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5.4 Satellite-Aided and Autonomous Underwater Navigation
for Ocean Observing
Autonomous surface vehicles and other surface or surface-piercing vehicles such as
gliders and buoys make use of the US Global Positioning System (GPS), a mainstay
for earth surface location and tracking in many applications including ocean observing. The system consists of a satellite constellation orbiting at 20,200 km altitude in
6 orbital planes with 27 operational satellites and additional spare units in orbit.
Each satellite carries multiple highly precise atomic clocks which allow a precise
geographical fix computation upon interrogation of at least three satellites in direct
line-of-sight by a surface platform. Data from additional satellites reduces the
uncertainty perimeter and permits calculation of altitude above the geoid. Since the
GPS satellites do not provide data relay, the autonomous platform must be provided
with alternate telemetry solutions such as cellular networks and/or any of the satellite communications systems cited above. For accurate navigation, technical details
for civilian GPS applications are available online at http://www.gps.gov/technical/
ps/2008-SPS-performance-standard.pdf (US Department of Defense 2008).
The GPS system as noted previously is widely used for navigation by autonomous surface craft, gliders, and free drifting profiling buoys. Moreover, data buoys
are commonly fitted with GPS receivers to confirm that the buoy remains within its
designated watch radius and has not lost its mooring.
For mobile underwater vehicles, radio frequency-based remote navigation aids
such as GPS are precluded because seawater is largely opaque to radio waves.
Inertial navigation, accurate timekeeping plus knowledge of orientation and acceleration in three axes, provides a means for calculating vehicle trajectories in both
time and space. Three-axis fluxgate magnetometer compasses are the mainstay for
determination of instrument orientation within the earth’s magnetic field. These
instruments incorporate orthogonal copper windings around a magnetic core.
Current induced in the windings depends on the orientation within the earth’s magnetic field. Single or multiple three-axis array magnetometer assemblies thus provide accurate readings of the magnetic field. Microelectromechanical devices
incorporate micro-machined cantilevered beams whose deformation provides the
acceleration data. Modern inertial instruments incorporating multiple inertial accelerometers, tilt sensors, and vibrating gyroscopic sensors, usually mounted on a
single board, constitute off-the-shelf inertial measurement systems.
Supplementing these measurements with acoustic means provides additional
navigation and positioning data. Doppler velocity logs incorporate ADCP current
measurements and acoustic bottom tracking as well as inertial navigation systems
providing effective navigation at altitudes (distance from bottom) of up to 200 m.
Bottom mounted acoustic beacons can provide additional reference for precise navigation within defined fields.
5 Signal Conditioning, Data Telemetry, Command Signaling and Platform Positioning…
5.4 Satellite-Aided and Autonomous Underwater Navigation
for Ocean Observing
Autonomous surface vehicles and other surface or surface-piercing vehicles such as
gliders and buoys make use of the US Global Positioning System (GPS), a mainstay
for earth surface location and tracking in many applications including ocean observing. The system consists of a satellite constellation orbiting at 20,200 km altitude in
6 orbital planes with 27 operational satellites and additional spare units in orbit.
Each satellite carries multiple highly precise atomic clocks which allow a precise
geographical fix computation upon interrogation of at least three satellites in direct
line-of-sight by a surface platform. Data from additional satellites reduces the
uncertainty perimeter and permits calculation of altitude above the geoid. Since the
GPS satellites do not provide data relay, the autonomous platform must be provided
with alternate telemetry solutions such as cellular networks and/or any of the satellite communications systems cited above. For accurate navigation, technical details
for civilian GPS applications are available online at http://www.gps.gov/technical/
ps/2008-SPS-performance-standard.pdf (US Department of Defense 2008).
The GPS system as noted previously is widely used for navigation by autonomous surface craft, gliders, and free drifting profiling buoys. Moreover, data buoys
are commonly fitted with GPS receivers to confirm that the buoy remains within its
designated watch radius and has not lost its mooring.
For mobile underwater vehicles, radio frequency-based remote navigation aids
such as GPS are precluded because seawater is largely opaque to radio waves.
Inertial navigation, accurate timekeeping plus knowledge of orientation and acceleration in three axes, provides a means for calculating vehicle trajectories in both
time and space. Three-axis fluxgate magnetometer compasses are the mainstay for
determination of instrument orientation within the earth’s magnetic field. These
instruments incorporate orthogonal copper windings around a magnetic core.
Current induced in the windings depends on the orientation within the earth’s magnetic field. Single or multiple three-axis array magnetometer assemblies thus provide accurate readings of the magnetic field. Microelectromechanical devices
incorporate micro-machined cantilevered beams whose deformation provides the
acceleration data. Modern inertial instruments incorporating multiple inertial accelerometers, tilt sensors, and vibrating gyroscopic sensors, usually mounted on a
single board, constitute off-the-shelf inertial measurement systems.
Supplementing these measurements with acoustic means provides additional
navigation and positioning data. Doppler velocity logs incorporate ADCP current
measurements and acoustic bottom tracking as well as inertial navigation systems
providing effective navigation at altitudes (distance from bottom) of up to 200 m.
Bottom mounted acoustic beacons can provide additional reference for precise navigation within defined fields.
5 Signal Conditioning, Data Telemetry, Command Signaling and Platform Positioning…