Figure 83: Strap-Down Ins Generic Architecture - Applanix POS MV V5 Installation And Operation Manual

POS MV V5 Installation and Operation Guide
Theory
of Operation
The navigation platform, though not a physical entity as was the gimballed platform, exists in the
navigation computer as an IMU-to-navigation frame DCM. For this reason, the geographic navigation
frame is sometimes called the mathematical platform.
Figure 83 shows the functional architecture of a strap-down INS. The arrangement contains an inertial
sensor assembly comprising a triad of accelerometers and a triad of gyros. Each of the three orthogonal
axis of the IMU body frame includes one accelerometer and one gyro.
The inertial sensor processor collects the digitised accelerations and angular rates from the inertial
sensor assembly and compensates these for temperature-dependent sensor errors.
Together, the inertial sensor assembly and the inertial sensor processor comprise the Inertial
Measurement Unit (IMU). In POS MV, the IMU is a separate self-contained unit that interfaces to the PCS
through a data interface cable.
x , , x , ,
y z y z
Gyros
Inertial Sensor Strapdown
Vx , ,
Vx , ,
y z Processor y z Navigator
Accelerometers
Inertial Sensor Inertial Sensor
Assembly Temperatures
Alignment
Algorithm
Inertial Measurement Unit (IMU)
Navigation and Alignment
Processor
Navigation Solution

Figure 83: Strap-Down INS Generic Architecture

The compensated inertial measurements pass to the navigation and alignment processor, which
implements the alignment algorithm and computes the INS navigation solution.
The alignment algorithm initializes the navigation equations, implements levelling and gyrocompassing,
and obtains a mathematical platform alignment.
In POS MV, the navigation and alignment processor is located inside the PCS.
The strap-down INS 'navigates' the inertial sensors and computes the roll, pitch and heading angles of
the inertial sensor assembly relative to the geographic navigation frame.
Generally, the housing of the INS (the IMU in POS MV) is assumed to be aligned with the inertial sensor
frame. With the inertial sensor installed so that its x, y and z axis coincide with the forward, starboard and
down directions of the vessel hull respectively, the INS will measure the vessel roll, pitch and heading
directly. Otherwise, the INS must apply a constant installation offset transformation to the computed roll,
pitch and heading to transform the inertial sensor assembly attitude to the hull attitude.
B-9
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