Ryan Malloy
cd338c3c64
Replace the sat_code git submodule (lib/sat_code/) with vendored sources in src/sgp4/. The upstream .cpp files are renamed to .c — the code is valid C99 with zero C++ features. This eliminates the g++ and -lstdc++ build dependencies. Adds 518 Vallado test vectors (AIAA 2006-6753-Rev1) as a 13th regression suite to verify byte-identical numerical output. Updates all documentation (CLAUDE.md, DESIGN.md, 11 MDX pages, Dockerfile) to reflect the new layout and pure-C compilation.
151 lines
5.0 KiB
C
151 lines
5.0 KiB
C
/* Copyright (C) 2018, Project Pluto. See LICENSE.
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*
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* sgp4.c - Near-earth satellite propagator (orbital period < 225 min)
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*
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* Implements the SGP4 model from STR#2 (Lane & Hoots 1979), as unified
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* in STR#3 (Hoots & Roehrich 1980) Chapter 6 (pp. 25-32).
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*
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* SGP4_init(): Computes drag coefficients c1..c5, d2..d4, and the
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* "simple" vs full mode flag (perigee < 220 km truncates to linear
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* variation in sqrt(a) and quadratic in mean anomaly).
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* STR#3 Eqs. 6-1..6-17, Vallado Rev-1 Section III.
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*
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* SGP4(): Secular updates for gravity + atmospheric drag, then delegates
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* to sxpx_posn_vel() for Kepler solution and short-period corrections.
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* STR#3 Eqs. 6-18..6-32, Vallado Rev-1 Eqs. 18-22.
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*/
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#include <math.h>
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#include <stdio.h>
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#include "norad.h"
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#include "norad_in.h"
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#define c1 params[2]
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#define c4 params[3]
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#define xnodcf params[4]
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#define t2cof params[5]
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#define p_aodp params[10]
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#define p_cosio params[11]
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#define p_sinio params[12]
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#define p_omgdot params[13]
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#define p_xmdot params[14]
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#define p_xnodot params[15]
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#define p_xnodp params[16]
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#define c5 params[17]
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#define d2 params[18]
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#define d3 params[19]
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#define d4 params[20]
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#define delmo params[21]
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#define p_eta params[22]
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#define omgcof params[23]
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#define sinmo params[24]
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#define t3cof params[25]
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#define t4cof params[26]
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#define t5cof params[27]
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#define xmcof params[28]
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#define simple_flag *((int *)( params + 29))
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#define MINIMAL_E 1.e-4
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#define ECC_EPS 1.e-6 /* Too low for computing further drops. */
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void SGP4_init( double *params, const tle_t *tle)
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{
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deep_arg_t deep_arg;
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init_t init;
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double eeta, etasq;
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sxpx_common_init( params, tle, &init, &deep_arg);
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p_aodp = deep_arg.aodp;
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p_cosio = deep_arg.cosio;
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p_sinio = deep_arg.sinio;
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p_omgdot = deep_arg.omgdot;
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p_xmdot = deep_arg.xmdot;
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p_xnodot = deep_arg.xnodot;
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p_xnodp = deep_arg.xnodp;
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p_eta = deep_arg.aodp*tle->eo*init.tsi;
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// p_eta = init.eta;
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eeta = tle->eo*p_eta;
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/* For perigee less than 220 kilometers, the "simple" flag is set */
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/* and the equations are truncated to linear variation in sqrt a */
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/* and quadratic variation in mean anomaly. Also, the c3 term, */
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/* the delta omega term, and the delta m term are dropped. */
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simple_flag = ((p_aodp*(1-tle->eo)/ae) < (220./earth_radius_in_km+ae));
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if( !simple_flag)
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{
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const double c1sq = c1*c1;
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double temp;
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simple_flag = 0;
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delmo = 1. + p_eta * cos(tle->xmo);
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delmo *= delmo * delmo;
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d2 = 4*p_aodp*init.tsi*c1sq;
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temp = d2*init.tsi*c1/3;
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d3 = (17*p_aodp+init.s4)*temp;
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d4 = 0.5*temp*p_aodp*init.tsi*(221*p_aodp+31*init.s4)*c1;
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t3cof = d2+2*c1sq;
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t4cof = 0.25*(3*d3+c1*(12*d2+10*c1sq));
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t5cof = 0.2*(3*d4+12*c1*d3+6*d2*d2+15*c1sq*(2*d2+c1sq));
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sinmo = sin(tle->xmo);
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if( tle->eo < MINIMAL_E)
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omgcof = xmcof = 0.;
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else
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{
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const double c3 =
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init.coef * init.tsi * a3ovk2 * p_xnodp * ae * p_sinio / tle->eo;
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xmcof = -two_thirds * init.coef * tle->bstar * ae / eeta;
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omgcof = tle->bstar*c3*cos(tle->omegao);
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}
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} /* End of if (isFlagClear(SIMPLE_FLAG)) */
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etasq = p_eta * p_eta;
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c5 = 2*init.coef1*p_aodp * deep_arg.betao2*(1+2.75*(etasq+eeta)+eeta*etasq);
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} /* End of SGP4() initialization */
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int SGP4( const double tsince, const tle_t *tle, const double *params,
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double *pos, double *vel)
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{
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double
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a, e, omega, omgadf,
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temp, tempa, tempe, templ, tsq,
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xl, xmdf, xmp, xnoddf, xnode;
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/* Update for secular gravity and atmospheric drag. */
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xmdf = tle->xmo+p_xmdot*tsince;
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omgadf = tle->omegao+p_omgdot*tsince;
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xnoddf = tle->xnodeo+p_xnodot*tsince;
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omega = omgadf;
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xmp = xmdf;
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tsq = tsince*tsince;
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xnode = xnoddf+xnodcf*tsq;
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tempa = 1-c1*tsince;
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tempe = tle->bstar*c4*tsince;
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templ = t2cof*tsq;
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if( !simple_flag)
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{
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const double delomg = omgcof*tsince;
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double delm = 1. + p_eta * cos(xmdf);
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double tcube, tfour;
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delm = xmcof * (delm * delm * delm - delmo);
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temp = delomg+delm;
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xmp = xmdf+temp;
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omega = omgadf-temp;
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tcube = tsq*tsince;
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tfour = tsince*tcube;
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tempa = tempa-d2*tsq-d3*tcube-d4*tfour;
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tempe = tempe+tle->bstar*c5*(sin(xmp)-sinmo);
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templ = templ+t3cof*tcube+tfour*(t4cof+tsince*t5cof);
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}; /* End of if (isFlagClear(SIMPLE_FLAG)) */
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a = p_aodp*tempa*tempa;
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e = tle->eo-tempe;
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/* A highly arbitrary lower limit on e, of 1e-6: */
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if( e < ECC_EPS)
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e = ECC_EPS;
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xl = xmp+omega+xnode+p_xnodp*templ;
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if( tempa < 0.) /* force negative a, to indicate error condition */
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a = -a;
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return( sxpx_posn_vel( xnode, a, e, p_cosio, p_sinio, tle->xincl,
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omega, xl, pos, vel));
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} /*SGP4*/
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