Background and Status of the Water-Vapour Radiometer at Effelsberg A. Roy U. Teuber R. Keller

Background and Status of the

Water-Vapour Radiometer at Effelsberg

A. RoyU. TeuberR. Keller

The Troposphere as Seen from Orbit

Method: Synthetic Aperture Radar (Earth Resources Satellite)Frequency: 9 GHzRegion: GroningenInterferograms by differencing images from different days

5 km

5 km

Internal waves in a homo-genously cloudy troposphere

A frontal zone Convective cells

0 mm

-100 mm

100 mm

Hanssen (1997)

Troposphere Seen by VLBI at 86 GHz

Water-Vapour Radiometry Basics

WVR Performance Requirements

Opacity MeasurementAim: measure tropospheric opacity with enough accuracy

to correct visibility amplitude to 1 % (1 )Info: median tropospheric transmission at 22 GHz

at Effelsberg in the period 03Jul to 04Mar was 0.07.WVR spec: absolute calibration accuracy 14 % (1 )

thermal noise per measurement 2.7 K.

Tropospheric Phase CorrectionAim: coherence = 0.9 after correction

requires / 20 path length noise after correction, ie 0.18 mm for = 3.4 mmrequires root-two better at each end of baseline (0.12 mm => 28 mK)

Info: tropo path / tropo brightness = 4.5 mm/K at 22.2 GHztypical water line strength = 35 K

WVR spec: thermal noise 14 mK in 3 sgain stability: 3.9 x 10-4 in 300 s

The Scanning 18-26 GHz WVR for Effelsberg

= 18 to 26 GHz = 900 MHzNchannel = 25Treceiver = 200 K = 0.025 s per channel = 61 mK per channelsweep period = 5 s

The Scanning 18-26 GHz WVR for Effelsberg

Front-end opened

Control unit

First light, April 2002, Bonn

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)

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(Data courtesy Dr. S. Crewell, Uni Bonn)

Gain Calibration

Measure: hot load sky dip at two elevations noise diode on/off

Derive: Tsky Treceiver gain

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

WVR Control Panel

WVR Panorama of Bonn

Move to Effelsberg

March 20th, 2003

WVR Panorama of Effelsberg

Scattered Cumulus, 2003 Jul 28, 1300 UT

Storm, 2003 Jul 24, 1500 UT

Validation of Opacity Measurement

Opacity Statistics at Effelsberg

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

VLBI Path Comparison, 3 mm VLBI, April 2004

VLBI Phase Correction Demo

Demonstration by Tahmoush & Rogers (2000) 3C 273Hat Creek – Kitt Peak86 GHz VLBI

400 s

4 mm

path

● RMS phase noise reduced from 0.88 mm to 0.34 mm after correction.● Coherent SNR rose by 68 %.

VLBI phase

WVR phase

Australia Telescope Phase Correction Demo

● Validate phase correction (3 mm VLBI from 2004 April 16-20)

● Validate zenith total delay using geodetic VLBI (July? campaign)

● Software development: (Rottmann, FP6 RadioNet, started May 3)data paths into CLASS and AIPSdata archiveonline (web-based) real-time display

● Investigate limitations on calibration accuracy

● Hardware development: (once usefulness established)temperature stabilization: further improvementsspillover: reduce with new feed?integration time efficiency: Data acquisition system upgradebeam overlap: move to prime focus receiver boxes

Future Developments

● WVR installation complete; WVR now running continuously

● Data available from Alan Roy

● Opacities agree with those from 100 m RT

● Validation of phase-correction data within weeks

● Web-based display & archive access coming soon

● 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