Aggressor Shoot-Back Simulation

The present invention relates generally to the field of training devices and their component features, and more specifically to such devices that offer interactive simulation having responsive graphics components and systems.

The requirement to maintain a high state of readiness during austere budget times and to simulate close combat training effectively has placed new requirements on the training device community. Increased use of small echelon military-style operations to perform counter-terrorist and anti-drug strikes, and to affect tactical law enforcement functions have placed unique and renewed emphasis on simulation and training. Heretofore, strategies and tactics were rudimentary. Likewise, simulators were straight-forward and basic. Recently, the skills required for successful close combat have been perfected, an have outpaced the ability of previously existing training devices to simulate the scenario. Needed were training devices that would allow trainees to practice and rehearse close combat training exercises such as low intensity conflict, light infantry, SWAT and security operations with an unsurpassed level of realism and feedback. Typical events might include security operations, hostage rescue, shoot-no-shoot, ambush training situations and routine law enforcement operations in a common team scenario environment.

Current simulator-based team trainers use technology which restricts both realism in tactical training situations and ability for thorough performance measurements. For example, aggressor images are not removed from the training scenario when the trainee successfully directs his or her simulated fire at the image, a feature if included that would simulate the aggressor in the real world who is disabled as a threat by accurate fire. In addition to directly affecting the training of the team member who is encountering the aggressor image, the training of other members as individuals and together as a team are negatively affected if the aggressor image is allowed to remain in the training scenario.

Further, current trainers do not require trainees to seek sensible cover and concealment during the scenario. Team trainers currently available permit the trainees to engage targets while fully exposed to on-screen aggressors since here is no aggressor shoot-back capability in the prior art.

Additionally, the prior art tracking systems for determining the aiming point of the trainees' weapons is limited to collecting data only at trigger-pull. As a result, continuous weapon position data is not available for replay, analysis, and feedback. There is also a substantial delay between trigger-pull and data collection that is inherent and proportional to the number of trainees in the team trainer.

Commercially available infrared spot tracking systems typically consist of a Charge Coupled Device (CCD) video camera interfaced to a digital frame grabber operating at standard video rates. A suitable lens system images the tracking area (i.e., video projection screen) onto the CCD imaging sensor. The frame grabber digitizes each frame of video data collected by the CCD camera. This data is further processed with digital signal processing hardware as well as proprietary software algorithms to find the position coordinates of the imaged IR spot.

The CCD imaging sensor consists cf a two-dimensional matrix of discrete photodiode elements. A 10-bit (1024 horizontal elements.times.1024 vertical elements) CCD imaging sensor has over one-million individual photodiode elements that convert the incident illumination into a proportional quantity of electrical charges. The electrical charges are sequentially transferred to a readout stage. At the readout stage, each electrical charge is converted into a proportional voltage signal. This voltage is further amplified to give a low impedance output video signal.

For accurate tracking and trigger-pull synchronization, the position coordinates of each weapon should be updated at least every 3 milliseconds with a resolution of 10 bits. The CCD-based tracking system discussed above requires over 30 milliseconds to sequentially sample the weapon position coordinates, which is too long for its application to multiple trainees.

The present invention and its related component systems improve the effectiveness and realism for training a weapon fire team in a simulator environment.

The goal of the development effort that led to the present invention was to introduce new technology and techniques which can improve current team training system technology. The new developments include an interactive and high speed weapon tracking system in a training system that allows trainees to engage disappearing aggressor targets which are presented on a large video projection screen.

The objectives were to overcome the disadvantages of the prior art and provide an improved team trainer, develop apparatus and a method to remove aggressor targets which are hit as a training scenario progresses, develop apparatus and a method which allows aggressor targets to engage and disable trainees who do not take appropriate cover, and design a weapon tracking system that continuously and accurately provides weapon aimpoint coordinates for up to 9 trainees.

The development effort met the stated objectives. Aggressor targets are instantly removed from a training scenario as they are disabled by weapon fire from trainees. Also, an array of infrared emitting diodes is placed above the projection screen and a detector harness is used to detect a modulated infrared beam from this array, which increases tactical realism by requiring trainees to seek appropriate cover when engaged by aggressor targets. An innovative weapon tracking system that generates accurate weapon position data at over 300 Hz has been developed which is capable of continuously tracking weapon aiming points for each of a plurality of trainees. Increased realism and effectiveness in simulator-based weapons team training can be realized through implementation of the new techniques and technology that are disclosed herein. Continuously tracking weapon aiming points for all members of a fire team expands performance measurement and playback capabilities. Training effectiveness and realism also are increased by instantly removing disabled aggressors from the training scenario and requiring trainees to take appropriate cover when an aggressor returns fire. Results include an increase in communication and awareness between members of the team. In contrast, previous training systems did not require trainees to seek appropriate cover. Also, aggressor targets were not removed from the progressing training scenario when they were successfully engaged and disabled by trainees.

FIG. 1 is a diagram of a combat team trainer.

FIG. 2 shows an Infrared Spot Tracker Imaging Diagram of the preferred embodiment.

FIG. 3 is a block diagream of a combat team trainer.

FIG. 4 shows a Timing Sequence for Tracking Weapon Aim points.

FIG. 5 shows a Shoot-back IRED Array Mounted Horizontally Above projection Screen.

FIG. 6 is a Block Diagram of IR Detection Circuitry.

FIG. 7 shows the Sound System Components and Speaker Configuration of the preferred embodiment.

FIG. 8 shows a Two Dimensional PSD Structure.

FIG. 9 shows a One Dimensional PSD Structure.

FIG. 10 shows an Infrared Spot Tracker Block Diagram.

FIG. 11 shows a DC Coupled PSD/Transimpedance Amplifier Configuration.

A preferred embodiment of the present invention is shown in FIG. 1. The training device accommodates training for a plurality of military or law enforcement trainees in a common-threat scenario The trainees 10 interact with 100 inch video projection screen 12 set up in a training exercise room. The video projection screen 12 displays both live video targets and graphics overlay from video projector 14 and a video disk player under computer 26 control. Each trainee has a weapon that is equipped with a collimated source 16 of infrared (IR) energy, an infrared emitting diode (IRED). The IRED is collimated to maximize the IR energy transferred from the weapon to the projection screen 12 while minimizing the IR beam 18 divergence. The collimated infrared source 16 is aligned with the trainee's weapon and places a small infrared spot 20 on the video projection screen 12 corresponding to the location the trainee is pointing his weapon. The infrared sources 16 are sequentially modulated in a time-multiplexed mode by the system computer 26 to both identify the active weapon among the plurality of trainees and to improve signal detection.

A high-speed, low cost, infrared spot tracker 22 determines the continuous X and Y position coordinates of each weapon. The optical system for the infrared spot tracker 22 (IST) views the entire video projection screen 12 from a distance of approximately 12 feet, as configured in the preferred embodiment. The infrared spot 20 imaged onto the projection screen 12 surface is optically transferred or reimaged to a corresponding location on the Position Sensing Detector 24 (PSD) as shown in FIG. 3. The PSD and associated electronic circuitry is located within the IST enclosure. The system computer 26 determines the position coordinates of the infrared spot on the PSD and consequently the video projection screen 12 as well.

The high-speed PSD-based infrared spot tracker 22 generates the continuous position coordinate data of each weapon in less than 3 milliseconds; in contrast, a typical CCD-based tracker would require over 16 milliseconds. Due to the high-speed tracking capability of the PSD-based tracker 22, the training device allows for accurate tracking and trigger-pull synchronization for up to nine trainees.

The system computer 26 shown in FIG. 3 synchronizes the time-multiplexed enable signal for each weapon with the 12-bit analog to digital conversion of the IST position data. Once the system computer 26 knows the position coordinates of a weapon, it can compare that data to the stored coordinates of active targets 28 on the projection screen 12 at the time of trigger pull. If the IST position data matches the coordinates of a target on the projection screen 12, a hit is recorded for that weapon. A high-speed video graphics board utilizing "active windows" enables the targets 28 to disappear when hit without affecting the ongoing scenario.

The trainees are encouraged to take sensible cover as they would in the real world while engaging targets 28 displayed on the video projection screen 12. Each trainee wears a Multiple Integrated Laser Engagement System (MILES) torso harness 30 containing infrared detectors and an alarming device to indicate if he has been hit by an on-screen aggressor. The on-screen aggressor shoot-back is simulated by using an array of infrared emitting diodes (IREDs) located above the video projection screen 12. Each IRED is pointed in a particular sector within the training exercise room so that all exposed areas are within the field of fire of the on-screen aggressors. The individual IREDs are turned on and off by the system computer 26 corresponding to where the on-screen aggressor is pointing his weapon. If a trainee does not take cover while in the field of fire of the on-screen aggressors he will be illuminated with infrared energy. The infrared detectors positioned on the MILES torso vest will detect the incident IR energy and activate an alarm to indicate that the trainee has been shot by the on-screen aggressor. Once a trainee has been hit he is considered dead and his weapon is disabled.

After a training session is over, the video scenario is played back in slow motion. The system computer 26 shows the continuous pointing location of each weapon by graphically displaying color coded icons representing the continuous IST position data stored by the system computer 26 during the actual training session Hit and miss shot locations are indicated by changing the color of the icons.

A complete sound system 32 has also been developed to simulate the actual acoustical training environment of each scenario. An Analog/Digital sampler digitizes, stores and plays back the background sounds as well as the synchronized gun shot sounds corresponding to the trainees and the on-screen aggressors. The sampler is under the control of a Musical Instrument Digital Interface (MIDI) port interfaced to the system computer 26 for proper timing and synchronization.

Several software programs control both training scenario development and presentation for the training device. A source code of the programs, written in C language under the MS-DOS operating system, are attached hereto. Computer software control of the optical disc player allows automated scenario development and rapid aggressor selection. Control of the weapon tracking system hardware provides continuous tracking of each weapon's aimpoint and status. Various functions of the video graphics adapter allow interactive control of the on-screen aggressors. Commands transmitted by MIDI (Musical Instrument Digital Interface) board 34 provide sound effects as each scenario progresses. Synchronous control of the training device system hardware based on the scenario content creates the training session.

Moving video footage from an optical disc player 36 generates the training device aggressor threat. Scenario development begins with formulation of a script for the aggressor force. The script describes aggressor actions including timing and movement within the camera's field of view. Creating aggressors that will disappear when hit imposes some restrictions on the video recording process. Scenario constraints include maintaining a stationary camera, restricting overlap of aggressor targets 28, and sustaining consistent lighting However, these constraints enable instant feedback through disappearing targets 28 and increase flexibility in aggressor selection.

Creating aggressor targets 28 which disappear when hit requires consistency in background and lighting of the video image. These factors are crucial during portions of a scenario where aggressor targets 28 are visible and engageable. Each scenario's moving video can be sectioned into segments in which an aggressor appears into view, engages the trainee, and then takes appropriate cover. Dividing a scenario's moving video footage into sections maximizes optical disc storage by eliminating nonessential video. During each section, camera stability and lighting consistency allow the video graphics adapter to add or remove aggressor targets 28 as a training session progresses.

Depending on the type of scenario, movement of the camera may be necessary to recreate the threat situation. For example, a security force clearing a building would maneuver through the building. Therefore, maneuvering the camera is necessary to produce this type of scenario. To allow for this type of camera movement the scenario script specifies locations where aggressor engagements occur. Before aggressors are introduced into the scenario, the camera position is fixed at a designated location which maintains a consistent background. From this location, multiple aggressor actions are recorded. The camera is then maneuvered to the next designated area and the process is repeated. Recording multiple aggressor actions at each location enables the training session to branch based upon the trainee performance. These video segments of aggressor engagement and camera movement are edited and transferred to optical disc.

After transferring a scenario's video segments to optical disc, a program generates a detailed description or map of each segment. This program automates this mapping process by using a user-friendly menu system, graphical overlays under mouse control, and optical disc control functions. The mapping process generates a data file specifying each video segment. First, the optical disc is scanned to locate the start frame for a video segment. Once located, the number of aggressor targets 28 is identified and entered. For each target, a rectangle is drawn around the area which covers the complete exposure or path of the aggressor target during the video segment. This rectangle defines the live video window used to interactively control each aggressor. By single stepping the optical disc both hit areas and shootback directions are identified for each frame of the video segment. Specifying a unique filename for each segment's mapping data creates a data base which describes every video segment applicable to a specific training scenario.

The purpose of the detailed mapping process is to allow a video graphics adapter to interactively present the aggressor force during a training scenario. The optical disc player composite video output is converted to an analog RGB signal for input to the video graphics adapter. The video graphics adapter is configured for a 756.times.972 pixel display buffer which is capable of storing two high resolution video frames, each containing 756.times.486 pixels. The video graphics adapter performs real-time capture of the video image at 16 bits per pixel. This 16 bit per pixel format allows the display of both live video and high resolution graphics. Addition and removal of aggressor targets 28 is accomplished by opening and closing live video windows within the captured video image. Closing a live video window while using a double buffer drawing technique allows instantaneous removal of the aggressor target.

Software control of the tracking system hardware allows each weapon to be continuously monitored during a training scenario. The hardware is comprised of the newly developed infrared spot tracker 22 based on a position sensing detector 24 (PSD), an analog to digital (A/D) conversion board 38, a high powered infrared emitting diode (IRED) mounted on each weapon, and control electronics. Each weapon's aimpoint, trigger switch position, selector switch position, and magazine reload indicator are sampled at approximately 60 Hz.

A periodic interrupt procedure controls the weapon tracking process. The A/D board is configured to acquire the IST's four analog outputs with direct memory (DMA) data transfer, which requires minimal CPU overhead. A programmable interval timer provides timing signals which sequence the process. The programmable interval timer is configured to generate both a 3 millisecond periodic interrupt (rate generator) and a 2 millisecond one shot delay. Activated every 3 milliseconds, an interrupt service procedure controls the weapon tracking process.

The timing sequence for a two weapon system is shown in FIG. 4. At the start of the first interrupt cycle weapon one's IRED is activated and the programmable oneshot is retriggered. After 2 milliseconds the infrared spot tracker's four analog outputs have settled and reflect the horizontal and vertical position of weapon one's aimpoint. Simultaneously, the programmable oneshot output gates the A/D board to acquire the tracker's four outputs. Each output is converted at 50 kHz to 12-bit digital values. During approximately 640 microseconds the four outputs are sampled eight times and the 12-bit results are DMA transferred into a data buffer. Upon entry of the second interrupt cycle, weapon one's IRED is turned off and the A/D data buffer is monitored. Comparing data buffer elements to a voltage threshold determines the presence of the weapon one's infrared spot 20. If detected, the tracker's raw data is averaged. Calculations are then used to determine weapon one's non-scaled horizontal and vertical positions. In addition, weapon one's switch positions are updated. Similarly, weapon two's IRED is modulated during interrupt cycle three and positional data is generated during cycle four.

Repetition of this interrupt sequence provides continuous update of each weapon's aimpoint and switch status. Two techniques enable this tracking process to require minimal CPU overhead. First, multiple conversions of the tracker's four analog outputs are performed with DMA data transfer. Second, a periodic interrupt procedure, essentially a background task, performs both tracking system controls and basic position data calculations.

A simple weapon zeroing procedure is used to find coefficients and offsets for two first order equations. These equations are then used to convert the raw tracking system data to x and y screen coordinates. However, this technique does not maximize the accuracy and stability of the tracking system hardware. In order to increase the accuracy of the tracking system, the weapon alignment algorithm would be adjusted to account for the tracker's viewing angle, the tracker's lens distortion, and the video projector's linearity; and, the tracker's imaging optics would be optimized to increase accuracy. Furthermore, increasing the A/D conversion rate to acquire more samples and improving data conditioning algorithms would improve tracking system performance.

As described for the preferred embodiment, the training device is configured as a two weapon system. However, the weapon tracking process is expandable. Additional weapons can be added while achieving sufficient sampling rates, up to 9 weapons at greater than 30 Hz. Also, a larger field of view can be covered through the use of multiple infrared spot tracker 22s.

The training device system computer 26 controls the presentation of each scenario's moving video footage through a RS-232 communication link to the optical disc player. Synchronizing the moving video footage to the simulation software provides an event timing mechanism. Each moving video segment is synchronized by initiating the optical disc playback operation and monitoring a vertical sync counter on the video/graphics board. During optical disc playback the current frame number is instantly accessible by reading this counter. This provides an accurate and efficient method for synchronizing the simulation software. In comparison, polling the optical disc player through the RS-232 port requires too much time and CPU overhead.

During a training scenario various segments of moving video footage are presented to the trainees. Target mapping and hit areas are read from a data file located on ramdisk. The simulation is controlled by synchronizing scenario mapping data to the interrupt generated rifle tracking data. An aggressor target is removed from the training scenario when a trainee succesfully fires his weapon within the hit area defined for the current video frame. Weapon sound effects are generated based on both rifle tracking data and aggressor target shoot-back data. Weapon aim points, shot locations, and status are continuously stored for each trainee during the training session. After a training session is completed, this information is provided to the trainees for review.

The performance of each trainee is evaluated based on the number of rounds expended, the number of aggressor targets hit, and a visual replay of each weapon's movement with shot locations. Upon completion of a training scenario a replay function performs a slow-motion display of the scenario with graphical overlays. During replay a different colored circle represents each weapon's aim point. Shot locations are indicated by changing the weapon's aim point color and briefly pausing the video playback. An aggressor target hit, a semi-automatic fire miss, or an automatic fire miss is indicated by changing the aim point color to red, blue, or green respectively The ability to continuously track each weapon's movement enhances both individual and team performance measurements.

The implementation of the aggressor shoot-back system consist of three subsystems: 1) the shoot-back bar 40, consisting of a horizontal IRED array located just above the video projection screen, 2) the IRED modulator/driver circuitry 42 located in the proximity of the system computer and coupled to bar 40 by lines 44, and 3) the infrared detection circuitry located on the back of the MILES torso vest, and shown in FIG. 6. The system computer 26 turns the IRED modulator/driver on and off in synchronization with the on-screen aggressors action. If the on-screen aggressor is shooting his weapon towards a particular sector then the corresponding IRED is enabled and modulated, forcing the trainee to take cover while in that sector.

The shoot-back bar comprises nine IREDs with built in lenses placed horizontally across the top of the video projection screen 12. FIG. 5 illustrates the shoot-back IRED array used to simulate aggressor shoot-back. The individual IREDs are mounted on a ball and socket swivel mount for optimum adjustment. The IREDs have a half intensity beam angle of less than six degrees. The small beam angle is suitable for the other dimensions of the training device and allows the individual IREDs output energy to be strategically directed throughout the training exercise room.

The IREDs are modulated by a 1.6 Khz square wave when enabled by the system computer 26. Modulating the IREDs allows the driver circuit to pulse more current through the IRED for a higher output power as well as increasing the detectivity of the low level IR signal by the detection circuitry.

The modulator circuit consist of a LM555 timer integrated circuit operating in the astable oscillating mode. The TTL output voltage of the LM555 timer supplies the gate voltage for an Enhancement Mode Junction Field-Effect Transistor (EMJFET) which then sources the required current to IRED.

The IR detection circuitry is shown in FIG. 6 and comprises eight infrared detectors connected in parallel and strategically placed on the MILES torso vest. The original MILES electronics is replaced with specific electronics to detect the modulated infrared energy from the IRED array used to simulates shoot-back.

A low noise transimpedance amplifier converts the output current from the photodetectors into a proportional voltage. The infrared signal voltage is amplified and filtered with a fourth order bandpass filter. The output signal from the bandpass filter is rectified and demodulated with a lowpass filter. The lowpass filtered signal is compared to a reference voltage to determine if the trainee was hit by an on-screen aggressor. If a sufficient signal is detected to indicate a hit, then an alarm sounds to indicate to the trainee he has been killed. The alarm is latched with an SCR; therefore, the trainee must disable his weapon to utilize a "key" to turn off the hit indicator alarm. Sound effects are generated during a training scenario. The major sound system components and speaker configuration is shown in FIG. 7. They provide sounds of the various weapons being fired by both the trainees and their on-screen adversaries. Also, background sounds are generated to increase realism during a training scenario. The heart of the sound system 32 is a digital sampler module. The sampler digitizes, stores and plays back sound effects under the control of a MIDI (Musical Instrument Digital Interface) port. A MIDI controller card is installed in the system computer 26. During a scenario the computer sends appropriate commands to the sampler via the MIDI interface. The sampler creates the appropriate sounds and sends to amplifiers which drive foreground and background sounds. Mixers are used to control dispersion between the foreground and background.

Using a sampler module with an external storage device allows a multitude of sound effects to be available for increasing realism in training. The sampler uses both a 3.5 inch 800 Kbyte floppy drive and an 80 Mbyte SCSI hard disk to store digitized sounds. Depending on sample rate, the 80 Mbyte SCSI disk can hold as much as an hour or more of sampled sounds that can be mixed and sequenced by the sampler to generate essentially unlimited amounts of audio feedback. The sounds that are digitized and recreated by the sampler come from a variety of sources. Some may be selected from commercially available sound effects available on compact disk. Some are recorded in the field using both regular and DAT tape recorders. Still others may be synthesized. For use in the training device, the sounds were edited and sometimes normalized before being digitized. Realism and variation in training scenarios is enhanced by adding computer controlled sound effects.

An infrared spot tracking system is used in the training device to determine the continuous X and Y position coordinates representative of where each trainee is pointing his weapon.

To overcome the disadvantages of CCD-based tracking systems, a low-cost, high-speed, IST was developed utilizing a two-dimensional lateral-effect photodiode, the Position Sensing Detector 24 (PSD). The PSD is not a discrete charge transfer device such as the CCD, but rather a continuous analog output device. In contrast to other types of position sensing photo devices such as CCD detectors, the PSD offers higher resolution, faster speed, larger dynamic range, and simpler signal processing.

The PSD is a photoelectronic device utilizing the lateral photo-effect to convert an incident light spot into continuous position data. Its two-dimensional PSD structure is shown in FIG. 8. The lateral photo-effect occurs because of the diffusion properties of separated charge carriers along a uniformly or nonuniformly irradiated p-n junction. The current diffusion in a fully reversed-biased p-n junction occurs primarily due to the external collection of generated charge carriers through finite loading impedances. For a two-dimensional fully reversed-bias PSD with zero loading impedance, there is an analytical linear relationship between the output current and the light spot position along the pertinent axis.

The basic construction of a two-dimensional lateral-effect PSD consist of p and n doped layers of silicon forming a p-n junction. The front side of the PSD is an implanted p-type resistive layer with two lateral contacts placed opposite each other. The back side of the PSD is an implanted n-type resistive layer with two lateral contacts placed orthoganol to the contacts on the front side. The p and n layers are formed by ion implantation to ensure uniform resistivity. As an example, high resistivity silicon can be implanted with boron on the front side and with phosphorus on the back side. The p-n junction is light sensitive; therefore, incident light will generate a photoelectric current which flows through the ion implanted resistive layers. Electrodes are formed at the edges of the PSD by metalization on the ion-implanted resistive layers. Transimpedance amplifiers serve as a finite load impedance to convert the generated charge carriers to a position dependent voltage.

The two-dimensional lateral-effect PSD used in the design of the IST is able to detect an incident light spot position on its rectangular surface with a maximum non-linearity of 0.1 percent.

The photoelectric current generated by the incident light flows through the device and can be seen as two input currents and two output currents. The distribution of the output currents to the electrodes determines the light position in the Y dimension; the distribution of the input currents determines the light position in the X dimension. The current to each electrode is inversely proportional to the distance between the incident light position and the actual electrode due to the uniform resistivity of the ion implanted resistive layers. FIG. 9 shows a one-dimensional PSD position model that illustrates how simple algebraic equations determine the incident light spot position. This model assumes a zero ohm load impedance and a theoretically uniform implanted resistive layer.

In FIG. 9 the distance between electrodes 1 and 2 is 2L, and the uniform resistance is R. The distance from electrode 1 to the position of the incident light spot is L-X, and the resistance is R.sub.1. The distance from electrode 2 to the position of the incident light spot is L+X, and the resistance is R.sub.2. The photocurrents produced at electrodes 1 and 2 are proportional to the incident input energy and inversely proportional to the uniform resistant path from the incident light to the electrodes. The total photocurrent produced by the input energy is I.sub.0. The sum of the output currents I.sub.1 and I.sub.2 is equal to I.sub.0. From FIG. 10, we can derive the following equations, ##EQU1## The resistance of R.sub.1 and R.sub.2 is proportional to the linear distance that R.sub.1 and R.sub.2 represent since the resistive layer is uniform. In general, the resistance of a given material is given by ##EQU2## where, p=resistivity of the material in ohms.meter

L=length of material in meters

A=area of material in meters.sup.2

IF we now define R.sub.1 and R.sub.2 with respect to p, L, and A we obtain the following expressions: ##EQU3## Substituting equations (4) and (5) into equations (1) and (2), the output currents I.sub.1 and I.sub.2 can now be written as: ##EQU4## We can eliminate the dependance of equations (6) and (7) on I.sub.0 by dividing the difference of I.sub.1 and I.sub.2 by the sum of I.sub.1 and I.sub.2. We can now solve for the X position (X.sub.pos) of the incident input energy relative to the chosen coordinate system shown in FIG. 10. ##EQU5## Substituting equations (6) and (7) into equation (8) gives ##EQU6## Equation (9) gives the linear position of the incident energy source independent of its intensity. This feature is very important since the intensity of the focused energy source on the PSD surface is rarely constant in a typical application. The two-dimensional PSD position model operates analogous to the one-dimensional PSD position model except that there are now two uniform resistive layers and four electrodes. The top resistive layer is used to divide the output currents into I.sub.y1 and I.sub.y2. The bottom resistive layer is used to divide the input currents into I.sub.x1 and I.sub.x2. The four currents I.sub.y1, I.sub.y2, I.sub.x1, and I.sub.x2 determine the x and y position coordinates of the incident energy source analogous to the one-dimensional case. The X position coordinate is given by ##EQU7## The Y position coordinate is given by ##EQU8## Equations (10) and (11) clearly show that we may obtain the X and Y position coordinates of an incident energy spot focused onto the PSD surface by a simple manipulation of the output photocurrents.

Since it is the magnitude of the photocurrents that we wish to manipulate we can represent the four output currents with four output voltages as long as we preserve the magnitude information. We now have the following design equations: ##EQU9##

The design of the analog electronic subsystems for the PSD-based tracker 22 is dependent on the amount of reflected IR energy collected and focused onto the PSD surface. This energy is a function of the IR energy source mounted on the weapon, the projection screen reflectivity, the angle of incidence of the collimated energy source 16 to the projection screen, and the collecting optics used to collect and focus the reflected IR energy onto the PSD surface.

FIG. 10 shows that the electronic design of the IST consists of six functional blocks. The lens system 46 as previously discussed images the video projection screen 12 onto the PSD surface, thereby imaging the modulated IR spot on the PSD accordingly. The PSD's photo-voltaic effect converts the modulated IR energy focused on its surface into four separate photocurrent outputs. The voltage representation of the magnitude of the photocurrent outputs are used to calculate the spot position on the PSD surface according to equations (12) and (13).

The photocurrent outputs from the PSD electrodes are terminated into low noise transimpedance amplifiers. FIG. 11 illustrates a typical dc coupled transimpedance configuration with a bias potential for reverse biasing the p-n PSD junction. In this configuration, the PSD views a load impedance Z.sub.L (s) defined as ##EQU10## where, A(s) is the amplifier's open-loop transfer function

w is the modulating frequency of the IRED

R.sub.f is the feedback resistor

C.sub.f is the feedback capacitor

To maximize the lateral photo-effect and the linearity of the PSD output currents, the terminating load impedance Z.sub.L (s) should be much less than the position sensing sheet resistance of the PSD [3]. As can be seen from equation (14), this limits the magnitude of the feedback resistor and the modulating frequency of the IRED for optimum performance.

The transimpedance amplifier converts the generated charge carriers from the PSD to a representative voltage. The output voltage of the transimpedance amplifier is given by, ##EQU11## where, V.sub.o (s) is the output voltage of the amplifier

I(s) is one of four modulated PSD output currents

V.sub.n (s) is the total output noise voltage including shot noise, thermal noise, and amplifier noise

V.sub.os (s) is the total offset voltage due to the dark current and amplifier bias currents

The low-level dc coupled output voltage from the transimpedance amplifier is band-pass filtered with a wideband fourth order Butterworth filter. The band-pass filter suppresses the unwanted background signal (e.g., room lights), the reverse bias voltage, the offset voltage, and the output noise from the transimpedance amplifier while amplifying the 10 KHz IR signal from the trainee's weapon.

The band-pass filtered signal is further amplified and converted back to a modulated dc voltage level by a precision full-wave rectifier circuit. The dc restoration enables the original dc modulated 10 KHz photocurrent magnitude information to be retained as a dc modulated 20 KHz time varying voltage with a nonzero average.

The full-wave rectified signal is low-pass filtered (demodulated) with a fourth-order Bessel filter to remove the ac Fourier components of the waveform while retaining the dc magnitude information. A cutoff frequency of 500 Hz was chosen to minimize the transient response of the low-pass filter while still allowing for adequate filtering.

The analog output voltages from the low-pass filters are used to calculate the incident spot position relative to the PSD surface according to the following equations: For the X position coordinate, ##EQU12## and for the Y position coordinate, ##EQU13## where, V.sub.x1, V.sub.x2, V.sub.y1, and V.sub.y2 are the analog output voltages representing the photocurrent magnitude information from the PSD.

The high-speed analog to digital converter board converts the analog output data from the IST to a 12-bit digital signal. The system computer 26 performs the simple calculations to determine the X.sub.pos and Y.sub.pos coordinates of the IR spot based on equations (16) and (17). An algorithm based on statistical averaging and position probablity is performed over a number of samples to increase the effective resolution to 10 bits.

From the foregoing description, it may readily be seen that the present invention comprises new, unique and exceedingly useful apparatus which constitutes a considerable improvement over the prior art. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the present invention may be practiced otherwise than as specifically described. ##SPC1##