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WAYPOINT SHIPBOARD PHASED-ARRAY RADARS Requirements, technology and operational systems By Dimitris V. Dranidis The sight of huge planar arrays aboard warships has been with us for a good two decades now, beginning with the fleet introduction of the Aegis system and more recently expanding with the current European naval construction programs. At this point, most defence analysts agree that such radar systems are going to become the standard norm not only in naval construction programs, but also throughout the various military branches. In the naval section, this obviously begs the common-sense question by the observer & taxpayer: Ships have done quite satisfactory with plain-old rotating mechanical radar sets for over half a century by now, so what’s the deal with these funky new, slick and ultra-expensive gismos? Given that naval forces worldwide have to answer that awkward question in times of constantly reduced budget appropriations, it is worth taking a closer look at the subject of phased-array radars: the principles behind the technology, the problems it is meant to overcome and some of the operational systems of the past, the present and the near future. The problem For modern shipboard AAW systems, the most critical performance requirement is the ability to successfully counter saturation attacks: such attacks may include numerous aircraft and particularly anti-ship missiles converging from multiple directions in close coordination, with the clear intention of overwhelming the defences. The successful engagement of each of these targets by the AAW system(s) requires their precise tracking so that useful fire-control grade can be supplied to the ship’s overall combat system. Conventional mechanically-scanned 2D or 3D radars achieve this tracking by correlating successive radar echoes for each target. This function is often referred to as “Track-While-Scan” (TWS) and is usually performed for multiple targets at the same time, the system’s computational power permitting. Obviously, the higher the sweep rate of the radar, the finer- grained the tracking information is going to be for each air target. In mechanically-scanned radars, the rotation speed of the radar antenna and the update rate of target information (often referred to as “data rate”) are obviously identical. An SPY-1 radar antenna However, the data quality required for the successful control of anti-aircraft weapons dictates very high data rates, much higher than the rotation speed of typical mechanical-scan radars. If the data rate is not increased, targets of high speed or high agility are virtually impossible to engage. The obvious solution, spinning the radar antenna faster, entails a significant drawback: In pulse and pulse-doppler radars (ie. The vast majority of mechanically-scanned radar systems), the ability to detect targets at long range is directly relevant to the total electromagnetic energy reflected back to the receiver from the target (in more detail, it is proportional to the transmitter’s PRF and the time duration of the target’s presence within the main lobe [radar beam] of the transmitter). As the radar antenna spins faster, it has less time to gather the reflected energy – thus, the target detection range shrinks dramatically, particularly for targets with reduced radar signature or under the cover of surface clutter. This places the AAW system designer between a rock and a hard place: he has to accept either short-range penalty or poor fire-control solution. Strategic phased-array radars such as this PAVE PAWS unit have been in service for decades, but their great expense has until recently prevented them from being mass-employed in tactical military branches The answer to this problem, and the accepted practice for most current warships, is to provide separate radars dedicated to the target-tracking function. This creates a clear separation of duties: the surveillance radar performs the initial target detection and low-quality tracking, and then passes this data to the tracking radar, which performs the high-quality tracking and fire-control operation (frequently providing illumination for radar-guided weapons). But this solution, while perfectly adequate for the “single incoming target” scenario, is severely handicapped in a saturation attack scenario: as the maximum multi-target ability is equal The magazine of the computer Harpoon community - http://www.harpoonhq.com/waypoint/ WAYPOINT to the number of dedicated tracking & illumination radars, and with a very finite number of these systems being installed on the ship (typically 2 for frigates & destroyers, 2-4 on cruisers etc.), it is quite easy to overwhelm the ship’s defences with multiple simultaneous attacks. Clearly, what is needed is a way to provide target-tracking data of very high quality (sufficient for weapons guidance) while at the same time being able to do this for a very large number of targets, and without sacrificing long-range performance. The technology The requirement for the concurrent high-quality tracking of a large number of targets essentially dictates breaking off from the shackles of mechanical scan: the need for high data-rate means that the radar beam has to jump between widely-separated targets near-instantaneously in order to quickly provide updates on their status, something impossible for a mechanical antenna. On the contrary, this is perfectly possible if the management of the radar beam is instead handled electronically, by antennas formed by multiple independent transmitters, spaced at predefined regular intervals. Understanding how this works calls for a small diversion into EM physics (fear not, the principle is simple). Let us assume that we have a flat radar antenna composed of regularly-spaced transmitters. All transmitters emit the same signal. For each transmitter, the signal follows the typical sinusoidal pattern, with a maximum and a minimum amplitude value. Naturally, as the multiple transmitter elements are tightly stacked, there is strong interference between them. In our case, this is intentional and welcome: according to the principle of constructive interference, the electromagnetic energy received at a point in space from two or more closely-spaced radiating elements is at a maximum when the energy from each radiating element arrives at a point in phase (concurrently). These “pulse- intersection” points, if joined together, form an apparent (virtual) dimensional plane. The vector axis of the main lobe of the transmitted pulse (i.e, the main radar beam) is always perpendicular to that apparent plane (wave front) of the electromagnetic field generated by the transmitters. The magazine of the computer Harpoon community - http://www.harpoonhq.com/waypoint/ itially, this principle was applied to 3D Now, when all elements transmit in the same phase (as happens with mechanical-scan antennas), the plane of the EM field is parallel to that of the antenna, and the main lobe will be staring right forward (boresight). By shifting the order of transmission however (essentially altering the relative phase of the EM pulse on each transmitter, i.e. making some of the transmitters shoot their pulse slightly ahead or after others), the apparent EM plane rotates, and the main lobe can be steered around, right out to the maximum scan limits of the antenna. Because this is an electronic, not mechanical operation, the steering of the main beam is near- instantaneous, thus fulfilling the need for rapid updates between targets. Phased-array technology finds its simplest form of implementation in single-dimension scanning systems. This graph demonstrates how 3D radars like the SPS-48 series use the vertical steering of the beam to stabilize against ship movement by forming a “virtual” horizontal axis independent of the true boresight axis. A similar technique is used by E-3 Sentry AWACS aircraft to stabilize the beam when banking to turn. In mechanical-scan radars. These use an antenna formed by multiple horizontal slotted waveguides, each of them being an independent transmitting element. By altering the phase of the RF pulse transmitted by each waveguide, the WAYPOINT beam can be steered on the vertical axis in order to provide altitude data on multiple targets – but horizontal scanning still depends on the antenna spin. Radars that employed this technique include the APY-1/3 of the E-3 Sentry AWACS aircraft, as well as most 3D air-search radars installed on USN ships in the 70s and 80s (most notable being the SPS-48 family) as well as most modern air-surveillance radar sets. This single-dimension scanning was adopted as an interim step because of the high cost of independent transmitters and the general immaturity of the technology at the time. With the rapid cost decrease of such transmitters however, it became practically feasible to populate an antenna with multiple elements both horizontally and vertically. This in turns means that the radar beam can be steered on both axes, thus eliminating the need for mechanical scan altogether. The ability to provide a high data-rate on a large number of targets at sufficient range is not the only advantage of ¾ Because of the near-instant redirection of the main beam, a single radar unit can perform multiple functions ¾ U return ¾ A electronic-scan arrays. Other benefits include: concurrently: For example long-range air surveillance, low-rate tracking of neutrals or suspected contacts, high-rate tracking of confirmed hostiles and radar illumination of hostiles within weapon parameters. These duties typically require the flawless cooperation of several different mechanical-scan radar sets to be successfully performed, as previously described. While obviously beneficial to a ship of any size, this consolidation of capabilities is particularly important where hard upper limits on size & displacement are present, thus limiting the number of sensors that can be installed (as is normally the case with most naval forces). s cess. D luable t f e h b a s o t i o i p 1 Many r keep sea does rea pon detecting a potential target, a mechanical-scan radar system typically waits for a second sweep This advertising graph from Lockheed Martin emphasizes the multifunction capability of the SPY-1 radar; similar advantages are offered by most phased-array systems o that it can correlate the two echoes, extract course & speed information and start a new tracking pro epending on the sweep rate (typically not very fast for long-range surveillance sets), this wastes va ll mechanical-scan radars typically create secondary beams (sidelobes) in addition to the main beam ime against an incoming enemy aircraft or weapon. Furthermore, if for any reason the target denies one or a ew sweeps during tracking (if, for example, the targeted aircraft temporarily drops under the horizon or masks ven for a few seconds behind an island or mountain range) the radar will typically drop the track1 and will ave to start all over again, with the same inherent delays. This means that, by periodically somehow reaking the track (denying LOS, jamming, beaming etc.) an enemy aircraft or weapon has a good chance of pproaching dangerously close without being successfully tracked and engaged. Phased-array radars can tart a track immediately upon detecting a target, since they can instantly reposition the beam upon it instead f waiting for the next sweep. Furthermore, while they too can be disrupted by much the same track-breaking echniques, they can counter them much more effectively: for example, the main beam can be instructed to ncrease its dwell time on the direction that the threat was last detected (at the expense of reduced scanning n other, non-threatening sectors), so that when eventually it does (unavoidably) reappear, it will be mmediately re-detected and the track will recommence instantly. roduced. These sidelobes are highly undesirable as they represent both a significant prize for enemy ELINT adars have a “track memory” feature to counter such problems, and for some time after loosing track they will rching in the expected direction of the threat, trying to re-acquire. This however is only effective if the target ppear soon. The magazine of the computer Harpoon community - http://www.harpoonhq.com/waypoint/ WAYPOINT assets and also a source of vulnerability to jamming. Electronic-scan arrays produce sidelobes of relative lower signal strength; they are therefore less vulnerable to such exploits. ¾ Like most electromechanical devices, mechanical arrays are vulnerable to own vibration and have strict maintenance requirements for the effective operation of their moving parts. Electronic arrays have no moving parts and suffer from no vibration of their own. They are also inherently better stabilized against the ship’s motion: mechanical antennas have complex multi-axis stabilization systems to compensate (themselves a source of maintenance & reliability headaches), whereas electronic arrays simply steer the main beam counter to the detected motion. ¾ Mechanical-scan radar systems typically have several single points of failure inherent in their design: if the servo motors for the antenna spin fail, the radar is out. If the stabilizing servos fail, the tracking data quality gets so bad that the radar is good as out. If the antenna itself gets bend out of shape even partially, the radar is out. The list goes on. ¾ Electronic-scan sets, while not impervious to sub-system failures or damage (particularly with regards to the RF pulse-generator tubes), are typically more resistant to them. This is partly a result of the modular structure of the antenna itself, and of the disposable nature of the independent transmitter elements: if any of the transmitters fails, the others will take over. This means that the antenna can have a significant portion of its elements destroyed (e.g. from the fragments of an ARM detonation) and still be able to function, albeit at a reduced capability. ¾ Mechanical-scan antennas are the hardware in hand? In any of these cases, the hardware has to ¾ Electronic-scan systems, while having their own physical limitations with regards to transmitted power, scan designed under a certain set of operational assumptions, which in turn drive the technical specifications: the radar will transmit a pulse of such and such energy and frequency, with a given PRF, forming a beam of a given width (the physical size & shape of the radar antenna is precisely formed over these requirements), rotating at some set speed (thus having a fixed data rate) etc. etc. Now, what if the adversary uses a new jamming technique or employs different technical characteristics than those predicted? What if the land- or sea- clutter is greater than expected? What if the tactical circumstances call for a higher data-rate than the “standard”? What if years of actual operational employment show that the desired technical specs are different than those of be redesigned and physically rebuilt. coverage etc, are considerably more flexible on their operation within these limits. Their technical characteristics are largely driven by the controlling software rather than the underlying hardware. This means that by altering the software code, the same piece of hardware can be modified/enhanced to adjust to new threat environments. The system’s software-driven nature also increases the tactical flexibility of its The magazine of the computer Harpoon community - http://www.harpoonhq.com/waypoint/ WAYPOINT The magazine of the computer Harpoon community - http://www.harpoonhq.com/waypoint/ he next step: AESA he technology described so far assumes a system comprised of a number of transmitter modules, but only one he benefits, however, fully justify the extra costs & complexity: instead of forming a single main beam and ast, present and future platforms: Failures and successes PG-59/Typhon he first attempt for an operational phased-array system commenced on 1958 as a US Navy venture. The aim was to operational employment: The precise characteristics of the transmitted beam can be altered on-the-fly to suit the tactical situation at hand, rather than arbitrary pre-assumptions based on imperfect intelligence. T T receiver module, forming a single main beam. Such antennas are often described as passive electronic-scan arrays. The next evolutionary step in this technique is to make each of the transmitters a receiver in its own right, essentially constructing an antenna formed by thousands of independent (but coordinated) transmitter-receiver (T/R) modules. Such systems are referred to as active electronic-scan arrays (AESA). This technique obviously requires a much higher level of electronics integration and is more expensive to develop than the previous solution, which is why systems embodying this principle have only very recently began reaching operational status. T electronically steering it around, the presence of multiple T/Rs allows the forming of multiple independent main beams, each assigned to its own task. So, instead of having the single beam being time-shared between the various duties (surveillance, tracking, fire-control etc.) and hop from one target to another, we can assign each of the beams to a specific function or permanently “stick it” to follow a specific target, while other duties & targets are handled to the other beams. This “true” multi-function capability opens up other potential uses: for example, since it is possible to transmit two or more signals with completely different characteristics concurrently, it is perfectly feasible to have the radar antenna double-up as a powerful jammer. AESAs are also inherently less power-hungry, as each of the T/R modules transmits a low-power EM pulse, the beams being formed by the intersections of the pulses (This contrasts to the very powerful EM pulse being transmitted by passive electronic-scan systems). This is a significant consideration when thinking about the applicability of such a technology on platforms with a limited power budget. P S T develop an extremely advanced SAM system called Typhon, which would offer significantly enhanced multi-target abilities than the existing family of Terrier, Tartar and Talos systems. These systems, while technological marvels in their own right, were never designed to handle the then-emerging threat of huge numbers of bombers and anti-ship missiles entering service with the Soviet Naval Aviation (AV-MF). The intended solution to this problem was track-via- missile guidance (adopted much later successfully in the land-based Patriot SAM), in which radar signals were received by the missile, but forwarded and processed on the surface ship with its much greater available computing power. The system structure called for a radar component able to perform all the different duties (including fire-control & terminal target illumination on multiple contacts) concurrently. It was therefore sensible that the first venture for an operational phased-array radar (PAR) began with this project: The heart of the new system would be the massive SPG-59 electronically scanned tracking radar, which could track multiple targets and intercept missiles. This would be matched to an extremely advanced missile, able to intercept both fast aircraft and missiles out to 110nm. As soon as the technological R&D commenced in earnest, the hurdles began materialising: the state of the art was ven wors