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/*
* gc.c
*
* Great Circle. This program is used to determine bearing
* and range to two stations given latitude and longitude.
*
* Ver 1.07 By S. R. Sampson, N5OWK
* Public Domain (p) June 1993
*
* Ref: "Air Navigation", Air Force Manual 51-40, 1 February 1987
* Ref: "ARRL Satellite Experimenters Handbook", August 1990
*
* Usage examples:
*
* gc n 35.19n97.27w 0s0e (Moore to Prime/Equator)
* gc n 35.19N97.27W 38.51n77.02W (Moore to Washington D.C., mixed case)
* gc n 33.56n118.24w 55.45n37.35e (L.A. to Moscow)
* gc n 35N70W 35N71W (No decimal points used, all uppercase)
*
* Modified the program to incorporate short and long path information
* from the Satellite Handbook. This version also takes into consideration
* the two points being close enough to be in the near-field, and the
* antipodal points, which are easily calculated. These last points were
* made in discussions with John Allison who makes the nice MAPIT program.
*
* Compile GNU C with: cc -O gc.c -o gc -lm
*/
/* Includes */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
/* Defines */
#define RADIAN (180.0 / M_PI)
/* Globals */
struct {
double miles; /* arc length for 1 degree, various units of measure */
char *text;
} Units[] = {
{ 60.0, "Nautical Miles"},
{ 111.2, "kilometers"},
{ 69.1, "Statute Miles"}
};
/* Simple Declare, No Prototypes */
/*
* Error routine
*/
void err(type)
int type;
{
switch(type) {
case 1:
printf("\007Latitude Out of Range (90N to 90S)\n");
break;
case 2:
printf("\007Longitude Out of Range (180W to 180E)\n");
break;
case 3:
printf("\007Minutes Out of Range (0 to 59)\n");
}
exit(-1);
}
/*
* Convert Degrees and Minutes to Decimal
*/
double dm2dec(n)
double n;
{
double t;
t = (int)n;
n -= t;
n /= .60;
if (n >= 1.0)
err(3);
return (n + t);
}
/*
* Parse the input line
*
* dd(.mm)[NnSs]ddd(.mm)[EeWw]
*/
void parse(s, lat, lon)
char *s;
double *lat, *lon;
{
register char *i, *t, *e;
e = s + strlen(s);
for (i = s; i < e; ++i) {
switch (*i) {
case 'n':
case 'N':
*i = '\0';
t = i + 1;
*lat = atof(s);
break;
case 's':
case 'S':
*i = '\0';
t = i + 1;
*lat = -atof(s);
break;
case 'e':
case 'E':
*i = '\0';
*lon = -atof(t);
break;
case 'w':
case 'W':
*i = '\0';
*lon = atof(t);
}
}
*lat = dm2dec(*lat);
*lon = dm2dec(*lon);
if (*lat > 90.0 || *lat < -90.0)
err(1);
if (*lon > 180.0 || *lon < -180.0)
err(2);
/* Prevent ACOS() Domain Error */
if (*lat == 90.0)
*lat = 89.9;
if (*lat == -90.0)
*lat = -89.9;
}
void main(argc, argv)
int argc;
char **argv;
{
double tmp, arc, cosaz, az, azsp, azlp, distsp, distlp;
double QTH_Lat, QTH_Long, DEST_Lat, DEST_Long, Delta_Long;
int units;
if (argc != 4) {
fprintf(stderr, "\nUsage: gc units station1 station2\n\n" \
"This program computes Great Circle Bearing and Range\n" \
"given the latitude and longitude (degrees and minutes).\n\n" \
"You must input the lat/long of the two stations.\n" \
"The output will then be relative from station1 to station2.\n\n" \
"Input the two station lat/longs using the following format:\n\n" \
"\tdd.mmHddd.mmG lead/lagging zeros can be left out.\n\n" \
"d = Degrees, m = Minutes, H = Hemisphere (N or S), " \
"G = Greenwich (W or E)\n\n" \
"units is 'n' for Nautical, 'k' for kilometers, and 's' for " \
"Statute.\n\n");
exit(1);
}
/* Process the command line data */
switch (argv[1][0]) {
case 'k':
case 'K':
units = 1;
break;
case 's':
case 'S':
units = 2;
break;
case 'n':
case 'N':
default:
units = 0;
}
parse(argv[2], &QTH_Lat, &QTH_Long);
parse(argv[3], &DEST_Lat, &DEST_Long);
QTH_Lat /= RADIAN; /* Convert variables to Radians */
QTH_Long /= RADIAN;
DEST_Lat /= RADIAN;
DEST_Long /= RADIAN;
Delta_Long = DEST_Long - QTH_Long;
tmp = (sin(QTH_Lat) * sin(DEST_Lat)) +
(cos(QTH_Lat) * cos(DEST_Lat) * cos(Delta_Long));
if (tmp > .999999) {
printf("Station points coincide, use an Omni!\n\n");
exit(0);
} else if (tmp < -.999999) {
/*
* points are antipodal, he's straight down.
* So take 180 Degrees of arc times 60 nm,
* and you get 10800 nm, or whatever units...
*/
printf("Station is equal distance in all Azimuths " \
"(antipodal)\n%.0f %s\n\n",
(Units[units].miles) * 180.0,
Units[units].text);
exit(0);
} else {
arc = acos(tmp);
/*
* One degree of arc is 60 Nautical miles
* at the surface of the earth, 111.2 km, or 69.1 sm
* This method is easier than the one in the handbook
*/
/* Short Path */
distsp = (Units[units].miles) * (arc * RADIAN);
/* Long Path */
distlp = ((Units[units].miles) * 360.0) - distsp;
}
cosaz = (sin(DEST_Lat) - (sin(QTH_Lat) * cos(arc))) /
(sin(arc) * cos(QTH_Lat));
if (cosaz > .999999)
az = 0.0;
else if (cosaz < -.999999)
az = 180.0;
else
az = acos(cosaz) * RADIAN;
/*
* Handbook had the test ">= 0.0" which looks backwards??
*/
if (sin(Delta_Long) < 0.0) {
azsp = az;
azlp = 180.0 + az;
} else {
azsp = 360.0 - az;
azlp = 180.0 - az;
}
/* Computations complete, show answer */
printf("Short Path Bearing is %03.0f Degrees for %.0f %s\n",
azsp, distsp, Units[units].text);
printf(" Long Path Bearing is %03.0f Degrees for %.0f %s\n",
azlp, distlp, Units[units].text);
exit(0);
}
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