Intelligence, Surveillance, and Reconnaissance - Spawar

176
INTELLIGENCE, SURVEILLANCE, AND RECONNAISSANCE
addressing, ranging, channel estimation, power control, and adaptive
modulation [13]. Reliability is enhanced through the implementation of
negative acknowledgements, range-dependent timers, retries, and automatic
repeat requests [14].
At the network layer, routing and navigation are accomplished through
embedded data structures distributed throughout the network. Critical
source-to-destination connectivity can be monitored through the use of
receipt utility packets. The Seaweb server maintains these data structures,
supports network configurability, manages network traffic at the gateways,
and provides the graphical user interface for client workstations at
manned command centers [15].
Localized clusters of autonomous assets (e.g., sensors, crawlers, and
divers) assimilate through the formation of a Seaweb acoustic local area
network (Sealan) covering an undersea region up to 5 km2 . A central
Sealan node collects high-bit-rate, digital acoustic transmissions from the
Sealan peripheral nodes. The sophisticated central node receives Sealan
traffic with the processing gains afforded by coherent processing, adaptive
equalization, spatial diversity, and beamforming. Received data pass
to a host computer for local-area data fusion. Through asymmetric lowbit-rate
backlinks, the central node can adapt the peripheral nodes to an
evolving search-space situation and otherwise performs networking overhead
functions, including ranging, localization, navigation, and remote
control. The peripheral nodes may use the low-bit-rate link format for
peer-to-peer communications, including network-layer token passing and
ranging. For undersea communications to beyond the local area, the
Sealan central node uses Seaweb wide-area networking to cue neighboring
Sealan central nodes, and to report events of interest to distant command
centers by using Seaweb’s decentralized wide-area networking.
Sealan acoustic networks operate in a higher frequency band and with
different transducers. Therefore, the central node can maintain simultaneous
Sealan and Seaweb communications without suffering interference.
The centralized approach for local-area networks is identified as a solution
consistent with the desire for information convergence and with the
constraints of undersea acoustic propagation. The central node orchestrates
the overall performance of the local sensors and communications
and is the local interface to the wide-area deployment. Since high-bit-rate
transmitter and low-bit-rate receiver are simple, the peripheral nodes can
be inexpensive, small, and plentiful.
EXPERIMENTAL IMPLEMENTATIONS
Seaweb is implemented as Navy-restricted firmware operating on commercial
modem hardware produced by Small Business Innovation
Research (SBIR) contractor Benthos, Inc. Racom buoys are gateway
nodes that have successfully incorporated line-of-sight FreeWave radios,
cellular digital packet date (CDPD) telephone modems, Iridium satellite
communications (SATCOM) modems, and Global Positioning System
(GPS) receivers. Sublink capability has been implemented easily on
WQC-2 and BSY-1 submarine sonar. A Seaweb server resides at manned
command centers and is the interface to the undersea network.
The power of Seaweb connectivity was successfully and dramatically
demonstrated during recent Navy and international experiments. Figure 2
is a chart of the Fleet Battle Experiment India (FBE-I) Seaweb network, a