The complementary solution
Acoustic and Inertial positioning principles in combination is ideal, since they
have complementary qualities. Acoustic positioning is characterised by relatively
high and evenly distributed noise and no drift in the position, whilst Inertial
positioning has very low short-term noise and relatively large drift in the position
A HAIN computer onboard the vessel interfaces the Inertial Measurement Unit (IMU)
via a serial line, and the Acoustic Positioning system (HiPAP or HPR) via an Ethernet
interconnection. The HAIN position reference system provides:
- Improved acoustic position accuracy The HAIN system will typically improve
the accuracy some 2-3 times. Example: If the "ping to ping" deviation is 6 meters,
the HAIN will reduce this to approximately 2 meters.
- Higher position update rate The HAIN calculates a new position every 1 second regardless of water depth.
- Extends operational depth capabilities Since both the accuracy and the position
update rate are improved, the HAIN allows operation in deeper waters.
- Longer transponder-battery lifetime The HAIN position update rate allows
slowing down the acoustic update frequency. This will result in less "ping"
per hour, and thereby longer battery duration.
- Position update during acoustic drop-out The HAIN gives continuity in position
output, even though the acoustic position should fail to operate in periods
of limited time.
Hydroacoustic Aided Inertial Navigation - HAIN
The HAIN positioning system can be used on any vessel equipped with an acoustic
positioning system. Inertial Measurement Unit - IMU The IMU contains three accelerometers
and three Fibre Optic Gyro that measure the vessel’s accelerations and rotation
in three axes very accurately.
The HAIN computer executes the navigation algorithm, which consists of strap-down
navigation equations and a Kalman filter. The unit is interfaced to an IMU and to
an acoustic positioning operator station. The HAIN computer receives the aiding
positions (latitude and longitude) from the acoustic positioning operator station
and will limit the position drift that is inherent in inertial navigation systems.
Vessel position, attitude, speed and expected accuracy of the data are sent back
to the operator station at a 1 Hz update rate.
The HAIN system is operated from an acoustic positioning operator station. The
information received from the HAIN computer is displayed and sent to external computer(s).
The operator station can request status information from the HAIN computer to be
displayed, which helps the operator to check the system in real-time.
Data logging can be done on two levels:
- The HAIN computer is logging all measurements on its hard disk. These data can be post-processed.
- The APOS can log measured and calculated vessel positions, attitude and velocity on its hard disk.
All measurements and positions in the log files are time stamped. The operator station clock and the HAIN computer clock are both synchronised with the GPS clock (1pps).
HAIN combines the acoustic measurements and the readings from the IMU sensor
installed onboard in an optimum way. The navigation equations update the vessel
position, velocity, heading and attitude almost continuously based on the readings
from the IMU. The Kalman filter corrects these values when new acoustic positions
are available. This gives improved position accuracy compared to the acoustic measurements,
as illustrated in the figure below.
The simulation parameters used in the figure are:
- HiPAP SSBL angle accuracy: 0.1° in x and y
- dGPS position white / coloured noise: 0.15 m / 0.1 m North and East