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 – pietropaolo.bertagnolio@ingv.it2/14

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 – pietropaolo.bertagnolio@ingv.it3/14

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 – pietropaolo.bertagnolio@ingv.it4/14

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 – pietropaolo.bertagnolio@ingv.it5/14

Functional scheme

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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 – pietropaolo.bertagnolio@ingv.it7/14

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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 – pietropaolo.bertagnolio@ingv.it9/14

Radiation pattern

Half-Power Beam Width (HPBW) = 3.5°Sidelobes < -40 dB below main lobeCross-polarization < -24 dB below main polarization

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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 – pietropaolo.bertagnolio@ingv.it11/14

Chopper mirror

With small absorber bar With dielectric (DELRIN) sheet

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it12/14

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 – pietropaolo.bertagnolio@ingv.it13/14

Wobbler performance

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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 – pietropaolo.bertagnolio@ingv.it15/14

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 – pietropaolo.bertagnolio@ingv.it16/14

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 – pietropaolo.bertagnolio@ingv.it17/14

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 – pietropaolo.bertagnolio@ingv.it18/14

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 – pietropaolo.bertagnolio@ingv.it19/14

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”)