The WVR at Effelsberg
Download
1 / 43

The WVR at Effelsberg - PowerPoint PPT Presentation


  • 116 Views
  • Uploaded on

The WVR at Effelsberg. Alan Roy Reinhard Keller Ute Teuber Dave Graham Helge Rottmann Walter 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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' The WVR at Effelsberg' - cybil


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

The WVR at Effelsberg

Alan Roy Reinhard Keller

Ute Teuber Dave Graham

Helge Rottmann Walter 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

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 K Measured: 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)




Move to Effelsberg

March 20th, 2003



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





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 mK Sramek (1990), VLA structure functions

95 mK Sault (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 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






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





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



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