ComNav Vector G2/G2B Installation & Operation Operation
Document PN 29010078 V2.0 - 40 -
DRAFT #3 – 29 May 2009
GPS Signal Reception & Processing
The G2 contains a high performance GPS “engine”, containing twin GPS receivers and
two multipath-resistant antennas. One antenna is designated as the Primary GPS
antenna, while the other is designated as the Secondary. In the G2B, there is also a
Radiobeacon antenna (located with the Primary GPS antenna).
The G2 tries to find four or more GPS satellites in the visible sky, and then uses the
Radio Frequency signals it receives from those satellites, and the GPS timing & ranging
information carried on the signals, to compute its current position, heading, speed, and
other Navigation information.
There are two main factors that affect how well the G2 operates:
• Satellite visibility … “a good view of the sky”
• Satellite signal quality
Determining Position
The Global Positioning System is essentially a timing-based position triangulation
system.
The G2’s GPS receivers automatically and continuously try to receive RF signals from all
GPS satellites that are visible in the sky. To determine its own geographic position, the
G2’s GPS receivers use a complex algorithm incorporating satellite locations and ranges
to each satellite. The ranges are calculated by determining how long it takes for the RF
signals transmitted from each visible GPS satellite to reach the G2’s two antennae, using
information carried on those RF signals.
If the signal from a visible satellite is of sufficient quality (i.e., strong enough, and with low
noise & errors), the G2’s receivers are able to decode the information on the RF signals,
and thus to calculate the range to that satellite.
Reception of good-quality signals from at least four GPS satellites allows the G2’s GPS
receivers to compute accurate, high-precision 3-dimensional position coordinates.
Positions computed by the G2 are based on the RF phase centre of the Primary antenna
(see page 41).
Differential Corrections
There is al
wa
ys some error in GPS positions, from any brand of GPS receiver. Errors
can be caused by many things: Selective Availability, atmospheric effects, timing errors
and satellite orbit perturbations are some examples.
An ordinary GPS receiver is often specified to be accurate only to about 10 metres for
position, because of such errors. But the Vector G2 is more sophisticated than ordinary
GPS receivers, and so is inherently much better than that – 2.5 metres or less, when
operating in what is referred to as Autonomous mode.
But, to improve the accuracy of position computations even more, the G2 can also
acquire and use Differential GPS (DGPS) correction data, from one of several possible
sources:
• Space-Based Augmentation System (SBAS) satellites, if the G2 is located where
one (or more) operational SBAS satellites are visible to it, and if the SBAS data
being transmitted by those satellite(s) is valid for the G2’s location.
The G2 automatically scans for and tracks SBAS satellites. It has two-channel
tracking capability, which enhances its ability to maintain a lock on a specific
SBAS satellite when more than one satellite is in view, and thus provides more
consistent tracking.
Current SBAS systems that the G2 can use include WAAS, EGNOS and MSAS;
other compatible systems may be operational in future – SNAS, GAGAN, etc.
• An external data source sending RTCM SC-104 DGPS data to the G2 (via its
RS-232 Port B or Port A).
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