In a missile-hardware-in-the-loop simulation, compo- nents of missile hardware are connected with a computer (or multiple computers) in a closed loop, and the simulation is operated in real time. The hardware provides the actual complex, nonlinear response characteristics of the guidance and control, and the computer simulates the aerodynamic and dynamic response of the missile, which cannot be reproduced by an actual missile in the laboratory. In a typi- cal missile-hardware-in-the-loop simulation, an arrange- ment of electromagnetic sources provides target and countermeasures stimuli much like those experienced by a missile in an operational environment (Ref. 10).
8-4.1.1 Substituting Missile Hardware
Any of the components of the guidance and control sys- tem of a missile may be included as hardware in a simula- tion. These include the seeker, signal processor, onboard computer, autopilot, and control servos. In most missile- hardware-in-the-loop simulations, however, not all of the guidance and control components are included as hardware.
Only those that are critical to the objectives of the simula- tion are included.
Typically, missile-hardware-in-the-loop simulations are arranged so that any or all of the missile hardware can be removed from the simulation loop and replaced temporarily by mathematical digital or analog models (Ref. 13) that per- mit simulation checkouts to be performed without the unan- ticipated effects and uncertainties associated with hardware performance and interfaces. These mathematical models of the components are often simplified versions that permit calculations to be performed in real time for real-time checkout of the simulation.
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Two basic modes of missile-hardware-in-the-loop simu- lation are defined: missile-seeker-in-the-loop simulations and missile-seeker-electronics-in-the-loop simulations. The difference between them depends on the type of hardware components employed.
In many applications sections of actual production mis- sile hardware are used in the simulation as shown in Fig. 8- 12. Sometimes, however, especially in the early phases of the development of a missile, actual seeker hardware is not available. In this case prototypes of the actual (or proposed) electronic signal processing circuits are substituted in the simulation, whereas other pertinent seeker functions-such as optical components and gyros for electro-optical (EO) seekers, and antennas, radomes, and rate gyros for RF seek- ers-are modeled mathematically.
8-4.1.1.1 Missile-Seeker-in-the-Loop Simulation Missile-seeker-in-the-loop simulation includes an actual physical missile seeker and physical electromagnetic radia- tion sources that simulate the target, background, and coun- termeasures for the seeker to view (Ref. 10). Basically, this means that the seeker optics and gyro for IR seekers, or radome, antenna and rate gyros for RF seekers, and all the guidance signal processing are represented in the simulation by the actual missile hardware. For EO missiles the sources of radiation power may include blackbody radiators and arc lamps. These sources produce power in the correct spectral regions and are imaged onto the missile seeker dome by appropriate optical lenses and mirrors (Ref. 14). For RF seekers the scenes are generated by RF antennas and reflec- tors. The RF power of the various sources that makeup the scene can be processed to give the proper Doppler and other effects, such as scintillation and glint.
The missile-seeker-the-loop simulation mode permits evaluation of actual missile hardware in a realistic environ- ment with the seeker acquiring and tracking real radiation that has the spectral characteristics of real target signatures and that zooms in image size and moves in relation to the seeker boresight axis (Ref. 15).
Figure 8-12. Examples of Production Hard- ware Employed in Simulation (Ref. 14)
Advantages of missile-seeker-in-the-loop simulation are that optics, gyros, radomes, antenna patterns, and inertial instruments do not have to be modeled mathematically and validated. Thus missile-seeker-in-the-loop simulation pro- vides a high degree of realism and credibility to the simula- tion. However, complex scenes are precluded by inherent limitations in present-day scene generators. Another disad- vantage of missile-seeker-in-the-loop simulation is that actual missile hardware is not always available, particularly during the early development phases of a missile.
8-4.1.1.2 Missile-Seeker-Electronics-in-the-Loop Simulation
The only hardware included in missile-seeker electron- ics-in-the-loop simulations is electronics. For example, the actual (or an electronic equivalent) seeker electronic proces- sor is included as hardware, but the optics and gyro of the seeker are modeled mathematically. No electromagnetic radiation is generated in a missile-seeker-electronics-in-the- loop simulation because there is no physical seeker to sense it. instead the target, background, and countermeasures scene is prerecorded and played back as electronic signals to a missile-seeker-electronics-in-the-loop simulation by a special electronic scene simulator. The electronic scene sim- ulator is capable of recording real scene data obtained by field measurements of actual targets against actual back- grounds and with actual countermeasures (Ref. 14). The scene signals, played back to the seeker electronics in the simulation, appear as they would if they were coming from the seeker detector itself.
Missile-seeker-electronic packaging provides special breakout points in their circuits that permit monitoring of various signals and also permit subcomponents (seeker detector, gyro torquer, guidance filter’s, etc.) to be bypassed and the corresponding mathematical models inserted in their places (Ref. 12). The electronics are typically arranged on circuit cards in a chassis suitable for mounting in a rack.
This configuration provides a multitude of test points not accessible for monitoring when the complete seeker guid- ance and control assembly is used and allows easier evalua- tion of seeker performance during the simulation. Also the lower density packaging affords better cooling for the elec- tronics than is possible with production packaging.
The major advantage of the missile-seeker-electronics-in- the-loop simulation mode over the missile-seeker-in-the- loop mode is the complexity and authenticity with which the target and countermeasures scene can be simulated and presented to the missile guidance electronics. This capabil- ity is essential for a realistic evaluation of imaging and pseudoimaging seekers that scan target and countermea- sures scenes. Another important advantage is that electron- ics are easily modified for parameter optimization and design tradeoff studies early in a missile system develop- ment cycle. A significant advantage in the cost and schedul- ing of computer runs is elimination of a cooling period,
which is required in the missile-seeker-in-the-loop mode between each simulated flight to prevent the seeker from overheating (Ref. 14).
8-4.1.2 Positioning Missile Hardware
In missile-seeker-in-the-loop simulations, it is essential that the seeker boresight axis have the angular freedom of motion it would have in an actual flight. In addition, the proper angular positions and rates between the hardware seeker boresight axis and the hardware missile body axis should be maintained, and if missile roll is included in the simulation, the hardware missile body should be rolled at the simulated rates. To permit the guidance and control hardware to experience physically the simulated angular rates, at least the section of missile hardware containing the seeker is mounted on a missile-positioning unit (MPU), also called a rotational motion simulator, or a flight table (Ref.
16). Dynamic inputs to the MPU, originating from a real- time dynamic simulation, enable hardware components mounted on the MPU to experience real-world rotational rates and angular positions during a simulated flight (Ref.
17).
The MPU supports and rotates the missile hardware about three rotational axes-yaw, pitch, and roll. When a missile is rolled during the simulation run, electrical slip rings are used to provide electrical power to the seeker, to allow monitoring, and to make available selected functions and signals from the hardware-such as the control-surface deflection commands-for use in the simulation. Cryogenic cooling of an IR seeker detector is provided by routing inert gas from a large, external tank through high-pressure plumbing to the missile seeker mounted in the MPU. This capability eliminates the need to mount a cryogenic reser- voir directly on the MPU and thus reduces the mass loading on the MPU and the need for frequent recharging of a smaller reservoir. The MPU is servo driven, usually electro- hydraulic, and receives its angular position and rate com- mands from the solution of the rotational equations of motion in the flight simulation. Thus the missile hardware experiences the rotational motion predicted by the mathe- matical simulation.
8-4.1.3 Closing the Loop With Missile Hardware
In a missile-seeker-in-the-loop simulation, the missile hardware seeker detects the radiation emitted from the sources in the scene simulation, performs the target tracking internally to the hardware, and processes the seeker signals, and thus generates electronic commands to the missile con- trol surfaces, This electronic signal is sent by hardwire to the computer where mathematically simulated control sur- faces respond to the command. Based on simulated control- surface motion and on the missile aerodynamic characteris- tics, the missile flight is simulated in the computer by using the equations of motion. inputs to the MPU are derived from the calculated Euler angles (Ref. 12), which define missile attitude, and the MPU updates the attitude of the missile hardware. In the computer the simulated target posi- tion is compared with the simulated missile position to obtain line-of-sight angles. Electrical commands based on the calculated line-of-sight angles are sent to the target scene simulator, which updates the target and countermea- sures positions in the scene. The signatures of targets, decoys, and jammers, stored in computer memory as func- tions of aspect angle, are used to compute the new radiation characteristics of each of the components in the scene.
Finally, the hardware seeker responds to the updated scene, and thus the simulation loop is closed.
The sequence is similar in a missile-seeker-electronics- in-the-hop simulation except that preprocess scene data are supplied directly to the hardware electronics by a target image simulator. The hardware electronics responds to the scene data by providing signals to the mathematical model to rotate the simulated seeker head and to deflect the simu- lated control surfaces. The simulation of missile and target positions and attitudes and the calculation of new line-of- sight angles proceed as in a missile-seeker-in-the-loop sim- ulation. Finally, the target image simulator responds to the new range and line-of-sight data with a new set of prepro- cessed scene data which is sent to the hardware electronics, and thus the simulation loop is closed.
Fig. 8-13 shows the simulation loop in--block diagram form for the missile-seeker-in-the-loop mode, and Fig. 8-14 shows the loop for the missile-seeker electronics-in-the- loop mode.
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Figure 8-13. Flight Simulation Employing Actual Missile Seeker in the Loop (Ref. 14)
Figure 8-14. Flight Simulation Employing Seeker Electronics in the Loop (Ref. 14)
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