Snodar
Snodar at Dome A
A new site-testing instrument, Snodar (Surface layer NOn-Doppler Acoustic Radar), is designed to measure the height and intensity of the atmospheric boundary layer on the Antarctic plateau. The nature of the atmospheric boundary layer above the Antarctic plateau is of interest both to atmospheric scientists and to astronomers wishing to plan future optical telescopes. Snodar works by sending an intense acoustic pulse into the atmosphere and listening for backscatter off inhomogeneities resulting from temperature gradients and wind shear. The theory of operation is very similar to that of the well known underwater sounding techniques of SONAR.
Data collection began on 10 February 2008, and early indications are that the instrument is working well and providing a clear measurement of the boundary layer thickness. A diurnal variation in the boundary layer is visible.
Snodar is a monostatic acoustic radar with a minimum sampling height of 5m, a range of at least 100m, a vertical resolution of 1m. Snodar operates at frequencies between 5kHz and 15kHz. Such high frequencies propagate relatively well in the low temperature of the Antarctic atmosphere.
A typical day of Snodar data showing a clear boundary-layer height diurnal-variation
Snodar uses a single horn-loaded compression driver as both transmitter and receiver, and a 1.5m off-axis parabolic dish to collimate the acoustic beam. The signal is acquired using a USB sound card. A PC/104 computer performs the signal processing in real time. Snodar runs autonomously, storing raw data on USB flash disks for retrieval the following summer, while uploading processed data via the Iridium satellite network. The Iridium communication also allows Snodar to be remotely controlled.
Snodar dish and compression driver (credit: Colin Bonner)
Snodar is guarded from background noise by a heavily damped enclosure. The inside of enclosure is lined with material that absorbs nearly all the sound frequencies that can interfere with its correct operation.
Colin Bonner with the Snodar enclosure (credit: Michael
Ashley)
Contacts
Primary designer and constructor: Colin Bonner
Principal Investigator: Professor Michael Ashley
Participating institutions in alphabetical order
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University of Auckland, NZ |
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University of New South Wales, Australia |