21. Naval Systems:Naval Radars and Multi Mission Combat Systems

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(Published in Defence and Security of India, May 2013)

Naval Systems:Naval Radars and Multi Mission Combat Systems

 

The discovery of reflection of radio waves from solid objects was studied by Heinrich Hertz in 1886; subsequently Huelsmeyer in 1904 was the first to patent a detector (“Telemobiloscope”) which could detect metallic objects at a distance using radio waves. In 1922 Taylor and Young showed that range and bearing of ships can be determined using radio waves even in low visibility and darkness, but the US Navy did not accept their novel idea. Many patents were granted and work carried out in secrecy in a number of countries during the ensuing period till in 1934 the British decided to use it very effectively to detect and thereafter shoot down hostile German aircraft  by using a prototype made by Watson Watts. Automatic tracking of aircraft in azimuth and bearing and subsequently in range was also accomplished during the WWII itself. Though it took over 45 years since the initial discovery for the Radar to prove its tremendous utility in military and civil applications, Radar has continued its dominance as a formidable sensor for the past eight decades. Post WWII a major improvement was to introduce moving target indicator (MTI) function by using Doppler Effect, by which it was possible to discriminate between a stationary and a moving target. This was followed by the Phased array antenna technology which involved 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.

The aim of this article is to provide the discernible reader with  a bird’s eye view of the naval radar world covering major relevant aspects, in as simple terminology as feasible.

Classification of Radars

A radar system comprises of a transmitter, a receiver, a signal processor and an antenna. Radar is used in a range of diverse applications in the civil and military field. The applications include, weather sensing, air traffic control, navigation, target detection, acquisition and tracking, missile and gun direction, air borne systems, research and so on.  Military radars can be classified in many ways; for example based upon the type of platform, i.e. land based, ship borne or air/space borne; or mission based for example, early warning, tracking, fire control, weather etc; or they may be classified based upon radar characteristics like wave form, frequency used, type of antenna etc. Most prevalent classification is according to frequency or wave form utilised.

Frequency Based Classification. The frequencies that have been longest in use are in the band 3MHz to 300MHz. Over the horizon radar (OTH), i.e. radars which can detect targets beyond the radar horizon and the early warning radars use the high frequency (HF) band 3MHz to 30MHz (eg. Russian Woodpecker and US Navy’s AN/TPS-71 Relocatable OTH radar). The accuracy in this type of radars, however gets comprised 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 also. 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 and therefore are limited to very short ranges and have applications in civil/police/research requirements.

Wave Form Based Classification. The radars can also be classified as continuous wave (CW) or pulsed radars. As the name depicts, CW radars emit continuous waves and therefore have distinct transmitting and receiving antennas. CW radars can determine the radial velocity and bearing of the target accurately and are used for missile guidance. The pulsed radars use a sequence of modulated wave pulses and can be further classified in to; high & medium pulse repetition frequency (PRF) radars used for velocity determination, and low PRF radars used for range finding purposes.

At this juncture it would be appropriate to briefly discuss some features which affect the detection, range, bearing and display of targets in naval radars.

Features Affecting Detection, Range, Bearing and Display of Targets 

The maximum detection range of the target is affected by the frequency of operation, since lower frequencies attenuate less, therefore lower the frequency higher the range of the radar. It depends also upon the peak power output, i.e. more the power higher the range. Further longer the pulse length, more the power radiated on to the target hence longer the range. Since the difference between two transmitted pulses should be more than the maximum designed range of the radar to avoid echoes from different targets from overlapping, the maximum measurable range of the radar is also a factor of the pulse repetition rate (PRR). In addition the range is  affected by target characteristics & aspect presented (large or small, material, RCS etc), and the antenna rotation rate (lower rotation rate increases chances of smaller targets to be detected at maximum range). Radars with longer wavelengths suffer lesser attenuation at sea and therefore can detect targets at a range longer than radars with a shorter wave length.

The minimum detection range of radar is dependent mainly upon its pulse length and is ideally given by half of the pulse length of the radar (150 meters per microsecond of pulse length). It is however affected by inherent electronic circuit delays, side lobe echoes, sea clutter and the design of the vertical beam width. The range accuracy is dependent upon the precision in measuring the interval between the transmitted pulse and the received echo. Further it is dependent upon the fixed inherent errors, constancy of voltage, variations in transmitted frequency, incorrect calibration and interpretation of the target. Range resolution or the clarity, in lay terms, of separation between two targets on same bearing but nearby in range mainly depends upon the length of the transmitted pulse. The targets cannot be distinguished if they are on the same bearing unless they are separated by a distance more than half the pulse length. For example radar with pulse length of 0.05 microseconds would be able to distinguish targets which are more than 7.3 meters apart only. The target separation on the display also depends upon the proper adjustment of the gain of the receiver unit, size of the CRT spot, and range scale selected.

The bearing accuracy is dependent mainly upon how narrow the horizontal beam width is, which in turn provides clearer definition of the target. It also depends upon the size of the target, its rate of movement, parallax errors etc. Bearing resolution is the ability of the radar to separately present two targets at the same range but on different but close bearings. The bearing resolution is dependent upon the width of the horizontal beam, as the target blip on the display suffers an angular aberration up to the size of the horizontal beam width. Further two targets at the same range must be angularly separated by more than the horizontal beam width to be clearly discernible on the display. Thus for a 2 deg beam width, targets at a range of 8km should be separated by at least 320 meters. Hence a narrower horizontal beam provides a better bearing resolution.

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 eg. 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 eg. 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 defence, 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.

Multi Mission Combat Systems.            

Multi mission combat systems have been designed for operation from naval ships as a total weapon system from detection to final destruction of the threats emanating at sea from air, surface and subsurface. Such a system is expected to search and track multiple targets and also carry out weapon direction by use of a sophisticated radar/sonar and competent decision making component.  By use of this design a naval ship can effectively carry out anti-air, anti-surface and anti-submarine operations concurrently. The most effective and proven multi mission combat system which deserves mention here, with reference to the usage of sophisticated radar, is the Aegis weapon system.

“… and among them went bright-eyed Athene, holding the precious aegis which is ageless and immortal: a hundred tassels of pure gold hang fluttering from it, tight-woven each of them, and each the worth of a hundred oxen.”

Iliad, Homer, Martin Hammond’s Translation

 

Aegis weapon system comprises of the AN/SPY-1 type radar, the Command and decision suite, Mk 99 Fire control system and Standard Missile family of weapons. The  AN/SPY-1 type radar, is an advanced multi function phased array, auto track and detect  radar which can track more than 100 targets at over 190km and also carry out missile guidance functions. It communicates with the Standard Missile for mid course correction using an RF link, terminal guidance is carried out by the AN/SPG 62 radar. The target is handed over by the AN/SPY-1 type radar to the AN/SPG 62 radar, which is a continuous wave, illuminating I/J Band fire control radar. The Aegis  is fitted on many of US Naval cruisers, destroyers and frigates. The current modifications include Commercial Off the Shelf (COTS) networking system infrastructure and Multi mission signal processor. This would also simultaneously address ballistic missile defence (BMD) along with anti-air warfare against multi mission threats. The Aegis BMD is capable of neutralising short to intermediate range ballistic missiles with Standard Missile-3 (SM-3), further it can destroy short range ballistic missiles in terminal phase with Standard Missile -2. Equivalent anti-submarine capabilities are also part of the Aegis weapon system.

Radars with 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 recently delivered to India will 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 also classify surface targets at long ranges.

Future Trends

       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 broad band in the public arena. These are likely to impact the defence sector and soon such mass produced devices (albeit manufactured to stricter specifications) would be available for defence use. Thus the trend is a reversal of defence requirement based technology development to mass commercialisation 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 defence.

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 utilising 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 till now. Quantum radar is currently at the concept stage.

Conclusion

Radar technology has by and large kept pace with the miniaturisation as well as digitisation of electronic components. It has also been possible for the radar designers to meet the changing multi mission requirements of modern naval warfare post the cold war. Today, using single multi function radar, a ship can track and attack emerging fast cruise missile and aircraft threats, ballistic missile attacks, swamp attacks by fast small craft in littorals and also carry out various missile & gunfire surface warfare functions.The versatility and adaptability of the radar technology has thus ensured its continued relevance to the naval designers and war fighters.

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