Instrumentation

Inertial Navigation Systems
18
Inherent Errors
The irregular shape and composition of the earth, the movement of the earth through space
and other factors provide further possible sources of error. Such errors vary from system to
system depending upon the balance achieved between accuracy on one hand and simplicity of
design, reliability, ease of construction and cost of production on the other.
INS Control and Display Panels
There are many makes and models of INS currently on the market. The “state-of-the-art”
trend is towards a single control/display unit with a standard keyboard, but with a single small
video screen (rather than the various individual LED windows which are shown in the following
illustrations). With the modern video screen presentation, the loading and extraction of
information is achieved by selecting a “page number”, with each page (which is displayed on
the screen) dealing with associated functions. One major advantage of this type of system is
that hundreds or even thousands of waypoints can be “stored” in the machine memory. These
waypoints (normally airway reporting points such as VORs and intersections) are automatically
loaded from a master disc, which is supplied and regularly updated by specialist agencies.
Because of the high capital investment which was involved in the last generation of INS
systems, and because they are proving to be extremely reliable, you are perhaps more likely to
encounter the traditional type of control/display units described below. Another good reason
for considering this system, rather than the modern one, is that the JAA examination questions
are based on the older type of INS.
Figure 18.16 Mode Selector Unit
In any event, please appreciate that the following paragraphs are intended only as a general
guide and not as a definitive operating instruction for any particular model of INS.
Inertial Navigation Systems 18
The traditional INS system employs two panels for control and display. The simpler of the two,
the mode selector panel, is shown at Figure 18.16.
The function of the mode selector panel is straightforward:
• In the standby mode the power is supplied to all parts of the system. It is normal to insert
the start position (the aircraft’s ramp position in lat/long to the nearest tenth of a minute
of arc) whilst the equipment is in this mode.
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