Part B | 16.3
392 Part B Autonomous Ocean Vehicles, Subsystems and Control
Latency: some homing methodologies introduce
significant latency to the relative position measurement. This increases the ambiguity of the current
state of the vehicle, and delays the vehicle’s ability
to correct for disturbances. In many cases, latency
is introduced by signal processing inside the sensor
rather than the physics of the sensing technology.
An accurate navigation system onboard the AUV
can be particularly useful for docking, allowing the vehicle to navigate between homing system updates, to
compensate for latency, and to detect environmental influences like currents.
16.3.1 Acoustic Homing
Acoustic signals propagate over long distances under
water and, consequently, are the basis of homing for
a number of docking systems. Numerous acoustics navigation methodologies exist, and are often grouped by
the size of the transducer array required to achieve
a position fix, e.g., long baseline, short baseline, and
ultrashort baseline navigation systems. The most commonly used configurations are transponding in nature,
in that one system (usually the vehicle) emits an interrogation ping, and the other system (usually the dock)
replies. Range is determined by measuring the round
trip travel time of the acoustic signal. Practical homing systems used by AUVs have acquisition distances
ranging from a fraction a kilometer to many kilometers.
The performance depends on both the characteristics of
the acoustic homing systems, principally its operational
frequency, and the nature of the environment.
The physics of sound in the underwater environment creates a range of challenges. These include the
speed of sound, sound refraction due to sound speed
variations, interactions of sound with the seafloor and
sea surface, ambient noise, and the absorption properties of seawater [16.27]. In complex environments,
like shallow water, reverberation and the time varying
nature of the ocean surface and interior may cause individual pulses in the received signal to fade or amplify
from ping to ping. In signal processing terms, the channel impulse response function is both complicated and
time varying. The net effect is that measured properties
of a received ping, for example, time of arrival of the
leading pulse, may appear to jump in a non-Gaussian
way [16.28]. This is a well-known problem for undersea
acoustic navigation systems, and a variety of methods
have been developed, ranging from methods to reject
the jumps [16.29] to other approaches that seek to use
the complex nature of the arrival as a probe of the undersea environment sound speed environment [16.30].
Most often, however, the appearance of a multi-path as
the separation between source and receiver increases,
which becomes yet another factor limiting the useful
range of the system.
The update rate and latency of an acoustic system
homing are bounded by the finite speed of sound underwater. For the case when the AUV interrogates the
dock, and the dock replies, the round-trip for a vehicle
at 1:5 km is approximately 2 s. While the latency cannot be improved, the update rate can be increased by
simply accelerating the update rate as the vehicle gets
closer to the dock. After all, a fast update rate is really
only necessary in the terminal stages of homing.
Ultra-Short Baseline Acoustic Systems
Ultra-short baseline (USBL) acoustic homing systems
make a phase-sensitive measure of the difference in
time of arrival of an acoustic pulse at several hydrophones [16.31]. The difference in time of arrival can
be used, in turn, to compute the direction of propagation
of the pulse. The phase-sensitive nature of USBL allows
comparatively small hydrophone arrays, employing hydrophone separations less than half a wavelength of
the pulse. The small size of the hydrophone array,
for example, less than 7:5 cm for a 10 kHz signal, allows practical systems with arrays that could fit in
the palm of a person’s hand. By combining USBL
with standard acoustic ranging methods, a vehicle carrying a small USBL can measure the distance and
direction to a dock equipped with a transponder. Consequently, a wide number of AUV homing systems have
used USBL as the primary homing sensor, including
Light [16.32], Singh et al. [16.5], Stokey et al. [16.8], de
Goede et al. [16.33], Allen et al. [16.34], and McEwen
et al. [16.2].
USBL system integration on an AUV can require
careful calibration. Commercial USBL arrays are usually characterized in acoustic facilities to quantify the
as-built performance of the array. It is not unusual to
require an additional calibration step once the USBL
system is mounted on a vehicle, and even to require the
addition of materials like acoustical absorbing foam to
ensure adequate performance [16.35]. Sources of error
include acoustic refraction by any enclosure used for
the USBL array, as well as interfering acoustic reflections from vehicle components.
Short Baseline Systems
In contrast to ultra-short baseline systems, short baseline systems (SBL) dispense with the phase comparison
and only determine arrival time difference. For monofrequency pulse detection systems the electronics are
simple, most often an in-band energy detector of a hardlimited acoustic signal. However, the accuracy of such
arrival-time schemes are not as good as phase-sensitive
392 Part B Autonomous Ocean Vehicles, Subsystems and Control
Latency: some homing methodologies introduce
significant latency to the relative position measurement. This increases the ambiguity of the current
state of the vehicle, and delays the vehicle’s ability
to correct for disturbances. In many cases, latency
is introduced by signal processing inside the sensor
rather than the physics of the sensing technology.
An accurate navigation system onboard the AUV
can be particularly useful for docking, allowing the vehicle to navigate between homing system updates, to
compensate for latency, and to detect environmental influences like currents.
16.3.1 Acoustic Homing
Acoustic signals propagate over long distances under
water and, consequently, are the basis of homing for
a number of docking systems. Numerous acoustics navigation methodologies exist, and are often grouped by
the size of the transducer array required to achieve
a position fix, e.g., long baseline, short baseline, and
ultrashort baseline navigation systems. The most commonly used configurations are transponding in nature,
in that one system (usually the vehicle) emits an interrogation ping, and the other system (usually the dock)
replies. Range is determined by measuring the round
trip travel time of the acoustic signal. Practical homing systems used by AUVs have acquisition distances
ranging from a fraction a kilometer to many kilometers.
The performance depends on both the characteristics of
the acoustic homing systems, principally its operational
frequency, and the nature of the environment.
The physics of sound in the underwater environment creates a range of challenges. These include the
speed of sound, sound refraction due to sound speed
variations, interactions of sound with the seafloor and
sea surface, ambient noise, and the absorption properties of seawater [16.27]. In complex environments,
like shallow water, reverberation and the time varying
nature of the ocean surface and interior may cause individual pulses in the received signal to fade or amplify
from ping to ping. In signal processing terms, the channel impulse response function is both complicated and
time varying. The net effect is that measured properties
of a received ping, for example, time of arrival of the
leading pulse, may appear to jump in a non-Gaussian
way [16.28]. This is a well-known problem for undersea
acoustic navigation systems, and a variety of methods
have been developed, ranging from methods to reject
the jumps [16.29] to other approaches that seek to use
the complex nature of the arrival as a probe of the undersea environment sound speed environment [16.30].
Most often, however, the appearance of a multi-path as
the separation between source and receiver increases,
which becomes yet another factor limiting the useful
range of the system.
The update rate and latency of an acoustic system
homing are bounded by the finite speed of sound underwater. For the case when the AUV interrogates the
dock, and the dock replies, the round-trip for a vehicle
at 1:5 km is approximately 2 s. While the latency cannot be improved, the update rate can be increased by
simply accelerating the update rate as the vehicle gets
closer to the dock. After all, a fast update rate is really
only necessary in the terminal stages of homing.
Ultra-Short Baseline Acoustic Systems
Ultra-short baseline (USBL) acoustic homing systems
make a phase-sensitive measure of the difference in
time of arrival of an acoustic pulse at several hydrophones [16.31]. The difference in time of arrival can
be used, in turn, to compute the direction of propagation
of the pulse. The phase-sensitive nature of USBL allows
comparatively small hydrophone arrays, employing hydrophone separations less than half a wavelength of
the pulse. The small size of the hydrophone array,
for example, less than 7:5 cm for a 10 kHz signal, allows practical systems with arrays that could fit in
the palm of a person’s hand. By combining USBL
with standard acoustic ranging methods, a vehicle carrying a small USBL can measure the distance and
direction to a dock equipped with a transponder. Consequently, a wide number of AUV homing systems have
used USBL as the primary homing sensor, including
Light [16.32], Singh et al. [16.5], Stokey et al. [16.8], de
Goede et al. [16.33], Allen et al. [16.34], and McEwen
et al. [16.2].
USBL system integration on an AUV can require
careful calibration. Commercial USBL arrays are usually characterized in acoustic facilities to quantify the
as-built performance of the array. It is not unusual to
require an additional calibration step once the USBL
system is mounted on a vehicle, and even to require the
addition of materials like acoustical absorbing foam to
ensure adequate performance [16.35]. Sources of error
include acoustic refraction by any enclosure used for
the USBL array, as well as interfering acoustic reflections from vehicle components.
Short Baseline Systems
In contrast to ultra-short baseline systems, short baseline systems (SBL) dispense with the phase comparison
and only determine arrival time difference. For monofrequency pulse detection systems the electronics are
simple, most often an in-band energy detector of a hardlimited acoustic signal. However, the accuracy of such
arrival-time schemes are not as good as phase-sensitive