NORTHROP GRUMMAN NAVITWIN IV, Operation, Installation And Service Manual

Page: 49 / 164

NAVITWIN IV 056360/E
GAS Heading Substitution 3-1
Chapter 3: Grounding Avoidance System (GAS)
The NAVITWIN IV may be incorporated into a grounding avoidance sys-
tem (GAS) according to DNV class notation NAUT-AW requirements.
In this context, “grounding avoidance system” refers to the whole of all
bridge navigation (sub-)systems required to keep the heading distribu-
tion operating in all steering modes in case of failure of the active head-
ing reference.
3.1 GAS Heading Substitution
The NAVITWIN IV’s function within a grounding avoidance system is to
generate a substitute heading which is transmitted to the track control-
ler or INS when the active heading source has failed, in track steering
mode (auto steering mode) and as well in manual steering mode.
The purpose of heading substitution is to prevent an immediate failure
of the track controller or INS as a result of the heading failure.
Once triggered, the heading substitution will continue even when the
operator switches to the manual steering mode to select one of the
remaining heading sources as the active reference.
The GAS mode will only be triggered and a substitute heading provided
to the track controller / INS if all of the following conditions apply:
• GAS heading substitution has been enabled in the NAVITWIN IV ser-
vice setup.
• A gyro / other (true) heading source is active.
• The active source fails (no valid data received).
• A secondary gyro / other (true) heading source is available.
• The difference between the last known heading and the heading of
the secondary source does not exceed the GAS threshold = maxi-
mum heading difference as defined in the NAVITWIN IV service
setup.
3.2 GAS Automatic Switch Over Functionality
In case of a heading failure of the active heading source, the switch over
unit (SOU) is automatically triggered to switch the heading distribution
abruptly to the secondary source and new heading data are immediately
transmitted to all the external devices.
Note GAS heading substitution is intended as a temporary measure, available
in auto steering mode and manual steering mode, for the operator to
gain an overview of the steering situation after a source failure of head-
ing source 1 has occurred.
Note The NAVITWIN IV provides GAS functionality in auto steering mode and
manual steering mode.
An active gyrocompass or other true heading source and another none
active true heading source with a heading below GAS threshold are man-
datory for the GAS functionality.