% -----------------------------------------------------------------------------
%
%                              procedure sgp4
%
%  this procedure is the sgp4 prediction model from space command. this is an
%    updated and combined version of sgp4 and sdp4, which were originally
%    published separately in spacetrack report #3. this version follows the
%    methodology from the aiaa paper (2006) describing the history and
%    development of the code.
%
% Author: 
%   Jeff Beck 
%   beckja@alumni.lehigh.edu
%   1.0 (aug 7, 2006) - update for paper dav
% original comments from Vallado C++ version:
%   author        : david vallado                  719-573-2600   28 jun 2005
%
%   inputs        :
%     satrec    - initialised structure from sgp4init() call.
%     tsince    - time eince epoch (minutes)
%
%   outputs       :
%     r           - position vector                     km
%     v           - velocity                            km/sec
%     return code - non-zero on error.
%                    1 - mean elements, ecc >= 1.0 or ecc < -0.001 or a < 0.95 er
%                    2 - mean motion less than 0.0
%                    3 - pert elements, ecc < 0.0  or  ecc > 1.0
%                    4 - semi-latus rectum < 0.0
%                    5 - epoch elements are sub-orbital
%                    6 - satellite has decayed
%
%   locals        :
%     am          -
%     axnl, aynl        -
%     betal       -
%     COSIM   , SINIM   , COSOMM  , SINOMM  , Cnod    , Snod    , Cos2u   ,
%     Sin2u   , Coseo1  , Sineo1  , Cosi    , Sini    , Cosip   , Sinip   ,
%     Cosisq  , Cossu   , Sinsu   , Cosu    , Sinu
%     Delm        -
%     Delomg      -
%     Dndt        -
%     Eccm        -
%     EMSQ        -
%     Ecose       -
%     El2         -
%     Eo1         -
%     Eccp        -
%     Esine       -
%     Argpm       -
%     Argpp       -
%     Omgadf      -
%     Pl          -
%     R           -
%     RTEMSQ      -
%     Rdotl       -
%     Rl          -
%     Rvdot       -
%     Rvdotl      -
%     Su          -
%     T2  , T3   , T4    , Tc
%     Tem5, Temp , Temp1 , Temp2  , Tempa  , Tempe  , Templ
%     U   , Ux   , Uy    , Uz     , Vx     , Vy     , Vz
%     inclm       - inclination
%     mm          - mean anomaly
%     nm          - mean motion
%     nodem      - longi of ascending node
%     xinc        -
%     xincp       -
%     xl          -
%     xlm         -
%     mp          -
%     xmdf        -
%     xmx         -
%     xmy         -
%     nodedf     -
%     xnode       -
%     nodep      -
%     np          -
%
%   coupling      :
%     getgravconst
%     dpper
%     dspace
%
%   references    :
%     hoots, roehrich, norad spacetrack report #3 1980
%     hoots, norad spacetrack report #6 1986
%     hoots, schumacher and glover 2004
%     vallado, crawford, hujsak, kelso  2006
%  ----------------------------------------------------------------------------*/

function [satrec, r, v] = sgp4(satrec,tsince);

   % /* ------------------ set mathematical constants --------------- */
   twopi = 2.0 * pi;
   x2o3  = 2.0 / 3.0;
   %  // sgp4fix divisor for divide by zero check on inclination
   temp4    =   1.0 + cos(pi-1.0e-9);

   %  // sgp4fix identify constants and allow alternate values
   global tumin mu radiusearthkm xke j2 j3 j4 j3oj2  
   vkmpersec     = radiusearthkm * xke/60.0;
   
   % /* --------------------- clear sgp4 error flag ----------------- */
   satrec.t     = tsince;
   satrec.error = 0;

   % /* ------- update for secular gravity and atmospheric drag ----- */
   xmdf    = satrec.mo + satrec.mdot * satrec.t;
   argpdf  = satrec.argpo + satrec.argpdot * satrec.t;
   nodedf  = satrec.nodeo + satrec.nodedot * satrec.t;
   argpm   = argpdf;
   mm      = xmdf;
   t2      = satrec.t * satrec.t;
   nodem   = nodedf + satrec.nodecf * t2;
   tempa   = 1.0 - satrec.cc1 * satrec.t;
   tempe   = satrec.bstar * satrec.cc4 * satrec.t;
   templ   = satrec.t2cof * t2;

   if (satrec.isimp ~= 1)
       delomg = satrec.omgcof * satrec.t;
       delm   = satrec.xmcof *...
           ((1.0 + satrec.eta * cos(xmdf))^3 -...
           satrec.delmo);
       temp   = delomg + delm;
       mm     = xmdf + temp;
       argpm  = argpdf - temp;
       t3     = t2 * satrec.t;
       t4     = t3 * satrec.t;
       tempa  = tempa - satrec.d2 * t2 - satrec.d3 * t3 -...
           satrec.d4 * t4;
       tempe  = tempe + satrec.bstar * satrec.cc5 * (sin(mm) -...
           satrec.sinmao);
       templ  = templ + satrec.t3cof * t3 + t4 * (satrec.t4cof +...
           satrec.t * satrec.t5cof);
   end

   nm    = satrec.no;
   em    = satrec.ecco;
   inclm = satrec.inclo;
   if (satrec.method == 'd')
       tc = satrec.t;
       [satrec.atime,em,argpm,inclm,satrec.xli,mm,...
           satrec.xni,nodem,dndt,nm] = dspace(...
           satrec.d2201,satrec.d2211,satrec.d3210,...
           satrec.d3222,satrec.d4410,satrec.d4422,...
           satrec.d5220,satrec.d5232,satrec.d5421,...
           satrec.d5433,satrec.dedt,satrec.del1,...
           satrec.del2,satrec.del3,satrec.didt,...
           satrec.dmdt,satrec.dnodt,satrec.domdt,...
           satrec.irez,satrec.argpo,satrec.argpdot,satrec.t,...
           tc,satrec.gsto,satrec.xfact,satrec.xlamo,satrec.no,...
           satrec.atime,em,argpm,inclm,satrec.xli,mm,...
           satrec.xni,nodem,nm);
   end % // if method = d

   if (nm <= 0.0)
%       fprintf(1,'# error nm %f\n', nm);
       satrec.error = 2;
   end
   am = (xke / nm)^x2o3 * tempa * tempa;
   nm = xke / am^1.5;
   em = em - tempe;

   % // fix tolerance for error recognition
   if ((em >= 1.0) || (em < -0.001) || (am < 0.95))
%       fprintf(1,'# error em %f\n', em);
       satrec.error = 1;
   end
   if (em < 0.0)
       em  = 1.0e-6;
   end
   mm     = mm + satrec.no * templ;
   xlm    = mm + argpm + nodem;
   emsq   = em * em;
   temp   = 1.0 - emsq;
   nodem  = rem(nodem, twopi);
   argpm  = rem(argpm, twopi);
   xlm    = rem(xlm, twopi);
   mm     = rem(xlm - argpm - nodem, twopi);

   % /* ----------------- compute extra mean quantities ------------- */
   sinim = sin(inclm);
   cosim = cos(inclm);

   % /* -------------------- add lunar-solar periodics -------------- */
   ep     = em;
   xincp  = inclm;
   argpp  = argpm;
   nodep  = nodem;
   mp     = mm;
   sinip  = sinim;
   cosip  = cosim;
   if (satrec.method == 'd')
       [ep,xincp,nodep,argpp,mp] = dpper(...
           satrec.e3,satrec.ee2,satrec.peo,...
           satrec.pgho,satrec.pho,satrec.pinco,...
           satrec.plo,satrec.se2,satrec.se3,...
           satrec.sgh2,satrec.sgh3,satrec.sgh4,...
           satrec.sh2,satrec.sh3,satrec.si2,...
           satrec.si3,satrec.sl2,satrec.sl3,...
           satrec.sl4,satrec.t,satrec.xgh2,...
           satrec.xgh3,satrec.xgh4,satrec.xh2,...
           satrec.xh3,satrec.xi2,satrec.xi3,...
           satrec.xl2,satrec.xl3,satrec.xl4,...
           satrec.zmol,satrec.zmos,satrec.inclo,...
           satrec.init,ep,xincp,nodep,argpp,mp);
       if (xincp < 0.0)
           xincp  = -xincp;
           nodep = nodep + pi;
           argpp  = argpp - pi;
       end
       if ((ep < 0.0 ) || ( ep > 1.0))
%           fprintf(1,'# error ep %f\n', ep);
           satrec.error = 3;
       end
   end % // if method = d

   % /* -------------------- long period periodics ------------------ */
   if (satrec.method == 'd')
       sinip =  sin(xincp);
       cosip =  cos(xincp);
       satrec.aycof = -0.5*j3oj2*sinip;
       % // sgp4fix for divide by zero with xinco = 180 deg
       if (abs(cosip+1.0) > 1.5e-12)
           satrec.xlcof = -0.25 * j3oj2 * sinip * (3.0 + 5.0 * cosip) /...
               (1.0+cosip);
         else
           satrec.xlcof = -0.25 * j3oj2 * sinip * (3.0 + 5.0 * cosip) /...
               temp4;
       end;
   end
   axnl = ep * cos(argpp);
   temp = 1.0 / (am * (1.0 - ep * ep));
   aynl = ep* sin(argpp) + temp * satrec.aycof;
   xl   = mp + argpp + nodep + temp * satrec.xlcof * axnl;

   % /* --------------------- solve kepler's equation --------------- */
   u    = rem(xl - nodep, twopi);
   eo1  = u;
   tem5 = 9999.9;
   ktr = 1;
   % //   sgp4fix for kepler iteration
   % //   the following iteration needs better limits on corrections
   while (( abs(tem5) >= 1.0e-12) && (ktr <= 10) )
       sineo1 = sin(eo1);
       coseo1 = cos(eo1);
       tem5   = 1.0 - coseo1 * axnl - sineo1 * aynl;
       tem5   = (u - aynl * coseo1 + axnl * sineo1 - eo1) / tem5;
       if(abs(tem5) >= 0.95)
           if tem5 > 0.0
               tem5 = 0.95;
           else
               tem5 = -0.95;
           end
       end
       eo1    = eo1 + tem5;
       ktr = ktr + 1;
   end

   % /* ------------- short period preliminary quantities ----------- */
   ecose = axnl*coseo1 + aynl*sineo1;
   esine = axnl*sineo1 - aynl*coseo1;
   el2   = axnl*axnl + aynl*aynl;
   pl    = am*(1.0-el2);
   if (pl < 0.0)
%       fprintf(1,'# error pl %f\n', pl);
       satrec.error = 4;
       r = [0;0;0];
       v = [0;0;0];
   else
       rl     = am * (1.0 - ecose);
       rdotl  = sqrt(am) * esine/rl;
       rvdotl = sqrt(pl) / rl;
       betal  = sqrt(1.0 - el2);
       temp   = esine / (1.0 + betal);
       sinu   = am / rl * (sineo1 - aynl - axnl * temp);
       cosu   = am / rl * (coseo1 - axnl + aynl * temp);
       su     = atan2(sinu, cosu);
       sin2u  = (cosu + cosu) * sinu;
       cos2u  = 1.0 - 2.0 * sinu * sinu;
       temp   = 1.0 / pl;
       temp1  = 0.5 * j2 * temp;
       temp2  = temp1 * temp;

       % /* -------------- update for short period periodics ------------ */
       if (satrec.method == 'd')
           cosisq                 = cosip * cosip;
           satrec.con41  = 3.0*cosisq - 1.0;
           satrec.x1mth2 = 1.0 - cosisq;
           satrec.x7thm1 = 7.0*cosisq - 1.0;
       end
       mrt   = rl * (1.0 - 1.5 * temp2 * betal * satrec.con41) +...
           0.5 * temp1 * satrec.x1mth2 * cos2u;
       su    = su - 0.25 * temp2 * satrec.x7thm1 * sin2u;
       xnode = nodep + 1.5 * temp2 * cosip * sin2u;
       xinc  = xincp + 1.5 * temp2 * cosip * sinip * cos2u;
       mvt   = rdotl - nm * temp1 * satrec.x1mth2 * sin2u / xke;
       rvdot = rvdotl + nm * temp1 * (satrec.x1mth2 * cos2u +...
           1.5 * satrec.con41) / xke;

       % /* --------------------- orientation vectors ------------------- */
       sinsu =  sin(su);
       cossu =  cos(su);
       snod  =  sin(xnode);
       cnod  =  cos(xnode);
       sini  =  sin(xinc);
       cosi  =  cos(xinc);
       xmx   = -snod * cosi;
       xmy   =  cnod * cosi;
       ux    =  xmx * sinsu + cnod * cossu;
       uy    =  xmy * sinsu + snod * cossu;
       uz    =  sini * sinsu;
       vx    =  xmx * cossu - cnod * sinsu;
       vy    =  xmy * cossu - snod * sinsu;
       vz    =  sini * cossu;

       % /* --------- position and velocity (in km and km/sec) ---------- */
       r(1) = (mrt * ux)* radiusearthkm;
       r(2) = (mrt * uy)* radiusearthkm;
       r(3) = (mrt * uz)* radiusearthkm;
       v(1) = (mvt * ux + rvdot * vx) * vkmpersec;
       v(2) = (mvt * uy + rvdot * vy) * vkmpersec;
       v(3) = (mvt * uz + rvdot * vz) * vkmpersec;
   end % // if pl > 0

       % // sgp4fix for decaying satellites
        if (mrt < 1.0)
   %         printf("# decay condition %11.6f \n",mrt);
            satrec.error = 6;
        end

   global idebug dbgfile
   if idebug
       debug7;
   end

   return;