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A generalized scheme to retrieve wet path delays from water vapor radiometer measurements applied to European geodetic VLBI network. Jung-ho Cho 1,2 , Axel Nothnagel 2 , Alan Roy 3 , and Ruediger Haas 4 1 Korea Astronomy and Space Science Institute

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slide1

A generalized scheme to retrieve wet path delays

from water vapor radiometer measurements applied to

European geodetic VLBI network

Jung-ho Cho1,2, Axel Nothnagel2, Alan Roy3, and Ruediger Haas4

1Korea Astronomy and Space Science Institute

2Geodetic Institute of the University of Bonn

3Max Plank Institute for Radio Astronomy

4Onsala Space Observatory of Chalmers Technical University

Purpose: To check the possibility of improvement in VLBI positioning results

introducing WVR WPD instead of estimation

  • WVR: Water Vapor Radiometers
  • WPD: Wet Path Delay

4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide2

Contents

  • Tropospheric delay in VLBI
  • Water vapor monitoring instruments
  • WVR network & WVR inter-comparison campaign
  • WPD retrieval scheme of four European VLBI sites
  • Results
  • Concluding remarks

2/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide3

Tropospheric delay in VLBI

  • Water vapor contents in troposphere are highly variable even in a short period

as well as long period.

  • It causes an unpredictable tropospheric path delay of radio signal propagation.
  • Although its size of 10~30cm is relatively small, water vapor is one of the
  • biggest pending problem in the space geodesy techniques.
  • Especially in VLBI, global scale network is normally used.
  • That means the tropospheric condition of each site is different enough.
  • But it is not enough to get stable 1mm-precision with conventional estimation.
  • We need to find a proper instrument that can be used as monitoring the water

vapor in troposphere directly.

Daily variance of water vapor contents in troposphere

John W. Birks

Elgered (1993)

L = S n ds – G

L = S (n – 1) ds + S - G

3/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide4

Water vapor monitoring instruments

Instruments

Strong points

Weak points

VLBI

+ Vertical distribution

+ Temporal resolution

+ The most direct way

+ Continuous observation

+ Global observation

+ Good resolution for ocean

+ Temporal resolution

+ Continuous observation

+ Free from raining

+ Possible to profiling

  • - Expensive & sporadic observation
  • - Drift while ascending
  • - Spatial resolution
  • - Instrumental calibration
  • - Saturation by dew and rain
  • - IR: Invisible in cloudy condition
  • - Microwave: Land area,
  • Temporal resolution
  • - Vertical distribution
  • - Calibration for absolute IWV
  • Beginning stage

N.A.

N.A.

(Future)

  • Radiosonde
  • Ground- based

WVR

  • Satellite-based

WVR or IR sensor

  • Ground-based

GPS

  • Space-borne GPS

4/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide5

Water vapor absorption model and its observations by WVRs

Westwater et al. (2004)

Elgered (1993)

  • MICAM (WVR Inter-comparison Campaign)
    • Dutch weather service facility in Cabauw
    • Eight WVR, Radar, Ceilometers, Radiosonde
    • Separation btw. WVR: 30m
    • Total freq.: 47 different freq.

5/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide6

European geodetic VLBI & WVR network

Astrid, 20.7/31.4 GHz,

37 sessions

Radiometrics, 23.8/31.4 GHz,

1 session

25 freq., 18.8~25.7 GHz,

1 session

JPL D2, 21.0/31.4 GHz

9 sessions

IEEC, Barcelona (europa.ieec.fcr.es/.../ recerca/gnss/euro_net.gif)

6/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide7

Water vapor sensing instruments collocated at Wettzell

  • Campaign period: April 11~19, 2005
  • Wettzell fundamental station, Germany
  • Instruments
    • 3 ETH series WVR instruments
    • 2 from BKG & 1 from ETH, Zurich
    • 2 Radiometrics
    • 1 from Univ. BW & 1 from TU Dresden
    • Sun spectrometer from ETH Zurich
    • Radiosondes launched with balloons
    • GPS & VLBI
  • VLBI session
    • R1 and R4 analysed by TU-Vienna
    • GPS observations analysed by IGS

7/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide8

WVR WPD retrieval scheme

Step II. Absolute calibration

● Receiver temp. (Trec)

Step I. Raw measurements

● Instrument gain

● Surface meteorological data

● Gain temp. coefficient

● Detector voltages on sky

● THot & TCold

● Spillover correction

● 2.7K CMB

● Linearization of Tb

Step III. WPD retrieval

● Inversion coefficients (GPS)

- Radiometers  PWV or ZIWV

- GPS  WPD

- Relationship btw PWV & WPD

● Inversion coefficients (RS)

- DSS65 & Effelsberg  WPD

●Self inverted WPD

- Onsala60 & Wettzell

● GPS aided calibration

● Locality: Radiosonde

An alternative WVR WPD retrieval scheme

(a plan)

Integrated WVR WPD retrieval scheme

(applied this study)

8/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide9

Geodetic VLBI data processing and analysis

WVR Calibration & Inversion Process

WVR WPD

DBCAL

ZWD

VLBI database

Use

WVR correction?

Yes

SOLVE

No

Dry part: NMF or CFA

Wet part: WVR

Dry part: NMF or CFA

Wet part: Estimation

Standard Sol.

WVR Sol.

Analysis

● WPD residual of SOLVE estimates

● Baseline evolution

● Changes and Concentration of vertical

components of baseline vectors

before/after using WVR corrections

WLSQ

Regression

9/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide10

Results; WPD residuals of SOLVE estimates

10/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide11

Results; Onsala-Wettzell baseline

5.2 ± 17.2 (mm)

-1.6 ± 21.9 (mm)

11/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide12

Results; DSS65-Wettzell baseline

Standard solution

WVR/Resch

WVR/Johansson

-5.8 ± 14.9 (mm)

-33.8 ± 12.8 (mm)

-28.8 ± 18.4 (mm)

12/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide13

Results; Effelsberg

NMF dry model only

NMF dry model + Tahmoush & Rogers

Comparison of vertical components btw standard solution (left) and WVR solution (right)

13/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide14

Results Summary

14/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide15

Concluding remarks

  • Concluding remarks
    • Impacts of adopting WVR WPD as a tropospheric calibration are shown
      • Four WVR data of European geodetic VLBI network are collected
      • Three different kinds of WPD retrieval methods are applied and results are compared
    • Alternative WVR WPD retrieval method is planed
      • New approach with mixture of GPS and WVR for WPD calibration
  • Future Task
    • Verification of the GPS aided WVR WPD calibration

15/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

slide17

Supplementary slides

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide18

Study summary

● European geodetic VLBI network

Operation: 1990~present

Application: Monitoring of local tectonic motion & glacial rebound etc.

● Motive

To check the possibility of improvement in VLBI positioning results

introducing WVR WPD instead of model calibration

● Primary obstacle

Unpredictable water vapor contents in troposphere

● Solution

Theoretical model, Radiosonde, WVR, GPS etc.

● Aim

Check the impact of WVR calibration on the quality of the results of the

European VLBI network and plan generalized WVR WPD retrieval scheme

as a proposal

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide19

Primary error sources of the WVR WPD

Error item

Error item

Error sources

Error sources

Characteristics

Characteristics

Gain error & drift

Offset error

Theoretical brightness temp.

Theoretical opacity

Coefficient error

Different elevation mask btw. stations

Physical obstacle

Radio interference

Inaccurate hydrostatic part

modeling

Unstable behavior of raw data

Drift while observing

Laboratory values; 5~10% error

for 20~32 GHz frequencies

5% of opacity model uncertainty

Non-unique mapping problem

Inconsistent tropospheric delay under 5deg. of elevation mask

Causing site-dependent error

Depending on the precision of surface met. measurements

  • Instrumental

calibration

  • Brightness temp.

modeling

  • WPD retrieval

algorithm

  • Elevation mask
  • Observation

circumstances

  • Model

uncertainty

Primary error sources of the GPS WPD

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide20

Contemporary WVR instruments

Westwater et al. (2004)

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide21

Path Delay from Various Inversion Method I

●Classical inversion coefficients: Resch (1983) & Keihm (1995)

Assuming that 31.4 GHz frequency has only continuum emission

20.7 GHz frequency has water vapor line and continuum

We can get the water vapor component by subtracting scaled 31.4 GHz from 20.7 GHz

Then convert from brightness temp. to PD using scale factor

PD = Cr1 + Cr2 Tb1 + Cr3 Tb2 Madrid and Effelsberg

Tb1: Brightness Temp. for 20.7 GHz, Tb2 : Brightness Temp. for 31.4 GHz

●Include Locality & Seasonal variation: Johansson (1993)

PD = Cj1 [ 1 + Cj2 COS(t – Cj3) – Cj4 (Tb – Cj5) ]  Madrid

t: DOY, Tb = [ (f2/f1)2 Tb1’ – Tb2 – Tbg],

Tb1’: Brightness Temp. for 21.0 GHz, Tbg: Cosmic Background Temp.

●Many-channel inversion method: Tahmoush & Rogers (2000)

Measure spectrum from 18 GHz to 26 GHz in 30 channels with sweeping radiometer

Separate continuum from line emission by fitting a frequency-squared baseline and a

van Vleck-Weisskopf water vapor line profile

PD = Ctr Tb-peak Effelsberg

Tb-peak: Water vapor spectral line intensity at 22.235 GHz

slide22

Path Delay from Various Inversion Method II

●Scale factor using sophisticated atmospheric models: Pardo & Cernicharo

(1988-2005), Liebe (1989)

Models include many atmospheric chemical constituents

Many hundreds of transitions and their Einstein rate coefficients

Multiple layers in atmosphere, each with T, P, partial pressure water vapor,

Cloud liquid water, Aerosols

●Optical depth(): Liljegren (1994)  Investigating

PWV = Cl1 + Cl2b1 + Cl3b2

b1: Brightness Temp. for 23.8 GHz, b2 : Brightness Temp. for 31.4 GHz

+Relationship btw. PWV and PD: Delgado et al.(ALMA MEMO No. 451) 

An idea using PWV from a lot of method using GPS and WVR together

It may can be a generalized WVR WPD retrieval method because almost

every WVR has identical PWV retrieval method. So we can spare time to

get the site-and-instrument dependent WVR WPD retrieval method and

just use simple value of relationship btw. PWV and PD.

For example Wettzell Radiometrics uses the value of 6.50 i.e. PD = 6.5*PWV.

Then we can use GPS PD as a reference PD value. There are so many studies

on proof of GPS PD accuracy and precision compared with WVR PD. So we

can adjust the value compared with WVR PD and GPS PD for each site.

This is my idea but it will be shown as a plan in 2006 IVS meeting.

slide23

Water vapor sensing instruments collocated at Wettzell

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide24

Results; Wettzell

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide25

Results; Onsala60-DSS65 baseline

Standard solution

WVR/Resch model

WVR/Johanssen model

Euro-63

The Onsala60-DSS65 baseline result shows relatively big degradation of WRMS after introducing

WVR data. But we have to note that there are only four sessions included. This means that the

Onsala60-DSS65 result is easily changed by a single value.

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide26

Summary of the multi session results

The UD (Up-Down) components have been computed with respect to the standard solution. Therefore

the reference UD component is set to zero and the other results are reported relative to this. The average

Vertical components are all smaller when WVR data has been used.

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

slide27

Design of low-cost radiometer

  • WP 2600 Description of work
  • Design a low cost microwave radiometer

for automatic, high accuracy LWP

measurement

  • Estimation of cost for different levels of

LWP accuracy

  • Development of a calibration concept to

Guarantee low maintenance

(Rose & Crewell, 2002)

Results

  • Flexible radiometer design
  • Several improvements from MICAM
  • Low maintenance every 3 months

4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006