a generalized scheme to retrieve wet path delays from water vapor radiometer measurements applied to...

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 Astronomy4Onsala Space Observatory of Chalmers Technical University

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

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

Contents

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

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

L = S n ds – G L = S (n – 1) ds + S - G

Elgered (1993)

Tropospheric delay in VLBI

John W. Birks

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


Water vapor monitoring instruments

Radiosonde

Ground- based WVR

Satellite-based WVR or IR sensor

Ground-based GPS

Space-borne GPS

Radiosonde

Ground- based WVR

Satellite-based WVR or IR sensor

Ground-based GPS

Space-borne GPS

Strong points Strong points Instruments Instruments

+ 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

+ 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

Weak points Weak points

- 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

- 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

VLBI

N.A.

N.A.

(Future)


Elgered (1993)

Water vapor absorption model and its observations by WVRs

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.

Westwater et al. (2004)


JPL D2, 21.0/31.4 GHz9 sessions

Radiometrics, 23.8/31.4 GHz,1 session

Astrid, 20.7/31.4 GHz, 37 sessions

25 freq., 18.8~25.7 GHz, 1 session

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

European geodetic VLBI & WVR network


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


● Detector voltages on sky ● THot & TCold

● Instrument gain

● Inversion coefficients (RS) - DSS65 & Effelsberg WPD

● Self inverted WPD - Onsala60 & Wettzell

● Locality: Radiosonde

● Gain temp. coefficient

Step I. Raw measurements

Step II. Absolute calibration

Step III. WPD retrieval

● Linearization of Tb

● Surface meteorological data

● Receiver temp. (Trec)

● Spillover correction

● 2.7K CMB

Integrated WVR WPD retrieval scheme(applied this study)

● Inversion coefficients (GPS) - Radiometers PWV or ZIWV - GPS WPD - Relationship btw PWV & WPD

● GPS aided calibration

An alternative WVR WPD retrieval scheme(a plan)


WVR WPD retrieval scheme

VLBI database

WVR WPD

Dry part: NMF or CFA Wet part: Estimation

Dry part: NMF or CFAWet part: WVR

Standard Sol. WVR Sol.

Use WVR correction?

DBCAL

SOLVE

ZWD

No

Yes

Analysis● WPD residual of SOLVE estimates ● Baseline evolution● Changes and Concentration of vertical components of baseline vectors

before/after using WVR corrections

WLSQRegression

WVR Calibration & Inversion Process

Geodetic VLBI data processing and analysis


Results; WPD residuals of SOLVE estimates


Results; Onsala-Wettzell baseline


5.2 ± 17.2 (mm) -1.6 ± 21.9 (mm)

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)

Results; Effelsberg

NMF dry model only

NMF dry model + Tahmoush & Rogers


- 60

- 50

- 40

- 30

- 20

- 10

0

- 60

- 50

- 40

- 30

- 20

- 10

0

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

Status/Instrument Retrieval method

Impacts

DSS65 First operation: WAVEFRONT(‘96) JPL D2 type, 21.0/31.4 GHz Advanced WVR was developed for more precise atmospheric calibration Euro-session No. : 9 (‘99~ )

Resch(‘83) PD = Cr1 + Cr2 Tb1 + Cr3 Tb2

Johansson(‘93) PD = Cj1 [ 1 + Cj2 COS

(t – Cj3) – Cj4 (Tb – Cj5) ]

20~30 mm reduction in vertical Concentration was changed Resch: -2 mm Johansson: 3 mm Small changes in baseline-rate & WRMS

Effelsberg

First operation: Dec. ‘04 25 channel WVR: 18 GHz ~ 26 GHz Mounted on top of the Antenna Euro-session No. : 1 (‘05~ )

Tahmoush & Rogers(‘00 ) PD = Ctr Tb-peak

~15 mm increment in vertical Reference: Effelsberg

Onsala60

First operation: ‘90 Astrid, 20.7/31.4 GHz Konrad WVR was developed for meteorological project Euro-session No. : 37 (‘90~ )

Johansson(’93) PD = Cj1 [ 1 + Cj2 COS

(t – Cj3) – Cj4 (Tb – Cj5) ]

~7 mm reduction in vertical concentration was degraded ~5 mm Small changes in baseline-rate & WRMS

Wettzell First operation: ‘97 (ETH series) WVR comparison campaign: Apr. ’05 Chosen WVR instrument: Radiometrics, 23.8/31.4 GHz Euro-session No. : 1 (‘05~ )

Radiometrics self-inversion program

Investigating

Results Summary


Concluding remarks

Future Task Verification of the GPS aided WVR WPD calibration

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


Thank you for your attention.

Supplementary slides

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

● 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

Study summary


Primary error sources of the WVR WPD


Instrumental calibration

Brightness temp. modeling

WPD retrieval algorithm

Elevation mask

Error sources Error item

Gain error & drift Offset error

Theoretical brightness temp.Theoretical opacity

Coefficient error

Different elevation mask btw. stations

Characteristics

Unstable behavior of raw dataDrift 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

Primary error sources of the GPS WPD

Observation circumstances

Model uncertainty

Error sources Error item

Physical obstacle Radio interference

Inaccurate hydrostatic partmodeling

Characteristics

Causing site-dependent error

Depending on the precision of surface met. measurements

Contemporary WVR instruments


Frequencies Characteristics Developer

NOAA/ETLDual-channel Radiometer

Dual-channel20.6 (23.87) and 31.65 GHz

• Less affected by rain drops• Internal calibration using three switches named Hach• Tip-cal calibration once per week

NOAA ETL, USA

MWR(Microwave Radiometer)

Dual-channel23.8/31.4 GHz

• Portable WVR • Calibration: Noise diode or Tip-cal method• Dew blower and moisture detector

Radiometrics co., USA

TROWARA(Tropospheric Water Vapor Radiometer)

Dual-channel21/31 GHz

• Continuous observation for IWV and LWP• Internal calibration every an hour using Tipping curve

IAP, Bern Univ., Swiss

MTP5(Meteorological Temp. Profiler)

Single-channel61 GHz

• Measure temperature from the surface to 600m altitude• Solid-state Dicke type super heterodyne receiver (1KHz)• Bandwidth: 2GHz

Attex co., Russia

MWP(Microwave

Profiler)

12-channel5ch.- 22~30GHz 7ch.- 51~59GHz

• Portable WVR, 32kg.• Self correction for frequency drift error• Measure infrared temp., Tsurface, H, and P

Radiometrics cp., USA

MICCY(Microwave

Radiometer for Cloud

Cartography)

22-channel10ch.- over 22.235GHz 10ch.- under 60GHz2ch.- around 90GHz

• Single-sideband total power radiometer• Heterodyne receiver filter bank design• Internal calibration using highly stable noise diode

MIUB, Germany

HATPRO(Humidity and Temp. Profiler)

14-channel20~60 GHz

• Total power radiometer that can detect directly to receiver• Each receiver & frequency are designed as filter bank• Flexible channel bandwidth• Reducing IF interference: high stability and accuracy

Radiometer Physics GmbH

ASMUWARA(All-Sky Multi-Wavelengt

h adiometer)

9-channel18~151 GHz

• Wideband Thermal infrared Radiometer• Equipped Camera and rain-drop sensor as well

IAP, Bern Univ.,Swiss

GSR(Ground-based

Scanning Radiometer)

23-channel50~380 GHz

• Modification version of WVRs in North pole region• New set of thermally stable calibration targets

NOAA ETL, USA

Westwater et al. (2004)

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 continuumWe can get the water vapor component by subtracting scaled 31.4 GHz from 20.7 GHzThen convert from brightness temp. to PD using scale factorPD = 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 radiometerSeparate continuum from line emission by fitting a frequency-squared baseline and a van Vleck-Weisskopf water vapor line profilePD = Ctr Tb-peak Effelsberg Tb-peak: Water vapor spectral line intensity at 22.235 GHz

Path Delay from Various Inversion Method II

● Scale factor using sophisticated atmospheric models: Pardo & Cernicharo (1988-2005), Liebe (1989)

Models include many atmospheric chemical constituentsMany hundreds of transitions and their Einstein rate coefficientsMultiple layers in atmosphere, each with T, P, partial pressure water vapor,Cloud liquid water, Aerosols

● Optical depth(): Liljegren (1994) InvestigatingPWV = 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.


Water vapor sensing instruments collocated at Wettzell

Results; Wettzell


- 30.0

- 20.0

- 10.0

0.0

10.0

20.0

30.0

40.0

Up East North

Medicinawith WVR

- 100.0

- 80.0

- 60.0

- 40.0

- 20.0

0.0

20.0

Up East North

Nyales20with WVR

- 10.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Up East North

Onsala60with WVR

- 20.0

- 10.0

0.0

10.0

20.0

30.0

40.0

Up East North

Svetloewith WVR

Standard solution WVR/Resch model WVR/Johanssen model

Euro-63

The Onsala60-DSS65 baseline result shows relatively big degradation of WRMS after introducingWVR data. But we have to note that there are only four sessions included. This means that theOnsala60-DSS65 result is easily changed by a single value.

Results; Onsala60-DSS65 baseline


The UD (Up-Down) components have been computed with respect to the standard solution. Thereforethe reference UD component is set to zero and the other results are reported relative to this. The averageVertical components are all smaller when WVR data has been used.

Summary of the multi session results


Design of low-cost radiometer


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

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)

a generalized scheme to retrieve wet path delays from water vapor radiometer measurements applied to...

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