Post on 13-Jan-2016
description
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