Security Research Review

Evaluating India's Land Based Missile Deterrent

Sanjay Badri Maharaj and Arun Vishwakarma

India's land based missile deterrent comprising of the Prithvi and Agni families of missiles has been the subject of much commentary post 1998. This article attempts to bring together all available data in the public domain on this issue. We also present summaries of available information on the re-entry vehicle technology available to India, warhead options, production and deployment issues. Lastly we present an estimate of the total size of the deterrent. 

Contents 
Introduction
The Prithvi SRBM
Agni Missile Family
Warhead Options
Production and Deployment
References and Footnotes

Introduction

India ’s land-based missiles can be divided into the Prithvi and Agni families of missiles. The broad dimensions and line drawings of the same are presented below for easy reference.

Figure 1: Line Drawings of the main missile families 

The Prithvi SRBM

The Prithvi is a mobile battlefield support system with a range of 150-330 km and a payload of 500 kg to 1,000 kg.1 The system is mounted on an 8 x 8 wheeled transporter-erector-launcher (TEL), based on a Czechoslovak TATRA truck chassis.2 The utility of these tactical SSMs depends on the nature of the payload carried and on the accuracy. The Prithvi has an ability to carry a heavy conventional payload as well as adequate for a nuclear, biological or chemical warhead but based on all the information published so far, India is currently deploying the system mainly with conventional warheads.

Many interchangeable warhead types are to be fitted to the Prithvi, including a high explosive (HE) monolith, a pre-fragmented anti-personnel warhead, cluster submunition bomblets for use against armour, incendiary, runway denial munitions and allegedly the ‘Type 77’ fuel-air explosive (FAE).3 The missile is supposed to be able to manoeuvre up to 15 degrees in flight as well as being ‘highly accurate’.4  Published figures give the CEP of the Prithvi as less than 250m at maximum range;5 however, other estimates from fairly authoritative sources, give the CEP as 100m.6 The discrepancy in these two figures has not been fully explained. It should be pointed out, however, that in the early years of the flight test program, many skeptical analysts had a tendency to downplay India ’s abilities in the missile field. It is unfortunate that these tendencies still continue, in some Western circles at any rate to this day, in spite of evidence to the contrary.  Another more interesting report from the last two tests of the Prithvi gave the CEP of the missile as an incredible 10m!7 The latter figure was achieved at a range of 67 km after the missiles were fired from pre-surveyed sites. If the system is fired from an unsurveyed site, the accuracy is estimated at 150m at a range of 150 km.

An advanced version of the basic Prithvi has recently been tested. On March 19, 2004 , the ‘Prithvi-III’ was tested.8 This has a range of 250-330 km and a payload of between 500 and 1,000 kg.9 Of even greater significance is that it used INS-GPS integrated avionics and its CEP is estimated at a mere 50m at maximum range.10 This could be due to the fact that a terminal guidance system is being developed for this version of the Prithvi.11 This could also be due to the fact that the missile was fired from a pre-surveyed launch site.

A more advanced Prithvi-III was tested on October 27, 2004 when it was launched from a launch platform simulating an underwater launch.12 The Prithvi-III is a solid-fuel, medium range, ballistic missile (MRBM) of about 350 km range with 1,000 kg payload; the range with a 500 kg payload would be significantly higher. It is a two stage, solid fuel, road-mobile, short-range surface-to-surface missile. Sagarika and Prithvi-III are two different names for the same missile. This medium-range missile can also be launched from a submerged submarine (Project K-15).13, 14

The initial user of the Prithvi is the Indian Artillery Corps with the 333 Missile Group being the first unit to receive the type.15 The group consists of three SSM sub-groups, each with four launchers and a support sub-group with warhead change vehicles and missile reloads. Survey and meteorological sections are also attached. A 222 Missile Group has subsequently been raised, to be followed eventually by 444 and 555 Missile Groups.16

It is not known how many reloads will be allocated per launcher. This will only be known by the Corps and Army commanders. The numbers will, of course, depend on the roles assigned to the Prithvi in Indian Army service.17 Although the Indian Army has issued manuals on the deployment of the Prithvi, its use has not been made clear. It is clear, however, that the Indian Army believes the Prithvi to be a viable tactical battlefield weapon. The Regiment of Artillery has started training personnel to operate the system and has developed a sophisticated simulator to assist in this task. 

Nonetheless, there are some major concerns regarding the availability of adequate storage space for the missiles already produced. Based on available data, some 100-150 Prithvi SRBMs have been produced, but how many have been issued is unclear.18 During the 1999 Kargil confrontation, it has been alleged that 4 Prithvi SS-250 were fitted with nuclear warheads to guard against any Pakistani nuclear blackmail. Prithvi is essentially a long range artillery system with deep reach behind enemy lines, degrading enemy war making capability (in conjunction with special force units operating deep behind the enemy rear) at the start of hostilities, preventing the enemy from consolidating at a marshalling area for attack / counter attack. Its heavy payload, range and accuracy allows the army to take out command centres, and fragmented sub-munitions can take out thin-skinned enemy vehicles over a select area. Press reports indicate that the Prithvi-III MRBM also has a theatre nuclear role.

Prithvi Missile Systems are gradually being inducted into the IAF. These missiles are a longer-range version meant for SEAD (against enemy runways and SAM batteries). Three squadrons along with the associated ground support equipment are planned for induction along with the Prithvi Tactical Air Centre (TAC).19

Agni Missile Family

The Agni missile family is envisaged to be the mainstay of the Indian missile-based deterrence. The Agni family will continue to grow, providing breadth of payload and range capabilities.

The Agni missile family consists of (in chronological order of development):

• Agni-TTB (Technology Test Bed);

• Agni-II (IRBM);

• Agni-I (MRBM);

• Agni-IIA (referred hereafter as Advanced Technology variant) awaiting testing;

• Agni-III (IRBM) awaiting testing.

The Agni-TTB (Technology Test Bed)

Compared to the Prithvi, the ‘Agni’ is a much larger system with a range of 2,500 km and a payload of 1,000 kg.24 The original ‘Agni’ was an amalgam of the Prithvi and the SLV-3 booster. The ‘Agni’-TTB was a cheap test vehicle to develop re-entry and guidance technology for use on a more advanced platform.25

The last test of the basic ‘Agni-TTB’, which took place on February 19, 1994, appeared to be a major technical breakthrough for India . The system tested included a manoeuvrable re-entry vehicle for increased accuracy with terminal guidance.26 This terminal guidance system is reported to be comprised of a scanning correlation optical system based on a scanning focal plane homing head in the infrared and millimetric wavelengths of the electromagnetic spectrum.27 It is not known if this terminal guidance system has been fully developed.

Considerable uninformed comment exists regarding the fact that Agni was only tested to a range of 1,450 km. No missile actually needs to be tested to full range. It is possible to lift or depress the trajectory of the missile to simulate a longer range.28

Dr Abdul Kalam stated that the missile could be fully deployed within two years.29 Dr Kalam also asserted that the ‘Agni’ was ready for serial production while some simultaneous development flights aimed at achieving a much greater performance were undertaken.30 He claimed that no further test flights were necessary for the basic ‘Agni’ system.

The Agni-TTB programme apparently ran its course with the development and proving of crucial technology for full-fledged multi-staged long-range ballistic missiles, including re-entry and navigation avionics. This missile remained in engineering status and it is believed that none were released to the military, although during the Kargil imbroglio, a few units were made ready as nuclear deterrence. This missile is believed to not exist anymore, having been superseded by the Agni-II that has been on line production and operationalised.

The Agni-2 IRBM    

The Agni-II was first tested on April 11, 1999 . This missile has a theoretical maximum range of some 3,000 km with a 1,000 kg payload. Tested to a range of over 2,000 km, the Agni-II has an all-solid propellant system – though a liquid-fuelled configuration is also available and an improved guidance system – incorporating a Global Positioning System (GPS) has been alleged.31 A repeat test took place on January 17, 2001 , after which the missile was cleared for production. It is possible that a production capacity – under utilised at present – exists for 12 Agni-II missiles per year.32 In the January 17, 2001 test, it was claimed to have covered a range of over 2,100 km with a 700 kg warhead. As this test was described as being in operational configuration, it might be assumed that the Agni-II will be deployed with a 700 kg warhead. The range differential between the first and second tests can be explained by the use of a different trajectory and flight profile.

Technical Aspects

Agni-II is a 2-stage missile; both the stages are solid fuelled.

Propulsion

First Stage

The Agni-II first stage is largely the same as that of Agni-TD. However, Agni-II is believed to use a more energetic fuel, similar to that used on PSLV’s booster stage with an Isp of 269 (vacum) and 237(sea level). The first-stage solid motor case is made of high-strength 15CDV6 steel and is fabricated using conventional rolling and welding techniques. Agni-TD used AP-Al-PBAN composite propellant, but Agni-II onward, DRDO switched over to HTPB [hydroxyl-terminated polybutadiene]. The booster motor is one metre in diameter and ten metres long. It has a propellant mass of 9 tonnes and mass fraction of 0.865 (estimated). The stage has 3 segments propellant grain, with internal star configuration33 (for increased thrust during initial boost phase) and a loading density of 78 per cent. It is case bonded with a liner system between propellant and insulation.

The motor’s nozzle is built from 15CDV6 steel; a carbon-phenolic thermal protection system is used for the convergent throat, high-density graphite is used for the throat, and carbon and silica-phenolic lining is used in the fore end and aft end of the divergent.*

Second Stage

Agni-II second stage is solid fuelled and weighs about 4,200 kg. It has 1-metre diameter, about 4.8 metre length and is connected to the booster stage via a vented interstage. Its case is likely made of the same material as the booster stage (high-strength 15CDV6 steel) for ease of manufacturing. The stage has flex nozzles for thrust vectoring for trajectory control.

Manoeuvring Re-Entry Vehicle: Agni RV-Mk2

The Agni’s re-entry vehicle is designed to ensure that the temperature inside the vehicle does not exceed 60°C, a condition necessary to protect the warhead and electronic systems placed inside. The Agni’s Manoeuvring Re-entry Vehicle (MRV) is made of multi-directionally carbon fibre woven reinforced carbon-carbon composite material.34 The 0.8m diameter and 4 metre long re-entry vehicle consists of five sections. Each of these sections is made up of a two-layer composite construction. The inner layer is made up of a carbon/epoxy filament mould constructed on a CNC winding machine. The inner layer is designed to bear structural loads. The outer layer is made up of carbon/phenolic filament wound construction, and cured in an autoclave at 7 bar pressure.35 The outer ablative layer ensures high thermal robustness for shock and temperature extreme.

The 1980 vintage RV was reportedly originally designed to be able to carry a BARC developed boosted nuclear weapon of 200kT yield weighting 1,000 kg (1980 vintage design). After making room for a new and lighter Indian Thermo-nuclear weapon payload (1995 vintage design), the MRV has room for about 200 kg (estimated) liquid fuel in pressurised vessels. For velocity correction though, approximately 50-80 kg is estimated to be sufficient. At least one variant type uses a set of solid fuelled cartridges for velocity trimming.

The Re-entry Vehicle (RV) is reported to have an attitude control system and aerodynamic manoeuvre fins, that also make missile defence more difficult.36 Unconfirmed reports suggest that an improved optical or radar terminal phase correlation system has been developed to provide accuracy of around 40m CEP, although later reports have suggested that the accuracy was around 100 to 200m CEP. The RV largely inherits the basic shape, design and technology of the earlier Mk.1 RV of the Agni-TD.

The Agni is unlike long-range missiles developed by western countries, where the RV is purely a passive ballistic load whose accuracy depends on the launching vehicle’s exact insertion into the desired sub-orbital trajectory. Large inaccuracy associated with first generation RV involved spinning the RV for greater stability during atmospheric re-entry. Second-generation western missiles were mostly MIRV and the accuracy was greatly improved by use of a payload bus with a HAM velocity correction package for more accurate sub-orbit insertion. It also allowed individual MIRV payload to be imparted slightly different velocity so that each could be independently targeted to a different target albeit in the vicinity of each other. As before, the RV continued to be passive and purely ballistic.

Agni-RV Mk-2 is more advanced than western RVs, for it embodies propulsion, navigation and control all the way to the target. The RV re-enters at an altitude of 100 km at a shallow angle with a gliding trajectory.37

• The manoeuvring fins allow:

     – Trajectory error to be determined late into the flight and corrected using aerodynamic force during re-entry.38

     – Execute non-ballistic trajectory to make interception more difficult.

     – Overcome any perturbation due to high altitude atmospheric disturbance.

     – Enable use of body lift at hypersonic velocity to glide the missile over longer range,39 reducing the thermal and physical stress40 at a modified Max-Q point.

     – Support a wider range of payload weight and configuration.

     – Terminal manoeuvre dive for a more acute target interdiction angle improving CEP.

• Velocity correction package for greater precision; using an integrated High Altitude Motor (HAM) (liquid fuelled or a set of solid-fuel cartridges) to correct impulse variance of solid fuelled stages and subtle launch trajectory perturbation.

• Depending on actual payload configuration, the HAM fuel load can be increased to trade range for a lighter / compact weapon.

• The larger volume allows more sophisticated ABM counter measures.

Avionics, Navigation and Control

The Agni family of missiles uses a strap-down INS system for flight control and navigation.

The Agni introduced a new concept by adopting MIL-STD-1553 databus for all on-board communication and control device interconnections (mainly INS system, Flight Control Computer, actuators and sensors).41 It is the standard that is adopted in new civilian and military aircraft (circuit routing and device mounting) and all the software in the Agni-II has been designed around this bus. DRDO sources claim that this reduces the number of connections and also making the missile more rugged. However, some missile analysts feel that a standard databus may not be the best path to follow. It is said that a customised databus is better because in a standard databus, one tends to use off-the-shelf electronic devices whose performance may not be optimal. However, most modern missiles are moving towards digital buses using commercial off-the-shelf technology, which enables affordable sub-system replacement or enhancement.

Accuracy

Agni-II navigation and aiming utilises an advanced ground based beacon system using TDOA (Time Delay Of Arrival) technique similar to GPS42 that constantly provides missile flight position and velocity update that has been proven in test flights.43  The TDOA system reportedly improved the accuracy by three times.44

India has demonstrated a measure of mastery in navigation sensors and flight control through its space programme. The placement accuracy in GTO (involving powered flight of 1,000 seconds, much of it in sub-G or gravity free environment) is a far more complicated and delicate matter45 than that of the sub-orbital trajectory of an IRBM. Thus the GSLV-D2 and GSLV-F01 GTO Apogee accuracy of 1965 PPM46,47 and 361 PPM48,49 respectively, compares with Agni-II’s 40 metres CEP at IRBM range with 13 PPM accuracy.

It is worthwhile to note that the INS error differs for a ballistic missile versus an aircraft. Ballistic missile accuracy is only dependent on the INS accuracy up to the point when rocket fuel is expended (100 seconds for Agni-II) and it exits the atmosphere (90 km altitude), after that the trajectory is purely ballistic that is predetermined and easily computed. On the other hand, the INS in a combat aircraft requires continuous operation of IMU(Inertial Measurement Unit) and navigation computer throughout the flight during which the error keeps building as IMU sensors drift.

A ballistic missile that can update its position and velocity from auxiliary means can completely eliminate the built up error from INS and continue flight at the precise predetermined path, if necessary correcting the launch error by using:

• Small velocity correction thruster package and / or

• Aerodynamic manoeuvring during re-entry (this requires active RV configuration with integrated INS and control system).

The Agni-II reportedly makes use of both the above techniques.

The Agni-II exits atmosphere and expends the second stage at an altitude of 120 km and at a distance of about 150 km. This allows the ground based TDOA system to operate well within Indian territory and at close range (i.e. robustness against Electronic Warfare interference). The missile maintains LOS (line of sight) well beyond apogee.

The overall accuracy is a cumulated sum of:

• Accuracy of determining geographic coordinate of target and launcher;

• Accuracy of hitting the designated coordinates that is determined by the missile’s navigation and control system.

Launching the Agni from a surveyed site is one aspect of item 1 above. Sub-metre target coordinates using national surveillance assets (aerospace sensors etc) would largely address the accuracy of target coordinate designation.

Figure 2: Error Probability

Long-range ballistic missile (passive RV) targeting error is typically spread in highly elliptic pattern. The CEP is thus adversely biased by a wide error spread in longitudinal axis (due to shallow incidence angle). The Agni’s active manoeuvring RV with onboard IMU (INS) and control system can perform terminal manoeuvres to correct error and make a more accurate top attack profile using a greater incidence angle,50  significantly reducing the longitudinal spread and overall CEP.

Terminal Guidance

With the confusion over the maximum range of Agni-II comes further perplexity whether the Agni-II is actually fitted with any form of terminal guidance system. This is not an easy question to dismiss with a glib negative answer. The RV of the Agni-II is fitted with some rather prominent manoeuvring fins which permit the warhead to perform porpoising manoeuvres to evade and confuse enemy defences, implying built-in navigation, IMU and control system.

Agni-II is fitted with a basic strap-down inertial navigation system rather than with a more advanced (but expensive and less robust) gimballed or platform one. This is by no means a mean achievement and does not by itself mean that the Agni-II has poor accuracy.51 It would mean, however, that the need for some kind of terminal guidance system is necessary – especially true since DRDO claims that the CEP for Agni-II is three times lower than that of the earlier Agni variant with CEP figures as low as 40 metres being mentioned.

DRDO stated, especially after the first Agni-II test, that it had tested a terminal guidance system that dramatically enhanced accuracy. For adjustments to missile trajectory during flight, which allow for higher accuracy, the second stage booster has a flex nozzle that enables a change in the thrust vector direction. The flex nozzle technology was validated in the third stage motor of India ’s Polar Satellite Launch Vehicle. Furthermore, it was alleged that the re-entry vehicle employs a terminal guidance radar operating in the C and S bands. Finally, re-entry adjustments have been optimised through on-board control software that allows re-entry velocity trimming.

There were also early reports that a terminal guidance system based on ISRO technology was to be employed. This terminal guidance system is reported to be comprised of a scanning correlation optical system based on a scanning focal plane homing head in the infrared and millimetric wavelengths of the electromagnetic spectrum.  However, it is not known whether this advanced terminal guidance system has been fully developed. Allegations about the employment of a GPS assisted terminal guidance were effectively denied by DRDO scientists as the external control of the GPS network was thought to be a liability.

DRDO’s terminal guidance claim is at times belittled by certain quarters – often anti-DRDO naysayers – who speak in terms of the superior Chinese and North Korean guidance systems and their use by Pakistan . Such comments, usually unsupported by any meaningful evidence cannot be given weight on merit, but they do put renewed emphasis on the need for DRDO to be more forthcoming about the guidance system of the Agni-II. DRDO has nothing to lose and everything to gain by explaining a bit more about the guidance technologies used for Agni-II. At stake is the credibility of the DRDO scientists who have made statements to this effect at earlier dates.

Range

The first point of confusion regarding Agni-II is as to what its maximum range is.  As will be shown in this paper, the range is greatly influenced by use or non-use of thrusters on the RV (required for velocity trimming) for propulsion as a HAM (High Altitude Motor). There seems to be room in the RV for about 200 kg fuel(solid or liquid) after allowing for a long but lightweight TN weapon.  This RV integrated HAM is referred to as the half stage after the 2 solid fuelled stages. This stage provides a disproportional increase in range for lighter RV payload. Thus development of a lightweight nuclear payload is paramount to the missile’s range.

Table 1 : Agni Missile Details 

When the Agni-II was first launched, the then Defence Minister, George Fernandes indicated that the maximum range of the Agni-II was 3,000 km. Since then, ranges from 2,000 km to 2,500 km have been bandied about while Dr Kalam, at Aero-India 1998, stated that Agni-II had a maximum range of 3,700 km!

The range of 2,000 km can be excluded as the system has been tested to greater range in both 1999 and 2001. Given the test to 2,300 km in 1999 and 2,100 km in 2001, with an apparently lighter payload would indicate that a variation in trajectory was used and it may be possible to extrapolate some more accurate estimates of Agni-II’s maximum range.

It would appear that Agni-II has a theoretical ability to hit a target 3,000 km away with a 1,000 kg overall payload (a 250 kg RV deadweight and a 750 kg warhead). It is suggested that a 200 kiloton ‘boosted fission’ warhead was earlier developed for the Agni system when it was on the drawing board in the late eighties, however after Pokhran-II series of nuclear tests in May-1998, the 200 kT boosted fission design has clearly given way to a 200-300 kT two stage TN design that is expected to be much lighter. From the tables below, one can see that a number of permutations and combinations are available to DRDO based on the existing Agni-II design and Indian propulsion technology.

Table 2: Agni Missile Performance

Range changes can be made by either varying the payload or by altering the engine configuration.

Given the available data, it is clear that Agni-II has a maximum range of somewhere in excess of 3000 km, and possibly as high as 3,500 km with a 1,000 kg payload. Greater range with a lighter payload, however, requires the RV to be qualified for higher re-entry velocity and corresponding Max-Q for thermal stress.

As the backbone of the Indian land-based nuclear deterrent, the real significance of the Agni-II is the fact that it is both road and rail mobile. This is an indication of India ’s desire not to put its missiles into vulnerable silos. The mobility of the Agni-II, combined with the sheer physical size of India renders a mobile IRBM a very secure and survivable delivery system. Furthermore, Raj Chengappa alleges that in one of the tests, the Agni-II was tested with a nuclear weapon assembly – minus the plutonium core – mounted in the warhead assembly area to ensure that all systems, including safety locks, would work.57