EFFICIENT FLARE COUNTERMEASURES AGAINST INFRARED GUIDED THREATS
By Yeondeog Koo, Unseob Jeong, and Wonseok Choe
Portable infrared (IR) guided missiles have been the biggest threat to aircraft, especially low-speed helicopters, for the last several decades. The development of IR-based seekers for missile guidance has been an especially active research field, resulting in various emerging and/or advanced technologies, such as spin scan, conical scan, Rosetta scan, and image seekers . Likewise, numerous aircraft countermeasures (CMs) against these threats, including flares, infrared countermeasures (IRCM), and directed infrared countermeasures (DIRCM), have been developed. And as aircraft have had counter-measuring equipment installed, the seekers have adopted infrared counter-countermeasures (IRCCM).
There are currently three major IRCCM capabilities: intensity rise time, line-of-sight (LOS) rate change, and spectral distribution discrimina- tion [1–5].
- Intensity Rise Time – Although an aircraft has constant IR intensities, flares have abruptly increasing intensities right after burning. This increase is to get sufficient intensities before separating from the aircraft. Accordingly, a missile seeker has an intensity rise time trigger, enabling the seeker to recognize a flare when its intensity increases abruptly and discontinue tracking it.
- LOS Rate Change – IRCCM seekers can distinguish an aircraft-fired flare as a decoy by measuring the angular velocity of two objects when a flare separates rapidly from the aircraft right after firing. Thus, to delude the seeker, the aircraft must operate and launch flares in an effective manner.
- Spectral Distribution Discrimination – Seekers have IRCCM to detect flares by comparing the intensity ratios of an aircraft and flares in the near- and mid-IR regions. An aircraft has relatively constant intensities in the two IR regions, but flares have high intensities in the near IR. Therefore, flares have been developed recently to increase the intensities in the mid-IR region, providing similar ratios to an aircraft.
This following sections discuss effective flare operation methods against LOS rate change of IRCCM. First, we introduce the experimental approaches that were conducted to characterize flight features of flares and an aircraft regarding the LOS rate change of IR seekers. The first set of experiments, flare firing, was performed to acquire trajectory and speed characteristics of flares with different conditions. The focus of the second set of experiments was on LOS rate change measurement using an actual IR seeker. Based on the analysis of these experiments and their resulting data, several flare firing management approaches are suggested to maximize aircraft survivability.
Flare Firing Experiments
For this study, flare trajectories from an actual firing test were studied, while previous studies have relied on simulation data [3, 6]. In the case of the simulation, the model was designed such that the flare weight and velocity decrease as the simulation time elapses, while the acceleration value reaches its maximum at burn-out time. For this study, the actual flare range, speed, and acceleration were obtained by the firing test.
In the experiment, conventional-type flares were fired to the horizontal direction from a hovering helicopter, and video was taken in the direction perpendicular to the flare motion direction. Based on this video, the effective time of the flare was measured to be approximately 4 s. The video was subdivided into 0.1-s intervals, and the distance was measured horizontally and vertically using some reference value of helicopter specifications for 4 s. The trajectory is shown in Figure 1, with just flare movements and no helicopter velocity elements. The trajectory equations were approximated by using polynomial interpolation. The horizontal and vertical speeds are shown in Figure 2, where the initial speed is 50 m/s and the final horizontal speed is almost 0.
Relative Speeds of Helicopter and Flares Viewed From the Missile
Assuming the helicopter speed to be 120 nm (60 m/s), the relative speeds of the helicopter and the flares viewed from the incoming missile were analyzed for 4 s, with the flare firing angle varying from 0 to 150° (see Figures 3–5). The angles of the incoming missile were also supposed to be from 0 to 150°. In the analysis, the flare speeds obtained in the experiments were applied, being attenuated for 4 s.
In the figures, the relative speed of the helicopter and flares can be seen to decrease as flares are fired in the direction similar to the helicopter movement, regardless of missile viewing angles. The speed is approximately 30 m/s, which is 50% of the helicopter speed (60 m/s), except when firing backward. The maximum values are similar to helicopter speed (60 m/s). The relative horizontal positions of the two were measured for 4 s in different firing angles (see Figure 6). The vertical values are changed to 40 m for 4 s in Figure 1.
The values in Figure 6 were used to measure the length of time that the target and flares simultaneously stay in the field of view (FOV) of a missile, assuming 2° (see Table 1). The longer the time in the FOV, the better the decoying. One reason is that the smaller the separation rate of the flares and target, which is LOS rate change, the better it is against the IRCCM seeker. Another reason is that the seeker cannot track a target normally when seeing the target and the flares simultaneously. Table 1 shows the longer time length as the flare firing angle gets closer to the helicopter heading direction.
Table 1 Length of Time Staying in the FOV of Missile (Target and Flare Simultaneously)
LOS Rate Change Experiment
The trajectory sensing performance and LOS rate change function of an IR missile were tested in the laboratory (the setup of which is shown in Figure 7). The seeker used in the experiment was one of the units under test. Two IR sources were located 10 m from the seeker and heated to 2,000 °C. The heat sources areas, which were assumed to be a helicopter and flare, were changed to control IR intensities. It is generally known that flare IR intensities are 2–10 times greater than helicopter intensities in near IR . In some experiments, the intensities were reported to be 50–100 times greater. In this study, the IR intensities of the flare varied between 10 and 110 times greater than the intensities of the helicopter.
The helicopter IR source, which has a weak intensity, was fixed in a location, and the flare source was moved along the tangential direction of the seeker with different speeds and intensity. The seeker was checked to determine whether to track the strong IR source, the flare. The maximum speed at which the tracker could keep tracking the weak source was recorded at different flare intensities. In the experiment, the angular velocities were increased according to two intensities ratios. The measured angular velocities and LOS rate changes are shown in Figure 8.
In studying the trajectories of flares fired from a helicopter, the separation speed viewed from an incoming missile was found to be approxi- mately 30 m/s, which is half of the helicopter speed (except when fired in backward directions). The actual LOS rate change tested in the laboratory was betweem 2.3 and 6.3°/s. By applying these two values mathematically, we can conclude the flare decoy distance is 270–750 m from the helicopter. Thus, the flare can delude an incoming missile if the missile is further than this distance from the aircraft.
In a high-speed aircraft, the flare firing speed is relatively small compared to the aircraft. Therefore, the separation speed of the two will be the same as the aircraft speed regardless of firing angles. Assuming the aircraft speed to be 300 m/s, the flare decoy distance is 10 times that of a helicopter. Therefore, considering the missile effective range (3,000–5,000 m), it is nearly impossible to delude the missile by flare. To increase the survivability of high-speed aircrafts, installing flares with improved aerodynamic characteristics could be considered to minimize air resistance and increase flare speed near to the aircraft speed. These effects can increase the effectiveness by decreasing the separa- tion speed [5, 7].
The ultimate measure to protect an aircraft against IR missiles is to fire flares several times repeatedly. IR seekers, seeing flare appearance, operate in three steps: flare detection, CCM tracking, and normal tracking [1–3]. If a flare is fired again before the seeker transits back to the normal tracking mode (in the case of exceeding the LOS rate change), it is considered possible to delude the seeker. The time interval is predicted to be related to the signal process period of the seeker, which is a unique specification for each seeker. Considering the aircraft-speed-calibrated flight trajectory of flares , two flares per 0.1 s will be able to mask the aircraft detection by the LOS rate change function.
The number of times the two rounds of flares with a 0.1-s firing interval are suitable and the amount of time interval that is needed are related with the missile flight time and effective flare time. Flares should be effective during missile flight. For example, if we assume the missile flight time to be 6 s, two rounds of flares with a 0.1-s interval should be fired and repeated two times more with a 1-s interval between repetitions.
Generally, passive missile warning sensors are widely used because of their covertness, but they also have the disadvantage of not knowing the missile distance and arrival time. Adding active sensors can control the numbers of flares firing because the arrival time is known. This addition/control will increase the effectiveness and also reduce false alarms.
Based on the experimental results from obtaining the relative flare speeds separated from a helicopter under various conditions and measuring the LOS rate change function, the following conclusions regarding effective flare firing techniques are made. For low-speed aircraft, such as helicopters, firing one round of flare can be effective against LOS rate change-based IRCCM seekers. With aircraft opera- tional factors, such as safety, not considered, firing in the forward direction showed better results than firing in the backward direction. In the case of high-speed aircraft, IRCCM seekers can easily differentiate flares from aircraft due to high LOS rate change. Installing aerody- namically designed flares can help reduce the detection probability. Finally, for both low- and high-speed aircraft, firing two rounds of flares with a 0.1-s interval and repeating the firing several times is considered to be highly effective.
ABOUT THE AUTHORS
Dr. Yeondeog Koo works for the Agency for Defense Development in South Korea. He was the project manager for the Mission Equipment Package research and development (R&D), including Aircraft Survivability Equipment (ASE) for helicopters, and he is experienced in R&D and the test and evaluation (T&E) of various ASE, including the computer, missile warning system, radar warning system, laser warning system, and chaffs and flares.
Dr. Unseob Jeong is currently the team leader for the Agency for Defense Development’s Electronic Warfare Systems and is a former project manager for the ASE system R&D for helicopters.
Mr. Wonseok Choe manages and develops the Electronic Warfare Experiment Laboratory for the Agency for Defense Development.
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