Snodar
(Surface layer NOn-Doppler Acoustic Radar)
A new instrument to measure the height of the Atmospheric boundary
layer on the Antarctic plateau.
Colin S. Bonner
[email protected]
Michael C.B. Ashley, Jon S. Lawrence, Daniel Luong-Van, John W.V. Storey
What is a SODAR?
S. Bradley, et al “Sodar calibration for wind energy applications,” March 2005.
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Why build a new SODAR?
South Pole 2001…
• We needed it…
• The single purpose of Snodar is to measure the height of
the boundary layer in Antarctica to ±1m.
– Obtaining a calibrated CT2 profile is a bonus.
… Dome C 2003
• Snodar was not intended to measure CV2 or wind
profiles.
– Snodar is truly monostatic and cannot measure CV2,
however it is possible to construct wind profiles.
…and Dome A 2008 ???
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
System overview
16bits
USB PC sound card, 96kHz 24bits.
Class-D power amplifier.
Hardware loop back.
Hardware triggered low
noise solid state switch.
Custom 12-bit programmable gain preamplifier.
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Hardware
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
Operating frequency is a function of atmospheric absorption, scattering
due to turbulence and background noise.
- Background noise  1/f, but unknown at Dome A.
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
An Antisocial instrument?
Snodar
D.M. Howard, J. Angus, “Acoustics and psychoacoustics”. 2006
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
We initially looked at a phased array with inertia-driven Piezoelectric
speakers.
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Acoustics
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Software
The software is simple and runs in user space – not kernel space and
does not require a real-time operating system.
- Uses POSIX threads and ALSA API.
- Hardware loop back allows software synchronization of signals and
hardware relay triggering reduces timing constraints.
- On site data reduction and compression (500x reduction, 2x comp.).
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Performance - Noise
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Performance - Example data
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Calibration
Calibrate Snodar using another instrument, but what?
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Calibration
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Calibration
Calibrate Snodar using another instrument, but what?
Direct in situ measurement of CT2 with differential
thermocouples:
CT2 = <[T(x) – T(x+r)]2> / r2/3
K
x2
<...>
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Calibration
Differential thermocouple
•Measurement BW 0.5 Hz to 300 Hz.
•Type E thermocouples with 13ms time
constant (FW05).
• r = 0.75m
• H = 5.75m
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
Calibration
1.2
1.2
SNODAR
Microthermal
1
Microthermal C2 [k 2 m-2/3]
1
0.8
T
0.6
T
C2 [k 2 m-2/3]
0.8
0.4
0.2
0
0.6
0.4
0.2
0
0
20
40
60
80
Time [Seconds]
100
120
0
0.2
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
0.4
0.6
0.8
SNODAR C2T [k 2 m-2/3]
1
1.2
Noise equivalent CT2
-4
10
-5
10
-6
C2
T
10
-7
10
-8
10
-9
10
-10
10
10
20
30
40
50
60
Height [m]
70
80
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)
90
100
Plans for the future
- More calibration.
- Dome A - again.
- Dome C.
- Possible collaboration with the British Antarctic Survey.
Acknowledgments
We would like to thank Elena Masciadri, Stuart Bradley, the
entire Chinese PANDA traverse team.
Optical Turbulence Astronomy meets Meteorology, 15-18 September 2008, Sardegna (Italy)