|Last update: Tuesday, 30-Sep-2008 16:11:53 CEST|
Practical results of Direction Finding method applied on a 4 antenna linear array Wera
byYves Barbin, Pierre Broche, Jean-Charles de Maistre, Philippe Forget et Joël Gaggelli
CNRS & University du Sud Toulon Var, LSEET-LEPI, BP20132, F-83957 LA GARDE, France
A 16 MHz 16 antenna receiving array is spread over a 15Om wide area, which must be well opened to the sea, and "homogeneous" for electromagnetic radiation. Sea surface current measurements might concerned very crowded areas where it is very difficult to find room to install such an array. The deployment time is also a strong limitation when fast and short campaigns are required. The Codar is a very elegant solution for such situations.
Here, we present the accomodation of the WERA radar with a four antenna receiving array (30m wide).
The Direction Finding processing (MUSIC Method in this case) sharpness "compensates" for the lack of angular resolution. At each distance range, the azimuthal localization (Direction of Arrival) is applied for each meaningfull doppler frequency line. The number of sources (one or two in this case) is automatically selected.
For a not too far range distance (good signal level), and depending on radial current diversity, up to one hundred doppler lines feeds the all direction bragg spectral interval. These lines may contain up to 150 primary or secundary sources that are mapped over a 29 cells 5° azimutal grid (-70°,+70°). Some cells ('angular directions') may remain blank (no radial speed value).
The spatial coverage depends on the sea state (mainly the radial
diversity), differential directional attenuation (presence of a hill in
front of the radar), signal to noise ratio, interferences...
A 4 antenna Wera is associated to a classical 12 antenna Wera for the NW Mediterranea (Golfe du Lion) LSEET-LEPI Campaign. Direction finding (DF) radial maps from the first site are combined with Beam Forming (BF) maps from the other site to build current maps that regularly cover more than a thousand square kilometers.
Some possible degraded situations were also investigated (operation on 3 contiguous or not contiguous antenna array, operation on 2 successive antenna only).
The DF method can also be applied to the large 12 antenna network for sharp mapping of current fronts (with an arbitrary 1° grid).