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

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Chapter 5 / Missile State Observer
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Chapter 5
MISSILE STATE OBSERVER
Centralised, distributed and hybrid architectures, and the advantages and
disadvantages of track and measurement fusion, are discussed. For airdefence
a hybrid system is the natural choice since by necessity the ground
radar is a distributed node of the full state observer. §4 and §5 together
provide the full state observation algorithm. §4 deals with ground tracking,
and §5 continues with the up-link to the centralised state observation in the
missile. Here the up-linked IMM target track and missile plots are fused
with missile sensor measurements.
The sensor measurement characteristics taken from §3, and the up-link
interface are described. The missile filter is a conventional EKF whose state
space, state partitioning, and launcher initialisation, are defined. The
process model is made as simple as possible so that it can be updated using
high frequency IMU measurements for seeker steering and airframe
stabilisation.
The fusion of correlated target tracks uses only the added information
between IMM up-links. Innovations and linearised measurement matrices
are defined for serially processing the remaining sensor data. Latency and
sensor moment arms are not considered at this stage. Moment arms are
essentially deterministic if one ignores error cross-coupling. Similarly,
known time delays can be handled using time stamped data and backward
state propagation when forming measurement innovations.
Pseudo-measurements are used to constrain the process model. This
approach provides flexibility linked to manoeuvre detection using IMM
mode probabilities and multi-rate scheduling that is important in practical
implementations. Quaternion normalisation, missile dynamics (assuming a
full set of normalised aerodynamic derivatives), circular and goal-orientated
PN target trajectories are introduced in this fashion.
Conventional EKFs, although flexible and simple to implement compared
with many other filters, can become ill-conditioned if the measurements are
non-Gaussian. Integrity features commonly used to de-sensitise an EKF
whose tenants have been violated are explored and performance metrics
introduced for use in §9.
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