Instrumentation

18
Inertial Navigation Systems
• In the alignment mode the platform is levelled and aligned (gyro compassed), and when
these processes are complete and READY NAV illuminated. The equipment can now be
switched into the Nav mode, and the aircraft is free to taxi without degrading the accuracy
of the INS.
There may be occasions when full navigation computing is not available but the gyros are
serviceable. This could be in the event of a computing malfunction so that earth rate and
transport wander corrections cannot be calculated or it could be after an alignment failure
in flight.
However, on many aircraft the gyros are used as primary attitude information as well as
for inertial navigation and it may be possible to retain gyro information. This is done by
selecting ATT REF on the MSU.
Selecting ATT REF disconnects computing and loses alignment, if this has not already
happened anyway. The accelerometers now act as gravity switches, as they do during the
levelling phase of alignment and the gyros become gravity-tied in the long term - earth
gyros. The system now gives attitude information and a limited form of heading. The gyros
are normally very accurate, but there is no correction for earth rate and transport wander
and the heading needs to be reset periodically to an independent (usually magnetic) source.
In effect, the gyros are acting as a super-accurate form of DGI and as an attitude indicator.
Should the aircraft electrical supply to the INS cease for any reason the INS will automatically
switch to its own battery pack. For as long as a satisfactory level of power is being supplied by
the internal battery, the INS BATT light will be illuminated on the Control and Display Unit. As
the power from the battery starts to fail, the BATT warning light on the Mode Selector Unit will
illuminate, indicating that the INS is about to fail. If you are half way across the Pacific Ocean
at this time, this could spoil your whole day, since of course the INS cannot be re-levelled and/
or re-aligned in flight (for this the aircraft must be stationary, and the exact position known).
18 Inertial Navigation Systems
The control/display unit (CDU) is shown at Figure 18.17.
The reader who has completed
his or her studies of the radio
syllabus will undoubtedly
notice the similarity between
this CDU and the control/
display panel of a similar
vintage VLF/Omega receiver.
Although the inputs for the
two equipments are vastly
different, the presentation
of navigational information
to the pilot is more or less
identical in both cases.
Figure 18.17 Control and display unit (CDU)
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