(Published in SP’s Military Year Book 2015)
We may produce at will, from a sending station. an electrical effect in any particular region of the globe; we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed.
— Nikola Tesla, ‘The Problem of Increasing Human Energy’, The Century (Jun 1900)
Sensors ensure the survivability of a warship at sea during peacetime as well as hostilities. Warships at sea are buzzing with inputs from a multitude of sensors. A warship’s basic sensors are those whose outputs are required for practically all operations at sea. These include meteorological sensors, conductivity, temperature & density sensors, communication sensors, ships speed sensors or logs, depth sensors or echo sounders and satellite signal receivers. Apart from these, a ship utilizes Radar and Sonar for its peacetime and combat operations.
Meteorological Sensors. A warship requires accurate measurement of wind speed and direction, temperature, pressure, humidity and other local environmental parameters. This is required for various tasks including flight operations, gunnery, rocket and missile firings etc. AGIMET is one of the manufacturers for such systems.
Speed Log. For measurement of a ship’s transversal and longitudinal speed, single and dual axis speed logs as well as dual axis doppler logs, are available. The speed logs provide ship’s speed, drift speed and angle at all times and in any depth. Raytheon Anshutz manufacture some of the popular ship’s logs.
Conductivity, temperature, and density (CTD) are used extensively for the measurement of temperature and salinity, as also for deriving parameters of density and speed of sound. Teledyne RDI Citadel CTDs fall under this category.
The Expendable Bathythermograph(XBT). It is used by warship to obtain an ocean temperature versus depth profile. It is useful for anti-submarine warfare (ASW) by warships and for anti ship warfare by submarines. Lockheed Martin Sippican has manufactured over 5 million XBT’s since the 1960’s.
Echo Sounder. Data consisting of the immediate depth and a record of soundings are required for navigation. Kongsberg’s EN 250 is one such navigation echo sounder.
Communication Systems. Navies use visual, sound, and electrical means for communications. Telecommunication includes in its ambit transmission, emission, signals, images, sounds, and intelligence information by visual, oral, wire, radio, or other electronic systems. Since these systems, fundamentally sense electromagnetic radiation these also come under the overall ambit of vital sensors for the Navy.
Satellite Signal Receivers for Communication and Navigation. As far as communication systems are concerned, use of satellites is fairly well understood and is common knowledge with deep inroads made by mobile telephony and internet. Methods of navigation have changed throughout history. Satellite navigation using radio signals from satellites for determining position have enhanced the mariner’s ability to complete his voyage safely and expeditiously. Modern integrated systems take inputs from various ship sensors, electronically and automatically chart the position, and provide control signals required to maintain a vessel on a preset course.
Radar has continued its dominance as a formidable sensor in both the civil and military domains. Post WWII a major improvement was to introduce moving target indicator (MTI) function by using Doppler Effect, where in it was possible to discriminate between a stationary and a moving target. This was followed by the Phased array antenna technology involving dynamic beam forming by combined operation of a number of individual transmitting elements. Strides in digital signal processing led to development of the synthetic aperture radar and consequently to high-resolution imagery.
Frequency Based Classification for the Navy. The frequencies that have been longest in use are in the band 3MHz to 300MHz. Over the horizon radar (OTH), and the early warning radars use the high frequency (HF) band 3MHz to 30MHz (e.g. Russian Woodpecker and US Navy’s AN/TPS-71 Re-locatable OTH radar). The accuracy in this type of radars however is compromised while gaining the range advantage. Very long-range early warning radars use the very high frequency (VHF) band in the range of 30MHz to 300MHz, or the ultra high frequency (UHF) band 300MHz to 1GHz, this band is very useful in detection and tracking of ballistic missiles. Frequency band 1GHz to 2GHz (L band) is used in naval applications of long-range air surveillance. The SMART-L naval radar has a phased array with 24 elements; it has a maximum range of 400km against patrolling aircraft and 65km against an incoming missile. The band 2GHz to 4GHz (S band) is used for Air Borne Warning and Control Systems (AWACS), Boeing E-767 AWAC aircraft uses the AN/APY-2 Pulse Doppler radar, it can determine the velocity of the target as well as distinguish between airborne and maritime targets from ground interference and sea clutter. The band 4GHz to 8GHz (C band) is used for weapon guidance; these are small but highly precise radars. An example is the TRS -3D naval radar for weapon guidance and surveillance, it uses a phased array in 3D for simultaneous detecting and tracking of multiple targets up to a range of 200km. It is designed for detecting sea skimming missiles and attack helicopters. The band 8GHz to 12.5GHz (X band) is used for maritime navigation and airborne radars. The naval Active Phased Array Multifunction Radar (APAR) works in this frequency band, it is capable of automatic detection and tracking of low-level sea skimmers up to 75km and is designed for carrying out terminal guidance requirements of ESSM and SM-2 missiles. The higher frequency bands from 12.5GHz to 40GHz are subject to very high attenuation, therefore are limited to very short ranges, and have applications in civil/police/research requirements. Some prominent radar systems are-
-Enterprise Air Surveillance Radar (EASR) is a development program for replacement for the SPS-48 and SPS-49 air surveillance radars currently on board US Navy’s amphibious ships and aircraft carriers by the 2020. Northrop Grumman has been awarded an 18-month contract for the study of the EASR requirement. The new radar system will utilize technologies from the AN/TPS-80 Ground /Air Task-Oriented Radar (G/ATOR) program.
-Empar (European Multifunction Phased Array Radar) is a G-band, multifunction, active phased array radar being developed by Selex for the Italian Navy and French Navy. Its rotating antenna at 60 rpm provides continuous surveillance, tracking, and weapons fire control. The Empar radar system will be integrated on the Horizon frigates ordered by Italy and France and the Italian Navy’s Conte di Cavour.
-Raytheon’s AN/SPY-5 is an X-band multi-tracking, target-illuminating system for surface combatants that can simultaneously search, detect, and precisely track multiple surface and air threats. The SPY-5 is an open architecture, phased-array radar system, providing an advanced self-defense solution for small and large surface ships operating in the littorals and other maritime environments. It is compatible with all digital combat management systems, and the radar’s range, accuracy, and beam agility enable the full performance of the Evolved Sea Sparrow Missile (ESSM).
Some Specific Types of Radars
Stealth Radars – Low Probability of Intercept Radars (LPI). LPI radars transmit weak signals, which are difficult to detect by an enemy intercept receiver. This capability is attained by the use of specific transmitter radiated waveform, antenna, & scan patterns and power management features. The LPI radars are continuous wave, wide bandwidth radars emitting low power signals. This makes LPI radars difficult to detect by passive radar detection systems. Such radar is used in Super Hornet aircraft of the US Navy.
2D, 3D, and 4D Radars. A 2D radar provides range and azimuth information about the target. 3D radar, in addition provides the elevation information. These are of two types namely; Steered beam radars, which steer a narrow beam through a scan pattern to generate a 3D picture, for e.g. AN/SPY-1 phased array radar on Ticonderoga class of guided missile cruisers; and the Stacked beam radars which transmit and receive at two different angles and deduce the elevation by comparing the received echoes, for e.g. The ARSR-4 radar with a range of over 250 miles.
4D radar is Pulse-Doppler radar capable of 3D functions and determines a target’s radial velocity as well. This type of radar has great applicability in defense, since it can detect targets by removing hostile environmental influences such as electronic interference, birds, reflections due to weather phenomenon etc. In addition, a 4D radar uses much less power and thus helps in stealth function. TRS-4D surveillance radar with Active Electronically Scanned Array (AESA) technology is in use by the German Navy.
Radars – Indian Navy
Indian Navy has various types of indigenous and imported radars. Among the indigenous radars, it has L Band surveillance radar RAWL MK II &III; F Band combined warning and target indication radar RAWS 03 Upgrade, 3D surveillance radar Revathi and navigation radar APARNA etc. Among the imported radars, it has a mix of radars from both the east and the west. Some of the imported radars are; MF-Star 3D phased array radar,MR-760 Fregat M2EM 3-D,MR-90 Orekh fire control radar, Signaal D Band radar,MR-310U Angara air surveillance radar, MR-775 Fregat MAE air surveillance radar, Garpun-Bal fire control radar, MR-352 search radar etc. The P8i Maritime patrol aircraft be operating AN/APY-10 multi function, long-range surveillance radar, capable of operating day and night under all weather conditions. It provides mission support for ISR, anti-surface and anti-submarine warfare. It has both Synthetic Aperture Radar (SAR) and Inverse SAR capability, the Inverse SAR can detect, image and classify surface targets at long ranges.
Some of the indigenous Radars manufactured by BEL, India are-
-L- Band Surveillance Radar, RAWL02 Mk-III, is long-range L band surveillance radar for detection of air and surface targets. It has a roll and pitch stabilized antenna platform, Synthesizer controlled transmitter with TWT amplifier, state of art video extractor track management system based on COTS technology, low noise receiver combined with split pulse and matched dynamic range compression, ECCM capability and a range of 270 Km.
-3D Surveillance Radar, REVATHI, is a state-of-the-art, S-band, Track-While-Scan (TWS) radar designed to effectively play the role of a medium range surveillance radar mounted on a stabilized platform for detection of air and surface targets. It has ECCM features, integrated IFF Mk XI , stabilization against roll & pitch, and remote transmission of data of tracks & plots over LAN for interface with external systems.
-Active & Passive Radar for Navigation & Attack (APARNA), is designed to detect surface targets, furnish target data to weapon computer for missile firing at these targets in the autonomous mode from the ship. The radar system is provided with two transmitter–receiver channels i.e. the first or main channel and the second or navigational channel. The two channels differ in transmitter peak power, pulse width etc.
Future Trends in Radar Technology
Some of the discernible future trends in radar technology are-
Commercial off the Shelf Components (COTS). New technologies are being developed rapidly in the commercial sector for low cost manufacturing processes of RF and microwave devices due to very heavy penetration and demand of smart mobiles and broadband in the public arena. These are likely to influence the defense sector and soon such mass produced devices (albeit manufactured to stricter specifications) would be available for defense use. Thus, the trend is a reversal of defense requirement based technology development to mass commercialization driven innovation. A wide range of Gallium Arsenide (GaAs) Monolithic Microwave Integrated Circuits (MMICs), RF power amplifiers, and other RF devices already developed in the commercial sector have direct applications in Radars and other RF devices in defense.
Cognitive Radar. The term cognitive radar implies a radar that has tremendous transmit/receive adaptivity and diversity along with high performance inbuilt intelligent computing. With the inclusion of environmental dynamic database and knowledge-aided co-processor, it is feasible to add new sources of information, which facilitate additional adaptivity. Currently new generation cognitive radars are at the design stage.
Quantum Radar. Quantum illumination has been tested up to a distance of 90 miles and it is believed that soon it will be possible to establish much longer ranges utilizing this principle of bouncing photons off a target and comparing them with their unaltered twin. It has been observed that the amount of information so gathered is much more than that available through conventional RF beam reflection from objects. Since energy, quanta behave both as a wave and as a particle; it would be possible to design quantum radar. It is expected that such quantum radar would provide a many fold increase in information parameters and data about the target than has been feasible until now. Quantum radar is currently at the concept stage.
We were told that it was impossible to grapple with submarines, but methods were found … Many things were adopted in war which we were told were technically impossible, but patience, perseverance, and above all the spur of necessity under war conditions, made men’s brains act with greater vigour, and science responded to the demands.
— Winston Churchill, 1935
Sonar systems have benefited enormously with the advances in digital electronics, and signal processing. Many algorithms applicable to radar systems have been adapted in sonar. Use of Synthetic aperture methods in sonar has increased the quality of image and robustness of the system. Use of multiple transducer sensors and sophisticated beam forming techniques adapted from improvements in target detection in radar has yielded similar benefits in sonar.
-Thales Underwater Systems has developed and produced Sonar 2087.
It has been designed to be a variable depth, towed active and passive Sonar system that performs in conjunction with Sonar 2050 bow-mounted active sonar on UK’s Type 23 frigates. Digital technology in signal processing and COTS hardware has been used extensively. It is claimed that S2087 will be suitable for both, littoral environments and Deep Ocean.
-Raytheon has developed the AN/SQQ-90 tactical sonar suite for the US Navy’s DDG 1000-class multi-mission destroyer. The AN/SQQ-90 comprises of the AN/SQS-61 hull-mounted high-frequency sonar, AN/SQS-60 hull-mounted mid-frequency sonar, and the AN/SQR-20 multi-function towed array sonar and handling system.
-Atlas Elektronik will supply Active Towed Array Sonar, ATAS to the Indian Navy, which will equip the Delhi and Talwar class ships. ATAS would be subsequently manufactured in India under cooperation with BEL.
-EdgeTech, has delivered 12 advanced side scan sonar systems (mine warfare) for the Indian Navy.
Indigenous Sonars – Indian Navy
Indigenous Sonars held by the Indian Navy are manufactured by BEL. Two important Sonars manufactured by BEL are the Advanced Active cum Passive Integrated Sonar System (HUMSA NG) and the Integrated Submarine Sonar (USHUS).
-HUMSA-NG is an advanced Active cum Passive integrated sonar system to be fitted on a wide variety of Indian Navy platforms such as the Project 17, Project 15A and Project 28 class ships. HUMSA-NG is an advanced version of the existing HUMSA sonar presently fitted on P16, P15, Ranjit, and Talwar Class of ships. The HUMSA (NG) is designed for enhancing the system performance, reliability, and maintainability. It is capable of detecting, localizing, classifying, and tracking sub-surface targets in both active and passive modes. The system provides simultaneous long-range detection in active and passive modes. The sonar is capable of localization and automatic tracking of up to eight targets in both active and passive modes.
-Integrated Submarine Sonar (USHUS) is used to detect, localize, and classify underwater submerged and surface targets through passive listening, interception of signals and active transmissions of acoustics signals. Its passive sonar has preformed beams in azimuth and in three vertical directions using ASICS. It can auto track six targets and its active sonar has CW and LFM modes of transmission. Its intercept sonar can provide early warning long range target detection, all round coverage in three bands, FFT, and Spectral processing. The underwater communication system has multiple mode acoustic communication in dual frequency to meet NATO and other requirements, voice, telegraph, data, and message modes of operation. Its obstacle avoidance sonar is a high frequency short range sonar with rectangular transducer array and its transmission covers three sectors of 30° each.
ASW Sensors on Naval aircraft
These are of two types namely acoustic and non-acoustic sensors. The non-acoustic sensors include radars, electromagnetic emission sensors, magnetic anomaly detectors (MAD), and infrared receivers. Many Air ASW radars employ multiple radar frequencies, transmission patterns, scan speeds, pulse lengths, and noise reduction techniques. These radars are lightweight, and in addition to ASW operations, they are utilized for surface surveillance, and navigation. Some prominent radar systems used on board Naval ASW aircraft include the AN/APS-137 (S-3B, also P-3Cs), AN/APS-124 (SH-60B), and AN/APS-115 (P-3C). As far as MAD sensors are, concerned naval ASW aircraft use the AN/ASQ-81 MAD system. Its advanced version using digital processor based system the AN/ASQ-208 has already been fitted on a few P-3C aircraft. ASW aircraft EM systems are designed to search mainly for radar signals. EM systems on naval ASW aircraft include the AN/ALQ-142 on the SH-60B Seahawk, the AN/ALR-76 on the S-3B Viking, the AN/ALQ-78 and AN/ALR-66 series on the P-3C Orion. Among the infrared sensors either Infra-Red Detection System (IRDS) or Forward Looking Infra-Red (FLIR) are used. These are used for ASW as well as surface surveillance roles. As an illustration the sensor package for the Sikorsky SH-60 Seahawk includes; second generation integrated AAS-44 Forward-Looking Infrared (FLIR) system for expanded night vision and HELLFIRE targeting capability, new APS-147 multi-mode radar with long/short range search Inverse Synthetic Aperture Radar imaging and periscope detection modes, integrated AQS-22 Airborne Low Frequency Sonar with expanded littoral and deep-water capability including concurrent dipping sonar and sonobuoy processing capability, advanced ALQ-210 Electronic Support Measures (ESM) system for passive detection, location and identification of emitters.
Advances in Submarine Sensors
Advances in submarine sensors include Acoustic Rapid COTS Insertion (ARCI) this takes in to account the applicability of advances in commercial technology to acoustic sensors. With the same sonar arrays, ARCI has demonstrated significant improvement in performance of sonar. ARCI has been designated as the baseline sonar system for the VIRGINIA Class SSN. Another is the development of High Frequency Sonar especially for utilization in the littorals. It would provide detailed information about the undersea environment. Conformal sonar arrays make available an optimally sensor coated submarine with improved stealth. Conformal Acoustic Velocity Sonar (CAVES) would be replacing the Wide Aperture Array technology in the VIRGINIA Class submarines.
Fielding of unmanned underwater vehicles (UUVs) with advanced sensors and weapons, form SSNs would allow SSN to gain access to denied areas like mined waters, very poor acoustic conditions, or extremely shallow water. Missions that the UUVs would be performing include Intelligence, Surveillance & Reconnaissance (ISR), Mine Warfare (MIW), underwater sensing and mapping. The Long-term Mine Reconnaissance System (LMRS) with UUVs would significantly enhance a submarine’s mine hunting capabilities.
Future Trend – Consolidated Antennas and Sensors
A warship requires concurrent functioning of various navigation, combat, and communication systems. Thus, information flow is necessitated between various systems and equipments for e.g. a warship’s navigation and combat systems require information of ship’s course, speed, water depth, and geographical position. The sensors have to feed different systems simultaneously in an integrated manner. This implies in tandem functioning of different systems in a coordinated and unified manner. This is a formidable task since systems are highly complex, diverse electronic units sourced from multiple sources with different standards. The integration unit should be able to comprehend the language of different units, extract relevant information, and feed it to systems in the acceptable format. It should have flexibility to integrate upgrades and new equipment. In addition communication technology developments to provide ever-increasing requirements of multiple bands and bandwidths, foresee a need for large rotating antennas. These pose several problems on board warships like space availability, electromagnetic interference and increase in ships radar signature. The trend is tilting towards development of single unit consolidating antennas and sensors. Thales Netherlands is developing its integrated sensor and communications suite, which will house radio and data-link communication systems, radar and electro-optical subsystems and IFF in a single unit. The US Navy has awarded 18 contracts to develop integration and management technology for radio frequency radar and communications functions. The objective of the advanced multifunction radio frequency concept is the integration of radar, electronic warfare and communications into a common set of apparatus with signal and data processing, signal generation and display hardware.
Thus from the above it can be appreciated that the field of sensors for utilization on a warship is an ever expanding one with new features and capabilities adapted from the commercial world being added practically every hour. There are going to be phenomenal additions to the features and capabilities of various war ship sensors by end of this decade.