/**
 * @file     utc2llt.c
 * @brief    Sample program of Earth day(UTC) to LLT conversion
 * @author   Yukio Yamamoto
 * @date     2013-09-25
 */

/*
 * 1. Viking Lander 2 landing point
 *   type: Planetographic
 *   latitude:   47.967 deg N
 *   longitude: 225.737 deg W
 *
 *   cf. Geometric Calibration of Viking Lander EDR Images
 *   http://pds-geosciences.wustl.edu/viking/vl1_vl2-m-lcs-2-edr-v1/vl_0001/geom/geominfo.htm
 *
 *
 * 2. Mars coordinate
 *   Longitude is positive to the west if the spin is prograde (Earth, Moon, and
 *   Sun are exceptions). The longitude et2lst_c() requires must be 225.737 [deg W],
 *   not -225.737 [deg E].
 *   
 *   In addition, et2lst_c() provides local time on 24 hour clock, not on SOL,
 *   therefore the output is corrected from Earth day to Mars SOL.
 *
 *   See also et2lst_c() in CSPICE API:
 *   http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/et2lst_c.html
 *
 * 3. Usage
 *   Requirements:
 *     naif0010.tls, pck0010.tpc, de421.bsp must be put in the execution directory.
 *
 *   Compile the program:
 *     gcc -o utc2llt utc2llt.c -I(somewhere)/cspice/include (somewhere)/cspice/lib/cspice.a -lm
 *
 *   Execute the program:
 *     ./utc2llt YYYY-MM-DDThh:mm:ss
 *
 *   Example:
 *     ./utc2llt 1976-09-03T22:43:43
 */
#include <stdio.h>
#include <stdlib.h>
#include <SpiceUsr.h>

#define TIMLEN 51
#define AMPMLEN 51
#define MARS 499
#define TYPE "PLANETOGRAPHIC"
#define VL2_LON 225.737

int main(int argc, char* argv[]) {
  SpiceChar ampm[AMPMLEN], time[TIMLEN];
  SpiceDouble et, rlon;
  SpiceInt hr, mn, sc;
  double earth_sec, mars_sec;
  
  furnsh_c("naif0010.tls");
  furnsh_c("pck00010.tpc");
  furnsh_c("de421.bsp");
  
  utc2et_c(argv[1], &et);
  
  rlon = VL2_LON * rpd_c();
  et2lst_c(et, MARS, rlon, TYPE, TIMLEN, AMPMLEN,
	   &hr, &mn, &sc, time, ampm);

  earth_sec = hr * 3600 + mn * 60 + sc;
  mars_sec = earth_sec * 88775.244/ 86400;
  
  hr = (int)(mars_sec / 3600);
  mars_sec -= hr * 3600;
  mn = (int)(mars_sec / 60);
  mars_sec -= mn * 60;
  sc = (int)mars_sec;
  
  printf("%6.2f degrees W "
	 "planetographic longitude is %02d:%02d:%02d\n",
	 VL2_LON, hr, mn, sc);
  
  return EXIT_SUCCESS;
}