//perform calculation at time for given TLE lines
//if you can get at objects from the JS use the commented version
//this will make GetPos() and GetVel redundant
/*public Vectors calculate(int time, string line1, string line2){
TLE_DATA sat = new TLE_DATA();
sat.TLE_DATA_INIT(line1, line2);
return satvec(sat, timeutil.UnixTimeToDayNumber(time),timeutil.UnixTimeToDayFrac(time));
}*/
public void calculate(int time, string line1, string line2){
TLE_DATA sat = new TLE_DATA();
sat.TLE_DATA_INIT(line1, line2);
results = satvec(sat, timeutil.UnixTimeToDayNumber(time),timeutil.UnixTimeToDayFrac(time));
}
//get day fraction
public double EpochToFract(TLE_DATA tle){
return tle.TE - (int)tle.TE;
}
//calculate stalite position at DN, TN
private Vectors satvec(TLE_DATA sat,int DN, double TN){
//calculate average precession rates
double N0 = sat.MM/86400; //Mean Motion rad/s
double A0 = Math.Pow((GM/N0/N0),(1.0/3.0)); //semi major axis km
double B0 = A0*Math.Sqrt(1 - sat.EC*sat.EC); //semi minor axis km
double SI = Math.Sin(sat.IN);
double CI = Math.Cos(sat.IN);
double PC = RE*A0/(B0*B0); //precession constant rad/day
PC = 1.5*J2*PC*PC*sat.MM; //precession constant rads/day
double QD = -PC*CI; //precession rate rad/day
double WD = PC*(5*CI*CI-1)/2; //Perigee precession rate rad/day
double DC = -2*(sat.M2)/sat.MM/3; //REM Drag coeff. (Angular momentum rate)/(Ang mom) s^-1
//Sidereal and Solar data. NEVER needs changing. valid until year ~2015
//double YG = 2000;
//double G0 = 98.9821; //GHAA, year YG Jan 0.0
// UPDATED BY ALEX: Sidereal and Solar data. Valid to year ~2030
double YG = 2014;
double G0 = 99.5828; //GHAA, year YG Jan 0.0
//Bring sun data to satellite epoch
double TEG = (timeutil.EpochToDay(sat) - timeutil.FNday(Convert.ToInt32(YG),1,0)) + timeutil.EpochToFract(sat); //Elapsed Time: Epoch - YG
//there is a wired bug in this line fixed by defining WE explicitly
double GHAE = degtorad(G0) + TEG*6.300388; //GHA Aries, epoch
//Time
double T = (DN - timeutil.EpochToDay(sat)) + (TN - timeutil.EpochToFract(sat)); //Elapsed Time since epoch, days
//Linear drag terms
double DT = DC*T/2;
double KD = 1 + 4*DT;
double KDP = 1 - 7*DT;
double M = sat.MA + sat.MM*T*(1- 3*DT); //Mean anomaly at YR,TN
//orbit number
int DR = Convert.ToInt32(M/(2*PI)); //strip out whole number of revs;
M = M - DR*2*PI; //M now in range 0 - 2pi
//double RN = sat.RV + DR; //current orbit number
//solve M = EA - EC*Sin(EA) for EA givne M, using newtons methood
double EA = M; //Initial solution
//declare here for use later
double D = 1;
double DNOM = 0;
double C = 0;
double S = 0;
while((Math.Abs(D) >= 1e-5)){
C = Math.Cos(EA);
S = Math.Sin(EA);
DNOM = 1 - sat.EC*C;
D = (EA- (sat.EC*S) -M)/DNOM; //change to EA for better solution
EA = EA - D; //by this ammount
}
//distances
double A = A0*KD;
double B = B0*KD;
//double RS = A*DNOM;
//calculate satalite position and velocity in plane of elipse
double Sx = A*(C - sat.EC);
double Sy = B*S;
double Vx = -A*S/DNOM*N0;
double Vy = B*C/DNOM*N0;
double AP = sat.WP + WD*T*KDP;
double RAAN = sat.RA + QD*T*KDP;
double CW = Math.Cos(AP);
double CQ = Math.Cos(RAAN);
double SW = Math.Sin(AP);
double SQ = Math.Sin(RAAN);
//plane . celestial coordinate transformtation, [C] = [RAAN]*[IN]*[AP]
double CXx = CW*CQ - SW*CI*SQ;
double CYx = CW*SQ + SW*CI*CQ;
double CZx = SW*SI;
double CXy = -SW*CQ - CW*CI*SQ;
double CYy = -SW*SQ + CW*CI*CQ;
double CZy = CW*SI;
//double CXz = SI*SQ;
//double CYz = -SI*CQ;
//double CZz = CI;
//Compute satalites position vector and velocity in clestial coordinates
double SATx = Sx*CXx + Sy*CXy;
double SATy = Sx*CYx + Sy*CYy;
double SATz = Sx*CZx + Sy*CZy;
double VELx = Vx*CXx + Vy*CXy;
double VELy = Vx*CYx + Vy*CYy;
double VELz = Vx*CZx + Vy*CZy;
//express position and velocity in Geocentric coordinates
//weired WE def bug also in this line fixed with explicit definition
double GHAA = GHAE + 6.300388*T; //GHA Aries at elapsed time T
C = Math.Cos(-GHAA);
S = Math.Sin(-GHAA);
Sx = SATx*C - SATy*S;
Sy = SATx*S + SATy*C;
double Sz = SATz;
Vx = VELx*C - VELy*S;
Vy = VELx*S + VELy*C;
double Vz = VELz;
Vector pos = new Vector(Sx, Sy, Sz);
Vector vel = new Vector(Vx, Vy, Vz);;
Vectors output = new Vectors(pos, vel);
return output;
}
//convert satilite epoch to day No
public int EpochToDay(TLE_DATA tle){
return FNday(tle.YE,1,0) + (int)tle.TE;
}
