The antennas are located on the surface and rotate with respect to a
source in the sky due the rotation of the earth. For aperture synthesis the
antenna positions are specified in a co-ordinate system such that the
separation of the antennas is the projected separation in plane normal
to the phase center. In other words, in such a co-ordinate system the
separation between the antennas is as seen by the observer sitting in
the source reference frame. This system, shown in Fig 10.1,
is the right-handed () coordinate system fixed on the surface of the
earth at the array reference point, with the (
) plane always
parallel to the tangent plane in the direction of phase center on the
celestial sphere and the
axis along the direction of phase center.
The
axis is along the astronomical E-W direction and
axis is
along the N-S direction. The (
) co-ordinates of the antennas are
the E-W and N-S components of position vectors. As the earth rotates, the
(
) plane rotates with the source in the sky, changing the
(
) coordinates of the antennas, generating tracks in the
-plane.
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In the above formulation, the co-ordinate of one antenna is with
respect to the other antenna making the interferometer, which is
located at the origin. If the origin is arbitrarily located and the
co-ordinates of the two antennas are
and
,
Eq. 10.1.3 becomes
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(10.1.5) |
Since only the relative positions of the antennas with respect to each
other enter the equations, it is only useful to work with difference
between the position vectors of various antennas in the (
co-ordinate system. The relative position vectors are called
``Baseline vectors'' and their lengths referred to as ``baseline
length''.
The source surface brightness distribution is represented as a
function of the direction cosines in the () coordinate system.
In Eq. 10.1.4 above,
is the direction cosine. The source
coordinate system, which is flat only for small fields of view, is
represented by (
). Since this coordinate system represents the
celestial sphere,
is not an independent coordinate and is
constrained to be
.