Introducing VESPA-22: a ground-based microwave spectrometer for
measuring middle atmospheric water vapour at polar latitudes
27 April 2012EGU General Assembly 2012
Pietro Paolo Bertagnolio, Giovanni Muscari, Irene Fiorucci and Massimo Mari
Istituto Nazionale di Geofisica e Vulcanologia, Rome, ItalyDepartment of Earth Sciences, University of Siena
Distributed under Creative Commons Attribution 3.0
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Our goal
To observe changes in the water vapour concentration profile in the stratosphere and mesosphere in the polar regions
Long-term (decadal trends)
Short-term (diurnal cycle)
With a new ground-based microwave spectrometer to measure the 22.235 GHz transition of water vapour as part of the NDACC network
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Main ideas• Balancing technique (signal-reference)• Front-end:
– Parabolic reflector– Uncooled heterodyne receiver– chopper with dielectric sheet– Wobbler for λ/4 shift
• Back-end:– Acqiris FFT spectrometer
• Calibration:– Noise diodes for on-line calibration– LN2 for absolute calibration
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Observation goals Instrument specifications
Observation angle
10°-15° Spectral resolution (B)
61 kHz
Signal-to-noise ratio (SNR)
115 Spectrometer bandwidth
1 GHz
Total integration time (ttot)
12 hrs (1 h if binned)
Antenna beamwidth (HPBW)
3.5°
Altitude range of profiles
20 - 80 km Effective observation time (t / ttot)
40%
Profile accuracy 15% System temperature (Tsys)
≈ 165 K
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Functional scheme
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
VESPA-22 (water Vapor Emission Spectrometer for Polar Atmospheres at 22 GHz)
Parabolic mirror
Choppermirror
Quarter-wavelength shift
Receiver
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Parabolic antenna
• Feedhorn from University of Navarra– Same design as IMK Karlsruhe– Length: 20 cm– HPBW: 12.5°
• Parabolic reflector from Thomas Keating Ltd.– Our design– Long axis: 60 cm– Total HPBW: 3.5°
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Radiation pattern
Half-Power Beam Width (HPBW) = 3.5°Sidelobes < -40 dB below main lobeCross-polarization < -24 dB below main polarization
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Diagrammi di radiazione e fase fra 21.2 e 23.2 GHzCampo lontano (4 m)
Antenna + specchio parabolicoCampo vicino (0.43 m)
Antenna
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Chopper mirror
With small absorber bar With dielectric (DELRIN) sheet
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Chopper performance
Dielectric material Balancing angle Added power wrt zenith
Plexiglas 43°
Small absorber bar
Delrin (2 mm) 28.8° 10.2%
Delrin (5 mm) 8.5%
Delrin (2+2 mm) 43° 5.2%
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Wobbler performance
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Setup di test
14
LNA
Amplifier 2IF Mixer To Back-end
Antenna
Amplifier
LO 1IF
LO 2IF
1IF Mixer
Sidebandfilter
IF filters
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Noise diode calibration
Cold body (LN2) Calibration sources
Hot body𝑇 𝑠−𝑇 𝑅=𝐺 (𝑉 𝑠−𝑉 𝑅 )=𝑇𝑁𝐷
𝑉 𝑁𝐷+𝑅−𝑉 𝑅(𝑉 𝑠−𝑉 𝑅)
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Noise diode calibration
0 5000 10000 15000
0.5
1
1.5
2
x 108
FFT Channels
FF
T C
oun
ts"Raw" Calibration Spectra
Cold Target (77 K)
Hot Target (295 K)Noise Diode 1 (84 K)
Noise Diode 2 (131 K)
Trec = 312 K
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
22.15 22.2 22.25 22.3-2000
-1000
0
1000
2000
3000
Frequency [GHz]
Bri
ghtn
ess
Te
mpe
ratu
re [m
K]
Calibrated Spectrum18-04-2012 13:20-17:20
Effective Integration Time 80'
No baseline subtracted
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
22.225 22.23 22.235 22.24 22.245-100
0
100
200
300
Frequency [GHz]
Brig
htne
ss T
empe
ratu
re [m
K]
Water Vapour Emission Line @ 22.235 GHz18-19/04/2012
Effective Integration Time 3h40'
2 sine waves subtracted
EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – [email protected]/14
Future work (now the fun starts…)• Improve baseline flatness:
– λ/4 wobbler instead of fixed shift– Delrin compensating sheet– Front-end optimization
• Improve sensitivity and Trec– Test single-sideband mixer
• Test with longer integration times from an high-altitude observatory (Gran Sasso)
• Set up inversion algorithm
Conclusions• Long-term monitoring of polar stratospheric water vapour is
needed• We designed and built a new 22-GHz spectrometer for polar
observations• We measured the first atmospheric spectra (“first light”)
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