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Version 2.1: Now with CPR decoding verified and improved
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#include <math.h> | ||
#include <stdio.h> | ||
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// | ||
//========================================================================= | ||
// | ||
// Always positive MOD operation, used for CPR decoding. | ||
// | ||
static int cprModInt(int a, int b) { | ||
int res = a % b; | ||
if (res < 0) res += b; | ||
return res; | ||
} | ||
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static double cprModDouble(double a, double b) { | ||
double res = fmod(a, b); | ||
if (res < 0) res += b; | ||
return res; | ||
} | ||
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// | ||
//========================================================================= | ||
// | ||
// The NL function uses the precomputed table from 1090-WP-9-14 | ||
// | ||
static int cprNLFunction(double lat) { | ||
if (lat < 0) lat = -lat; // Table is simmetric about the equator | ||
if (lat < 10.47047130) return 59; | ||
if (lat < 14.82817437) return 58; | ||
if (lat < 18.18626357) return 57; | ||
if (lat < 21.02939493) return 56; | ||
if (lat < 23.54504487) return 55; | ||
if (lat < 25.82924707) return 54; | ||
if (lat < 27.93898710) return 53; | ||
if (lat < 29.91135686) return 52; | ||
if (lat < 31.77209708) return 51; | ||
if (lat < 33.53993436) return 50; | ||
if (lat < 35.22899598) return 49; | ||
if (lat < 36.85025108) return 48; | ||
if (lat < 38.41241892) return 47; | ||
if (lat < 39.92256684) return 46; | ||
if (lat < 41.38651832) return 45; | ||
if (lat < 42.80914012) return 44; | ||
if (lat < 44.19454951) return 43; | ||
if (lat < 45.54626723) return 42; | ||
if (lat < 46.86733252) return 41; | ||
if (lat < 48.16039128) return 40; | ||
if (lat < 49.42776439) return 39; | ||
if (lat < 50.67150166) return 38; | ||
if (lat < 51.89342469) return 37; | ||
if (lat < 53.09516153) return 36; | ||
if (lat < 54.27817472) return 35; | ||
if (lat < 55.44378444) return 34; | ||
if (lat < 56.59318756) return 33; | ||
if (lat < 57.72747354) return 32; | ||
if (lat < 58.84763776) return 31; | ||
if (lat < 59.95459277) return 30; | ||
if (lat < 61.04917774) return 29; | ||
if (lat < 62.13216659) return 28; | ||
if (lat < 63.20427479) return 27; | ||
if (lat < 64.26616523) return 26; | ||
if (lat < 65.31845310) return 25; | ||
if (lat < 66.36171008) return 24; | ||
if (lat < 67.39646774) return 23; | ||
if (lat < 68.42322022) return 22; | ||
if (lat < 69.44242631) return 21; | ||
if (lat < 70.45451075) return 20; | ||
if (lat < 71.45986473) return 19; | ||
if (lat < 72.45884545) return 18; | ||
if (lat < 73.45177442) return 17; | ||
if (lat < 74.43893416) return 16; | ||
if (lat < 75.42056257) return 15; | ||
if (lat < 76.39684391) return 14; | ||
if (lat < 77.36789461) return 13; | ||
if (lat < 78.33374083) return 12; | ||
if (lat < 79.29428225) return 11; | ||
if (lat < 80.24923213) return 10; | ||
if (lat < 81.19801349) return 9; | ||
if (lat < 82.13956981) return 8; | ||
if (lat < 83.07199445) return 7; | ||
if (lat < 83.99173563) return 6; | ||
if (lat < 84.89166191) return 5; | ||
if (lat < 85.75541621) return 4; | ||
if (lat < 86.53536998) return 3; | ||
if (lat < 87.00000000) return 2; | ||
else return 1; | ||
} | ||
// | ||
//========================================================================= | ||
// | ||
static int cprNFunction(double lat, int fflag) { | ||
int nl = cprNLFunction(lat) - (fflag ? 1 : 0); | ||
if (nl < 1) nl = 1; | ||
return nl; | ||
} | ||
// | ||
//========================================================================= | ||
// | ||
static double cprDlonFunction(double lat, int fflag, int surface) { | ||
return (surface ? 90.0 : 360.0) / cprNFunction(lat, fflag); | ||
} | ||
// | ||
//========================================================================= | ||
// | ||
// This algorithm comes from: | ||
// http:https://www.lll.lu/~edward/edward/adsb/DecodingADSBposition.html. | ||
// | ||
// A few remarks: | ||
// 1) 131072 is 2^17 since CPR latitude and longitude are encoded in 17 bits. | ||
// | ||
int decodeCPRairborne(int even_cprlat, int even_cprlon, | ||
int odd_cprlat, int odd_cprlon, | ||
int fflag, | ||
double *out_lat, double *out_lon) | ||
{ | ||
double AirDlat0 = 360.0 / 60.0; | ||
double AirDlat1 = 360.0 / 59.0; | ||
double lat0 = even_cprlat; | ||
double lat1 = odd_cprlat; | ||
double lon0 = even_cprlon; | ||
double lon1 = odd_cprlon; | ||
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double rlat, rlon; | ||
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// Compute the Latitude Index "j" | ||
int j = (int) floor(((59*lat0 - 60*lat1) / 131072) + 0.5); | ||
double rlat0 = AirDlat0 * (cprModInt(j,60) + lat0 / 131072); | ||
double rlat1 = AirDlat1 * (cprModInt(j,59) + lat1 / 131072); | ||
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if (rlat0 >= 270) rlat0 -= 360; | ||
if (rlat1 >= 270) rlat1 -= 360; | ||
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// Check to see that the latitude is in range: -90 .. +90 | ||
if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90) | ||
return (-2); // bad data | ||
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// Check that both are in the same latitude zone, or abort. | ||
if (cprNLFunction(rlat0) != cprNLFunction(rlat1)) | ||
return (-1); // positions crossed a latitude zone, try again later | ||
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// Compute ni and the Longitude Index "m" | ||
if (fflag) { // Use odd packet. | ||
int ni = cprNFunction(rlat1,1); | ||
int m = (int) floor((((lon0 * (cprNLFunction(rlat1)-1)) - | ||
(lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5); | ||
rlon = cprDlonFunction(rlat1, 1, 0) * (cprModInt(m, ni)+lon1/131072); | ||
rlat = rlat1; | ||
} else { // Use even packet. | ||
int ni = cprNFunction(rlat0,0); | ||
int m = (int) floor((((lon0 * (cprNLFunction(rlat0)-1)) - | ||
(lon1 * cprNLFunction(rlat0))) / 131072) + 0.5); | ||
rlon = cprDlonFunction(rlat0, 0, 0) * (cprModInt(m, ni)+lon0/131072); | ||
rlat = rlat0; | ||
} | ||
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// Renormalize to -180 .. +180 | ||
rlon -= floor( (rlon + 180) / 360 ) * 360; | ||
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*out_lat = rlat; | ||
*out_lon = rlon; | ||
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return 0; | ||
} | ||
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int decodeCPRsurface(double reflat, double reflon, | ||
int even_cprlat, int even_cprlon, | ||
int odd_cprlat, int odd_cprlon, | ||
int fflag, | ||
double *out_lat, double *out_lon) | ||
{ | ||
double AirDlat0 = 90.0 / 60.0; | ||
double AirDlat1 = 90.0 / 59.0; | ||
double lat0 = even_cprlat; | ||
double lat1 = odd_cprlat; | ||
double lon0 = even_cprlon; | ||
double lon1 = odd_cprlon; | ||
double rlon, rlat; | ||
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// Compute the Latitude Index "j" | ||
int j = (int) floor(((59*lat0 - 60*lat1) / 131072) + 0.5); | ||
double rlat0 = AirDlat0 * (cprModInt(j,60) + lat0 / 131072); | ||
double rlat1 = AirDlat1 * (cprModInt(j,59) + lat1 / 131072); | ||
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// Pick the quadrant that's closest to the reference location - | ||
// this is not necessarily the same quadrant that contains the | ||
// reference location. | ||
// | ||
// There are also only two valid quadrants: -90..0 and 0..90; | ||
// no correct message would try to encoding a latitude in the | ||
// ranges -180..-90 and 90..180. | ||
// | ||
// If the computed latitude is more than 45 degrees north of | ||
// the reference latitude (using the northern hemisphere | ||
// solution), then the southern hemisphere solution will be | ||
// closer to the refernce latitude. | ||
// | ||
// e.g. reflat=0, rlat=44, use rlat=44 | ||
// reflat=0, rlat=46, use rlat=46-90 = -44 | ||
// reflat=40, rlat=84, use rlat=84 | ||
// reflat=40, rlat=86, use rlat=86-90 = -4 | ||
// reflat=-40, rlat=4, use rlat=4 | ||
// reflat=-40, rlat=6, use rlat=6-90 = -84 | ||
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// As a special case, -90, 0 and +90 all encode to zero, so | ||
// there's a little extra work to do there. | ||
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if (rlat0 == 0) { | ||
if (reflat < -45) | ||
rlat0 = -90; | ||
else if (reflat > 45) | ||
rlat0 = 90; | ||
} else if ((rlat0 - reflat) > 45) { | ||
rlat0 -= 90; | ||
} | ||
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if (rlat1 == 0) { | ||
if (reflat < -45) | ||
rlat1 = -90; | ||
else if (reflat > 45) | ||
rlat1 = 90; | ||
} else if ((rlat1 - reflat) > 45) { | ||
rlat1 -= 90; | ||
} | ||
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// Check to see that the latitude is in range: -90 .. +90 | ||
if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90) | ||
return (-2); // bad data | ||
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// Check that both are in the same latitude zone, or abort. | ||
if (cprNLFunction(rlat0) != cprNLFunction(rlat1)) | ||
return (-1); // positions crossed a latitude zone, try again later | ||
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// Compute ni and the Longitude Index "m" | ||
if (fflag) { // Use odd packet. | ||
int ni = cprNFunction(rlat1,1); | ||
int m = (int) floor((((lon0 * (cprNLFunction(rlat1)-1)) - | ||
(lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5); | ||
rlon = cprDlonFunction(rlat1, 1, 1) * (cprModInt(m, ni)+lon1/131072); | ||
rlat = rlat1; | ||
} else { // Use even packet. | ||
int ni = cprNFunction(rlat0,0); | ||
int m = (int) floor((((lon0 * (cprNLFunction(rlat0)-1)) - | ||
(lon1 * cprNLFunction(rlat0))) / 131072) + 0.5); | ||
rlon = cprDlonFunction(rlat0, 0, 1) * (cprModInt(m, ni)+lon0/131072); | ||
rlat = rlat0; | ||
} | ||
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// Pick the quadrant that's closest to the reference location - | ||
// this is not necessarily the same quadrant that contains the | ||
// reference location. Unlike the latitude case, all four | ||
// quadrants are valid. | ||
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// if reflon is more than 45 degrees away, move some multiple of 90 degrees towards it | ||
rlon += floor( (reflon - rlon + 45) / 90 ) * 90; // this might move us outside (-180..+180), we fix this below | ||
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// Renormalize to -180 .. +180 | ||
rlon -= floor( (rlon + 180) / 360 ) * 360; | ||
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*out_lat = rlat; | ||
*out_lon = rlon; | ||
return 0; | ||
} | ||
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// | ||
//========================================================================= | ||
// | ||
// This algorithm comes from: | ||
// 1090-WP29-07-Draft_CPR101 (which also defines decodeCPR() ) | ||
// | ||
// Despite what the earlier comment here said, we should *not* be using trunc(). | ||
// See Figure 5-5 / 5-6 and note that floor is applied to (0.5 + fRP - fEP), not | ||
// directly to (fRP - fEP). Eq 38 is correct. | ||
// | ||
int decodeCPRrelative(double reflat, double reflon, | ||
int cprlat, int cprlon, | ||
int fflag, int surface, | ||
double *out_lat, double *out_lon) | ||
{ | ||
double AirDlat; | ||
double AirDlon; | ||
double fractional_lat = cprlat / 131072.0; | ||
double fractional_lon = cprlon / 131072.0; | ||
double rlon, rlat; | ||
int j,m; | ||
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AirDlat = (surface ? 90.0 : 360.0) / (fflag ? 59.0 : 60.0); | ||
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// Compute the Latitude Index "j" | ||
j = (int) (floor(reflat/AirDlat) + | ||
floor(0.5 + cprModDouble(reflat, AirDlat)/AirDlat - fractional_lat)); | ||
rlat = AirDlat * (j + fractional_lat); | ||
if (rlat >= 270) rlat -= 360; | ||
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// Check to see that the latitude is in range: -90 .. +90 | ||
if (rlat < -90 || rlat > 90) { | ||
return (-1); // Time to give up - Latitude error | ||
} | ||
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// Check to see that answer is reasonable - ie no more than 1/2 cell away | ||
if (fabs(rlat - reflat) > (AirDlat/2)) { | ||
return (-1); // Time to give up - Latitude error | ||
} | ||
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// Compute the Longitude Index "m" | ||
AirDlon = cprDlonFunction(rlat, fflag, surface); | ||
m = (int) (floor(reflon/AirDlon) + | ||
floor(0.5 + cprModDouble(reflon, AirDlon)/AirDlon - fractional_lon)); | ||
rlon = AirDlon * (m + fractional_lon); | ||
if (rlon > 180) rlon -= 360; | ||
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// Check to see that answer is reasonable - ie no more than 1/2 cell away | ||
if (fabs(rlon - reflon) > (AirDlon/2)) | ||
return (-1); // Time to give up - Longitude error | ||
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*out_lat = rlat; | ||
*out_lon = rlon; | ||
return (0); | ||
} |
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