Artillery on target--trajectory correctable munitions. (Technology).
Many observers may wonder why it is necessary to alter the trajectory of a projectile after it has been fired, especially in these days of complex fire control and land navigation. The reason for the new munitions is inherent in the very nature of artillery.
Artillery is a weapon for area coverage. Although gunners would love the capability to precisely hit individual targets at long ranges, yet the many variables acting on indirect fire artillery projectiles after they are fired, both within the barrel and during flight, affect trajectory to the point that repeated strikes on the same spot are virtually impossible to achieve. Numerous firings therefore have to be made to cover a target area and produce the suppressive fire that is one of the gunner's main tasks. It follows that indirect fire artillery rarely employs single shots but massed salvoes or prolonged fire.
Accuracy degrading factors become very apparent at the long ranges that modern guns and howitzers can now achieve. In those days gone by when potential ranges were shorter, trajectories were also short in time scale terms. External variables such as wind, temperature, differences in propellant charge temperature and so on, were still present but had less time during which to inflict their trajectory alterations. It was therefore possible to anticipate that projectiles would reach the target and inflict their damage within a fairly limited area. This area, usually known as a footprint, becomes proportionally much larger when the longer and more extreme ranges are involved. Gunners are now thinking in terms of up to 50,000 metres, or even more thanks to some enhanced range projectile systems currently under development; in other words the oval shaped target footprint is larger, growing exponentially as ranges increase.
For any artillery projectile to reach a long range takes time. During a long trajectory the number of variables affecting the projectile will not only multiply but can inflict their degrading effects to a greater extent. By the time the projectile lands on target it will do so within a much larger footprint area than that produced at the shorter ranges. For instance, at 30,000 metres the Circular Error Probability (CEP) will be oval-shaped and measure approximately 400 x 100 metres, an appreciable area of real estate. When the 50,000-metre range or more is involved, the CEP becomes proportionally much greater. Any method of reducing that footprint area must be welcomed. The alternative is that the gunner will have to fire more and more projectiles to ensure the target area is neutralised. This is clearly an uneconomic and inefficient solution, but until recently it was the only one on offer.
What the gunner wants is some means of reducing the long range footprint area to what is acceptable at the shorter ranges. It has to be acknowledged that one's artillery projectiles will never fall on the same spot. The best that can be expected is that they fall within a limited area and fairly close together. The key is projectile location. If some form of knowing exactly where the projectile is during a trajectory and, just as important, where it should be, it then becomes possible for trajectory correction to be introduced.
Location, Location, Location
Several possible location solutions have been proposed, nearly all of them involving the Global Positioning System (GPS), which is already in widespread use for all manner of purposes. It is not generally realised that specially coded military GPS systems are capable of much greater position determining precision than is possible with commercial or general issue equivalents. Precise determinations of accuracy down to fractions of a metre are possible, although for practical reasons such precise performance is not really necessary. Yet it is on these coded GPS systems that gunners have come to rely.
Up until now, gunners have used GPS mainly for artillery survey purposes. Today they utilise GPS receivers and data transmitters that are not only miniaturised but can withstand the high acceleration and G forces that must be endured during firing and subsequent flight. This has already been achieved for proximity fuzes but, even so, GPS circuitry is still complex and, for the artillery task, has to remain operating reliably throughout today's longer trajectories. Packing such demanding equipment into a volume small enough so as not to impinge on the normal payload of an artillery projectile is quite a challenge, especially as the only available space is limited to the interior of a fuze body which still has to retain the usual fuze functions.
Although there were several other concerns involved, one of the first practical GPS artillery applications was proposed by Rockwell Collins of the USA. The company's GPS Artillery Engine was packed into a nose fuze body, one possible intention being that the Engine would constantly receive accurate GPS data throughout its flight and transmit that data back to a fire control centre for computation and display. The exact trajectory would then allow the control post personnel to detect where the projectile had landed and make the necessary on-carriage fire corrections to deliver the desired results. By itself the GPS Artillery Engine could thus have been a valuable fire control tool, but there was no way the projectile trajectory could be altered once it had commenced.
Rockwell Collins then joined an international team dedicated to developing the Star (Smart Trajectory Artillery Round), developments of the Artillery Engine then controlling the projectile trajectory in range terms. They teamed with Thales Missile Electronics, the Defence Evaluation and Research Agency (Dera-now Qinetiq) and BAE Systems, RO Defence to develop the Star to the operational level. The intention is that the Star fuze will, on firing, resemble a conventional nose fuze and will be installed into the existing standard thread fuze well with, it is intended, a minimum of intrusion into the projectile contents.
Most artillery corrected trajectory munitions work along the same pattern. The firing sequence is initiated by elevating the barrel to above the required firing angle, thereby ensuring that the resulting range is more than to the target area. At some point in the early section of the trajectory, or at its apex, the GPS receiver unit receives precise position data and calculates where the projectile is likely to land if left to complete its trajectory. Once in flight the round will utilise the GPS-based electronics to actuate pop-out drag inducing surfaces from the fuze body at the correct instant. Using microprocessors inside the fuze unit this is compared to where the projectile is intended to land. From this data it becomes possible to determine at which point the correction will be necessary to reduce the range and, with it, the resultant impact footprint. At precisely that point the drag-inducing surfaces around the fuze body are actuated by mechanical or explosive means. The surfaces disrupt the aerodynamic flow over the projectile body and slow it down, causing it to drop in a controlled manner onto the required target area.
A broadly similar method is to be employed by the French Samprass (Systeme d'Amelioration de la Precision de l'Artillerie Sol-Sol), under active development at Giat since 1998, in association with Sextant Electronique (GPS sub-system) and TDA (antennas and some electronics). The trajectory correction is introduced via what will probably be a microwave link to prevent easy jamming, either by a processor unit on the gun or by a special dedicated ground station. The Samprass project has, at the time of this writing, been put on hold.
Another generally similar proposal is the Swedish Bofors Defence Bromsa programme, with which the usual drag surfaces are replaced by a mechanism to blow off the fuze nose to induce drag at the appropriate instant. (It should not be forgotten that Bofors Defence is now owned by United Defense).
Yet another drag-inducing proposal has been forwarded by Diehl Munitionssysteme. Instead of fixed surfaces, a thick wire mesh 'skirt' is pushed out from the fuze body to introduce the required trajectory corrections. Diehl Munitionssysteme has teamed with Denel of South Africa with a view to combining this system with the Naschem Assegai family of 155 mm long-range projectiles, among other possible applications. Fired from a 52-calibre barrel, these projectiles can have a maximum range of up to 40,000 metres.
Not all current trajectory correction systems rely on GPS for projectile location determination. While gunners have come to value the positional accuracy information that GPS can provide, many of them keep at the back of their minds that, should serious hostilities commence, the precious GPS space satellites and the communication links with them could be amongst the first targets to attract attack. There could be a resulting loss of GPS data due to jamming or some form of interference with the communication links that will deliver misleading data.
One alternative to reliance on GPS is a ground based Doppler radar systems. As a substitute to the Samprass mentioned above, Giat is also developing the Spacido that relies on a Doppler Cinemometer. This again involves a fuze pattern body, the radar following the projectile trajectory once fired. The resultant positional data is then used to calculate and actuate, under ground control, the drag inducing surfaces at the correct instant. Although the Spacido type of system is slightly less precise than a GPS-based system, it is still more than adequate for many fire missions. It also has the advantage that the costs involved will be less than GPS-based systems. It has been forecast that future artillery systems will have access to a mix of both GPS and radar-based trajectory correction to provide the gunners with options to match any particular fire mission.
It will be noted that all the trajectory correction solutions mentioned above deal with range only. It would be ideal if range corrections could be somehow combined with line correction to improve on-target accuracy that much further. Although forms of line correction are currently under development in several countries, none of them have yet passed the early technical demonstration phases of development. Technical solutions may be already available (see below) but the problem will be to keep the resulting costs down to a level where combined line and range correction devices can be afforded in the quantities that will be required by field artillery engaging their normal run of targets. For special long-range fire missions, fully comprehensive trajectory correction and control systems are already under investigation.
It has already been forecast that some form of canard control surfaces or explosive or rocket 'nudgers' around the projectile body will be involved. The problem will be to make them operate consistently under field conditions at an economic cost. The more finesse that is given to any solution will be reflected in the final price to be paid. Not every nation will be able to foot the bill. While it is known that investigations into such 'all singing, all dancing' projects are currently in progress, the following two examples will provide an indication of what could be involved.
One involves an international collaboration programme between Bofors Defence of Sweden and several concerns in the United States. The American prime contractor is Raytheon, leading a team that includes General Dynamics Ordnance and Tactical Systems, Versatron, KDI, White Electronics Design Corporation, Allied Techsystems (ATK), Micropulse and, not the least, L3 (Interstate Electronics) which is in charge of the GPS and inertial measuring unit. This formidable team has combined to merge the techniques developed for the Bofors Trajectory Correctable Munition (TCM) and the Raytheon XM982 Excalibur.
The original TCM and Excalibur programmes differed fundamentally in concept. The TCM was originally conceived as an artillery projectile with GPS-based guidance. The Excalibur is an extended-range, autonomously guided projectile using a combination of a high glide ratio lifting body airframe and a combined Global Positioning System/Inertial Measuring Unit (GPS/IMU) guidance from L3. It hardly seems to be an artillery projectile, looking more like an air defence guided missile. The end result has been to combine the two, based around the XM982 Excalibur airframe. The Excalibur has been under active development since the mid 1990s, and it seems unlikely that the final result will differ much from the intended XM982 Excalibur objectives.
In operation the Excalibur projectile is loaded and fired conventionally, being compatible with all current propelling charges. Since the projectile is autonomously guided, precise on-carriage laying and charge zoning is not necessary. A slipping obturator allows for compatibility with all currently fielded and anticipated gun systems while keeping the spin rate imparted to the projectile at low levels. After leaving the muzzle, four aft-mounted stabilising/ lifting fins are deployed. Any imparted spin decays to a nominal rate and the fin-stabilised projectile follows a course to the upper part of the trajectory. It is during this part of the trajectory that the GPS unit is activated, calibrated and acquires the necessary satellite lock. At the apogee, four forward canard control surfaces are deployed and a navigation system autopilot is energised. Based on the input from the ballistic flight computer, the autopilot translates course correction demands into electrical impulses sent to the canard drives.
When the projectile reaches the target area it executes a terminal manoeuvre appropriate to the particular type of warhead it is carrying. Three types of payload are envisaged. It seems likely that the first model to be fielded will carry multiple dual-purpose (anti-personnel/anti-armour) submunitions, exactly how many has yet to be released. Another option was to have been two Sadarm autonomous anti-armour submunitions, but with the demise of that programme, and with Bofors Defence now in the American loop, this seems very likely to be changed to at least two Bonus submunitions.
The third type of Excalibur payload is perhaps the most intriguing. It is described as a unitary payload for the destruction of high value, protected spot targets such as bunkers or control centres. Concrete penetrating depths of 200 cm and more have been mentioned. Perhaps the most interesting point regarding this warhead is its expected high degree of delivery accuracy, especially as the projectile will have to impact at a precise angle to optimise target penetration. With the original Excalibur proposals it was expected that artillery systems with 39-calibre barrels could reach 40,000 metres, due mainly to the glide properties of the projectile. Longer-barrel systems can expect yet more range. The defunct XM2001/ XM2002 Crusader self-propelled 155 mm system was expected to reach up to 57,000 metres.
Another future system in the Excalibur class is the Giat Pelican. This is an elongated 141 mm diameter projectile carrying up to five Bonus (or similar) smart submunitions. It follows much the same trajectory correction lines as the Excalibur but it is anticipated that ranges of well over 80,000 metres will be achieved, thanks partially to rocket assistance.
Gunners today have much in the future to anticipate.
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|Author:||Gander, Terry J.|
|Date:||Apr 1, 2003|
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