The WVR at Effelsberg

The WVR at Effelsberg

Alan Roy Reinhard KellerUte Teuber Dave GrahamHelge Rottmann Walter AlefThomas Krichbaum

The Scanning 18-26 GHz WVR for Effelsberg

= 18.5 GHz to 26.0 GHz = 900 MHzChannels = 24Treceiver = 200 Ksweep period = 6 srms = 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

Ethernet data acquisition system Temperature regulation modules Control unit

March 16th, 2004

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 mmNeed: 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 mKNeed: absolute calibration 14 % (1 ) Measured: 5 %

WVR View of Atmospheric Turbulence

Absorber Zenith sky(clear blue, dry, cold)

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

19 to 26 GHz

el = 90 ◦ to 0 ◦

dete

ctor

ou

tpu

t

0 V

to 0

.3 V

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 GHzbeamwidth = 1.26◦

18.0 GHzbeamwidth = 1.18◦

WVR Beam Overlap Optimization

WVR – 100 m RT Beam Overlap forthree WV profiles

Atmospheric WV Profiles atEssen from Radiosonde launches every 12 h(courtesy Dr. S. Crewell, Uni Cologne)

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 2003186 mK per channel on absorber, scaled to 25 channelsdifference 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 mK typical elevation pointing jitter is 0.1◦

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

Beam mismatch: 145 mK measured by chopping with WVR between two sky positions with 4◦ throw, Aug 20034◦ = 120 m at 1.5 km and el = 60◦

66 mK to 145 mK Sramek (1990), VLA structure functions95 mK Sault (2001), ATCA 2001apr27 1700 UT

Other ? Spillover model errors, cloud liquid waterremoval, 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

Tantenna = 23 KTmoon = 220 K at 22 GHzBeam filling factor = 0.114

Beam efficiency = 92 %

23 K

Spillover Reduction

19 to 26 GHz19 to 26 GHz

el = 90 ◦ to 0 ◦

dete

ctor

ou

tpu

t

0 V

to 0

.3 V

WVR Path Data from 3 mm VLBI, April 2004

Time / UT hours

18 3024 36 42

Path

length

/ m

m

0

30

60

90

120

150

180

210

90°

45°

0°

Ele

vati

on

path length

elevation

VLBI Path Comparison, 3 mm VLBI, April 2004

VLBI Phase Correction Demo

NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17

420 s

3.4 mm

path

● Path rms reduced 1.0 mm to 0.34 mm● Coherent SNR rose 2.1 x

WVR phase

VLBI phase

No phase correction

EB phase correction

Coherence function before & after

EB+PV phase correction

VLBI Phase Correction Demo

NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17

420 s

3.4 mm

path

● Path rms reduced 0.85 mm to 0.57 mm● Coherent SNR rose 1.7 x

WVR phase

VLBI phase

No phase correction

EB phase correction

Coherence function before & after

VLBI Phase Correction Demo

NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17

420 s

3.4 mm

path

● Path rms saturated at 0.95 mm● Coherent SNR decrease 7.5 x

WVR phase

VLBI phase

Before phase correction at EB

After phase correction at EB

Coherence function before & after

VLBI Phase Correction Demo

NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17

● Coherence improves for most scans

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

0.0

1.0

2.0

Coherent integration time

Impro

vem

ent

fact

or

360 s240 s120 s0 s

Cloud Removal

NRAO 15086 GHz VLBI2004 April 17

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

EB WVR path time series Keep VLBI scan times only Subtract linear rate

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

0 mm

2 mm

1 mm

0 h 24 h 24 h0 h

UT UT

sun

rise

sun

rise

sun

set

sun

set

Absolute Calibration for Astrometry & Geodesy

120 km

Opacity Effects and the Mapping Function

Issues: TCP/IP time overhead

Issues: Temperature stability

20 mK

Physical temperature near LNA vs time

Tsys vs time

250 mK

3 min

Issues: Temperature stability

Solution: weaken thermal coupling between Peltier and RF plate

Effects: No more 3 min temperature oscillation Worse long-term temperature stability

Strong thermal coupling

Weak thermal coupling

Temperature vs time

Temperature vs time

5.5 C0.7 C

2.5 days

0.75 days

Issues: Noise Diode Stability

Calibrated with noise diode

Calibrated with temperature

Original data

Structure function of Tsys on absorber

Tsy

s rm

s /

K

0.1 K

1 K

Time / s100 1000 10000

Tsys vs time on absorber Calibrate using temp. Calibrate using noise diode

2.0 K

22 h

Issues: Beam Mismatch at Low Elevation?

● Software development: (Helge Rottmann, RadioNet)data paths into JIVE correlator, AIPS and CLASSimprove calibration accuracy (allow for opacity effects)

● Hardware development: temperature stabilization: better insulation, regulationreduce Tsys? Cooling?spillover: reduce with new feed?integration time efficiency: Data acquisition system upgradebeam overlap: move to prime focus receiver boxes?

Future Developments

● 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

Conclusions