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A generalized scheme to retrieve wet path delays

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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A generalized scheme to retrieve wet path delays

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. Results; WPD residuals of SOLVE estimates 10/15 4th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

  11. 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

  12. 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

  13. 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

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

  15. 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

  16. Thank you for your attention.

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

  18. 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

  19. 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

  20. Contemporary WVR instruments Westwater et al. (2004) 4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

  21. 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

  22. 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.

  23. Water vapor sensing instruments collocated at Wettzell 4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

  24. Results; Wettzell 4th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

  25. 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

  26. 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

  27. 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

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