3.2 Attitude data from INS
Attitude data, i.e. the roll-, pitch- and yaw- angles of the
sensor body axis to the terrestrial fixed coordinate system can
be directly measured by an inertial navigation system (INS)
belonging to the equipment of every larger aircraft. The eva-
luation of INS measurements is in most cases very difficult and
depends essentially on the physical model which describes the
systematic influences of a large number of physical effects.
The analysis of INS data presented in this chapter examine only
the registered attitude angles. Any systematic drift effects
caused by remaining errors in the gyros of the INS are not
taken into consideration. For this reason the filtered atti-
tudes cannot be estimated as absolute values, but must be rec-
tified for the systematic drifts. Then the estimated accuracies
of the measured angles are understood as the precision of one
reading of the gyro.
The data for the analysis come from two different dynamic
applications of INS. In the first example the attitude data of
an INS from a NASA Space Shuttle mission are presented. The
analysed data are registered during the metric camera experi-
ment of the ESA/NASA's D1 Spacelab mission on 2nd December
1983. The registrations took place every 2 seconds, the angles
are measured in degrees. The attitudes refer to the earth-fixed
Greenwich True of Date system. The registrations were subdi-
vided according to 21 photo-strips consisting between 120 and
800 points. As the estimated accuracies are very similar bet-
ween the strips, only the root mean square values from all the
strips are listed in table 4.
Table 4: INS attitude data from Space Shuttle
Estimated standard deviations in [deg]
YAW PITCH ROLL
observation noise On 0.0060 0.0048 0.0060
ARI-model errors oce 0.0019 0.0012 0.0014
filtered data ox 0.0027 0.0027 0.0031
Correlation coefficients of filtered data d-2 sec
r(1d) 0.71 0.62 0.67
r (2d) 0.46 0.39 0.47
r{ 3d) 0.18 0.10 0.15
r (4d) 0.13 013 0.13
The mean ARI process order in this case is (6,1). The estimated
accuracies are not very different between the three angles, so
that a common measurement precision can be assumed. The estima-
ted autocorrelation coefficients show a decrease within the
first five seconds.
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