Indian Journal of Geo-Marine Sciences

Vol. 39(4), December 2010, pp. 631-645

Role of oceanography in naval defence

J Swain, P A Umesh & M Harikrishnan

Naval Physical & Oceanographic Laboratory, Kochi 682 021, India

[E-mail: tsonpol@vsnl.com]

Received 20 August 2010; revised 21 December 2010

The study of oceans is an essential aspect from strategic, economic and ocean engineering points of view. Recent

advancements in SONAR (Sound Navigation and Ranging) technology for under water applications including naval defence

demand a clear understanding of the sound propagation in the ocean which is vital for detection of a target. As compared to

deep waters, the detection in coastal waters is relatively a challenging task. Detection becomes still complex in the littoral

waters. To ensure success in these highly complex oceanic regions, high resolution descriptions/predictions of past, current

and future conditions (hindcast, nowcast and forecast) as well the analysis of open ocean, coastal, and nearshore/littoral

zones around the Indian continent is an essential pre-requisite. At any given situation, the detection and discrimination of an

underwater target is highly dependant on the propagation characteristics of the medium, the surface and bottom boundaries,

leaving apart the efficiency of the SONAR system and the type of the target. It can be either a SONAR performance model

or an operational model for tactical warfare; one has to have the predicted sound speed for the 3-dimensional ocean

environment concerned since it is not always feasible to depend only on measurements. There are on-going programs of

collecting and analyzing ocean and atmospheric data and a wide range of research and development activities, some of

which are reviewed/presented in this paper with typical examples. In addition to in-situ measurements, significant progress

has been achieved in India by demonstrating the utility of satellite based remote sensing data for oceanographic research and

applications. By utilizing the existing and on-going experimental data from the regions of interest, it has become feasible for

implementation of an integrated “Naval Operational Ocean Prediction System” consisting of wave, tide, circulation and

internal wave models. The ocean environmental information predicted by these ocean models shall provide necessary inputs

to the SONAR range prediction models for routine operational use, tactical operations and simulating warfare scenarios.

[Keywords: Oceanography in naval defence, Naval operational prediction, Ocean prediction system, Nearshore

prediction]

Introduction

The oceans that make up three-quarters of the Earth's surface are realms of boundless energy. Oceans have been a source of food, the birthplace of weather systems that affect the continents, pathways for commerce, and fields for battle. Studying the world oceans, the atmosphere above it, and air-sea interaction

processes is the science of oceanography. Oceanography has been recognized as a formal scientific discipline for nearly one hundred and fifty years. However, finding practical applications (inventions) for commerce and war at sea, dates much further. Hence, oceanography means more than an

understanding of how the weather and climate changes over the globe. The history of both ocean exploration and ocean warfare is filled with examples of “environmental intelligence” with the invent of new weapons, sensors, ships and submarines from time to time. Nevertheless, understanding of underwater

environment is also the new frontier for major future discoveries for the benefit of the world at large

1.

Materials and Methods

Recent advancements in SONAR (Sound Navigation and Ranging) technology for under water applications including naval defence demand a clear understanding of the sound propagation in the ocean which is vital for detection of an enemy target. As a thumb rule, the winner is the one who detects first; no

matter how latest are the weapon systems the opponent is in possession. In the 1920s and 1930s, the scientific understanding on the behaviour of sound in the sea and its application to SONAR systems for anti-submarine warfare advanced slowly, and it was only with the emergence of a vastly increased

submarine threat at the onset of Second World War in 1939 that a major national effort was undertaken for the study of underwater acoustics. What emerged was a series of results that showed that the transmission of sound in the sea and in particular how effectively it could be used to detect submarines depended crucially

on how the temperature and salinity of the seawater varied with range and depth. It was found that sound

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rays bend underwater in ways that are intimately linked to the variation of the speed of sound from place to place, and that this could create "shadow zones" in which a target could hide.

These discoveries significantly widened the range

of oceanic phenomena which created interest among

oceanographers. In addition to concerns with water

depth, winds, waves and currents, the need to

measure and interpret underwater physical

parameters such as water temperature, salinity, and

sound speed at increasing depths, assumed major

importance. This required the development for new

kinds of instruments, new analysis techniques, new

ways of interpreting data, new modeling approaches,

state-of-the-art operational prediction capabilities

and in general, a substantial broadening of the

scientific disciplines needed in practice of

oceanography for naval applications. The present

paper shall highlight some of the aspects with

reference to the Indian waters.

Results and Discussion

Naval oceanography involves several major areas

of science: oceanography, meteorology, mapping,

charting, geodesy, astrometry (the science of

accurate astronomical measurements); and precise

time-keeping. There are on-going programs in

collecting and analyzing ocean and atmospheric

data spread over a wide spectrum of research

and development activities. The oceanographers

are investigating the nature and behaviour of the

oceans from every aspect. In addition, to customary

bathymetric surveys for bottom mapping, they also

collect data on the sub-bottom layers, the composition

and roughness of the ocean floor, as well the physical

and biological properties of seawater. Specialized

instruments are used to measure currents, waves,

ocean fronts, local variations of the Earth's

magnetic/gravitational fields, and acoustical

background noise. While these measurements have

traditionally been made from aircraft, buoys, and

ships at sea, there is an increasing emphasis on

the use of available space based platforms for a

wide variety of observational support. Ocean

research related laboratories and civilian technical

institutes/universities are major contributors to the

ocean environmental sciences, and dedicated efforts

are under way to translate their findings into research

applications for improving the accuracy and

timeliness of ocean prediction. The various issues

relating to the role of oceanography for naval defence

are further discussed below with some relevant

examples based on field measurements.

Oceanography and Naval Defence

In future, underwater acoustics will remain as the

principal means of detecting targets as the feasible

non-acoustic methods have limited range. When

compared to deep waters, the detection in coastal

waters is relatively a challenging task. The detection

becomes still complex in the littoral waters.

Therefore, the importance of “littoral warfare” today

is increasingly felt worldwide. At any given situation,

the detection and discrimination of an underwater

target highly depend on the propagation charac-

teristics of the medium as well as its surface and

bottom boundaries, leaving apart the efficiency of the

SONAR system and the type of target. To a large

extent, the geo-acoustic properties of the sea bottom

can vary in space, but can be considered as constant in

time although there can be short-term and seasonal

variability of the surface sediment layer, and the

suspended matter in particular. However, the surface

and the medium characteristics are highly variable in

coastal and littoral waters both in spatial and temporal

scales.

The sea surface wind, wind-induced surface gravity

waves and the associated sea-surface roughness are

of primary concern dealing with propagation of

sound in the surface duct. In terms of medium

characteristics, the spatio-temporal variations of

temperature, salinity with depth (or sound speed) are

the essential pre-requisites for the SONAR range

prediction model. Further, it could be either a

SONAR performance model or an operational model

for tactical warfare, where one needs to have the

predicted three-dimensional sound speed structure as

it is not always possible/feasible to depend solely on

measurements. However, measurements are required

for satisfying the initial and boundary conditions of

ocean environmental models which can supplement in

the post analysis of model predictions to support

ASW (Anti Submarine Warfare), surface warfare,

amphibious warfare and coastal surveillance.

Oceanographic Parameters for Naval Operations

To ensure success in the highly complex oceanic

regions, high-resolution descriptors of the past,

current and future conditions (hindcast, nowcast and

forecast) as well as analysis/monitoring of open

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ocean, coastal, and nearshore/littoral zones around

the Indian continent are very much essential. The

capability to derive and/or measure data in remote and

inaccessible part of our ocean providing suitable

analysis and prediction is a real need. Environmental

parameters which are of importance (direct and

indirect) for naval defence includes:

(i) Atmospheric: Weather (clouds, precipitation,

wind speed, wind direction and air temperature)

and marine boundary layer properties

(temperature, humidity, refractivity).

(ii) Oceanographic: Sea-state, tide, current (surface

and subsurface), internal waves, temperature,

salinity, turbidity, optical properties (vertical and

horizontal) and surf conditions.

(iii) Geophysical: Depth, beach slope, beach and

bottom sediment type/composition, sediment

layering and their geo-acoustic characteristics.

(iv) Acoustic: Scattering, ambient noise, reverbe-

ration and transmission loss.

(v) Gravity and Magnetic: Gravity anomaly,

bottom roughness, bottom strength/stability,

ambient magnetic and electrical fields.

(vi) Biological and Anthropogenic: Noise, optical

scattering, bioluminescence, fishing, offshore

activities, shipping and pollution.

In general, the above parameters directly or

indirectly contribute towards the oceanographic and

acoustic studies supporting the naval warfare.

However, the following discussions shall deal only

with the most important selected oceanographic

parameters.

Waves The wind-induced surface gravity waves have

several applications both in civilian and defence

sectors. In the context of military perspective, wave

information is vital in design of coastal and offshore

installations, jetties/harbors, optimum tracking of

surface and sub-surface vehicles, aircraft landing and

take off, towing of underwater arrays/bodies,

detection and discrimination of underwater targets/

objects, search and rescue operations during rough

weather and several other operations at sea. The

sea-state has a dominant role in the air-sea interaction

processes of the coupled ocean atmosphere system

for operational prediction of weather in advance.

Keeping all these in view, it is necessary to predict

the sea-state or the evolution of sea surface waves on

a routine basis for the region of interest.

There has been a lot of progress in the field

of wave modeling and prediction all over the

world2-11, 41-45

. In India, with the launch of Oceansat-I

(IRS-P4), extensive validations were performed with

the third generation wave model 3G-WAM for the

Indian Ocean region using the analysed wind fields

provided by the National Centre for Medium Range

Weather Forecast (NCMRWF), New Delhi. The work

carried out at NPOL through a collaborative research

programme between NPOL and Space Application

Centre (SAC) was reviewed12

. It was a part of the

IRS-P4, MSMR utilization programme. Under this

collaborative programme, performance evaluation

studies of 3G-WAM were carried out for the Indian

Ocean bounded in the geographical coordinates

encompassing 30°E to 120°E and 30°S to 30°N using

the analysed winds of NCMRWF as the forcing

mechanism. WAM runs were performed using six

hourly input wind fields in a 1.5°×1.5° grid

resolution. The outputs from WAM model such as

significant wave height, peak wave period, mean

wave period and mean wave direction were compared

with the time-series buoy measurements obtained

from National Institute of Ocean Technology (NIOT),

Chennai with few buoys and other available

measurements which is shown in Fig. 1.

NPOL has implemented WAM at Naval Operations

Data Processing and Analysis Centre (NODPAC),

Indian Navy, Cochin for operational wave forecasting

using NCMRWF winds since 2003. A sample output

of significant wave height (Hs) is shown in Fig. 2.

Evolution of 1-dimensional wave spectra at a selected

buoy location (DS1) for the period June-August 2002

and monthly mean wave spectra predicted by 3G-

WAM at the buoy location DS1 for the periods

Fig. 1—Map showing locations of wave measurements in the

Indian Seas utilized for WAM validations.

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May-August 2000 are depicted in Figs 3 and 4

respectively. The Year 2002 was an unusual event,

where the wave activity in general during June was

significantly higher compared with rest of the

monsoon period as seen in Fig. 3. Low wave activity

persisted during the July month normally considered

as the peak of south-west monsoon period. The

predicted mean monthly directional wave spectra at a

given location shown in Fig. 4 are able to indicate the

sea state which prevails at that location. Comparisons

between the predicted and observed wave parameters

for various case studies were found very encouraging.

Fig. 5 is an example of the WAM validations

carried out at NPOL utilising three hourly buoy

measurements. One of the important results to be

noted is that the model predictions of significant

wave height overestimates during the extreme wind

and wave conditions13,14,15,16

. In general, excluding

the extreme weather events, WAM predictions were

reasonably good for the south-west monsoon season

(May-September) periods in spite of the limitations

which were clearly known. These validation studies

Fig. 2—A sample WAM output of significant wave height (Hs) for Indian Ocean.

Fig. 3—Evolution of 1-dimensional wave spectra at a selected buoy location (DS1) during June-August 2002 (Swain et. al., 2005).

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Fig. 4—Mean wave spectra predicted by 3G-WAM at the NIOT buoy site [DS1] for May-August 2000

Fig. 5—Comparisons of observed and predicted wave parameters during May-August 2000 at DS1 location in the Arabian Sea

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have revealed that performance of WAM is

satisfactory and its predictions can be safely utilised

for various user applications in deep waters beyond

30 meters.

Tide

Tide is one of the most important hydrodynamic

parameter in coastal waters. It induces internal tides

(fluctuations of the internal temperature/density

layers) and largely influences the flow pattern from

surface to bottom in the shelf region. The Indian coast

experience semi-diurnal tide whose magnitude

gradually increases from south to north. The effect of

tides on the thermodynamic structure is also being felt

in deep waters (AMREX-II). Time-series current

measurements off Paradeep, east coast of India

revealed strong tidal reversals up to the bottom where

water depth was 51.5 m17

. During one of the field

experiment of NPOL, an echosounder was operated

continuously while the ship was anchored in the

absence of a tide gauge. The observed tidal range as

shown in Fig. 6 was about 1.5 m at the time-series

location, which is about 25 miles away from the coast.

Two distinct and regular crests were observed during

the day. The tidal height and the associated current at

different depths are valuable inputs for estimating the

pre-dominant component of coastal circulation and its

reversal during a tidal cycle. The observed current,

associated thermal structure and their variability

during the time-series measurements is discussed

further in the following sections in relation with the

tidal reversals.

As indicated above, tide is the most dominating factor in the coastal waters, which can directly or indirectly affect the oceanographic features leading to a considerable change in the medium properties

during its cycle. There are several models for the prediction of tide (astronomical) while some of them also predict the component of the meteorological tide

18.

Temperature and Salinity

As indicated earlier, the underwater sound

propagation depends on the environmental properties of the oceanic medium in addition to the surface and bottom boundary conditions. The spatio- temporal variations of temperature and salinity which determine the sound speed profile regulates the propagation of underwater sound. It is not

feasible to collect temperature and salinity profiles simultaneously from several locations by research ships. The simultaneous observations of temperature and salinity from different stations of a given location are limited to the number of ships involved in the field experiment (or scientific cruise). Generally,

when a single ship is involved spatial observations are taken in synoptic time intervals which depend on the speed of the ship and also the distance of separation between two stations. However, during analysis of the spatial data collected by a single ship or few ships, it is assumed that the observations are taken at a given

time simultaneously compromising on the short-term variability. Such measurements are alternatively possible by using moored buoys/sensors or through deployment of sensors similar to ARGO floats since the satellite data is restricted only to the sea-surface. Many such measurements and analysis is evident

from most of the published literature. However, temporal variations of temperature and salinity at a given location are possible from variety of measure-ment platforms (moored/anchored). One such measurement (Conductivity Temperature and Depth: CTD-yoyo) onboard INS Sagardhwani off Cochin,

west coast of India during 10-15 June 2003 (Fig. 7) demonstrates the variability of temperature and salinity during two tidal cycles (48 hours) at a coastal station. Such data sets can effectively improve the detection potential of SONAR stated above as sound is the best means of underwater detection for the

coming future. This was the first attempt by NPOL

Fig. 6—Time-series variability of water depth showing semi-diurnal

tide off Cochin, west coast of India

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(Mini-CTD yoyo) during which about 960 profiles of temperature and salinity were collected within a period of 48 hours by operating two Mini-CTD systems (measurement accuracies: depth ±0.02% of range 500 m, temperature ±0.01°C and salinity

±0.02 PPT) simultaneously. The water fall diagram as shown in Fig. 7 clearly

indicate the evolution of the temperature profiles due to the combined effect of diurnal heating/cooling, coastal currents, long-period waves, seasonal/climatic variations and other processes which occurs in the coastal ocean (tidal current appears to be the most predominant among all). The mixed layer depth (MLD) or the so called SONIC layer depth varies from almost 5 to 20 meters during the tidal cycle (semi-diurnal) which is very significant in acoustic detection using underwater systems. The effect is clearly seen up to the bottom (total depth 67 meters) as seen from the time-series currents, which is known to be very strong and a permanent feature in enclosed, semi-enclosed and estuarine waters

19,20.

The above time-series shown in Fig. 7, is a typical example of the short-term variability in coastal waters, the effect of which on SONAR range prediction is discussed later in this paper. Both temporal and spatial ocean variabilities are predictable using the state-of-the art ocean circulation models

21-24 with the help of measurements from all

available sources including satellites.

Currents

The sound speed profile in the ocean evolves with

the current. The following and opposing currents have significant influence on sound propagation and the effective detection range of a target. The ocean currents which are observed at various frequencies (periodicities) are responsible for the spatio-temporal variations of ocean properties like temperature and

salinity as discussed above. It is interesting to note that, the spatio-temporal variabilities of the ocean properties are in turn responsible for the varying ocean currents. It means that, the ocean is a dynamic system intimately coupled to the atmosphere where

the causes and effects of all its processes are dynamic and quasi-cyclic.

The variations of currents are shown in Fig. 8 (arrows/current vectors indicating speed and direction). This corresponds to the temperature variations as in Fig. 7 and the tidal cycle as in Fig. 6,

which is a typical example of coastal ocean variability in the Indian coasts. The current patterns clearly reveal the temperature variation particularly the MLD which need not exactly resemble each other since the flow vector is three dimensional depending on various other factors. The variability of currents due to the

tidal reversals both in magnitude (2 to 50 cm/second) and direction is the principal causative factor governing the sound speed profile.

Fig. 7—Water fall diagram showing variation of temperature from surface to 65m of water depth for a period of two days (X-axis), off

Cochin, west coast of India.

Fig. 8—Variations of observed temperature (minimum 19.7 near

bottom, maximum 30.2 degree Celsius at surface) and current for

two days (vectors indicate relative aspect ratio), off Cochin, west

coast of India

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Internal Waves

Oscillations of internal thermal layers within the ocean are very common in all the oceans, both in deep as well as shallow waters. These waves significantly

influence the sound transmission in the ocean causing fluctuations

25,26. The shallow water internal waves are

very complex and not well understood as the case in deep waters

27.

The variations of temperature and currents (speed and direction) as shown in Fig. 9 corresponding to the tidal cycle (Fig. 6) discussed before, are embedded

with low and high frequency variations (called as low and high frequency internal waves) which were demonstrated using band pass filters as indicated in the respective plots. The time-series temperature, current speed and direction (Figs 9a, 9c & 9d) clearly reveal the internal waves following the semi-diurnal

tide. Fig. 9b suggests the presence of high frequency internal waves with periods less than 3 hours (up to 15-30 minutes). These waves influence the propagation of sound in coastal waters.

A CTD yoyo experiment was conducted onboard ORV Sagar Kanya during her second cruise (17 July to 16 August 2002) - the Arabian Sea Monsoon experiment (ARMEX), Phase-I

28 an observational

study under the Indian Climate Research Programme (ICRP). The study of internal wave oscillations

(ARMEX programme onboard ORV Sagar Kanya) using CTD yoyo data, coincides the period where acoustic transmission experiment by NPOL onboard INS Sagardhwani was conducted. The objective of the acoustic transmission experiment was to study the effect of high-frequency (<1 cph) and low frequency (0.04 to 0.16 cph) internal waves which cause acoustic intensity fluctuations during its transmission in deep waters and prevailing rough weather conditions (south-west monsoon). The observed acoustic fluctuations in the surface duct could be due to wind, waves, internal waves and the prevailing current at the monitoring station. The depth variations of some typical isotherms plotted in Fig. 10 is based on the dataset collected during ARMEX which indicate internal wave periods ranging from 15 minutes to less than an hour (excluding higher frequencies) and also their vertical displacements (2 to 20 m) arising due to internal tides. The computed sound velocity using the observed temperature and salinity data reveals similar fluctuations as that during the yoyo period deciphering the presence of oscillations caused by internal waves. These oscillations are likely to influence the sound propagation

29 evident from the

observed internal wave magnitudes (Fig. 11) which shows variation predominantly from 2 to 5 m in a short span of 2 hours.

Fig. 9—Variations of observed isothermal layers (a) low passed time-series of temperature, (b) high passed time-series of temperature;

and observed time-series of current (c) speed & (d) direction for two days, off Cochin, west coast of India.

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Fig. 10—Fluctuation of selected isotherms plotted as time-series based on CTD yoyo at a selected location in the Arabian Sea

(Swain et. al., 2005).

Fig. 11—Frequency spectra for six selected isotherms (Swain et. al., 2005)

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The frequency spectra shown in Fig. 11 has

predominate peaks centered around 8 s with maximum

energy for all cases excepting the top 26°C isotherm.

In context of 26°C isotherms, high frequencies

oscillations are predominant. Average wave height and

the peak periods for the spectra shown in Fig. 11 varied

from 3.2 to 4.4 m and 7 to 40 s although the energy

levels are low for higher wave periods. This clearly

revealed the presence of high frequency internal

oscillations having intermediate periods between

the sampling frequency in the total length of the

time-series (CTD yoyo). Considerable progress has

been made to predict these waves with proper scientific

reasoning that was understood recently30

.

Oceanic Fronts and Eddies

Oceanic fronts and eddies are common

oceanographic phenomena which persists in various parts of the worlds oceans and play an important role in acoustic propagation

31,32. They have been reported

in the Arabian Sea and Bay of Bengal and also along the east and west coast of India by various investigators. A small scale front and eddy in the

coastal waters also play a significant role when the SONAR ranges of the order 5 to 15 kilometers. Large scale eddies are important for long range low frequency sound propagation. The density fronts that result due to variations in temperature and salinity in the coastal, nearshore and estuarine waters

influence the acoustic sensor performance. Suspended sediments may become trapped on these fronts resulting in turbid water columns which affect acoustic properties of the medium and thus the performance of acoustic sensors.

The above features are well identified and can be

routinely monitored using the satellite data. The spatial measurements (CTDs and hydrocasts) onboard research ships also form an important part of the database collected since several years of effort by many research organizations which have contributed towards the studies of fronts and eddies in the coastal

as well as deep waters. Presently, the most promising method for monitoring and predicting these phenomena is satellite measurements and models which can assimilate and predict their movements thereby its temporal evolution.

Nearshore & Estuarine waters

As stated earlier, the nearshore waters are highly

dynamic and mostly governed by the water

characteristics of the region/location and local

bathymetric conditions. Prediction in this region

(littoral zone) is a high challenging task. Waves feel

the bottom and break producing surf. The water depth

varies during the tidal cycle, which allow higher

waves to propagate during high tide from deep waters

and the water properties change accordingly. The

local winds (land and sea breezes), prevailing weather

conditions and freshwater discharges from land also

contribute towards the variability of the nearshore

waters considerably.

During one of the experiments, time-series and spatial observational data were collected from the coastal waters off Wheeler (up to 30 m water depth), north-east coast of India and the observed variabilities for a period of 24 hours is shown in Fig. 12. It may be

seen that the properties including water depth have varied dramatically during the semi-diurnal tide

20.

The observed salinity inside the estuarine waters varied from 15 to 31 PSU (includes all measurements time-series and spatial) which is due to the combined effect of diurnal as well as seasonal variations, tide

and mixing of fresh water discharged from the nearby rivers. The maximum current observed in the estuary was 1.2 m/s showing semidiurnal reversal along with the tide. The maximum tidal range measured at the outer Wheeler jetty was 3.1 m. The observed variations indicate a complex scenario for acoustic

propagation and detection in this region. Observed sea-state in the estuary was low

compared with the open coast/sea. The significant wave height varied from 0.2 to 0.9 m. One of the interesting features was the waves in the estuary were considerably high during the high tide compared to

low tide exhibiting a drastic variation of the sea-state during the day. This could be attributed due to the increased water depth as the waves are propagating from the deep waters. The spectral characteristics reveal single and multiple peaks (up to 3 peaks) with one or two high frequency peaks. The single peaked

swell spectra as shown in Fig. 13(f) was collected from a location which is open to the Bay of Bengal. The spectra show two prominent peaks which could be due to waves approaching the estuary from north-east and south-east directions, as there exists two open channels to the estuary from either side of the

observational point. The third peak noticed in the high frequency side of the spectrum is due to the presence of locally generated wind-waves within the estuary. The field experiment provided useful insights on the estuarine hydrodynamics and the predominance of the tidal currents. This information is essential for

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implementation in the hydrodynamic models and to validate the flow pattern within the estuary in presence of tidal variations (ebb and flood phase) prevalent inside the estuary.

Remote Sensing of Oceans Significant progress has been achieved in India

in demonstrating the utility of satellite based

remote sensing data for oceanographic research and

Fig. 12—Time-series plots of water depth, temperature, salinity, current speed, current direction at a location for 24 hours in Dhamara

estuary, north-east coast of India

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applications33

. Remote sensing of the oceans using

space based platforms is proven to be a feasible

alternative for obtaining the data in real-time for

operational utilization34

. Moreover, the present day

models are capable of providing information in the

three-dimensional ocean and atmosphere system over

a desired spatio-temporal resolution scales. This

cannot be obtained based on climatic database alone

real-time data alone or as simple the combination of

these two. Therefore, model based satellite data

assimilation for a variety of atmospheric and

oceanographic parameters can play a significant role

for naval defence, sea operations, routine exercises,

system evaluations, tactical operations and training of

personnel at sea.

In addition to the above, satellite remote sensing

can have several other feasible applications which are

well proven. The two-dimensional ocean wave

spectrum can only be measured from satellites (SAR

data) and it can provide the bathymetric data through

inverse modeling of waves and current besides the

surface signature of a submerged object or a

stationary underwater submarine most importantly in

the littoral waters. The blue-green laser based

measurements provide bathymetric information in

coastal waters and information on underwater moving

targets, submerged vessels in littoral and near shore

waters. Hydrodynamic signatures like surface wakes

and surface temperature anomaly due to moving ships

and submarines are detected by the satellites. The

satellites also provide information on ship traffic,

offshore operations/platforms, fishing boats, which is

the source of noise in the open sea. The measured sea-

bed microseism along the coast can provide

information on ships and submarines (coastal

surveillance) with the aid of satellite data on wind and

waves (source of ambient noise and surface

reverberation). Satellite based gravity and magnetic

anomaly detection can identify the movement of ships

or submarines. The satellite measurements can be

Fig. 13—Observed wave spectra from a selected location in Dhamara estuary, north-east coast of India

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effectively used to model the biological population in

the sea and its movement which contribute towards

biological noise in the sea. In the near future, satellite

remote sensing of the oceans will revolutionize our

understanding of the oceans.

High-resolution Ocean database As compared to deep waters, the detection of

underwater target in the coastal waters is relatively a

challenging task which demands high resolution

spatio-temporal information due to the rapid

variability in water properties35

. At any given

situation, the detection and discrimination of an

underwater target is highly dependent on the

propagation conditions of the medium as well its

surface and bottom conditions, leaving apart the

efficiency of the SONAR system and the type of the

target. It can be either a SONAR performance model

or an operational model for tactical warfare, one

needs to have knowledge on the predicted sound

speed for the 3-dimensional ocean environment. This

is true as it is not always feasible to depend only on

measurements as the surface and the medium

characteristics are highly variable both in space and

time scales in coastal and littoral waters. However,

field based measurements are essential for specifying

the initial and boundary conditions in ocean

environmental models which can supplement the post

analysis part of model predictions. Therefore, the

requirement of a high resolution database in space and

time need not be over emphasized for the coastal

ocean environment along the east and west coast of

India. One of the case studies utilizing the same data

as discussed before (Fig. 7) is used to study the short-

term variability of the predicted SONAR range.

The estimated/predicted SONAR ranges for three

selected frequencies (3, 9 and 14 KHz) for a source

depth of 5 m, FOM (Figure of Merit) 80 dB and target

depths 10 and 30 m as shown in Fig. 14 have revealed

that the dependence of SONAR detection range is

based on observed sound speed variations during the

two tidal cycles. Obviously, the detection range is

higher for the lower frequencies (3 and 9 KHz) as

compared with the higher frequency (14 KHz) but the

diurnal variations of the predicted range remains more

or less similar for all the three cases. The predicted

drastic changes in the detection range during tidal

reversals as observed from time-series data is very

essential for the operational requirements using under

water systems. It may be noted that, the over all

predicted range for three selected frequencies have

varied from 0 to over 50% of the average range

during a tidal cycle.

The composite plot of temperature (color

contours), tidal variation and SONAR detection range

(source 9.0 KHz, FOM 80 db, source depth 5 m,

target at 15 m) for 48 hours (Fig. 15) summarizes the

effect of environmental variations mostly the

temperature and salinity with the predicted detection

range assuming the bottom and surface parameter

unchanged. Kindly note that the graph shown for the

tidal variation in Fig. 15 indicates higher tidal level

which corresponds with higher depth scale, as the

water depth is plotted in negative scale. The observed

tidal range, while the ship was anchored is not

considered as the absolute tide at the observation

point. However, it clearly indicated the tidal cycle in

the coastal waters has certain time-lag compared with

the tides at Cochin. Comparing and considering the

extent of variations in temperature (and salinity) for

the corresponding tidal variation and the estimated

detection range demonstrates the necessity of short-

term predictions of the ocean parameters for

deployments of any under water systems and sensors.

Naval Operational Ocean Prediction System Unprecedented world political changes are

redefining national defense interests and altering the

research and development priorities of a nation.

Therefore, knowledge of the ocean, especially the

ocean acoustic properties of the Indian Seas (Arabian

Sea and Bay of Bengal) and coastal areas, is very

crucial and critical for our national defense. By

Fig. 14—Hourly variations of predicted SONAR range during two

tidal cycles for three selected source frequencies and source-target

depths

INDIAN J. MAR. SCI., VOL. 39, No. 4, DECEMBER 2010

644

utilizing the existing and ongoing experimental data

from the regions of interest, it is now feasible for the

implementation of an integrated “Naval Operational

Ocean Prediction System” which comprises the

waves, tides, circulation and internal wave models

being followed36-40

. The ocean environmental

information predicted by the integrated suite of ocean

models shall provide necessary inputs to the SONAR

range prediction models for routine operational use,

tactical operations and simulating warfare scenarios.

Acknowledgement

Authors express their sincere thanks to Director

and Group Head, Ocean Science Group, NPOL for

the facilities and support provided for this study.

Special thanks to all the cruise participants who were

responsible for the collection of field data utilized in

this paper.

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