ComNav Vector G2/G2B Installation & Operation Operation
Document PN 29010078 V2.0 - 41 -
DRAFT #3 – 29 May 2009
The Vector G2B can also use:
• Conventional terrestrial DGPS Radiobeacon signals.
Marine authorities in many countries around the world have installed networks of
Radiobeacon transmitters, which broadcast DGPS corrections valid in the vicinity
of each such transmitter.
As well, there is an increasing number of DGPS Radiobeacon transmitters
located well inland, which are usable for non-marine applications.
The G2B has a dual-channel Beacon receiver, able to operate in manual or
automatic tuning mode, or a database mode; the Radiobeacon receiver is always
operating, regardless of the DGPS mode of operation.
When using valid differential corrections, from any of the possible sources, the G2 can
improve its position computation to 1 metre or better accuracy.
Determining Heading
Satellite visibility & signal quality are again important to how well the G2 operates, in
computing heading. Signals from all visible satellites are used – even any not being used
in the position computations described above.
In other words … the more satellites which are visible, the better!
The Vector G2’s GPS engine uses both the data carried on the RF signal from GPS
satellites, and measurements of the RF carrier wave’s phase, to compute the location of
the Secondary GPS antenna in relation to the Primary GPS antenna, with a very high
(sub-centimetre) level of precision.
Each of the GPS receivers measures the phase of the RF signal carrier wave, at its own
antenna, from each of the GPS satellites it can see in the sky. The phase of the RF
waves at each receiver’s antenna depends on the distance, and the horizontal & vertical
angles, between the satellite and that individual antenna (this can be demonstrated easily
with a simple sketch of the physical geometry relationships).
The G2’s computed heading is the direction from the phase centre of the Primary
antenna to the Secondary antenna’s phase centre.
A “heading arrow” is embossed on the bottom of the G2’s enclosure. The Secondary
antenna is located directly above the arrowhead; the Primary antenna is at the other end.
That means that the arrow points in the direction of the computed heading. This will in
turn correspond to actual heading of the vessel, when the G2 is installed on (or is parallel
to) the vessel’s fore-aft centreline.
When the G2 is installed athwartships, the heading computation is still from the Primary
to the Secondary antenna, but then user-set configuration parameters (see Roll
Alignment, pa
ge 58) a
re applied, to adjust for the ±90° corre
ction required, so that the
heading data output correctly corresponds to the vessel’s actual heading.
Similarly, if the G2 is not installed exactly parallel to the vessel’s centreline – or not quite
athwartships – a user-set heading offset parameter is applied.
Note: because the actual phase of the RF waves from the satellites is
used, reflections of those waves from nearby surfaces can have a
significant adverse effect on the heading performance of the G2
The effect can be thought of as being similar to the “ghost” effect (a
faint copy of the image, shifted slightly on the screen) that can
sometimes be seen on over-the-air TV signals from distant stations.
On the G2, such “ghosting” – from reflections – can cause the
heading computation to become inaccurate, or to fail completely.
L That is why it is so important to mount the G2 as far away as
possible from “reflecting surfaces” (see page 18).
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