343 lines
11 KiB
Text
343 lines
11 KiB
Text
#****************************************************************************
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#
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# Derived from
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# A Digital Lead Computing Optical Sight Model
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# Anthony L. Leatham, et al
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# Air Force Academy
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# Sept. 1974
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# http://www.dtic.mil/dtic/tr/fulltext/u2/786464.pdf
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#
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# Appendex D Air Force Avionics Laboratory digital LCOS
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# Contains FORTRAN code that is used here converted into Nasal code.
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#
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# Original source for equations and code was from:
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#
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# Air-to-Air Gun Fire Control Equations for Digital Lead Computing Optical Sights
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# R.A. Manske
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# AFAL-TM-74-8-NVE-1
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# April 1974
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# Air Force Avionics Laboratory
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# Wring-Patterson AFB, Ohio
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#
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# Document location unknown.
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#
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# non-static inputs
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#
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# range in feet
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# rangeRate range rate in ft/sec
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# P angular pitch rate of aircraft body axis in radians/second
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# Q angular yaw rate of aircraft body axis in radians/second
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# R angular roll rate of aircraft body axis in radians/second
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# normalAcceleration normal acceleration in ft/sec/sec (-32.17 straight and level)
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# alpha angle of attack radians
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# trueAirSpeed true airspeed - feet per second
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# RHO air density in slugs per cubic foot
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#
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#********************************************************************************
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# ================================ Initalize ======================================
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# Make sure all needed properties are present and accounted
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# for, and that they have sane default values.
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# print("gunsight-computer.nas started");
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var propertyTreeRoot = "/controls/armament/gunsight/";
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var z_offset = 0;
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var y_offset = 0;
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var gunElevationRadians = 0;
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var ballisticCoefficienct = 0;
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var muzzleVelocity = 0;
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var harmonizationRange = 0;
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var range = 0;
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# constants
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var timeStep = getprop(propertyTreeRoot, "timeStep");
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# sight damping factor
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var SIG = getprop(propertyTreeRoot, "dampingFactor") * 0.4;
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var KSIG = 1.0 / (1.0 + SIG);
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var seaLevelAirDensity = 0.00238;
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var radiansPerDrgree = 0.0174532925;
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# Ballistic coefficient divided by sea level air density
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var KBRHO = 0;
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var cosGunElevation = 0;
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var sinGunElevation = 0;
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# Reciprocal of gun harmonization range
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var RRH = 0;
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var SQRV = 0;
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var RHO = 0;
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var RDC = 0;
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var LA = 0;
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var LE = 0;
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var VLS = 0;
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var rangeRate = 0;
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var VOS = 0;
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var VCM = 0;
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var VMC = 0;
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var rangeRateArray = [0, 0, 0, 0];
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var rRAindex = 0;
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var P = 0;
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var Q = 0;
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var R = 0;
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var normalAcceleration = -32.17;
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var maxElevation = getprop(propertyTreeRoot, "maxElevation");
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var maxAzimuth = getprop(propertyTreeRoot, "maxAzimuth");
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var initSightComputer = func()
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{
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# print("gunsite-computer.nas init()");
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# Get aircraft/sight specific info
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# Offset of gun in feet from sight line
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z_offset = getprop(propertyTreeRoot, "z-offsetFeet");
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y_offset = getprop(propertyTreeRoot, "y-offsetFeet");
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# Elevation of gun(s) above alpha
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gunElevationRadians = getprop(propertyTreeRoot, "gunElevationDegrees") * 0.0174532925;
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# Projectile ballistic Coefficient and muzzle velocity in feet per second
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ballisticCoefficienct = getprop(propertyTreeRoot, "ballisticCoefficienct");
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muzzleVelocity = getprop(propertyTreeRoot, "muzzleVelocity");
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# Range where the sight line is = the guns bore line
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harmonizationRange = getprop(propertyTreeRoot, "gunHarminizationRangeFeet");
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# range to target usually from an onboard radar system or a
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# manual system controlled by the aircraft crew
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range = getprop(propertyTreeRoot, "range");
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var seaLevelAirDensity = 0.00238;
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# Ballistic coefficient divided by sea level air density
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KBRHO = ballisticCoefficienct / seaLevelAirDensity;
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cosGunElevation = math.cos(gunElevationRadians);
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sinGunElevation = math.sin(gunElevationRadians);
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# Reciprocal of gun harmonization range
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RRH = 1.0 / harmonizationRange;
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SQRV = math.sqrt(muzzleVelocity);
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RHO = getprop("/environment/density-slugft3");
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VLS = KBRHO * RHO * SQRV * range;
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rangeRate = 0;
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VOS = muzzleVelocity + VLS + rangeRate;
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VCM = math.sqrt(VOS * VOS - 4.0 * (rangeRate) * VLS);
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VMC = math.sqrt(VOS * VOS - 4.0 * (rangeRate) * VLS);
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# print("gunsite-computer.nas initialization done");
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}
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var getRangeRate = func()
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{
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# print("gunsite-computer.nas getRangeRate");
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var oldRange = range;
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range = getprop(propertyTreeRoot, "range");
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rangeRateArray[rRAindex] = (oldRange - range) / timeStep;
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rRAindex = rRAindex + 1;
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if (rRAindex > 3)
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{
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rRAindex = 0;
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}
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var rangeRateSum = 0;
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forindex(i; rangeRateArray)
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{
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rangeRateSum = rangeRateSum + rangeRateArray[i];
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}
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var rangeRateAve = rangeRateSum / 4;
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setprop("/controls/armament/gunsight/rangeRate", rangeRateAve);
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return -1.0 * rangeRateAve;
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}
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var getAccelerationData = func()
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{
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# print("gunsite-computer.nas getAccelerationData()");
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P = getprop("/orientation/p-body");
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Q = getprop("/orientation/q-body");
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R = getprop("/orientation/r-body");
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normalAcceleration = getprop("/accelerations/pilot/z-accel-fps_sec");
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}
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var getTrueAirSpeed = func()
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{
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# print("gunsite-computer.nas getTrueAirspeed()");
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# from http://en.wikipedia.org/wiki/True_airspeed
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# TASKnots = A0 * M * sqrt(T/T0)
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# where A0 = speed of sound a standard sea level (661.47 Kts)
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# M = MACH speed of aircraft
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# T = static air temperature in Kelvin at aircraft position
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# T0 = temperature a standard sea level (288.15 K)
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var ambientTemperatureKelvin = getprop("/environment/temperature-degc") + 273.15;
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var MACH = getprop("/velocities/mach");
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var TASKnots = 661.47 * MACH * math.sqrt(ambientTemperatureKelvin / 288.15);
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# print("true airspeed = ", TASKnots);
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# return TAS in feet per second.
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return TASKnots * 1.68781;
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}
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var getAzimuthAndElevation = func()
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{
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# print("gunsite-computer.nas getAzimuthAndElevation");
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getAccelerationData();
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# print("normalAcceleration = ", normalAcceleration);
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rangeRate = getRangeRate();
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range = getprop(propertyTreeRoot, "range");
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var alpha = getprop("/orientation/alpha-deg") * radiansPerDrgree;
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var trueAirSpeed = getTrueAirSpeed();
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var totalInitialVelocity = trueAirSpeed + muzzleVelocity;
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var SQRV = math.sqrt(trueAirSpeed + totalInitialVelocity);
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var RHO = getprop("/environment/density-slugft3");
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var SLA = LA * (1.0 - 0.16667 * LA * LA);
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var SLE = LE * (1.0 - 0.16667 * LE * LE);
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var CLE = 1.0 - (LE * LE)/2;
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var CLA = 1.0 - (LA * LA)/2;
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var RCOS = 1.0/(CLA * CLE);
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var RDE = (-rangeRate - RDC * SLE) * RCOS;
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var RE = range * RCOS;
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# Approximation used for sqrt
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# If y=approx. square root of x, then SQTR(x) = 0.5 * (y + x/y) is very close
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SQRV = 0.5 * (SQRV + (trueAirSpeed + totalInitialVelocity)/SQRV);
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VLS = KBRHO * RHO * RE * SQRV;
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VOS = totalInitialVelocity - RDE - VLS;
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# Approximation used for sqrt
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# RDE = -rangeRate makes this calculation identical to other LCOSS
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RDE = -rangeRate;
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VCM = 0.5 * (VCM + (VOS * VOS - 4.0 * (trueAirSpeed + RDE) * VLS) / VCM);
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var VE = 0.5 * (VOS + VCM);
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var recipricalTimeOfFlight = VE / RE;
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var averageRelativeVelocity = VE + RDE;
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var rRange = 1.0 / range;
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var KH = 1.0 - range * RRH;
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var VN = averageRelativeVelocity - SIG * RDE * VLS * (averageRelativeVelocity + trueAirSpeed)/(VCM * averageRelativeVelocity);
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var PG = P * cosGunElevation - R * sinGunElevation;
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var RG = P * sinGunElevation + R * cosGunElevation;
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RDC = trueAirSpeed * (alpha + gunElevationRadians) *
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(totalInitialVelocity - averageRelativeVelocity) /
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(trueAirSpeed + totalInitialVelocity) +
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0.5 * normalAcceleration / recipricalTimeOfFlight;
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# WJ and WK are computed sight line angular rates
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var WK = (KH * y_offset * CLA * recipricalTimeOfFlight - VN * SLA) * rRange;
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var WJ = ((RDC - KH * z_offset * recipricalTimeOfFlight) * CLE - VN * CLA * SLE) * rRange;
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var LED = (WJ + SLA * PG - CLA * Q) * KSIG;
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var LAD = ((WK - SLE * (CLA * PG + SLA * Q)) / CLE -RG) * KSIG;
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LA = LA + LAD * timeStep;
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LE = LE + LED * timeStep;
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}
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var gunSightMain = func()
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{
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# print("gunSightMain()");
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if (getprop(propertyTreeRoot, "computer-on") == 1)
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{
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getAzimuthAndElevation();
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# LA and LE are azimuth and elevation angles of the computed sight line
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# LA and LE are in radians.
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# Compute PLA and PLE as negative mills
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# var PLA = -1000.0 * LA;
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# var PLE = -1000.0 * LE;
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# update reticle position
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# Under extreme conditions such as a spin the algorithm
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# can generate invalid results and return NaN for LA
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# and LE which causes the whole thing to break down.
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# If that happens set them to 0 (reinitialize LA and LE)
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# and continue on.
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# clamp LA and LE to max deflection values for the sight
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var tempLA = LA;
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var tempLE = LE;
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if (LA > maxAzimuth/1000)
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{
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tempLA = maxAzimuth/1000;
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}
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else if (LA < -maxAzimuth/1000)
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{
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tempLA = -maxAzimuth/1000;
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}
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if (LE > maxElevation/1000)
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{
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tempLE = maxElevation/1000;
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}
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else if (LE < -maxElevation/1000)
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{
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tempLE = -maxElevation/1000;
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}
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call(func setprop(propertyTreeRoot, "azimuth", tempLA), nil, var LEerr = []);
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if (size(LEerr))
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{
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tempLA = 0;
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setprop(propertyTreeRoot, "azimuth", tempLA);
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}
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call (func setprop(propertyTreeRoot, "elevation", tempLE), nil, var LAerr = []);
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if (size(LAerr))
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{
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tempLE = 0;
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setprop(propertyTreeRoot, "elevation", tempLE)
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}
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# print("gunSightMain loop");
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# print("LA (azimuth) = ", LA);
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# print("LE (elevation)= ", LE);
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# wait a while and do this again
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settimer(gunSightMain, timeStep);
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}
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}
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# listener for the the sight computer/gyro to be powered up or down.
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# Will start gunSightMain process when the gun sight computer/gyro is powered on.
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var listenGunSightGyroPower = func(i)
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{
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if (getprop(propertyTreeRoot, "computer-on") == 1)
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{
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initSightComputer();
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setprop(propertyTreeRoot, "gunsightComputerInitialized", 1);
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# print("gunsite-computer.nas power switched");
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gunSightMain();
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}
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else
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{
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setprop(propertyTreeRoot, "gunsightComputerInitialized", 0);
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}
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}
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setprop("/controls/armament/gunsight/gunsightComputerInitialized", 0);
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var L1 = setlistener("/sim/signals/fdm-initialized", func(i)
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{
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var L = setlistener("/controls/armament/gunsight/computer-on", listenGunSightGyroPower, 1, 0);
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}, 1, 0);
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