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Underwater Acoustic Navigation with the WHOI Micro-Modem
Sandipa Singh∗, Matthew Grund∗, Brian Bingham†, Ryan Eustice‡, Hanumant Singh∗, Lee Freitag∗
∗Woods Hole Oceanographic Institution
Woods Hole, MA 02543 USA
†Franklin W. Olin College of Engineering
Needham, MA 02492 USA
‡Johns Hopkins University
Baltimore, MD 21218 USA
Abstract— The WHOI Micro-Modem is a compact, low-power
acoustic transceiver that can provide both acoustic telemetry and
navigation. Its size and versatility make it ideal for integration in
autonomous underwater vehicles (AUVs). The modem supports
the use of both broadband and narrowband transponders for long
baseline navigation systems, has a modem-to-modem ranging
capability, and can be configured to provide synchronous one-
way ranging, when integrated with a precision clock. This paper
gives an overview of the different navigation systems supported
by the Micromodem and presents the results from field tests
conducted on the SeaBED AUV in deployments in Greece, the
Bluefin AUV, and whale localizations in the Stellwagen Bank
Marine Sanctuary.
I. INTRODUCTION
Acoustic navigation is a vital aspect of underwater ve-
hicle operations. The WHOI Micro-Modem, in addition to
providing high-rate underwater acoustic communications, also
provides signaling which is useful to an underwater vehicle’s
navigation system. It can be commanded to interrogate both
narrowband and broadband transponders, and provides travel
time information. These travel times can then be used to
derive vehicle position. In addition, the modem has a one-way
ranging capability derived from every communications packet,
when provided with an external clock reference, providing an
efficient telemtry and ranging solution.
This paper can be broadly divided into two sections. Sec-
tion II gives an overview of the system software, focusing on
the detector. It then goes on to briefly describe narrowband
and broadband LBL navigation, ranging and synchronous
navigation. Section III will deal with results from various
recent experiments.
II. SYSTEM DESCRIPTION
The Micro-Modem’s real-time embedded operating system
is implemented on a low-power fixed-point digital signal
processor, the Texas Instruments TMS320C5416 DSP. When
receiving, the analog input signal is sampled with a 12-bit A/D
converter. Passband data are demodulated and then passed to
a matched-filter detector. The detector can be configured to
process four separate signals simultaneously within the same
band. The detection scheme uses two configurable thresholds.
The noise threshold is used to reject impulse noise, while the
detection threshold is used to detect a peak from the matched
filter output which is normalized by the broadband noise power
Fig. 1. The SeaBED AUV, developed at Woods Hole Oceanographic
Institution, uses the Micro-Modem for telemetry and navigation. It currently
can perform narrow-band LBL ranging, and acoustic telemetry synchronous
ranging, which is used to correct its DVL asssisted dead-reckoning. SeaBED
is designed to make high resolution photo mosaics, do multi-beam bathymetry
and also carries a suite of chemical and oceanographic sensors.
estimate. This detector can be configured for a wide variety of
signals that can be used to support several navigation schemes.
The detector resolution is 125 microseconds.
All communications packets in the Micro-Modem are pre-
ceded by an FM sweep of 10 ms duration and a bandwidth
of 4 kHz. Table II-B shows some performance metrics for the
sweep in differing environments.
A. Broad band REMUS-based LBL navigation
The Micro-Modem can be used to interrogate up to four
REMUS transponders at center frequencies of 24 kHz and
25 kHz, using broadband PSK codes. On receiving replies, it
publishes one-way travel times which can be used to calculate
position using an active navigation solution. The interrogation
frequency is 26 kHz at a bandwidth of 4 kHz.
TABLE I
OBS ERVED DET ECTO R PERFO RMAN CE IN VARIOUS UNDERSEA
ENVIRONMENTS TO-DATE
Center frequency Channel (depth) Range
10 KHz 200m shelf 3.2 km
15 KHz 2200m offshore 3.6 km
25 KHz 10m Very shallow water 2 km
25 KHz 3m Surf Zone 800m
B. Narrow-band LBL navigation
The Micro-Modem also supports narrow-band transponders
which are used by the SeaBED, JASON and ABE vehicles.
It can be tuned to frequencies ranging from 10 kHz to 30
kHz, depending on the transponders’ characteristics. It can be
also configured to listen silently without interrogation using
an external reference for calculating travel times.
The travel-times measured by the modem can be used to
calculate position using active as well as passive navigation
schemes [1]. Active navigation uses fixed transponder loca-
tions as well as range from the unknown position to each
transponder. With three or more range nodes, a linear least
squares estimator is used. However with two nodes, the fix is
ambiguous and external information is utilized to choose be-
tween two possible solutions. The passive navigation scheme
uses time-difference of arrivals and transponder positions to
solve a linear system of equations for the unknown position.
This solution can be iteratively refined using a system based
on Taylor series expansion. With four or more nodes, a
unique solution can be obtained, but with three nodes, external
information is needed to constrain the solution.
C. Modem to Modem Ranging
The modem uses the ping command to range to any other
node. The ping, which is a short communications packet of
approximately 1 second duration, is sent out to the specified
node, which sends a return ping after a fixed amount of time,
thus enabling the originating node to calculate range.
D. Synchronous Telemetry and Ranging
The Micro-Modem provides precise arrival times of com-
munication packets utilizing a user-supplied Pulse-Per-Second
(PPS) reference clock signal [2]. The Micro-Modem uses its
onboard oscillator to make timing measurements for packet
arrivals between the PPS ticks. The packet itself contains
the originating node’s position information which is used
by the vehicle to estimate its position. The message format
for the data packet follows the Compact Control Language
specifications [3]. It consists of 32 bytes of data containing a
high resolution latitude and longitude position fix, the time of
that fix and the time of the packet transmission.
Synchronized clocks can also be exploited to calculate one-
way range. Synchronous transmission allows the modem to
transmit coincident with the PPS signal so that the arrival
time on a receiver with a synchronized clock can be used
for one-way ranging. With synchronous navigation, multiple
vehicles deployed in a network can simultaneously calculate
navigation fixes without the degradation associated with time-
division multiplexing of two-way transponder networks.
III. RES ULTS
This section presents results of several experiments in
which the Micro-Modem was succesfully used to provide
communication and navigation capabilities. In this paper, we
will focus on the navigation data from these experiments.
A. LBL navigation with SeaBed
The WHOI Micro-Modem has been installed on the SeaBed
AUV, shown in figure 1, which was recently deployed off
the coast of Andros, Greece on an archaeological cruise. The
Micro-Modem was used by the vehicle with two standard
Benthos narrow-band transponders with pulses centered at
10.5 and 11 kHz with a duration of 10 ms. Range data
thus collected was used to calculate a position fix for the
vehicle using an active navigation solution. Figure 2 shows
range data from the ship-mounted (topside) modem to the
transducer. Predicted data is the estimated range based on
the GPS locations and the estimated transponder location.
Residual error in range, calculated from GPS logs, is 1.59
m. Figure 3 shows similar data for the 10.5 kHz transponder
with an average residual error of 1.11 m.
Figure 4 shows tracking data for the vehicle during a
mission off the coast of Andros that lasted 3.5 hours and
was carried out at an average depth of 60 meters. It covered
an area approximately 2000m x 1500m. The blue tracklines
show unfiltered LBL data calculated using one-way travel
times between the vehicle and the two transponders. The red
line is Doppler Velocimeter (DVL) data used in conjunction
with a 3 axis north-seeking (IXSEA Octans) fibre-optic gyro
(FOG) system. As expected, this solution tends to drift over
time. Typically, LBL data will be filtered and fused with the
DVL-FOG estimate to obtain absolute position with both high
precision and accuracy [4].
B. Synchronous ranging with SeaBed
During recent sea trials of the SeaBed vehicle with the
R/V Tioga near Wood Hole in shallow water, synchronous
communications with the Micro-Modem were tested from the
vehicle to the ship (topside). All acoustic communications
traffic between the topside modem and vehicle modem orig-
inated on a PPS clock edge. Accurate synchronization of the
modem clock with a GPS receiver enabled both the vehicle and
the topside modem to calculate precise one-way ranges while
receiving vehicle data through the acoustic packets at the same
time. Ground truthing with a LINKQUEST USBL system
showed the one-way travel time errors to be of the order of
1 ms. The onboard drift of the PPS clock was 1 µs. These
ranges can be used along with DVL-FOG estimate to generate
position estimates that can be combined with vehicle LBL
navigation for higher precision and accuracy [5]. Synchronous
communications can thus eliminate the need for deployment
of transponders for vehicle navigation.
Fig. 2. Range data from the topside modem to the Alpha (11 kHz) transducer
plotted over time, in minutes. Predicted ranges calculated from GPS logs.
Average residual error in range is 1.59 m
Fig. 3. Range data from the topside modem to the Beta (10.5 kHz) transducer
plotted over time, in minutes. Predicted ranges calculated from GPS logs.
Average residual error in range is 1.11 m
C. Synchronous ranging with Bluefin AUV
Synchronous communications have been used by Bluefin’s
AOFNC-1 vehicle to augment tracking data from dead reck-
oning and GPS. Figure 7 shows test results from a single run
of the vehicle between two transducers mounted 1.5 m below
two kayaks, labeled K0 and K1. The kayaks defined a baseline
for the vehicle which is about 140 m long. Ranges obtained
from synchronous communications were used to filter dead-
reckoned trajectory data, initialized with GPS at the surface. It
can be seen that the range filtered data agrees with GPS data
better than the unfiltered, dead-reckoned tracks.
Eventually, one-way travel time data from the modem will
be used by the vehicle as part of a moving LBL navigation
system in which the kayaks will be moving in the water
Fig. 4. SeaBed LBL navigation: Blue lines show raw tracking data from the
modem using ranges from 2 transponders. Red lines are obtained from the
Doppler Velocitimeter and show considerable drift over time.
Fig. 5. Synchronous arrival times from the SeaBed AUV. The blue stars
are vehicle travel times and the red circles are travel times recorded by the
topside.
simultaneously with the vehicle.
D. Whale tracking and localization
Passive navigation is a useful technique for tracking and lo-
calizing marine mammals. The WHOI Micro-Modem has been
used to successfully track a humpback whale in the Stellwagen
Bank National Marine Sanctuary using the Realtime Acoustic
Tracking System (RATS) developed for Mark Baumgartner
of the Woods Hole Oceanographic Institution. This consists
of four buoys, each equipped with a Micro-Modem listening
passively for a 36 kHz pulse from a pinger that is attached
to the whale via a suction cup. The pinger has a pressure
sensor, and it varies the time between pings in proportion to
the depth. Using pseudo ranges from the buoys and absolute
depth values from the pinger, a position estimate for the whale
Fig. 6. A kayak autonomous surface craft (ASC), developed jointly by
MIT and Robotic Marine Systems, was used in a navigation aid role, to
provide synchronous navigation and ranging telemetry, using the WHOI
Micro-Modem.
Fig. 7. Bluefin AUV AOFNC1 tracks: Dead reckoning tracks in blue are
overlaid by green tracks, filtered by one-way travel time data. Red circles
indicate range fix from K0, while black circles indicate range fixes from K1.
in x,y and z can be arrived at using the passive navigation
scheme described in section II. The buoys also carry a GPS
and freewave modem so that detections and buoy positions
are reported in real time. This allows the observer to get close
to the animal even when it is submerged. Figure 8 shows 1
hour of tracking data collected on August 30, 2005 1800-1900
UTC on the humpback whale. Three distinct dives are clearly
visible during this tracking period.
IV. CONCLUSION
The WHOI Micro-Modem is being widely used by a variety
of vehicles as a low-power acoustic communication and nav-
igation system. Its compact size and low power consumption
makes it ideal for integration with AUVs. Future work includes
higher rate communication packets for synchronous com-
munications, networking capabilities and configurable power
Fig. 8. Humpback whale tracking data. Estimated whale positions are shown
as colored dots with the color indicating the depth of the animal. The gray
concentric circles are 500 meters apart. The lines indicate the trajectories of
the 4 tracking buoys over the 1-hour period. The green and red filled circles
indicate the positions of the buoys at the beginning and end of the period,
respectively.
control.
V. ACK NOWLEDGE ME N TS
The authors would like to thank Mark Baumgartner of the
Woods Hole Oceanographic Institution for providing whale
tracking data and Jerome Vaganay of Bluefin Robotics, Cam-
bridge Massachusetts, for providing synchronous ranging data
from the AOFNC-1 vehicle. The SeaBED portion of this work
was made possible by a grant from NSF under grant number
99868321.
REFERENCES
[1] L. Freitag, M. Johnson, M. Grund, S. Singh, and J. Preisig, “Integrated
Acoustic Communication and Navigation for Multiple UUVs,” in IEEE
Proceedings of Oceans, Honolulu, October 2001, pp. 2065–2070.
[2] L. Freitag, M. Grund, J. Partan, S. Singh, P. Koski, and K. Ball, “The
WHOI Micro-Modem: An Acoustic Communications and Navigation
system for Multiple Platforms,” in Proc. Oceans 2005, Sept. 2005.
[3] R. Stokey, “A Compact Control Language for Autonomous Underwater
Vehicles,” Woods Hole Oceanographic Institution, Tech. Rep., 2005.
[4] L. Whitcomb, D. Yoerger, H. Singh, and D. Mindell, “Towards precision
robotic maneuvering, survey and manipulation in unstructured undersea
environments,” Robotics Research - The Eighth International Symposium,
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[5] H. Singh, J. Bellingham, F. Hover, S. Lerner, B. Moran, K. von der Heydt,
and D. Yoerger, “Docking for autonomous ocean sampling network,”
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