get_el.cpp

/* Copyright (C) 2018, Project Pluto.  See LICENSE.  */

#include <string.h>
#include <stdint.h>
#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include "norad.h"

#define PI 3.141592653589793238462643383279502884197
#define TWOPI (2. * PI)
#define MINUTES_PER_DAY 1440.
#define MINUTES_PER_DAY_SQUARED (MINUTES_PER_DAY * MINUTES_PER_DAY)
#define MINUTES_PER_DAY_CUBED (MINUTES_PER_DAY * MINUTES_PER_DAY_SQUARED)
#define AE 1.0
                             /* distance units, earth radii */

/* TLEs have four angles on line 2,  given in the form DDD.DDDD.  This
can be parsed more quickly as an integer,  then cast to double and
converted to radians,  all in one step.    */

static int get_angle( const char *buff)
{
   int rval = 0;

   while( *buff == ' ')
      buff++;
   while( *buff != ' ')
      {
      if( *buff != '.')
         rval = rval * 10 + (int)( *buff - '0');
      buff++;
      }
   return( rval);
}

/* Converts the quasi scientific notation of the "Motion Dot Dot/6" or
"BSTAR" field to double.  The input will always be of the form

sdddddSe

   ....where s is blank or + or -;  ddddd is a five-digit mantissa;
S is + or - or blank;  and e is a single-digit exponent.  A decimal
point is assumed before the five-digit mantissa.  */

static double sci( const char *string)
{
   double rval = 0.;

   if( string[1] != ' ')
      {
      const int ival = atoi( string);

      if( ival)
         {
         rval = (double)ival * 1.e-5;
         if( string[7] != '0')
            {
            int exponent = string[7] - '0';

            if( string[6] == '-')
               while( exponent--)
                  rval *= .1;
            else
               while( exponent--)
                  rval *= 10.;
            }
         }
      }
   return( rval);
}


/* Does a checksum modulo 10 on the given line.  Digits = their
value, '-' = 1, all other chars = 0.  Returns 0 if ok, a negative
value if it's definitely not a TLE line,  positive if it's all OK
except the checksum.  This last was added because people sometimes
want to use TLEs without worrying about the checksum. */

int DLL_FUNC tle_checksum( const char *buff)
{
   int rval = 0;
   int count = 69;

   if( (*buff != '1' && *buff != '2') || buff[1] != ' ')
      return( -1);
   while( --count)
      {
      if( *buff > '0' && *buff <= '9')
         rval += *buff - '0';
      else if( *buff == '-')
         rval++;
      if( *buff < ' ' || *buff > 'z')           /* invalid character */
         return( -2);
      buff++;
      }
   rval -= *buff++ - '0';
   if( *buff > ' ')                 /* line unterminated */
      rval = -3;
   else
      {
      rval %= 10;
      if( rval < 0)
         rval += 10;
      }
   return( rval);
}

static inline int mutant_dehex( const char ichar)
{
   int rval;

   if( ichar <= '9' && ichar >= '0')
      rval = ichar - '0';
   else if( ichar >= 'A' && ichar <= 'Z')
      rval = ichar + 10 - 'A';
   else
      rval = -1;
   return( rval);
}

/* The "standard" SDP4 model fails badly for very high-flying satellites
(mostly,  but not always,  those with orbital periods of greater than
about a week).  Highly eccentric orbits are more likely to fail than
near-circular ones.  And of course,  hyperbolic orbits never work with
SGP4/SDP4.

   As a non-standard extension,  I'm simply storing state vectors for
such orbits,  using the following somewhat odd scheme :

1 40391U 15007B   15091.99922241 sxxxxxxxx syyyyyyyy szzzzzzzzH  9997
2 49391 [valid range, accuracy]  saaaaaaaa sbbbbbbbb scccccccc    0 8

   Epoch,  int'l & NORAD IDs are stored in the standard manner.  The
'ephemeris type' is H (rather than the otherwise universal 0).  The
xyz position and vx, vy, vz velocity are stored as 8-digit signed
base-36 integers,  hence a range of +/- 36^8 = about +/- 2.82x10^12.

  x, y, z are in meters,  and hence cover a range +/- 18.9 AU.
vx, vy, vz are in 10^-4 m/s,  range +/- 94% c.  The state vectors
are in the geocentric ecliptic plane of date.  See 'sdp4.cpp' for
a discussion of how they're actually used.  */

static double get_high_value( const char *iptr)
{
   int64_t rval = 0;

   assert( *iptr == '+' || *iptr == '-');
   if( *iptr == '+' || *iptr == '-')
      {
      int i, digit;

      for( i = 1; i < 9; i++)
         {
         digit = mutant_dehex( iptr[i]);
         assert( digit >= 0);
         rval = rval * (int64_t)36 + (int64_t)digit;
         }
      if( *iptr == '-')
         rval = -rval;
      }
   return( (double)rval);
}

/* Traditionally,  NORAD numbers were stored as five digits.  In 2020, new
detectors threatened to go past 100K objects;  the 'Alpha-5' scheme allows
the first byte to be replaced by an uppercase letter,  with I and O
skipped.  That gets us to 339999 :

https://www.space-track.org/documentation#tle-alpha5

   Note that Alpha-5 is referred to as a "stopgap".  Near the bottom of
the above link,  "space-track.org encourages users to switch to... XML,
KVN,  or JSON",  (partly) because these will handle nine-digit catalog
numbers.

   To go beyond the Alpha-5 limit of 340000 possible numbers and store
all nine-digit numbers in five bytes,  I have added options 3 and 4
below.  To do so,  we need a 'base64'-like scheme,  using all ten
digits,  26 uppercase and 26 lowercase letters,  and + and /.

   d = digit, L = uppercase letter,  x = any base64 character
   X = non-digit base-64 character

(1) ddddd = 'traditional' scheme provides 100000 combinations;
         Numbers 0 to 99999

(2) Ldddd = Alpha-5 scheme adds 240000
         Numbers 100000 to 339999;     A0000 to Z9999

(3) xxxxX = 64^4*54      = 905969664 more  (start of 'Super-5' range)
         Numbers 340000 to 906309663;  0000A to -----

(4) xxxXd = 64^3*54*10   = 141557760 more
         Numbers 906309664 to 1047867423;  000A0 and up
              (going slightly past the billion we actually need) */

static int base64_to_int( const char c)
{
   int offset;

   if( c >= 'A')
      {
      if( c <= 'Z')
         offset = 'A' - 10;
      else if( c >= 'a' && c <= 'z')
         offset = 'a' - 10 - 26;
      else
         return( -1);
      }
   else
      {
      if( c >= '0' && c <= '9')
         offset = '0';
      else if( c == ' ')
         return( 0);
      else if( c == '+')
         return( 62);
      else if( c == '-')
         return( 63);
      else
         return( -1);
      }
   return( c - offset);
}

static int get_norad_number( const char *buff)
{
   size_t i;
   int digits[5], rval = 0;

   for( i = 0; i < 5; i++)
      {
      digits[i] = base64_to_int( buff[i]);
      if( digits[i] == -1)       /* not a valid number */
         return( 0);
      }
   if( digits[4] > 9)      /* case (3): last char is uppercase */
      rval = 340000 + (digits[4] - 10)
            + 54 * (digits[3] + (digits[2] << 6) + (digits[1] << 12) + (digits[0] << 18));
   else if( digits[3] > 9)    /* case (4) above */
      rval = 340000 + 905969664 + digits[4] + (digits[3] - 10) * 10
            + 540 * (digits[2] + (digits[1] << 6) + (digits[0] << 12));
   else        /* last four digits are 0-9;  'standard' NORAD desig */
      {
      for( i = 1; i <= 4; i++)
         assert( (buff[i] >= '0' && buff[i] <= '9') || buff[i] == ' ');
      if( *buff > 'I')
         {
         digits[0]--;
         if( *buff > 'O')
            digits[0]--;
         }
      rval = digits[0] * 10000 + atoi( buff + 1);
      }
   return( rval);
}

static inline double get_eight_places( const char *ptr)
{
   return( (double)atoi( ptr) + (double)atoi(ptr + 4) * 1e-8);
}

/* Meteor 2-08                                                           */
/* 1 13113U          88245.60005115 0.00000076           63463-4 0  5998 */
/* 2 13113  82.5386 288.0994 0015973 147.1294 213.0868 13.83869004325321 */

#define J2000 2451545.5
#define J1900 (J2000 - 36525. - 1.)

/* parse_elements returns:
         0 if the elements are parsed without error;
         1 if they're OK except the first line has a checksum error;
         2 if they're OK except the second line has a checksum error;
         3 if they're OK except both lines have checksum errors;
         a negative value if the lines aren't at all parseable */

int DLL_FUNC parse_elements( const char *line1, const char *line2, tle_t *sat)
{
   int rval, checksum_problem = 0;

   if( *line1 != '1' || *line2 != '2')
      rval = -4;
   else
      {
      rval = tle_checksum( line1);
      if( rval > 0)
         {
         checksum_problem = 1;  /* there's a checksum problem,  but it's */
         rval = 0;              /* not fatal; continue processing the TLE */
         }
      }

   if( rval)
      rval -= 100;
   else
      {
      rval = tle_checksum( line2);
      if( rval > 0)
         {
         checksum_problem |= 2;  /* there's a checksum problem,  but it's */
         rval = 0;               /* not fatal; continue processing the TLE */
         }
      }

   if( !rval)
      {
      char tbuff[13];
      int year = line1[19] - '0';

      if( line1[18] >= '0')
         year += (line1[18] - '0') * 10;
      if( year < 57)          /* cycle around Y2K */
         year += 100;
      sat->epoch = get_eight_places( line1 + 20) + J1900
             + (double)( year * 365 + (year - 1) / 4);
      sat->norad_number = get_norad_number( line1 + 2);
      memcpy( tbuff, line1 + 64, 4);
      tbuff[4] = '\0';
      sat->bulletin_number = atoi( tbuff);
      sat->classification = line1[7];       /* almost always 'U' */
      memcpy( sat->intl_desig, line1 + 9, 8);
      if( !memcmp( sat->intl_desig, "     ", 5))
         {  /* usually 'analyst' object w/o international (COSPAR) desig; */
         int i, n = sat->norad_number;      /* set launch 000,  year/part */
                                            /* data mapped from NORAD #   */
         for( i = 7; i > 4; i--, n /= 26)
            sat->intl_desig[i] = 'A' + n % 26;
         sat->intl_desig[2] = sat->intl_desig[3] = sat->intl_desig[4] = '0';
         sat->intl_desig[1] = '0' + n % 10;
         sat->intl_desig[0] = '0' + n / 10;
         }
      sat->intl_desig[8] = '\0';
      memcpy( tbuff, line2 + 63, 5);
      tbuff[5] = '\0';
      sat->revolution_number = atoi( tbuff);
      sat->ephemeris_type = line1[62];
      if( sat->ephemeris_type == 'H')
         {
         size_t i;
         double *state_vect = &sat->xincl;

         for( i = 0; i < 3; i++)
            {
            state_vect[i]     = get_high_value( line1 + 33 + i * 10);
            state_vect[i + 3] = get_high_value( line2 + 33 + i * 10) * 1e-4;
            }
         return( 0);
         }

      sat->xmo = (double)get_angle( line2 + 43) * (PI / 180e+4);
      sat->xnodeo = (double)get_angle( line2 + 17) * (PI / 180e+4);
      sat->omegao = (double)get_angle( line2 + 34) * (PI / 180e+4);
      sat->xincl = (double)get_angle( line2 + 8) * (PI / 180e+4);
      sat->eo = atoi( line2 + 26) * 1.e-7;

      /* Make sure mean motion is null-terminated, since rev. no.
          may immediately follow. */
      memcpy( tbuff, line2 + 51, 12);
      tbuff[12] = '\0';
            /* Input mean motion,  derivative of mean motion and second  */
            /* deriv of mean motion,  are all in revolutions and days.   */
            /* Convert them here to radians and minutes:                 */
      sat->xno = get_eight_places( tbuff) * TWOPI / MINUTES_PER_DAY;
      sat->xndt2o = (double)atoi( line1 + 35)
                        * 1.e-8 * TWOPI / MINUTES_PER_DAY_SQUARED;
      if( line1[33] == '-')
         sat->xndt2o *= -1.;
      sat->xndd6o = sci( line1 + 44) * TWOPI / MINUTES_PER_DAY_CUBED;

      sat->bstar = sci( line1 + 53) * AE;
      }
   return( rval ? rval : checksum_problem);
}