1. Cryocooled Sapphire Oscillator Frequency Standards for the shortest VLBI Wavelengths Maria Rioja, Richard Dodson Yoshiharu Asaki John Hartnett Steven Tingay (or improving sensitivity by reducing coherence losses)

2. 1.Why need to improve frequency standard? 2.Description of Simulation Studies 3.Comparative Performance: Coherence losses for H-maser and CSO 4. Other Strategies to improve sensitivity: 4.1 WVR (co-located independent technique), 4.2 Frequency Phase Transfer (FPT) (simultaneous dual frequency observations) Contents

3. 2 H-maser G ood Weather V ery Good =ALMA-type weather VW=WVR@ALMA Why? The Quest for Sensitivity… More Stable Cryocooled Sapphire Oscilator Trop phase fluctuations  site with stable weather conditions H-maser instabilities  ultra stable Cryogenic Sapphire Oscilator (CSO)Clock

4. 3 Hartnett & Nand, 2010 Hartnett et al. 2012 Ultra-stable Cryocooled Sapphire Oscillator (CSO)

5. GenerateSynthetic with ARIS Dataset GEO Source/ antenna/ errors Trp/Ion Error TRP Fluctuation CLOCKFrequency (GHz) Source: Point Strong Ion Fluct: Nominal errors Single freq: 86 175 350 Array: -VLBA -EHT EOP Trp error: 3 cm Ion error: 6 TECU - VW - V very good - G good - T typical - P poor -CSO -H-maser Dual freq: 43 / 86 87 / 175 175 / 350 Simulations: Parameter Space (Asaki+2007)

6. (86 GHz, Good Weather,) (Worse weather)

7. Analysis with AIPS Self-Calibration (SC) X11 Frequency Phase Transfer (FPT) (Dual Freq.) X11 FPT + SC = Hybrid X11 (x 11) Solint: 0.1, 0.2,… 6 minutes MAP (x 11) Simulations: Data Analysis

8. Flux loss 4% Flux loss 20% Uncompensated residual phase fluctuations leads to Flux loss. Use Flux loss as a measure of coherence loss for comparative studies.

9. RESULTS:CLOCK noise only, all freq. H-maser CSO

10. RESULTS:CLOCK noise only, all freq. 0% 0.5% 10% 40% 86 GHz 175 GHz 350 GHz CSO 0% H-maser

11. RESULTS:CLOCK noise only, all freq.RESULTS: ATM noise only, all weathers, all freq. ASD_V=3*ASD_VW ASD_G = 2*ASD_V ASD_T = 2*ASD_G ASD_P = 2*ASD_T V G VW

12. RESULTS: ATM noise only, all weathers, all freq. 20% 86 GHz 80% G V P T VW

13. RESULTS: ATM noise only, all weathers, all freq. 175 GHz 50% 80% 20% 86 GHz 80% G V P T VW

14. RESULTS: ATM noise only, all weathers, all freq. 175 GHz 80% 20% 86 GHz 80% G V P T 350 GHz 20% 80% VW

15. 14 SUPERIMPOSED H-Maser vs. ATM noise, all weathers, all freq (zoomed). 10% 86 GHz 175 GHz 350 GHz H-maser Significance of H-maser noise Expected to increase at highest frequency (350 GHz) and with best quality weather conditions (V,VW); the CSO noise remains negligible in all Circumstances. VW V G P T

16. 15 RESULTS : CLOCK (H-maser/CSO-100MHz) + ATM (Very Good), all freq. 2% change 20% 86 GHz 6% change 175 GHz 350 GHz 20% change + CSO x H-maser Comparative Performance CSO Significant Benefit (i.e. increased sensitivity) @ 350 GHz with V quality weather conditions.

17. 16 INTERPRETATION of RESULTS: SENSITIVITY + H-maser + CSO Thermal only 20% increase sensitivity with CSO wrt H-maser @ 350 GHz, V weather

18. 17 86 GHz 20% 175 GHz 350 GHz RESULTS: CLOCK (H-maser/CSO-100MHz) + ATM (VW), all freq. 2% change 10% change 40% change + CSO x H-maser CSO Very Significant Benefit (i.e. increased sensitivity) @ 350 GHz with VW quality weather conditions.

19. 18 RESULTS : CLOCK (H-maser/CSO-100MHz) + ATM (G), all freq. 86 GHz175 GHz 350 GHz 20% 1% change 3% change 8% change + CSO x H-maser Comparative Performance CSO moderate benefit (i.e. increased sensitivity) @ 350 GHz with G quality weather conditions.

20. 19 +CSO, 8% IMPROVEMENTS WRT H-maser, G weather, @350 GHz (G trop. loss) H-maser+WVR, 50% +CSO+WVR, 70% Other Strategy(1): WVR to “upgrade” weather quality (G tropospheric loss) (V tropospheric loss, H-maser loss) (V tropospheric loss)

21. 20 0-5% 20% Hybrid analysis: FPT @low freq (0.5’) + SC@high freq (3’, 6’). FTP: Use Low Freq. Analysis to Guide High Frequency (“disciplined phases”). FPT & Hybrid Analysis, Very Good Weather FPT & Hybrid Analysis, Good Weather (43x2  ) 86GHz (87x2  ) 175GHz (175x2  ) 350GHz Other Strategy(2): Multi Frequency Observations + FPT analysis 86 GHz 175,350 GHz 86, 175 GHz 350 GHz Extended (hours!) coherence Time at all frequencies also with G Quality weather conditions.

22. Master Title21 Summary The stability of typical H-masers introduce significant coherence losses at submm wavelengths. Most noticeable in very best weather conditions. A CSO based frequency standard for submm VLBI benefits from superior stability which results in Increased coherence time. Our estimates are 20% increase in sensitivity at 350GHz with “Very Good” (i.e. ALMA-type) weather conditions; along with WVR, 40% increase is possible. WVR have the potential to upgrade `Good’ sites into `VeryGood’ sites, ideal for submm observations (maximum benefits along with CSO). Including Freq. Phase Transfer has great potential to increase coherence time (i.e. sensitivity) at submm wavelengths - requires dual frequency observations.