The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008
Therefore the H 2x 4 in Equation (10) is
[0^(1
0
[Q y x(l-%)-<p y Yr ff -i%)
0
(13)
The stochastic model of the Kalman filter and the parameters
for its tuning are designed according to the sensors’
specifications and the system configuration in the field tests.
(m/s 2 )
Bias<12mg,
DGPS
(Novatel RTK)
20
Hz
N, E, Alt
(m);
V xv , V z (m/s)
-2 cm
-0.05 m/s
Optic Flow
50
Hz
q q
(deg/s)
Scale <2%
LRF
25
Hz
LR (m)
scale error <1%
offset <10cm
Table 1. Data collected for the integrated navigation system
4. SYSTEM CONFIGURATION
4.1 UAV (Helicopter) Platform
Experiments were conducted on an RMAX unmanned
helicopter manufactured by Yamaha. It has 30kg payload with
endurance of approximately one hour, and comprising a
conventional helicopter design with a single main rotor and a
single tail-rotor for anti-torque compensation.
An autopilot system is devised for conducting closed loop
experiments, in which a control by telemetry approach is taken.
The sensor information is sent to a ground computer using a
radio link, which processes the sensor information, calculates
corresponding control inputs and sends these back to the
helicopter via another radio link. An onboard PC 104 computer
based on the Pentium III chipset is for vision processing
(Garratt, 2007).
4.2 Sensors
The sensors onboard the helicopter includes DGPS, INS
magnetometer, LRF and CCD camera (imaging payload). Their
locations are indicated in Figure 3. The data collected for the
proposed integrated GPS/INS/Vision navigation system are
listed in Table 1.
Figure 3. UNSW@ADFA RMAX in flight
Sensors
Data
Rate
Parameters
Accuracy
INS
(IMU700CB)
50
Hz
p q r (deg/s)
a x a v a z
Gyro: Scale <2%,
Bias <20deg/hr;
Acce: Scale <1%
An analogue Sony CCD camera is used as the image sensor.
The camera outputs a PAL standard composite video signal
which is captured by the frame grabber. The inertial sensors for
the RMAX were pre-fitted with an isolation system, including
three gyroscopes and three accelerometers with orthogonal axes.
Highly accurate carrier phase DGPS measurements were
available to provide a monitoring system to record the
helicopter motion during closed loop. Novatel OEM4-G2L GPS
cards are mounted adjacent to the RMAX flight computers. The
OEM4 cards are used with differential corrections from a
nearby base station fitted with another OEM4 card. The card
operates in a Real-Time Kinematic (RTK) positioning mode; to
provide 20Hz position and velocity with an accuracy of l-2cm.
Helicopters are dynamically unstable and require constant
control inputs to prevent them from diverging from a level
flying attitude. Accurate knowledge of attitude (pitch, roll and
yaw) is therefore vital to robust control. The RMAX helicopter
is fitted with an in-house system for attitude determination
(Garratt, 2007).
Figure 4. Laser scanning system
A LRF with a novel rotating mirror was integrated with the
RMAX helicopter as shown in Figure 4.1. Due to the
orientation of the axis of the mirror and the angle of the mirror
to the axis, the laser traces an elliptical cone shape on the
ground below. To obtain a sufficiently fast scan rate, the mirror
is spun at least 1500RPM or 25 cycles per second. The
rangefinder and encoder signals are read into a PC 104 with a
sample rate of 2 KHz. An AccuRange 4000 LRF from Acuity is
used for this project, with range accuracy of each point on the
ground is better than 2cm (Garratt et al., 2007)
4.3 Data Processing
The data are processed in integrated GPS/INS/Vision navigation
system described in Section 3. Two Kalman filters (KF) are