(JPF)×： using Ultra rapid orbit and clock (IGU)▲: using ultra rapid orbit and modified clock (IGUm)

Pairs

JPF -1.38 2.19 35-1.07 1.98 35

IGU -11.88 21.23 35

BIAS(mm)

R.M.S (mm)

IGUm

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SummarySummary

A Near Real-Time (NRT) analysis procedure of GPS stations in SE Asia is introduced.・The procedure uses “USUD” clock as a reference.・Retrieved GPS PWVs show comparable accuracy with those

obtained by using final orbits and clocks.→validations of other sites are needed.・Analysis of all GPS satellites’ clock offsets using global GPS

network allow us to analyze all GPS stations in near real-time.

→It can be applicable for ground-based GPS network for DA of global NWP model.

THANK YOU

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● Using GIPSY-OASIS II software package with ultra rapid orbit (IGU).● Clock correction by using “USUD” IGS station.● Retrieve PWV in every 10 minute within 2 hours after the observation for all 1,200 GPS

station in Japan.

Near Real-time GPS analysis in MRI

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Near Real-time GPS analysis in MRI● Using GIPSY-OASIS II software package with ultra rapid orbit (IGU).● Clock correction by using “USUD” IGS station.● Retrieve PWV in every 10 minute within 2 hours after the observation for all 1,200 GPS

station in Japan.

1) “USUD” clock offset is analyzed by PPP method with JPL rapid orbit, and then extrapolated 24 hours ahead.

2) Satellite clock offsets are analyzed by using “USUD” clock as a reference.

3) PPP is performed using corrected satellite clock as a reference

4) PWV RetrievalUsing ZTD and observed P

sfc, T

sfc .

Improvement of satellite clock offsets

IGU-JPL final orbit

MRI results-JPL final orbit

Our results show improvement in clock offset bias.Error variances near last edge of data also improved, error variances in first half became worse, though.In PPP analysis, average value of satellite clock offset is important.

Correction of clocks of all GPS satellites in view from Japan using GPS netowrk