THE USE OF SONARBELL CALIBRATION SYSTEM (SCS) FOR RADAR
AND SONAR ALIGNMENT TESTING
L. Symes
Subsea Asset Location Technologies (SALT) Ltd.
19 Portland Marina, Osprey Quay, Hamm Beach Road,
Portland, Dorset, DT5 1DX, UK
Tel/Fax: +44 (0) 1305 820321
Email: [email protected] Web: www.cesalt.co.uk
ABSTRACT
Radar and sonar alignment are critical to a platform to ensure that navigation and target position
provide an aligned maritime picture. In order to accept / prove these important criteria SALT have
developed a cost effective, carry with you system called SCS which allows a platform to check
alignment status wherever it is, including in theatre.
The system consists of a radar and sonar target designed to give a point target response allowing
very accurate positioning. Above water the equipment can be fitted with laser strobes and GPS
logging capabilities. Below the surface a passive sonar target is used to provide the same target
attributes as the radar system, built on the highly successful industry wide underwater location
target, SonarBell®.
The radar system will work for S and X Band radar systems allowing long and medium range testing.
The topside unit GPS logger allows the user to track the movement of the SCS. This can then be
uploaded to a variety of different file formats to view at a later date. For low light and dark
conditions laser strobes make the unit easily identified by optics, at ranges of up to approximately 3
miles in good weather conditions lasting a minimum of 30 hours. The radar target can typically be
tracked from 6 to 8 miles range.
The passive reflector SonarBell® can be tuned to give a high target strength return at multiple
frequency bands between 4 – 100kHz. The SonarBell® is pressure tested to just over 4000m and has
an operating range of approximately 6 to 8 miles (dependant on frequency).
In summary this system provides the capability to accurately align both radar and sonar targets in
any location frequently desired giving the platform operator peace of mind that both systems above
and below the sea level are aligned. The production SCS system has been tried and tested, is easy to
use, cost effective and requires minimal maintenance.
1.0 INTRODUCTION
For the first time a simple, cost effective, easily deployed
sonar and radar target is now available off the shelf to aid
with surface ship, testing and training through sonar
alignment and performance measurements. Radar and
sonar alignment are critical to a platform to ensure that
navigation and targeting provide coherent maritime
pictures. SALT’s man portable system allows a vessel to
check alignment and performance status at sea including in
theatre. The system consists of radar and sonar reflectors
designed to give point target responses for precise
positioning.
Radar
Enhancer
Laser strobe
Above water the system offers a range of options including:


Radar target enhancer operating on X and S band
allowing medium and long range testing
Eye safe laser strobe for low light/dark conditions
giving a range of 6km
Below water the SonarBell® provides a highly accurate
underwater sonar target offering:






A wide selectable frequency band from 4 to 100kHz
Differing target strengths available from -30dB to
-5dB
Detectable measurement ranges up to 16km
Depth rated to 4100m
Selectable deployment depths of 5m, 10m, 20m,
30m
Fitted with a custom made net for deployment
2.0 OPERATIONAL NEED
Buoy
Recovery
buoy
Having a complete system that can calibrate radar and sonar
systems but also allows alignment of both systems together
at the same time is crucial to the platforms performance.
The alignment of these important sensors ensures accurate
navigation, a coherent maritime picture and a reduction of
SonarBell
Fig 1 : SCS System
duplicate targets. Having a portable system that can be
deployed anywhere at anytime provides a flexible in theatre
system to prove functionality in varying environments. In the event of a system malfunction having
this portable system ready to deploy can save the mission time and money.
This system can be used throughout the life cycle of the vessel from acceptance and contractor sea
trials through mid life updates, system improvements and during platform deployments.
3.0 SONARBELL® TECHNOLOGY
The SonarBell® works through the constructive interference of sound waves when they meet within
the device after transiting the shell and core materials of the ball. The SonarBell® is made up of two
materials; the shell material can either be a hard Glass-Reinforced Plastic (GRP) for frequencies
above 90kHz or an Aluminium shell for frequencies below 100kHz, the core material is made out of a
silicone based gel which has been specially selected to sufficiently slow the speed of sound down so
that the sound waves around the shell meet at exactly the same point in time as the sound waves
through the core, to produce a focused return. As sound energy reaches the ball you will get an
initial reflection and scattering from the shell surface. The rest of the energy is sent in two
directions some energy will go around the shell, most will be focused through the silicone material
to the inside face of the back of the shell. This is where both paths will constructively interfere with
each other; this energy is then
focused back in the direction it came
from.
The SonarBell® has two highly
unique features in both the time and
frequency domain.
3.1 FREQUENCY DOMAIN
The SonarBell® can be tuned to have
optimum performance at a specific
frequency by changing the thickness
of the shell material. In doing so it
can be matched to the operating
frequency of any sonar therefore
reflecting a high target strength.
Figure 2 shows a typical frequency
response for a 120kHz SonarBell®.
For
this
particular
tuned
SonarBell® a 10dB to 15dB
separation between front and
focused echo is expected giving
the back echo a maximum
absolute target strength of -7dB.
Using this data the omnidirectionality of the SonarBell® can
be clarified by producing a polar
response (see Figure 3). This
Fig 2 : Frequency Response
Fig 3 : Polar Response
shows that the focused echo is relatively constant 360° around the sphere giving a 10 to 15dB
separation between front and back echo, as seen at resonance in the frequency response.
Using these unique features of the SonarBell® makes for a target that is not only tuned to the
operating sonar to give a high target strength when insonified, but when insonified the target has a
unique dual echo making it easy to distinguish from reverb, debris or rocks on the seabed.
Fig 4 : Time series response of a 50 and 200mm diameter of SonarBell showing the time
separation for differing sizes of SonarBell
3.2 TIME DOMAIN
As discussed above the SonarBell® has an initial front echo followed by a focused echo. The time
between the two echoes can be determined by changing the physical size of the ball giving the
acoustic diameter. Figure 4 shows the front and back echo separation for a 50mm and 200mm
SonarBell®.
For example depending on operational frequencies looking at the data from the 200mm diameter
ball that the separation is approximately 360µs whereas the 50mm diameter ball is approximately
90µs, therefore the smaller the size of the ball the shorter the separation and vice versa. This
unique feature can easily be picked up using any type of sonar, as long as the pulse lengths are
relatively short. If the pulse lengths are too long then the SonarBell® reflections will turn into one
elongated waveform/reflection. If this occurs the user would not be able to distinguish the front
from back echo although a high target strength will still be seen.
Using this technique different sized SonarBells could be used to mark different kinds of object on the
seabed or water column almost like an underwater bar-coding system. By calculating the distance
between the front and back echo, the sonar operator would be able to determine what size the
SonarBell® is, and in doing so they could relate this to a particular known object on the seabed.
3.3 RELATIVE SIZE VS DETECTION RANGE
The size of the SonarBell® denotes the reflected intensity of the target strength, the larger the
sphere the more sound energy can be captured and reflected back to the source. In other words at
a set frequency the larger the size of the ball the higher the target strength, the smaller the size of
the ball the lower the target strength. This can also be said for the detection ranges for different
sized SonarBells; the larger the size of the ball the longer the detection range will be, the smaller the
ball the shorter the detection range. Therefore the essential size of the SonarBell® depends very
much on the required target strength and detection range being sought. Typically SonarBells are
manufactured from 50mm diameter to 450mm diameter with the latter being used for low
frequency systems.
In conclusion the SonarBell® can be tuned to give a specific target strength at a specific frequency.
Knowing the target strength of the SonarBell® it can be utilised as a calibration target for the vessels
sonar system. Depending on the operating frequency of the sonar system the SonarBell® can give a
focused return at a single frequency or a high return across a broadband of frequencies. The
SonarBells relative size and focused return gives a point target when interrogated by a sonar system
allowing for accurate alignment of both sonar and radar.
For example a 450mm SonarBell® can be manufactured to provide a -5dB target strength sphere at
frequencies in the band of 2 – 10kHz proven by high resolution modelling and at sea measurements
using a bow mounted sonar.
4.0 RADAR
A high intensity target that has a high RCS (Radar Cross Section) intensity is needed to be able to
achieve the 6 – 8 mile range that is required to perform alignment tests. RCS is a measure of the
targets reflectivity and indicates that an object is more visible on radar. A point target is high in
intensity where all of the reflected energy is focused to a single point rather than a diverse target
where the reflected energy is spread over a larger area. A diverse target makes it harder for the user
to define the centre of the target which makes alignment much harder.
Therefore for this particular application a high RCS point target was needed. Fig 5 shows the
calculated RCS of five active and passive radar reflectors at differing elevation angles from 0 to 20
degrees. The ideal solution for the system would be a passive reflector which would result in lower
battery requirements and a simplified system. In order to select the most appropriate reflector
modelling was completed on RCS for a number of commercially available units.
Fig 5 : Modelled RCS Plots
Using this data the Sea-Me is shown to have the largest overall RCS. In order to prove this SALT
requested at sea performance checks on the five different models listed below in Table 1.
Reflector
Sea-Me
Tri-Lens Large
Tri-Lens Standard
Echomax 230
Firdell Blipper
Dimension
416 (L) x 50mm (D)
160 x 160 x 80mm
120 x 120 x 60mm
610 (L) x 248mm (D)
595 (L) x 240mm (D)
Weight
0.4kg
5.5kg
2.5kg
2.4kg
1.8kg
Active/Passive
Active
Passive
Passive
Passive
Passive
Table 1 : Commercially available radar reflectors
Each reflector was trialed against both X and S Band radar systems out to a range of 8 miles, it was
found that the Sea-Me gave the best detectable range which confirmed the modelling shown above
in Fig 5.
The SCS operates with both X and S Band radar systems. X Band radar operates between 8 – 12GHz
which allows for shorter wavelengths giving a higher resolution but a shorter range than S Band. S
Band radar operates between 2 – 4GHz and is more susceptible to weather conditions these systems
tend to be larger due to the transmit power needed.
A range of 6 – 8 miles needed to be achieved to allow for a greater alignment accuracy. For instance
if the radar and sonar is out of alignment by just 2 degrees at 10km the error could be as much as
350m (see Fig 6). Therefore the separation distance between vessel and target achieves better
alignment accuracy.
Fig 6 : 1 and 2 degree mis-alignment error in metres
Also the height of the system in water compared to the height of the ships radar will have an impact
on performance. For instance the height above sea water of the radar reflector is 1.5m, lets say the
radar is located 20m above sea level. In simple terms the range will be limited to line of sight,
depending on environmental conditions the line of site at 20m above sea level will be approximately
10 miles therefore 6 – 8 miles is achievable as an operating range for the SCS system.
5.0 SUB-SYSTEMS
As well as the SonarBell® for use with sonar and the radar enhancer there are bolt on sub systems
available.
5.1 LASER STROBE
A laser strobe is mounted on top of the SCS pole alongside the radar enhancer. The flare is designed
to work in low light and dark conditions and is connected to a control box that is attached to the
metal plate above the buoy. The control box has a light dependant sensor on it which turns the laser
on when covered or dark. The laser can be seen up to 4nm and depending on conditions it will
operate from 18 to 24 hours of continuous use. The head can be seen clearly with night vision
equipment and over optics and pose no threat to eyesight.
5.2 ONBOARD POWER SYSTEMS
The system includes a 24hr operating gel lead acid battery system to power both the radar enhancer
and laser strobe, this is protected inside an IP68 container with waterproof cables connecting the
power system to the head electronics. In addition an intelligent battery charger is supplied to
recharge and recondition the battery, this provides indication of charge status, evaluation mode and
fault cases.
5.4 BUOYANCY
Currently there is in excess of 30kg of buoyancy in seawater which allows addition of extra sub
systems as required by the customer for example an AIS Transponder and/or GPS tracking. A further
enhancement can be achieved by using different sized buoy systems which can increase or decrease
buoyancy resulting in increased or decreased size. The current buoy system was chosen as a
compromise between weight, reserved buoyancy and stability in sea states. The system has a small
recovery buoy attached to aid quick and effective recovery using the ships RIB. The system once
deployed is left unattended and allowed to drift while the surface vessel undertakes the
performance checks. On completion a simple evolution can be undertaken to recover the system.
6.0 SUMMARY
In summary this highly portable and modular system provides the fighting platform with a take
anywhere capability to test monitor and fully understand sensor performance while in theatre. Its
low cost low maintenance nature reduces load on crews and support systems resulting in the ship
being able to check its own performance quickly and effectively at anytime. This proven technology
now deployed on the latest UK RN Destroyers T-45 is a step change in both capability and cost
terms.
This system benefits from the highly innovative SonarBell® product originally designed in the UK
MoD research laboratories and exclusively licensed to Subsea Asset Location Technologies (SALT)
Ltd. The SonarBell® is in use in a wide range of military and commercial markets from fishing, oil and
gas and is a unique passive technology without any competitors.
REFERENCES
Smith, J; Emery, D; Williams, D; SECR Defence (2006) Acoustic Reflector, G.B. Pat. 2422282A
Smith, J; Emery, D; Williams, D; SECR Defence (2007) An Acoustic Reflector, G.B. Pat. 2437016Ab