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The object of the SINS selfalignment
is to determine
the direction cosine of the
transformation from the
body frame to the navigation frame,
namely, the elements of initial attitude
matrix, using the accelerometer
and gyro outputs. As the alignment
accuracy affects the accuracy of the
navigation system directly, one of
the most important requirements of
SINS alignment is high alignment
accuracy. In many practical
applications, SINS alignment also
requires high alignment speed and
the capability of self-determination,
especially for military applications.
At present, the alignment model given
by Bar-Itzhack and Bermant with the
observations of velocity, is widely
used in general stationary SINS selfalignment,
such as fast alignment
proposed by Jiang Cheng Fang and
De Jun Wan, multipositon alignment
presented by Jang Gyu Lee, Chan Gook
Park and Heung Won Park, while it
is not completely observable. In this
model the east gyro drift rate is only
weakly coupled to the velocities which
would serve as external information for
the purpose of alignment. So it is hard
to give more attention to alignment
accuracy and alignment speed during
stationary self-alignment using the
traditional initial alignment methods.
In this paper, a new self-alignment
approach based on the measurement
of IMU is proposed for SINS on
a stationary base. The approach
needn’t to carry out SINS navigation
calculation, thereby, it breaks
away from the model which is not
completely observable. Meanwhile, it
takes full advantage of specific force
and angular velocity information.which is the sensed output of IMU
as well as the characteristic of SINS
stationary alignment. In order to make
sure the high accuracy and speed of
alignment, a new alignment model
which is completely observable, is
established for the approach. On the
other hand, for the snake of selfalignment,
IMU measurement is used
as observations instead of velocities.
The main work of this paper is
following: firstly, coarse alignment
algorithm is presented. Secondly, a
new fi ne alignment model for SINS
stationary self-alignment is derived,
and the observability of the model is
analysed. Then, a modifi ed Sage-Husa
adaptive Kalman fi lter is introduced
to estimate the misalignment angles.
Finally, some computer simulation
results illustrate the effi ciency of the
new approach and its advantages,
such as higher alignment accuracy,
shorter alignment time, more selfcontained
and less calculation.
Coarse alignment
Normally, SINS initial alignment
process is divided into two phases, i.e.,
coarse alignment and fi ne alignment.
The purpose of coarse alignment is to
provide a fairly good initial condition
for the fi ne alignment processing.
For SINS stationary alignment, the
carrier is fi xed to the Earth. And
some characteristics of stationary
alignment are conclude as follow:
1) As a stationary carrier (ven ≡ 0),
the acceleration in navigation
frame (the local-level east, north
and up frame) equals zero, that is
an = 0 (1)
2) The angular velocity of the
body frame with respect to the Earth-Fixed frame also equals
zero, which can be written as
ωeb = 0 (2)
According to the characteristics of
SINS stationary alignment analyzed
as above with the defi nition of
specifi c force (ƒ=a−g), we have
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where g and ωie represent the magnitude
of gravity and Earth rate, respectively,ϕ is the local geographical latitude.
For the SINS, the accelerometer
and gyro output can be
expressed respectively as |
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| February 2007 |
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