The Navy Vol_70_No_4 Oct 2008 - Navy League of Australia

Reducing Snorting Risk
While li-ion batteries provide considerable power density
improvements they are still, currently, 20 times less efficient than
diesel fuel. Generation of electricity will still require snorting
to obtain oxygen to run the diesels. Snorting compromises
stealth by increasing broadband acoustic emissions and
exposing a detectable mast above the waterline. Considerable
effort has been made by Australian researchers and industry,
much of it highly classified, to counter the broadband acoustic
signature of the Collins. Reducing the emitted noise caused by
the running diesel motors through vibration control and active
cancellation has considerably reduced the acoustic snorting
signature of these boats.
Submarine mast detection is one of the most difficult roles
for radars requiring fast scanning rates and acute angular
relationships between the radar and the target. This means flying
at low altitude which considerably reduces the area available
for scanning compared to high altitude radars. Even the latest
Raytheon AN/APY-10 radar, formerly known as the AN/APS-
137B(V)5 and being fitted to the new Boeing P-8 Poseidon
MPA, is limited to a maximum range of 29nm against a target
The angular shapes of the masthead of the Kolmorgen Photonics Mast
Program (PMP) reduces the Radar Cross Section (RCS) of the submarine’s
above water interfaces improving operational stealth. (Kolmorgen photo)
with a Radar Cross Section (RCS) of at least one square metre
against a backdrop of 1m waves (Sea State 3).
RCS reducing designs similar to that used in stealth aircraft are
now being applied to the above water elements of submarine
masts. The Virginia class’s Kollmorgen Photonics Mast (PMP)
System has a geometrically shaped casing designed to reduce
RCS against periscope-hunting radars. Applied to the snorkel
and the risk of detection by radars is considerably reduced.
If RCS reduction is comparable to that applied to stealth
aircraft then detection range of an AN/APY-10 should be
reduced to the realm of under a single nautical mile, even for a
comparatively large above water snorkel.
By using more space and weight efficiently HTS motors/
generators and li-ion batteries could provide surplus space
for AIP systems. AIP systems don’t have the power density
to offer anything more than sustained loiter speeds but this
can be an important tool for diesel-electric submarines facing
‘hold down’ by overhead anti-submarine threats. However,
AIP does not have power densities much higher than current
li-ion batteries particular when the space and weight of the
additional generator is taken into account. AIP is likely to
be an unnecessary duplication of effort for the SEA 1000
future submarine.
Networked Submarines
Despite the improvements in power density and in reducing the
risk of snorting diesel-electric submarines will still struggle
to sustain high tactical speeds (not to be confused with the
maximum sprint speed) reducing their patrol area compared
to a nuclear submarine. Endurance at tactical speed is limited
by the battery while the nuclear submarine’s is only limited
by acoustics. The speed at which the submarine can detect a
threat before being detected itself is typically 20 knots for a
fourth generation USN boat like the VIRGINIA. This enables
the nuclear submarine to cover a patrol area up to five times
larger than a four-five knot sustained speed diesel-electric.
Using underwater networking with sonar equipped UUVs,
other submarines, surface ships and seabed arrays enables
a conventional submarine to significantly increase its patrol
coverage. Enabling networking is the TWC (through water
communications) ability of ASST (acoustic spread spectrum
technology) developed in Australia by Nautronix, now a
subsidiary of L-3 Communications. ASST enables high
bandwidth, discrete communications through water and
has enabled a range of new capabilities to be developed for
underwater operations.
Developed to provide a TWC solution for a low cost acoustic
monitoring array required to measure the Collins class’s
underwater signature, ASST spreads a transmitted message
over a comparatively large segment of the acoustic spectrum,
typically more than 10 times greater than would be required
for conventional narrowband communications. Spreading
a message across the spectrum transmitted between moving
objects is severely degraded by the Doppler Effect, which is
200,000 times stronger with acoustic transmission through
water than when compared to that of radio frequency (RF)
through air. L-3 Nautronix’s ASST technology is claimed to
overcome the Doppler effect through a proprietary method.
ASST provides the technology basis for Nautronix Acoustic
Sub-sea Hydro-Acoustic-Information-Link (NAS-HAIL)
developed under the Capability Technology Demonstrator
(CTD) program managed by the Australian Defence Science
and Technology Organisation (DSTO). NAS-HAIL is a
first generation ASST TWC and provides potentially low
probability intercept (LPI) communications and data transfer
levels of about 100 bytes per second, adequate for operational
THE NAVY VOL. 70 NO. 4 7