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In recent years, precision guided weapons play more and more important role in modern war. One of the greatest strengths of a precision strike missile is a reduction in the number of aircraft sorties required to destroy a target. One of the key contributors to the missile accuracy, lethality, and adverse weather capability of precision strike missiles is the advancements in missile seekers.
Strategy Analytics forecasts the global Smart Weapons (SW) market will grow to over $41.8 billion in 2025, representing a CAGR of 3.7%. A renewed emphasis on advancing Smart Weapon capabilities to counter evolving threats such as A2/AD (anti-access Area Denial) envelopes, combined with on-going demand from asymmetric wars and continued force modernizations in emerging countries is driving spending across the full range of Smart Weapons.
Precision guided weapons may include a variety of imaging or non-imaging sensors to detect and track potential targets. Sensors used to guide projectiles to an intended target are commonly referred to as seekers. Seekers may operate in various portions of the electromagnetic spectrum, including the visible, infrared (IR), microwave, and millimeter wave (MMW) portions of the spectrum. Some projectiles may incorporate multiple sensors that operate in more than one portion of the spectrum. A seeker that incorporates multiple sensors that share a common aperture and/or common optical system is commonly called a multimode seeker.
While optical systems (visible and IR) require clear atmospheric conditions for reliable operation, MMW imaging is relatively immune to weather conditions such as cloud, fog, snow, and light rain. The seeker can operate in low visibility and contaminated battlefield conditions, and is not susceptible to battlefield obscurants such as smoke, dust, flares and chaff.
RF Seeker technology
Active radar seekers are the most popular in all the current missile programmes owing to their flexibility of design and implementation to suit almost every mission requirement apart from all weather capability. This is primarily due to the
choice of waveform design, optimisation of receiver, and adaptability and flexibility offered by the digital signal processing techniques in vogue.
The seeker sends an electromagnetic signal at the speed of light towards the target that reflects it. The target echo then propagates back and is sensed by the antenna. Afterward, the processing unit analyses the received signal and estimates the new target direction with ecartometry techniques, finally the command system steers the missile accordingly. The antenna has the role to detect the echo signal and the processing unit to determine the target direction, they are complementary.
The most extensively employed configuration of active radar seekers so far realised is the coherent monopulse tracker with a gimbaled antenna structure. Thee antenna is mounted on a gyro-stabilised platform such that the antenna is isolated/decoupled from the body movement of the missile.
A signal is received by two antenna sub-array to calculate the ecartometry error. Then the signals go through transmitter and receiver modules which the main goal is signal amplification. Then the signals are mixed for transposition into baseband. The analog signals are converted into digital signals and analyzed in the Field Programmable Gate Array
(FPGA) module. This module carries out beamforming, detection, estimation of the target position, and target tracking. Finally based on the signal analysis, the command centre adapts its decision.
Additional target information in terms of range and/or relative velocity (Doppler frequency shift) is used to identify and track a desired target among the other targets including clutter, if any. The radar sensor, for most of the surface-to air missiles (SAMs) as well as air-to-air missiles (AAMs), is configured as a high PRF (HPRF) pulsed (Doppler) radar frequency essentially as a Doppler tracker apart from being a basic monopulse angle tracker.
Missile environment
In a realistic battle-space scenario, many factors impact upon the detection and estimation of the target. Firstly, the missile undergoes strong vibrations as it travels at several times the speed of sound; the seeker has to be robust
enough to endure them. Secondly, natural hurdles due to the environment such as fog, clouds, rain or snow, can affect the signal strength. The target may also use countermeasure techniques to affect the missile guidance. The target carries out jamming or can use a decoy or chaffs: a cloud of metal fibre filaments that prevent target detection.
Finally, the signal suffers from the noise in the receiver and various losses due to the electronic components imperfections. All of these effects have to be accounted during the seeker conception to make a more effective and resilient detection and estimation. Other factors include the low volume and weight, ease of maintenance, long storage
period after which the missile should be unaltered, etc.
The performance criteria of an RF-seeker
An RF-seeker system performs well when the quantity of information extracted from a pulse is high, for example: target range, speed, angular position, type of target etc. To achieve this goal, the two radar components, the antenna and the processing unit, should be high performing.
Frequency of operation
Frequency of operation varies from X-band to Ku-band for SAMs and AAMs to millimeter wave frequencies (35 GHz and 94 GHz) for SAMs in air defence role. Seekers for precision-guided munitions (PGMs) and antitank missiles (ATMs)
also operate at these millimeter wave frequencies. However, it may be noted that for antiship missiles (ASMs), ATMs and PGMs, the radar waveform is specially designed for slow-moving target detection for seeker stabilisation (strapped-down
and identification.
Radome
The variable thickness radomes as well as thin-walled radomes based on slip-cast fused silica / glass-filled polycarbonate (GFPC) technologies have been widely used with radome error slope variation compensated by special manufacturing process or by correction method incorporated in the angle error extraction circuit.
Antenna
That part of a transmitting or receiving system that is designed to radiate or to receive electromagnetic waves.
The antenna provides a transition between the electric and magnetic currents in a circuit and the electromagnetic signal in the free-space and reciprocally.
A microstrip patch antenna consists of a metallised pattern and a metallic ground plane, separated by a dielectric substrate. The metallised pattern order of magnitude is the wavelength. A slotted waveguide antenna is made of a metallic pipe with circular or rectangular sections, into which slots have been pierced. The slot order of magnitude is the half wavelength. Array antenna: An antenna comprised of a number of radiating elements the inputs (or outputs) of which are combined
The slotted planar waveguide array antenna weighing less than 500 g with an integrated microwave receiver, has been used in most of the latest conventional seekers.
Multi-Function RF Seeker Based on 3D Phased Array
Current RF seekers in use today have mechanical steerable antennas. In order to reduce the cost of the mechanical system and to significantly improve the performance of the missile seeker, the electrically controlled 3D antenna array is used. It results in a much more robust antenna which will be capable of steering much faster and more accurately than existing solutions. Furthermore, the antenna will provide an increased coverage and dwell time as a result of flexible beam steering. Additional degrees of freedom will allow it to carry out multiple tasks. Electronically steering of the radiation beam has many advantages over the traditional gimbaled system including faster beam steering time and increased space savings.
Stabilization System
The seeker system including its antenna assembly will be onboard the missile. Due to the missile trajectory corrections, the seeker antenna pointing to the target may get disturbed resulting in track loss. To avoid this track loss, it becomes
necessary to stabilize the antenna system in two planes. The fundamental role of stabilization loop in seeker application is to precisely follow the angular rate of the target. In order to achieve this requirement, it is essential to highly isolate the gimbaled antenna from the missile body motion due to the maneuvering of target or low frequency vibration during flight
The high precision-gimbaled mechanical system with low outline has been used with the gyro of the inertial navigation system providing reference for seeker stabilisation (strapped-down configuration).
Transmitter
The master oscillator power amplifier (MOPA) configuration using travelling wave tube (TWT) / klystron has been extensively used. The transmitter based on injection-locked magnetrons has been developed. Solid state power amplifier driver along with microwave power amplifier (tube-based) have been implemented following the concept of microwave power module (MPM) where efficiency is maximized.
Receiver
Almost all the seekers currently use state-of the-art triple-super heterodyne MMIC-based receivers with a very low-noise figure (< 2 dB).
Various information about the target is embedded in the signal echo: the range, the speed, the angular position and the target type. Important parameters for a seeker are the angular position: θ and ϕ, . θ is the elevation angle, it is
referenced from the axis Z. ϕ is the azimuthal angle, it is spinning around the axis Z and referenced from the axis X.
During its life time, the RF-seeker has two functioning phases. First the RF-seeker searches for the target, then the RF-seeker tracks it. During the tracking phase, the radar has prior information on the target angle. As the target moves, the error angle is calculated with ecartometry techniques. These include, the conical scan and the monopulse technique.
To use the monopulse technique, an antenna array emits a pulse signal towards the target. Then, in reception the array is divided in four quadrants. Comparison of the signals is realised in amplitude for the amplitude monopulse technique and in phase for the phase monopulse technique.
The performance criteria for an ecartometry technique is related to an accurate estimation of the target angular position that would be robust to the perturbations of the environment and the countermeasures.
Inverse monopulse technique
In these systems, the radar signal is split in two before it is sent to the antennas. The two paths include some form of encoding that remains intact after reflecting off the target. Polarization is a common solution. The signal is then re-mixed and sent out of the antenna.
Two antennas receive the mixed signal after reflecting off the target. Filters then split the received signal back into two components, and a comparison of relative strengths can be made as before. However, if the signals are directional, as in the case of polarization, there is no need to spin the antenna – the difference between the signals can be used to determine the directionality. In real-world systems, four antennas are used, two to compare left-right, and two for up-down.
The main advantage to this technique is that reflection off the ground randomizes the polarization of the signal. Some will be returned with the “proper” polarization, but the vast majority will end up being filtered out in the receivers. Even though the signal returned from a target aircraft may be tiny in comparison to the total ground reflection, after filtering it becomes visible again. This allows such radars to track targets below the fighter, giving it “look-down, shoot-down” capabilities.
The filtering also makes it much more difficult for electronic countermeasures to work effectively. Since only the signal with the matching polarity will make it through the filters, typical unpolarized pulses will normally be filtered out. To work against such a radar, the jammer has to either match the polarization of the signal, or broadcast so much signal that it randomly has enough energy with the correct polarization to get through the filters.
Finally, glint is significantly reduced. Glint occurs because the antennas are sensitive in only a single direction at a time, and as they spin they see signals from different parts of the aircraft. Monopulse receivers do not spin, and see the entire return at all times. Although they still see different signal strengths from different locations, this does not change as the missile approaches its target, so the missile is not being continually commanded to change direction. In testing, the majority of Skyflash missiles hit the target aircraft directly, compared to the original AIM-7’s conical scanning solution which brought the missile to within 20 to 30 metres (66–98 ft). Additionally, it was able to attack aircraft flying at 1,000 feet (300 m) altitude, a limit selected to allow tracking cameras to see the target. These tests demonstrated there was no practical lower altitude limit to the technique.
The downside to the inverse monopulse seeker is twofold. For one, it requires the radar on the launch platform to have monopulse encoding, or there will be no directional signal for the seeker to process. This links such missiles to their aircraft more tightly than the more generalist conical scanning systems which can be used with any radar the seeker can tune in. More importantly, the seeker is more complex and requires more electronics,
Signal Processing/Data Processing
Intensive application-specific integrated circuit- based digital signal processors have been developed to suit the application-specific requirements. However, in some radar seekers, wide dynamic range requirements have been met using crystal filter-based analog signal processing in the front high-frequency receiver section. A system-on-chip approach is being attempted.
Waveform Design
Currently highlmedium PRF pulsed waveform is widely used. However, in some seekers for ASM application, pseudo random phase-coded waveform has been used. The complex waveform design is the latest trend to combat electronic countermeasure techniques and provide automatic target recognition capability.
Changing requirements
With the advances in ECM techniques, stealth technology, and target operational performances, eg, manoeuvrability and speed, the following new requirements need to be considered for seeker design:
- Low peak power for reduced vulnerability of detection as well as less demanding power supply system
- High power aperture product for increased range
- Optimum waveform design for advanced ECCM features
- Wide bandwidth operation for providing frequency agility (pulse-to-pulset batch-to-batch, or in pseudo-random fashion)
- Multisensor data fusion being implemented through data link/INS-GPS
- Faster signal processing for large data handling and also for imaging
- Low radar cross section detection and tracking capability to meet stealth technology advancements
- Low weight / low volume high-density packaging and efficient thermal management for miniaturisation
- Use of commercially off the shelf (COTS) components for the development of low-cost seeker (as seeker costs 70 per cent of missile just for a price of kill)
Kongsberg to upgrade joint strike missiles with RF-seeker sensors
Kongsberg Defence Systems has received a contract from the Australian Department of Defence to install its RF-seeker sensors on joint strike missiles (JSM) that will be carried on-board F-35 fighter aircraft. Built by BAE Systems Australia, the RF-seeker sensor will allow the long-range precision strike missile to locate targets by their electronic signatures.
The potential upgrade will further strengthen the capabilities of JSM for the most challenging scenarios in a modern battlefield, Kongsberg stated. The JSM, which will be integrated for internal carriage on the F-35, is difficult to detect and stop even for the most advanced countermeasures and defence systems, Kongsberg stated. The missile uses a combination of advanced materials and can fly low, while following the terrain. It is said to be the only long-range sea and land-target missile that can be carried internally in the F-35 and will allow the aircraft to fight well-defended targets across long distances.
AFRL GBU-X (Flexible Weapons) programme
The GBU-X program is a cross-directorate AFRL initiative that seeks to mature key technologies that could enhance current weapons or lead to a new family of weapons made up of flexible, interchangeable, open system architecture components for sixth-generation aircraft.
It explores two primary areas of technology research, including the development of open systems architecture with common interfaces to facilitate rapid technology refresh and configuration of the munition system to meet individual mission needs, and cooperative engagement strategies using networked and selectable effects munitions for increased robustness to countermeasures and improved endgame performance over baseline inventory munitions.
The program is also examining supportability and affordability of a family of GBU-X weapons. Having an open architecture shall allow plug and play of sensors, reduce the need to develop and maintain vast number of disparate of weapon systems, resulting in lower USAF costs overall as well as reduce the time between when a weapon is developed and when it can be integrated onto a platform.
“Developing a common architecture that enables modular subsystems to achieve flexible weapons capability, while allowing us to refresh the technologies at the pace of better, more affordable and sustainable technologies as they are discovered and developed, is at the core of our mission,” said David Hayden, an AFRL researcher working on the project.
MMW wave seeker
The millimetric band seeker provides a high-resolution radar return image of the target, while the frequency gives a small beamwidth and therefore very high angular resolution and reduces unwanted clutter for the given antenna size, which is limited by the diameter of the missile.
The millimetre wave radar enables wideband operation, facilitating the use of very sophisticated electronic countermeasures. Millimetric radar attenuates more rapidly than conventional centemetric radar in rain, sleet and fog, but its advantage is high penetration, in comparison to infrared sensor systems when countermeasures are employed.
Automatic Target Recognition Technology
Automatic Target Recognition (ATR) technology for missile seekers homing on mobile targets is based on MMW image matching technology. An end-to-end ATR system used in operational environment requires four stages: image enhancement, target detection, target segmentation and target recognition. Images are preprocessed in the first stage for the best performance in the next three stages.
In the target detection stage non-target clutter in MMW images is rejected and potential targets are separated using Constant False Alarm Rate (CFAR) technologies that detect the targets while keeping the false alarm rate under a user defined threshold.
In the segmentation stage of ATR the output of CFAR detector is clustered to separate targets. Next, the detected target locations are clustered and segmented to obtain separate suspected targets. Clustering of the targets is achieved using morphological filters.
Target recognition is performed in the last stage of ATR processing. The role of target recognition is to accurately identify target of interest even in the presence of variability in target signatures. Only by accurate recognition of targets critical military operations, such as homing of fire-and-forget missile, can be achieved.
According to Hayden, the Air Force is interested in a mature automatic target acquisition approach that allows the Guided Smart Seeker to enter into closed-loop tracking without a human operator in the loop.
“One of the requirements we sought to meet was that the seeker possessed the ability to acquire targets and begin tracking them without human intervention,” Hayden said. “Intelligent target clustering is a capability that would give the seeker a more robust target tracking capability and reject any false alarms.”
BAE Systems has received a $117 million contract to produce next-generation missile seekers for the Long Range Anti-Ship Missile (LRASM) reported in July 2021
The seeker technology enables LRASM to detect and engage specific maritime targets in contested environments with less dependence on traditional navigation systems. The next-generation seeker design reduces overall missile costs.
“We’re committed to providing affordable systems that deliver unmatched capabilities to the U.S. and its allies,” said Bruce Konigsberg, Radio Frequency Sensors product area director at BAE Systems. “We’ve designed efficient seeker systems that are easier to build and test without compromising on performance.”
LRASM combines extended range with increased survivability and lethality to deliver long-range precision strike capabilities. LRASM is designed to detect and destroy specific targets within groups of ships by employing advanced technologies that reduce dependence on intelligence, surveillance and reconnaissance platforms, network links, and GPS navigation in contested environments.
Following design improvements conducted under a Diminishing Sources/Affordability contract, BAE Systems is producing next-generation seekers for Lots 4 and 5 that are more capable and easier to produce, with less-complicated manufacturing processes. The next-generation seekers have replaced obsolescent and limited-availability parts, dramatically reducing the system cost.
References and Resources also include:
https://core.ac.uk/download/pdf/333720359.pdf
