background and status of the water-vapour radiometer at effelsberg a. roy u. teuber r. keller
TRANSCRIPT
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