Operating Instructions and Installation Instructions

ComNav Vector G2/G2B Installation & Operation Operation

Document PN 29010078 V2.0 - 42 -

DRAFT #3 – 29 May 2009

Moving Base Station RTK

The technique of computing the location of the Secondary GPS antenna with respect to

the Primary antenna, when the Primary antenna is moving, is often referred to as Moving

Base Station Real-Time Kinematic (or as Moving-base-station RTK, or just RTK).

RTK technology is very sophisticated and requires a significant amount of real-time

computation – in essence “searching” for the correct solution, within a large number of

possible solutions, to the mathematical equations which represent the physical

relationships of the G2’s two antennae and each visible satellite.

The Vector G2’s GPS engine’s software is able to place a constraint on the RTK solution,

because the Secondary GPS antenna has a fixed separation of 0.50 metre, inside the

G2’s enclosure, from the Primary antenna.

This separation allows the RTK search (i.e., for the location of the Secondary antenna) to

be constrained to the surface of a sphere with a radius 0.50 metre, centered on the

location of the Primary antenna.

Figure 16 – Secondary Antenna Search Volume

Note: The G2’s Moving-base-station RTK algorithm uses only GPS

signals to calculate heading. No Differential corrections are used in

this calculation, and thus the current DGPS source & signal

conditions never affect heading accuracy.

Determining Speed and Other Navigation Information

The balance of the information that the G2 can output is in general computed from the

position and heading data, or obtained directly from data or other information provided by

the GPS satellites.

For example, speed

is determined from the moment-to-moment changes in the

computed position of the Primary antenna’s phase centre

In contrast, time & date are obtained from data carried on the RF signals from the

satellites.

As well, the G2 can output a wide variety of information on the status of the GPS

satellites – e.g., how many are currently in view – and on its own status.

… more correctly referred to as Speed Over Ground, since it is based on changes in position.

… and thus is less accurate at very slow speeds, and is invalid when the vessel is stationary (see page 60).