Geoscience Reference
In-Depth Information
Acquisition, reception and pre-processing units are mounted on a suspended frame inside a
protective metal container (60 cm, 50 kg). A PC controls the radar acquisition, being
synchronized to UTC time through a GPS or ethernet connexion, and is used for real-time
visualization of Doppler spectra and data storage. The 23 elements' square array antenna is
mounted on a tripod adjustable for site and azimuth, and can be easily dismantled for
transport. The 3 dB beam width is 9°, equivalent to site and azimuth resolutions of about 160
m at 1 km. The 300 W power consumption is provided through a small electric generator or
AC. Owing to its modularity and limited weight (~70 kg), the ensemble is easily
transportable, fits in a 4WD vehicle, and can be set up quickly in a volcanic environment.
This radar can thus be used for short-term scientific campaigns, as well as over the long
term for monitoring purposes.
A number of settings have been designed to be selectable to best adapt to the activity and
the sounding conditions. The pulse duration is selectable from 0.4 to 1.5 s so that the range
bin radial resolution can be chosen between 60 and 225 m according to the target
dimensions and the type of information searched for. The pulse repetition frequency can be
50, 100 or 200 s. The non ambiguous maximum range at a 100 microsecond repetition
frequency is 12 km. The gain attenuation can be varied by 50 dB through 10 dB steps to best
adapt to the eruption intensity. The format of the data stored on the PC hard disk can be
chosen in order to adjust the space memory consumption to the duration of the record
campaign: either the time series of the raw digitized signal can be recorded, i.e. after
coherent integrations in the time-domain, or alternatively only the spectra are saved, i.e.
after integrations in the frequency domain.
4.2 Echoing mechanism
A powerful short radio frequency pulse (duration ) is periodically transmitted into the
atmosphere through a switch and a directive antenna which concentrates the energy in a
narrow beam. Just after the pulse transmission, the switch connects the antenna to a radio
frequency receiver. If targets are located in the antenna beam, part of the pulse energy is
backscattered toward the antenna. These radar echoes are fed via a switch to the receiver for
amplification and filtering. At the receiver output, the electromagnetic signal is detected and
converted into digital data which are then processed and recorded.
Like in the case of atmospheric sounding, two main mechanisms give rise to radar echoes in
the case of volcanic targets (Sauvageot, 1992; Doviak and Zrnic', 1993; Dubosclard et al.,
1999): (i) Rayleigh (
D
< /4 ) or Mie scattering (
D
/4) from distributed targets, and (ii)
Bragg scattering from spatial irregularities of the refractive index induced by turbulent
eddies inside the hot jet, and supposedly of secondary importance in the volcanic case
because of the large reflectivity of tephra. In addition to the distance of the sounded
volumes, the radar reflectivity () is deduced from the intensity of the echo signal by using
the radar equation:
PC
r
(1)
r
r
2
where
P
r
is the echo power measured by the radar receiver and
C
r
a constant including the
radar parameters such as transmitted power, pulse duration, wavelength, antenna
