1. 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 2 Geodetic Institute of the University of Bonn 3 Max Plank Institute for Radio Astronomy 4 Onsala Space Observatory of Chalmers Technical University 4 th 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

2. Contents 2/15 4 th 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

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

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

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

6. JPL D2, 21.0/31.4 GHz 9 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 6/15 4 th 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 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

8. ● Detector voltages on sky ● T Hot & T Cold ● 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 T b ● Surface meteorological data ● Receiver temp. (T rec ) ● 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) 8/15 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006 WVR WPD retrieval scheme

9. VLBI database WVR WPD Dry part: NMF or CFA Wet part: Estimation Dry part: NMF or CFA Wet 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 WLSQ Regression WVR Calibration & Inversion Process Geodetic VLBI data processing and analysis 9/15 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

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

11. Results; Onsala-Wettzell baseline 11/15 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006 5.2 ± 17.2 (mm)-1.6 ± 21.9 (mm)

12. Results; DSS65-Wettzell baseline Standard solutionWVR/ReschWVR/Johansson 12/15 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006 -5.8 ± 14.9 (mm)-33.8 ± 12.8 (mm)-28.8 ± 18.4 (mm)

13. Results; Effelsberg NMF dry model only NMF dry model + Tahmoush & Rogers 13/15 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006 Comparison of vertical components btw standard solution (left) and WVR solution (right)

14. Status/InstrumentRetrieval methodImpacts 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 = C r1 + C r2 T b1 + C r3 T b2  Johansson( ‘ 93) PD = C j1 [ 1 + C j2 COS (t – C j3 ) – C j4 (T b – C j5 ) ]  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 = C tr T b-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 = C j1 [ 1 + C j2 COS (t – C j3 ) – C j4 (T b – C j5 ) ]  ~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 14/15 4 th IVS General Meeting, Concepcion, Chile, Jan. 9~11, 2006

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

16. Thank you for your attention.

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

18. ● 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 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

19. Primary error sources of the WVR WPD 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006  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 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 Primary error sources of the GPS WPD  Observation circumstances  Model uncertainty Error sources Error item Physical obstacle Radio interference Inaccurate hydrostatic part modeling Characteristics Causing site-dependent error Depending on the precision of surface met. measurements

20. Contemporary WVR instruments 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006 FrequenciesCharacteristicsDeveloper NOAA/ETL Dual-channel Radiometer Dual-channel 20.6 (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-channel 23.8/31.4 GHz Portable WVR Calibration: Noise diode or Tip-cal method Dew blower and moisture detector Radiometrics co., USA TROWARA (Tropospheric Water Vapor Radiometer) Dual-channel 21/31 GHz Continuous observation for IWV and LWP Internal calibration every an hour using Tipping curve IAP, Bern Univ., Swiss MTP5 (Meteorological Temp. Profiler) Single-channel 61 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-channel 5ch.- 22~30GHz 7ch.- 51~59GHz Portable WVR, 32kg. Self correction for frequency drift error Measure infrared temp., T surface, H, and P Radiometrics cp., USA MICCY (Microwave Radiometer for Cloud Cartography) 22-channel 10ch.- over 22.235GHz 10ch.- under 60GHz 2ch.- 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-channel 20~60 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- Wavelength adiometer) 9-channel 18~151 GHz Wideband Thermal infrared Radiometer Equipped Camera and rain-drop sensor as well IAP, Bern Univ., Swiss GSR (Ground-based Scanning Radiometer) 23-channel 50~380 GHz Modification version of WVRs in North pole region New set of thermally stable calibration targets NOAA ETL, USA Westwater et al. (2004)

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 = C r1 + C r2 T b1 + C r3 T b2  Madrid and Effelsberg T b1 : Brightness Temp. for 20.7 GHz, T b2 : Brightness Temp. for 31.4 GHz ● Include Locality & Seasonal variation: Johansson (1993) PD = C j1 [ 1 + C j2 COS(t – C j3 ) – C j4 (T b – C j5 ) ]  Madrid t: DOY, T b = [ (f 2 /f 1 ) 2 T b1’ – T b2 – T bg ], T b1’ : Brightness Temp. for 21.0 GHz, T bg : 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 = C tr T b-peak  Effelsberg T b-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 = C l1 + C l2  b1 + C l3  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. 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006 Water vapor sensing instruments collocated at Wettzell

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

25. Standard solutionWVR/Resch modelWVR/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. Results; Onsala60-DSS65 baseline 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

26. 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. Summary of the multi session results 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006

27. Design of low-cost radiometer 4 th IVS General Meeting Concepcion, Chile, Jan. 9~13, 2006 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)