Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Chapter 3 / Inertial Navigation
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SDINS using single-axis sensors are simpler to manufacture and maintain
and have generally replaced gimballed INS and dual-axis instruments.
Although autopilot and guidance functions require a low dynamic range,
typically 10 3 comparable with platform systems, for navigation and midcourse
guidance a dynamic range of 10 9 is required, i.e. from 0.01 deg/hr to
400 °/s.
The replacement of mechanical gimbals with SDINS software became
possible once gyroscopes could be mass produced with scale factors small
enough to cope with this wide dynamic range. Low scale factors are
inherent in optical instruments such as Ring Laser Gyroscopes (RLG), Fibre
Optical Gyroscopes (FOG) and resonating gyroscopes whose development
started in the early 60’s. Optical sensors also exhibit excellent linearity,
high bandwidths, insensitivity to high accelerations, and in the case of the
FOG they are relatively low cost and compact. These are dominant factors
in their development and rapid introduction into the aerospace environment,
not the mythical Mean Time Between Failures (MTBF) which is in fact
lower than for equivalent mechanical sensors due to their higher electronic
content which is comparably less reliable.
SDINS are particularly susceptible to rectified noise though its supports up
to 80-100 Hz. This effects short-term performance, an is an error source
often overlooked, and probably least understood, as it involves the local
flexure environment. The reference gyroscope and accelerometer inputs are
considered next in the context of master-slave TFA within a flexible
structure. TFA is the calibration of a low-grade missile INS using the more
accurate launcher INS velocity and attitude output to levels commensurate
the structural flexure. In the absence of flexure i.e. during ground
alignment, the slave INS can be levelled to an accuracy equivalent to the
accelerometer biases.
3.3.1 Reference Inertial Angular Rate
Figure 3-22 shows the frame of references used when determining the
reference input data to two IMUs. The master IMU-1 in the launcher is
referenced to point (u), and the slave IMU2 in the missile to point (m). In
the description of the master-slave IMU sensor inputs some of the
definitions given in §16 have been locally re-defined. The geodetic position
of IMU-1 is denoted by point (d) on the Earth’s surface, shown on the same
equatorial parallel plane as the Alignment frame origin at point (o) for
convenience. The IMU-1 and IMU-2 are aligned with the Launcher Body
frame (B) and the Missile Body frame (M) located at points (u) and (m)
respectively. For aircraft, the Wing and Pylon frames (W) and (P) originate
from point (n) located on the front face of the forward missile support. In
the absence of low frequency wing bending and flexure (W) are coincident
with (B). Pylon axes are defined relative to the wing and account for the
missile dispersion angle excluding harmonisation errors (small errors due to
pylon and rail manufacturing and fitting errors) and high frequency flexure.
3.3-2