The WVR at Effelsberg
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The WVR at Effelsberg. Alan RoyReinhard Keller Ute TeuberDave Graham Helge RottmannWalter Alef Thomas Krichbaum. The Scanning 18-26 GHz WVR for Effelsberg.  = 18.5 GHz to 26.0 GHz D  = 900 MHz Channels = 24 T receiver = 200 K sweep period = 6 s

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The WVR at Effelsberg

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The WVR at Effelsberg

Alan RoyReinhard Keller

Ute TeuberDave Graham

Helge RottmannWalter Alef

Thomas Krichbaum


The Scanning 18-26 GHz WVR for Effelsberg

 = 18.5 GHz to 26.0 GHz

D = 900 MHz

Channels = 24

Treceiver = 200 K

sweep period = 6 s

rms = 61 mK per channel

Features

 Uncooled (reduce cost)

 Scanning (fewer parts, better stability)

 Robust implementation

(weather-proof, temperature stabilized)

 Noise injection for gain stabilization

 Beam matched to Effelsberg near-field beam

 TCP/IP communication

 Web-based data access

 Improved version of prototype by Alan Rogers


The Scanning 18-26 GHz WVR for Effelsberg


The Scanning 18-26 GHz WVR for Effelsberg

Front-end opened

March 16th, 2004

Ethernet data acquisition system

Temperature regulation modules

Control unit


WVR Performance Requirements

Phase Correction

Aim:coherence = 0.9

requires   / 20 (0.18 mm rms at  = 3.4 mm) after correction

Need: thermal noise  14 mK in 3 s Measured: 12 mK = 0.05 mm

Need:gain stability 3.9 x 10-4 in 300 s Measured: 2.7 x 10-4

Opacity Measurement

Aim: correct visibility amplitude to 1 % (1 )

Need:thermal noise  2.7 KMeasured: 12 mK

Need:absolute calibration  14 % (1 )Measured: 5 %


WVR View of Atmospheric Turbulence

Zenith sky

(clear blue, dry, cold)

Absorber

12 h

1 h

● gain stability: 2.7x10-4 over 400 s

● sensitivity: 61 mK for τint = 0.025 s

(0.038 mm rms path length noise for τint = 3 s)


Typical Water Line Spectrum


WVR Panorama of Bonn


Move to Effelsberg

March 20th, 2003


WVR Panorama of Effelsberg


Spillover Cal: Skydip with Absorber on Dish

detector output

0 V to 0.3 V

el = 90◦ to 0◦

19 to 26 GHz


Gain Calibration

Measure: hot load

sky dip at two elevations

noise diode on/off

Derive: Tsky Treceiver gain


WVR Beamwidth: Drift-Scan on Sun

26.0 GHz

beamwidth = 1.26◦

18.0 GHz

beamwidth = 1.18◦


WVR Beam Overlap Optimization

Atmospheric WV Profiles at

Essen from Radiosonde

launches every 12 h

(courtesy Dr. S. Crewell, Uni Cologne)

WVR – 100 m RT

Beam Overlap for

three WV profiles


Scattered Cumulus, 2003 Jul 28, 1300 UT


Storm, 2003 Jul 24, 1500 UT


First Attempt to Validate Phase Correction


WVR Noise Budget for Phase Correction

Thermal noise: 75 mK in the water line strength, April 2003

186 mK per channel on absorber,

scaled to 25 channels

difference on-line and off-line channels

(34 mK in Feb 2004 due to EDAS hardware & software upgrade)

Gain changes: 65 mK in 300 s 2.7x10-4 multiplies Tsys of 255 K

Elevation noise:230 mKtypical elevation pointing jitter is 0.1◦

sky brightness gradient = 2.8 K/◦ at el = 30◦

Beam mismatch:145 mKmeasured by chopping with WVR between

two sky positions with 4◦ throw, Aug 2003

4◦ = 120 m at 1.5 km and el = 60◦

66 mK to 145 mKSramek (1990), VLA structure functions

95 mKSault (2001), ATCA 2001apr27 1700 UT

Other?Spillover model errors, cloud liquid water

removal, RFI, wet dish, wet horn

Total (quadrature):290 mK = 1.3 mm rms


Move to Focus Cabin

March 16th, 2004


WVR Beam Geometry

Beam overlap, April 2003

Beam overlap, April 2004


Optical Alignment using Moon

23 K

Tantenna = 23 K

Tmoon = 220 K at 22 GHz

Beam filling factor = 0.114

Beam efficiency = 92 %


Spillover Reduction

detector output

0 V to 0.3 V

el = 90◦ to 0◦

19 to 26 GHz

19 to 26 GHz


WVR Path Data from 3 mm VLBI, April 2004

210

180

150

120

path length

Path length / mm

90

90°

60

elevation

Elevation

45°

30

0

18

24

30

36

42

Time / UT hours


VLBI Path Comparison, 3 mm VLBI, April 2004


VLBI Phase Correction Demo

NRAO 150

Pico Veleta - Effelsberg

86 GHz VLBI

2004 April 17

No phase correction

VLBI phase

WVR phase

EB phase correction

path

3.4 mm

Coherence function before & after

EB+PV phase correction

● Path rms reduced 1.0 mm to 0.34 mm

● Coherent SNR rose 2.1 x

420 s


VLBI Phase Correction Demo

NRAO 150

Pico Veleta - Effelsberg

86 GHz VLBI

2004 April 17

No phase correction

VLBI phase

WVR phase

EB phase correction

path

3.4 mm

Coherence function before & after

● Path rms reduced 0.85 mm to 0.57 mm

● Coherent SNR rose 1.7 x

420 s


VLBI Phase Correction Demo

NRAO 150

Pico Veleta - Effelsberg

86 GHz VLBI

2004 April 17

Before phase correction at EB

VLBI phase

WVR phase

After phase correction at EB

path

3.4 mm

Coherence function before & after

● Path rms saturated at 0.95 mm

● Coherent SNR decrease 7.5 x

420 s


VLBI Phase Correction Demo

Coherence function after phase correction at EB divided by CF before phase correction

NRAO 150

Pico Veleta - Effelsberg

86 GHz VLBI

2004 April 17

2.0

Improvement factor

1.0

0.0

0 s

120 s

240 s

360 s

Coherent integration time

● Coherence improves for most scans


Cloud Removal

EB WVR path time series

Keep VLBI scan times only

Subtract linear rate

● Cloud contamination shows up as large scatter in the path lengths

NRAO 150

86 GHz VLBI

2004 April 17


VLBI Phase Correction Demo


VLBI Phase Correction Demo


Validation of Opacity Measurement


Path Length & Opacity Statistics at Effelsberg


Path Length Stability at Effelsberg

RMS path fluctuation over 120 s

vs hour of day - July

RMS path fluctuation over 120 s

vs hour of day - December

2 mm

1 mm

0 mm

0 h

24 h

0 h

24 h

sunset

sunrise

UT

sunrise

sunset

UT


Absolute Calibration for Astrometry & Geodesy

120 km


Opacity Effects and the Mapping Function


Issues: TCP/IP time overhead


Issues: Temperature stability

Physical temperature near LNA vs time

20 mK

3 min

Tsys vs time

250 mK


Issues: Temperature stability

Solution: weaken thermal coupling between Peltier and RF plate

Effects: No more 3 min temperature oscillation 

Worse long-term temperature stability 

Weak thermal coupling

Temperature vs time

Strong thermal coupling

Temperature vs time

0.7 C

5.5 C

0.75 days

2.5 days


Issues: Noise Diode Stability

Tsys vs time on absorber

Calibrate using temp.

Calibrate using noise diode

2.0 K

22 h

Structure function of Tsys on absorber

1 K

Original data

Calibrated with noise diode

Tsys rms / K

Calibrated with temperature

0.1 K

Time / s

100

1000

10000


Issues: Beam Mismatch at Low Elevation?


Future Developments

● Software development: (Helge Rottmann, RadioNet)

data paths into JIVE correlator, AIPS and CLASS

improve calibration accuracy (allow for opacity effects)

● Hardware development:

temperature stabilization:better insulation, regulation

reduce Tsys?Cooling?

spillover: reduce with new feed?

integration time efficiency:Data acquisition system upgrade

beam overlap: move to prime focus receiver boxes?


Conclusions

● WVR running continuously

● Phase correction of 3 mm VLBI has been demonstrated

(but in four experiments WVR made things worse.)

● Opacities agree with those from 100 m RT

● Zenith wet delays agree with GPS & radiosonde within 10 mm

● Web-based display & archive access available

●Radiometer stability is 2.7 x 10-4 in 400 s

● Radiometer sensitivity is 61 mK in 0.025 s integration time

http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html


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