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/**
* @file
*
* @date Created on Jan 9, 2025
* @author Attila Kovacs
*
* Example file for using the SuperNOVAS C/C++ library for determining positions for
* nearby (non-high-z) sidereal sources, such as a star.
*
* Link with
*
* ```
* -lsupernovas
* ```
*/
#define _POSIX_C_SOURCE 199309L ///< for clock_gettime()
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <novas.h> ///< SuperNOVAS functions and definitions
// Below are some Earth orientation values. Here we define them as constants, but they may
// of course be variables. They should be set to the appropriate values for the time
// of observation based on the IERS Bulletins...
#define LEAP_SECONDS 37 ///< [s] current leap seconds from IERS Bulletin C
#define DUT1 0.114 ///< [s] current UT1 - UTC time difference from IERS Bulletin A
#define POLAR_DX 230.0 ///< [mas] Earth polar offset x, e.g. from IERS Bulletin A.
#define POLAR_DY -62.0 ///< [mas] Earth polar offset y, e.g. from IERS Bulletin A.
int main() {
// SuperNOVAS variables used for the calculations ------------------------->
cat_entry star = CAT_ENTRY_INIT; // catalog information about a sidereal source
object source; // a celestial object: sidereal, planet, ephemeris or orbital source
observer obs; // observer location
novas_timespec obs_time; // astrometric time of observation
novas_frame obs_frame; // observing frame defined for observing time and location
enum novas_accuracy accuracy; // NOVAS_FULL_ACCURACY or NOVAS_REDUCED_ACCURACY
sky_pos apparent; // calculated precise observed (apparent) position of source
// Calculated quantities ------------------------------------------------->
double az, el; // calculated azimuth and elevation at observing site
// We'll print debugging messages and error traces...
novas_debug(NOVAS_DEBUG_ON);
// -------------------------------------------------------------------------
// Define a sidereal source
// Let's assume we have B1950 (FK4) coordinates...
// 16h26m20.1918s, -26d19m23.138s (B1950), proper motion -12.11, -23.30 mas/year,
// parallax 5.89 mas, radial velocity -3.4 km/s.
//
// NOTE, that make_cat_entry() expects radial velocities defined relative to the
// Solar-System Barycenter (SSB). But you can convert LSR-based velocities to
// the required SSB-based radial velocities using novas_lsr_to_ssb_vel() if needed.
// E.g. initialize with string coordinates to hours/degrees...
if(novas_init_cat_entry(&star, "Antares", novas_str_hours("16h26m20.1918s"), novas_str_degrees("-26d19m23.138s")) != 0) {
fprintf(stderr, "ERROR! defining cat_entry.\n");
return 1;
}
// Optionally, we might store the catalog information: catalog ID (5-chars max) and number.
novas_set_catalog(&star, "HIP", 80763);
// Set the proper motion
novas_set_proper_motion(&star, -12.11, -23.30);
// Set the parallax. Alternatively we could set distance with `novas_set_distance()` instead.
novas_set_parallax(&star, 5.89);
// Set radial velocity. Or we could set LSR velocity with 'novas_set_lsr_vel()' instead
novas_set_ssb_vel(&star, -3.4);
// -------------------------------------------------------------------------
// Convert to ICRS coordinates and wrap in a generic object structure.
if(make_cat_object_sys(&star, "B1950", &source) != 0) {
fprintf(stderr, "ERROR! configuring observed object\n");
return 1;
}
// -------------------------------------------------------------------------
// Define observer somewhere on Earth (we can also define observers in Earth
// or Sun orbit, at the geocenter or at the Solary-system barycenter...)
// Specify the location we are observing from
// 50.7374 deg N, 7.0982 deg E, 60m elevation (GPS / WGS84)
// (You can set local weather parameters after...)
if(make_gps_observer(50.7374, 7.0982, 60.0, &obs) != 0) {
fprintf(stderr, "ERROR! defining Earth-based observer location.\n");
return 1;
}
// -------------------------------------------------------------------------
// Set the astrometric time of observation...
// Set the time of observation to the current UTC-based UNIX time
if(novas_set_current_time(LEAP_SECONDS, DUT1, &obs_time) != 0) {
fprintf(stderr, "ERROR! failed to set time of observation.\n");
return 1;
}
// ... Or you could set a time explicily in any known timescale.
/*
// Let's set a TDB-based time for the start of the J2000 epoch exactly...
if(novas_set_time(NOVAS_TDB, NOVAS_JD_J2000, 32, 0.0, &obs_time) != 0) {
fprintf(stderr, "ERROR! failed to set time of observation.\n");
return 1;
}
*/
// -------------------------------------------------------------------------
// You might want to set a provider for precise planet positions so we might
// calculate Earth, Sun and major planet positions accurately. If an planet
// provider is configured, we can unlock the ultimate (sub-uas) accuracy of
// SuperNOVAS.
//
// There are many ways to set a provider of planet positions. For example,
// you may use the CALCEPH library:
//
// t_calcephbin *planets = calceph_open("path/to/de440s.bsp");
// novas_use_calceph(planets);
//
// accuracy = NOVAS_FULL_ACCURACY; // sub-uas precision
// Without a planet provider, we are stuck with reduced (mas) precisions
// only...
accuracy = NOVAS_REDUCED_ACCURACY; // mas-level precision, typically
// -------------------------------------------------------------------------
// Initialize the observing frame with the given observing and Earth
// orientation patameters.
//
if(novas_make_frame(accuracy, &obs, &obs_time, POLAR_DX, POLAR_DY, &obs_frame) != 0) {
fprintf(stderr, "ERROR! failed to define observing frame.\n");
return 1;
}
// -------------------------------------------------------------------------
// Calculate the precise apparent position (e.g. in CIRS).
if(novas_sky_pos(&source, &obs_frame, NOVAS_CIRS, &apparent) != 0) {
fprintf(stderr, "ERROR! failed to calculate apparent position.\n");
return 1;
}
// Let's print the apparent position
// (Note, CIRS R.A. is relative to CIO, not the true equinox of date.)
printf(" RA = %.9f h, Dec = %.9f deg, rad_vel = %.6f km/s\n", apparent.ra, apparent.dec, apparent.rv);
// -------------------------------------------------------------------------
// Convert the apparent position in CIRS on sky to horizontal coordinates
// We'll use a standard (fixed) atmospheric model to estimate an optical refraction
// (You might use other refraction models, or NULL to ignore refraction corrections)
if(novas_app_to_hor(&obs_frame, NOVAS_CIRS, apparent.ra, apparent.dec, novas_standard_refraction, &az, &el) != 0) {
fprintf(stderr, "ERROR! failed to calculate azimuth / elevation.\n");
return 1;
}
// Let's print the calculated azimuth and elevation
printf(" Az = %.6f deg, El = %.6f deg\n", az, el);
return 0;
}
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