observer.c File Reference

This module provides a set of functions that define an astronomical observer location, or ones that relate to observer location. More...

Functions
int	aberration (const double *pos, const double *vobs, double lighttime, double *out)
	Corrects position vector for aberration of light.
int	bary2obs (const double *pos, const double *pos_obs, double *out, double *restrict lighttime)
	Moves the origin of coordinates from the barycenter of the solar system to the observer (or the geocenter); i.e., this function accounts for parallax (annual+geocentric or just annual).
short	light_time (double jd_tdb, const object *restrict body, const double *pos_obs, double tlight0, enum novas_accuracy accuracy, double *pos_src_obs, double *restrict tlight)
	Computes the geocentric position of a solar system body, as antedated for light-time.
int	light_time2 (double jd_tdb, const object *restrict body, const double *restrict pos_obs, double tlight0, enum novas_accuracy accuracy, double *p_src_obs, double *restrict v_ssb, double *restrict tlight)
	Computes the geocentric position and velocity of a solar system body, as antedated for light-time.
int	make_airborne_observer (const on_surface *location, const double *vel, observer *obs)
	Populates an 'observer' data structure for an observer moving relative to the surface of Earth, such as an airborne observer.
int	make_gps_observer (double latitude, double longitude, double height, observer *obs)
	Initializes an observer data structure for a ground-based observer with the specified GPS / WGS84 location, and sets mean (annual) weather parameters based on that location.
int	make_gps_site (double latitude, double longitude, double height, on_surface *site)
	Initializes an observing site with the specified GPS / WGS84 location, and sets mean (annual) weather parameters based on that location.
int	make_in_space (const double *sc_pos, const double *sc_vel, in_space *loc)
	Populates an 'in_space' data structure, for an observer situated on a near-Earth spacecraft, with the provided position and velocity components.
int	make_itrf_observer (double latitude, double longitude, double height, observer *obs)
	Initializes an observer data structure for a ground-based observer with the specified International Terrestrial Reference Frame (ITRF) / GRS80 location, and sets mean (annual) weather parameters based on that location.
int	make_itrf_site (double latitude, double longitude, double height, on_surface *site)
	Initializes an observing site with the specified International Terrestrial Reference Frame (ITRF) / GRS80 location, and sets mean (annual) weather parameters based on that location.
short	make_observer (enum novas_observer_place where, const on_surface *loc_surface, const in_space *loc_space, observer *obs)
int	make_observer_at_geocenter (observer *restrict obs)
	Populates an 'observer' data structure for a hypothetical observer located at Earth's geocenter.
int	make_observer_at_site (const on_surface *restrict site, observer *restrict obs)
	Initializes an observer data structure for a ground-based observer at the specified observing site, and sets mean (annual) weather parameters based on that location.
int	make_observer_in_space (const double *sc_pos, const double *sc_vel, observer *obs)
	Populates an 'observer' data structure, for an observer situated on a near-Earth spacecraft, with the specified geocentric position and velocity vectors.
int	make_observer_on_surface (double latitude, double longitude, double height, double temperature, double pressure, observer *restrict obs)
int	make_on_surface (double latitude, double longitude, double height, double temperature, double pressure, on_surface *restrict loc)
int	make_solar_system_observer (const double *sc_pos, const double *sc_vel, observer *obs)
	Populates an 'observer' data structure, for an observer situated on a near-Earth spacecraft, with the specified geocentric position and velocity vectors.
int	make_xyz_site (const double *restrict xyz, on_surface *restrict site)
	Initializes an observing site with the specified Cartesian geocentric xyz location, and sets mean (annual) weather parameters based on that location.
int	novas_set_default_weather (on_surface *site)
	Sets default weather parameters based on an approximate global model to the mean annualized temperatures, based on Feulner et al.
int	obs_planets (double jd_tdb, enum novas_accuracy accuracy, const double *restrict pos_obs, int pl_mask, novas_planet_bundle *restrict planets)
	Calculates the positions and velocities for the Solar-system bodies, e.g.
int	obs_posvel (double jd_tdb, double ut1_to_tt, enum novas_accuracy accuracy, const observer *restrict obs, const double *restrict geo_pos, const double *restrict geo_vel, double *restrict pos, double *restrict vel)
	Calculates the ICRS position and velocity of the observer relative to the Solar System Barycenter (SSB).

Detailed Description

This module provides a set of functions that define an astronomical observer location, or ones that relate to observer location.

The following type of observer locations are supported:

1. Geodetic Earth-based observer locations: a. ground-based observer sites:
   • GPS locations via make_gps_observer()
   • ITRF / GRS80 locations via make_itrs_observer()
   • Cartesian x, y, z coordinates via make_xyz_site() and make_site_observer() b. airborne observers, via a location (e.g. make_gps_site(), make_itrf_site() or make_xyz_site()), and a ground velocity using make_airborne_observer().
2. Near Earth Orbit (NEO) locations, via make_observer_in_space() specifying momentary position and velocity w.r.t. the geocenter.
3. Virtual observer at the geocenter via make_observer_at_geocecenter().
4. Solar-system locations via make_solar_system_observer(), specifying a momentary barycentric (that is w.r.t. the SSB) position and velocity vector.

Once an observer is defined, it maybe used to set up an observing frame for a specific time of observation. Observing frames allow efficient and precise position calculations from an observer's point-of-view.

Date

Created on Mar 6, 2025

Author

Attila Kovacs and G. Kaplan

See also

timescale.c, frame.c, system.c, itrf.c

Function Documentation

aberration()

int aberration	(	const double *	pos,
		const double *	vobs,
		double	lighttime,
		double *	out )

Corrects position vector for aberration of light.

Algorithm includes relativistic terms.

NOTES:

1. This function is called by place() to account for aberration when calculating the position of the source.

REFERENCES:

1. Murray, C. A. (1981) Mon. Notices Royal Ast. Society 195, 639-648.
2. Kaplan, G. H. et. al. (1989). Astron. Journ. 97, 1197-1210.

Parameters

	pos	[AU] Position vector of source relative to observer
	vobs	[AU/day] Velocity vector of observer, relative to the solar system barycenter.
	lighttime	[day] Light time from object to Earth (if known). Or set to 0, and this function will compute it as needed.
[out]	out	[AU] Position vector, referred to origin at center of mass of the Earth, corrected for aberration. It can be the same vector as one of the inputs.

Returns

0 if successful, or -1 if any of the vector arguments are NULL.

See also

frame_aberration()

References novas_vlen().

bary2obs()

int bary2obs	(	const double *	pos,
		const double *	pos_obs,
		double *	out,
		double *restrict	lighttime )

Moves the origin of coordinates from the barycenter of the solar system to the observer (or the geocenter); i.e., this function accounts for parallax (annual+geocentric or just annual).

REFERENCES:

1. Kaplan, G. H. et. al. (1989). Astron. Journ. 97, 1197-1210.

Parameters

	pos	[AU] Position vector, referred to origin at solar system barycenter.
	pos_obs	[AU] Position vector of observer (or the geocenter), with respect to origin at solar system barycenter.
[out]	out	[AU] Position vector, referred to origin at center of mass of the Earth. It may be NULL if not required, or be the same vector as either of the inputs.
[out]	lighttime	[day] Light time from object to Earth. It may be NULL if not required.

Returns

0 if successful, or -1 if any of the essential pointer arguments is NULL.

See also

novas_make_frame(), light_time2()

References novas_vlen().

light_time()

short light_time	(	double	jd_tdb,
		const object *restrict	body,
		const double *	pos_obs,
		double	tlight0,
		enum novas_accuracy	accuracy,
		double *	pos_src_obs,
		double *restrict	tlight )

Computes the geocentric position of a solar system body, as antedated for light-time.

Parameters

	jd_tdb	[day] Barycentric Dynamical Time (TDB) based Julian date
	body	Pointer to structure containing the designation for the solar system body
	pos_obs	[AU] Position 3-vector of observer (or the geocenter), with respect to origin at solar system barycenter, referred to ICRS axes.
	tlight0	[day] First approximation to light-time (can be set to 0.0 if not readily available – it will then be computed as needed).
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
[out]	pos_src_obs	[AU] Position 3-vector of body, with respect to origin at observer (or the geocenter), referred to ICRS axes. It can be the same vector as either of the inputs.
[out]	tlight	[day] Calculated light time

Returns

0 if successful, -1 if any of the poiinter arguments is NULL, 1 if the algorithm failed to converge after 10 iterations, or 10 + the error from ephemeris().

See also

light_time2(), novas_make_frame()

References light_time2().

light_time2()

int light_time2	(	double	jd_tdb,
		const object *restrict	body,
		const double *restrict	pos_obs,
		double	tlight0,
		enum novas_accuracy	accuracy,
		double *	p_src_obs,
		double *restrict	v_ssb,
		double *restrict	tlight )

Computes the geocentric position and velocity of a solar system body, as antedated for light-time.

It is effectively the same as the original NOVAS C light_time(), except that this returns the antedated source velocity vector also.

NOTES:

1. This function is called by novas_geom_posvel() or place() to calculate observed positions, radial velocity, and distance for the time when the observed light originated from the source.

Parameters

	jd_tdb	[day] Barycentric Dynamical Time (TDB) based Julian date
	body	Pointer to structure containing the designation for the solar system body
	pos_obs	[AU] Position 3-vector of observer (or the geocenter), with respect to origin at solar system barycenter, referred to ICRS axes.
	tlight0	[day] First approximation to light-time (can be set to 0.0 if not readily avasilable – so it will be calculated as needed).
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
[out]	p_src_obs	[AU] Position 3-vector of body, relative to observer, referred to ICRS axes, components in AU.
[out]	v_ssb	[AU/day] Velocity 3-vector of body, with respect to the Solar-system barycenter, referred to ICRS axes.
[out]	tlight	[day] Calculated light time, or NAN when returning with an error code.

Returns

0 if successful, -1 if any of the pointer arguments is NULL or if the output vectors are the same or if they are the same as pos_obs, 1 if the algorithm failed to converge after 10 iterations, or 10 + the error from ephemeris().

See also

light_time()

novas_sky_pos()

Since

1.0

Author

Attila Kovacs

References bary2obs(), ephemeris(), NOVAS_BARYCENTER, NOVAS_FULL_ACCURACY, and novas_inv_max_iter.

make_airborne_observer()

int make_airborne_observer	(	const on_surface *	location,
		const double *	vel,
		observer *	obs )

Populates an 'observer' data structure for an observer moving relative to the surface of Earth, such as an airborne observer.

Airborne observers have an earth fixed momentary location, defined by longitude, latitude, and altitude, the same was as for a stationary observer on Earth, but are moving relative to the surface, such as in an aircraft or balloon observatory.

Parameters

	location	Current geodetic location, e.g. as populated with make_gps_site() or similar.
	vel	[km/s] Surface velocity.
[out]	obs	Pointer to data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL.

See also

make_itrf_site(), make_gps_site(), make_xyz_site()

make_gps_observer(), make_itrf_observer(), make_observer_at_site(), make_observer_in_space(), make_solar_system_observer(), make_observer_at geocenter(), novas_make_frame()

Since

1.1

Author

Attila Kovacs

References IN_SPACE_INIT, make_observer(), NOVAS_AIRBORNE_OBSERVER, and in_space::sc_vel.

make_gps_observer()

int make_gps_observer	(	double	latitude,
		double	longitude,
		double	height,
		observer *	obs )

Initializes an observer data structure for a ground-based observer with the specified GPS / WGS84 location, and sets mean (annual) weather parameters based on that location.

For the highest (μas / mm level) precision, you probably should use an ITRF location instead of a GPS based location.

Parameters

	latitude	[deg] Geodetic (GPS / WGS84) latitude north positive.
	longitude	[deg] Geodetic (GPS / WGS84) longitude east positive.
	height	[m] Geodetic (GPS / WGS84) altitude above sea level of the observer.
[out]	obs	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL (errno set to EINVAL), or if the latitude is outside of the [-90:90] range (errno set to ERANGE).

Since

1.5

Author

Attila Kovacs

See also

make_itrf_observer(), make_observer_at_site(), make_airborne_observer(), make_observer_in_space(), make_observer_at_geocenter(), make_solar_system_observer()

make_gps_site(), novas_set_default_weather(), novas_make_frame(), novas_geodetic_transform_site()

References make_gps_site(), and make_observer_at_site().

make_gps_site()

int make_gps_site	(	double	latitude,
		double	longitude,
		double	height,
		on_surface *	site )

Initializes an observing site with the specified GPS / WGS84 location, and sets mean (annual) weather parameters based on that location.

For the highest (μas / mm level) precision, you probably should use an ITRF location instead of a GPS based location.

Parameters

	latitude	[deg] Geodetic (GPS / WGS84) latitude; north positive.
	longitude	[deg] Geodetic (GPS / WGS84) longitude; east positive.
	height	[m] Geodetic (GPS / WGS84) altitude above sea level of the observer.
[out]	site	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL (errno set to EINVAL), or if the latitude is outside of the [-90:90] range (errno set to ERANGE).

Since

1.5

Author

Attila Kovacs

See also

make_gps_site(), make_xyz_site()

make_itrf_observer(), make_observer_at_site(), novas_set_default_weather()

References make_itrf_site(), novas_cartesian_to_geodetic(), novas_geodetic_to_cartesian(), NOVAS_GRS80_ELLIPSOID, and NOVAS_WGS84_ELLIPSOID.

make_in_space()

int make_in_space	(	const double *	sc_pos,
		const double *	sc_vel,
		in_space *	loc )

Populates an 'in_space' data structure, for an observer situated on a near-Earth spacecraft, with the provided position and velocity components.

Both input vectors are assumed with respect to true equator and equinox of date.

Parameters

	sc_pos	[km] Geocentric (x, y, z) position vector. NULL defaults to the origin
	sc_vel	[km/s] Geocentric (x, y, z) velocity vector. NULL defaults to zero speed.
[out]	loc	Pointer to earth-orbit location data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL.

See also

make_observer_in_space(), IN_SPACE_INIT

References in_space::sc_pos, and in_space::sc_vel.

make_itrf_observer()

int make_itrf_observer	(	double	latitude,
		double	longitude,
		double	height,
		observer *	obs )

Initializes an observer data structure for a ground-based observer with the specified International Terrestrial Reference Frame (ITRF) / GRS80 location, and sets mean (annual) weather parameters based on that location.

For the highest precision (μas level) applications you should make sure that the location provided here and the Earth-orientation parameters (EOP) used (in setting novas_timescale and in novas_make_frame()) are provided in the same ITRF realization. You can use novas_itrf_transform_eop() to change the ITRF realization for the EOP values, if necessary.

Parameters

	latitude	[deg] Geodetic (ITRF / GRS80) latitude; north positive.
	longitude	[deg] Geodetic (ITRF / GRS80) longitude; east positive.
	height	[m] Geodetic (ITRF / GRS80) altitude above sea level of the observer.
[out]	obs	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL (errno set to EINVAL), or if the latitude is outside of the [-90:90] range (errno set to ERANGE).

Since

1.5

Author

Attila Kovacs

See also

make_gps_observer(), make_observer_at_site(), make_airborne_observer(), make_observer_in_space(), make_observer_at_geocenter(), make_solar_system_observer()

make_itrf_site(), novas_itrf_transform_site(), novas_set_default_weather(), novas_make_frame()

References make_itrf_site(), and make_observer_at_site().

make_itrf_site()

int make_itrf_site	(	double	latitude,
		double	longitude,
		double	height,
		on_surface *	site )

Initializes an observing site with the specified International Terrestrial Reference Frame (ITRF) / GRS80 location, and sets mean (annual) weather parameters based on that location.

For the highest precision (μas level) applications you should make sure that the location provided here and the Earth-orientation parameters (EOP) used (in setting novas_timescale and in novas_make_frame()) are provided in the same ITRF realization. You can use novas_itrf_transform_eop() to change the ITRF realization for the EOP values, if necessary.

Parameters

	latitude	[deg] Geodetic (ITRF / GRS80) latitude; north positive.
	longitude	[deg] Geodetic (ITRF / GRS80) longitude; east positive.
	height	[m] Geodetic (ITRF / GRS80) altitude above sea level of the observer.
[out]	site	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL (errno set to EINVAL), or if the latitude is outside of the [-90:90] range (errno set to ERANGE).

Since

1.5

Author

Attila Kovacs

See also

make_gps_site(), make_xyz_site()

make_itrf_observer(), make_observer_at_site(), novas_set_default_weather(), novas_itrf_transform_site(), novas_itrf_transform_eop()

References on_surface::height, on_surface::latitude, on_surface::longitude, and novas_set_default_weather().

make_observer()

short make_observer	(	enum novas_observer_place	where,
		const on_surface *	loc_surface,
		const in_space *	loc_space,
		observer *	obs )

Deprecated

It is recommended that you use one of the more specific ways of initializing the observer data structure, e.g. with make_itrf_observer(), make_gps_observer(), make_observer_at_site(), make_airborne_observer() make_solar_system_observer(), ormake_observer_at_geocenter()`. This function will be available for the foreseeable future also.

Populates an 'observer' data structure given the parameters. The output data structure may be used an the the inputs to NOVAS-C functions, such as make_frame() or place().

Parameters

	where	The location type of the observer
	loc_surface	Pointer to data structure that defines a location on Earth's surface. Used only if 'where' is NOVAS_OBSERVER_ON_EARTH, otherwise can be NULL.
	loc_space	Pointer to data structure that defines a near-Earth location in space. Used only if 'where' is NOVAS_OBSERVER_IN_EARTH_ORBIT, otherwise can be NULL.
[out]	obs	Pointer to observer data structure to populate.

Returns

0 if successful, -1 if a required argument is NULL, or 1 if the 'where' argument is invalid.

See also

make_observer_at_geocenter(), make_itrf_observer(), make_gps_observer(), make_airborne_observer(), make_observer_in_space(), make_solar_system_observer()

References observer::near_earth, NOVAS_AIRBORNE_OBSERVER, NOVAS_OBSERVER_AT_GEOCENTER, NOVAS_OBSERVER_IN_EARTH_ORBIT, NOVAS_OBSERVER_ON_EARTH, NOVAS_SOLAR_SYSTEM_OBSERVER, observer::on_surf, in_space::sc_vel, and observer::where.

make_observer_at_geocenter()

int make_observer_at_geocenter

(

observer *restrict

obs

)

Populates an 'observer' data structure for a hypothetical observer located at Earth's geocenter.

The output data structure may be used an the the inputs to NOVAS-C functions, such as make_frame() or place().

Parameters

[out]

obs

Pointer to data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL.

See also

make_gps_observer(), make_itrf_observer(), make_observer_at_site(), make_airborne_observer(), make_observer_in_space(), make_solar_system_observer()

novas_make_frame()

References make_observer(), and NOVAS_OBSERVER_AT_GEOCENTER.

make_observer_at_site()

int make_observer_at_site	(	const on_surface *restrict	site,
		observer *restrict	obs )

Initializes an observer data structure for a ground-based observer at the specified observing site, and sets mean (annual) weather parameters based on that location.

Parameters

	site	Pointer to observing site data structure.
[out]	obs	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if the either argument is NULL (errno set to EINVAL).

Since

1.5

Author

Attila Kovacs

See also

make_itrf_observer(), make_gps_observer(), make_airborne_observer(), make_observer_in_space(), make_observer_at_geocenter(), make_solar_system_observer()

make_itrf_site(), make_gps_site(), make_xyz_site(), novas_make_frame()

References NOVAS_OBSERVER_ON_EARTH.

make_observer_in_space()

int make_observer_in_space	(	const double *	sc_pos,
		const double *	sc_vel,
		observer *	obs )

Populates an 'observer' data structure, for an observer situated on a near-Earth spacecraft, with the specified geocentric position and velocity vectors.

Both input vectors are with respect to true equator and equinox of date. The output data structure may be used an the the inputs to NOVAS-C functions, such as make_frame() or place().

Parameters

	sc_pos	[km] Geocentric (x, y, z) position vector.
	sc_vel	[km/s] Geocentric (x, y, z) velocity vector.
[out]	obs	Pointer to the data structure to populate

Returns

0 if successful, or -1 if the output argument is NULL.

See also

make_gps_observer(), make_itrf_observer(), make_observer_at_site(), make_airborne_observer(), make_observer_at_geocenter(), make_solar_system_observer()

novas_make_frame()

References make_in_space(), make_observer(), and NOVAS_OBSERVER_IN_EARTH_ORBIT.

make_observer_on_surface()

int make_observer_on_surface	(	double	latitude,
		double	longitude,
		double	height,
		double	temperature,
		double	pressure,
		observer *restrict	obs )

Deprecated

This old NOVAS C function has a few too many caveats. It is recommended that you use make_itrf_observer(), make_gps_observer(), or make_observer_at_site() instead (all of which set default mean annual weather parameters for approximate refraction correction), and optionally set actual weather data afterwards, based on the measurements available. This function will be available for the foreseeable future also.

Initializes an observer data structure with the specified ITRF / GRS80 location, and he specified local pressure and temperature. This old NOVAS C function does not set humidity. Thus, if humidity is needed for refraction correction, then you should set it explicitly after this call.

NOTES:

1. You can convert coordinates among ITRF realization using novas_itrf_transform(), possibly after novas_geodetic_to_cartesian() with NOVAS_GRS80_ELLIPSOID as necessary for polar ITRF coordinates.
2. You can convert ITRF Cartesian xyz locations to geodetic locations by using novas_cartesian_to_geodetic() with NOVAS_GRS80_ELLIPSOID as the reference ellipsoid parameter.
3. If you have longitude, latitude, and height defined as GPS (WGS84) values, you might want to use make_gps_observer() intead

Parameters

	latitude	[deg] Geodetic (ITRF / GRS80) latitude; north positive.
	longitude	[deg] Geodetic (ITRF / GRS80) longitude; east positive.
	height	[m] Geodetic (ITRF / GRS80) altitude above sea level of the observer.
	temperature	[C] Temperature (degrees Celsius).
	pressure	[mbar] Atmospheric pressure.
[out]	obs	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL, or if the latitude is outside of the [-90:90] range, or if the temperature or pressure values are impossible for an Earth based observer (errno set to ERANGE).

See also

make_itrf_observer(), make_gps_observer(), make_observer_at_site()

novas_set_weather(), novas_cartesian_to_geodetic(), novas_geodetic_to_cartesian(), NOVAS_GRS80_ELLIPSOID, novas_make_frame()

References make_observer(), make_on_surface(), and NOVAS_OBSERVER_ON_EARTH.

make_on_surface()

int make_on_surface	(	double	latitude,
		double	longitude,
		double	height,
		double	temperature,
		double	pressure,
		on_surface *restrict	loc )

Deprecated

This old NOVAS C function has a few too many caveats. It is recommended that you use make_itrf_site() or make_gps_site() instead (both of which set default mean annual weather parameters for approximate refraction correction), and optionally set actual weather data afterwards, based on the measurements available. This function will be available for the foreseeable future also.

Populates an 'on_surface' data structure, for an observer on the surface of the Earth, with the given parameters.

Note, that because this is an original NOVAS C routine, it does not have an argument to set a humidity value (e.g. for radio refraction). As such, the humidity is set to a a default mean annual value for the location. To set an actual humidity, set the output structure's field after calling this funcion.

NOTES

1. This implementation breaks strict v1.0 ABI compatibility since it writes to (initializes) a field (humidity) that was not yet part of the on_surface structure in v1.0. As such, linking SuperNOVAS v1.1 or later with application code compiled for SuperNOVAS v1.0 can result in memory corruption or segmentation fault when this function is called. To be safe, make sure your application has been (re)compiled against SuperNOVAS v1.1 or later.
2. You can convert coordinates among ITRF realization using novas_itrf_transform(), possibly after novas_geodetic_to_cartesian() with NOVAS_GRS80_ELLIPSOID as necessary for polar ITRF coordinates.
3. You can convert Cartesian xyz locations to geodetic locations by using novas_cartesian_to_geodetic() with NOVAS_GRS80_ELLIPSOID as the reference ellipsoid parameter.
4. If you have longitude, latitude, and height defined as GPS (WGS84) values, you might want to use make_gps_site() instead, and then set weather parameters afterwards as necessary.

Parameters

	latitude	[deg] Geodetic (ITRF / GRS80) latitude; north positive.
	longitude	[deg] Geodetic (ITRF / GRS80) longitude; east positive.
	height	[m] Geodetic (ITRF / GSR80) altitude above sea level of the observer.
	temperature	[C] Temperature (degrees Celsius) [-120:70].
	pressure	[mbar] Atmospheric pressure [0:1200].
[out]	loc	Pointer to Earth location data structure to populate.

Returns

0 if successful, or -1 if the output argument is NULL (errno set to EINVAL), or if the latitude is outside of the [-90:90] range, or if the temperature or pressure values are impossible for an Earth based observer (errno set to ERANGE).

See also

make_itrf_site(), make_gps_site(), make_xyz_site()

novas_set_default_weather(), novas_cartesian_to_geodetic(), ON_SURFACE_INIT, ON_SURFACE_LOC, NOVAS_GRS80_ELLIPSOID

References make_itrf_site().

make_solar_system_observer()

int make_solar_system_observer	(	const double *	sc_pos,
		const double *	sc_vel,
		observer *	obs )

Populates an 'observer' data structure, for an observer situated on a near-Earth spacecraft, with the specified geocentric position and velocity vectors.

Solar-system observers are similar to observers in Earth-orbit but their momentary position and velocity is defined relative to the Solar System Barycenter, instead of the geocenter.

Parameters

	sc_pos	[AU] Solar-system barycentric (x, y, z) position vector in ICRS.
	sc_vel	[AU/day] Solar-system barycentric (x, y, z) velocity vector in ICRS.
[out]	obs	Pointer to the data structure to populate

Returns

0 if successful, or -1 if the output argument is NULL.

See also

make_gps_observer(), make_itrf_observer(), make_observer_at_site(), make_airborne_observer(), make_observer_in_space(), make_solar_system_observer()

novas_make_frame()

Since

1.1

Author

Attila Kovacs

References make_in_space(), make_observer(), and NOVAS_SOLAR_SYSTEM_OBSERVER.

make_xyz_site()

int make_xyz_site	(	const double *restrict	xyz,
		on_surface *restrict	site )

Initializes an observing site with the specified Cartesian geocentric xyz location, and sets mean (annual) weather parameters based on that location.

For the highest precision (μas level) applications you should make sure that the site coordinates and the Earth-orientation parameters (EOP) used (in setting novas_timescale and in novas_make_frame()) are provided in the same ITRF realization. You can use novas_itrf_transform() or novas_itrf_transform_eop() to change the ITRF realization for the site coordinates and/or the EOP values, if necessary.

Parameters

	xyz	[m] Cartesian geocentric position.
[out]	site	Pointer to the data structure to populate.

Returns

0 if successful, or -1 if either of the arguments is NULL (errno set to EINVAL), or if the latitude is outside of the [-90:90] range (errno set to ERANGE).

Since

1.5

Author

Attila Kovacs

See also

make_gps_site(), make_itrf_site(), make_xyz_site()

make_observer_at_site(), novas_itrf_transform(), novas_itrf_transform_eop(), novas_set_default_weather()

References make_itrf_site(), novas_cartesian_to_geodetic(), and NOVAS_GRS80_ELLIPSOID.

novas_set_default_weather()

int novas_set_default_weather

(

on_surface *

site

)

Sets default weather parameters based on an approximate global model to the mean annualized temperatures, based on Feulner et al.

(2013), and scaling relations with altitude (up to 12 km).

Humidity is set to 70% at sea level (which is a typical value globally), and adjusted to decrease with altitude linearly at a rate of 7.5% per km up to 8000 meters. Above that a quadratic model is assumed, peaking at 45% at 14 km – based on the measured distribution by Mendez-Astudillo et al. (2021). Finally above 20.8 km, zero humidity is assumed.

These parameters are all very approximate, but in the absence of measured data, they represent a best guess default model of sorts.

REFERENCES:

1. Feulner, G., Rahmstorf, S., Levermann, A., and Volkwardt, S. (2013), Journal of Climate 26, 7136
2. Mendez-Astudillo, J., et al. (2021), Urban Heat Island (UHI) Mitigation (pp.43-59), DOI:10.1007/978-981-33-4050-3_3

Parameters

[in,out]

site

Site containing geodetic loation as input, and poulated with typical mean weather parameters for the output.

Returns

0 if successful, or else -1 if the site is NULL (errno is set to EINVAL).

Since

1.5

Author

Attila Kovacs

See also

make_itrf_site(), make_gps_site(), make_xyz_site(), make_itrf_observer(), make_gps_observer(), NOVAS_STANDARD_ATMOSPHERE

References on_surface::height, on_surface::humidity, on_surface::latitude, on_surface::pressure, and on_surface::temperature.

obs_planets()

int obs_planets	(	double	jd_tdb,
		enum novas_accuracy	accuracy,
		const double *restrict	pos_obs,
		int	pl_mask,
		novas_planet_bundle *restrict	planets )

Calculates the positions and velocities for the Solar-system bodies, e.g.

for use for gravitational deflection calculations. The planet positions are calculated relative to the observer location, while velocities are w.r.t. the SSB. Both positions and velocities are antedated for light travel time, so they accurately reflect the apparent position (and barycentric motion) of the bodies from the observer's perspective.

Parameters

	jd_tdb	[day] Barycentric Dynamical Time (TDB) based Julian date
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1). In full accuracy mode, it will calculate the deflection due to the Sun, Jupiter, Saturn and Earth. In reduced accuracy mode, only the deflection due to the Sun is calculated.
	pos_obs	[AU] Position 3-vector of observer (or the geocenter), with respect to origin at solar system barycenter, referred to ICRS axes.
	pl_mask	Bitwise (1 << planet-number) mask indicating which planets to request data for. See enum novas_planet for the enumeration of planet numbers.
[out]	planets	Pointer to apparent planet data to populate. The planets with non-zero mask bits will have have positions and velocities calculated. See enum novas_planet for the enumeration of planet numbers.

Returns

0 if successful, -1 if any of the pointer arguments is NULL or if the output vector is the same as pos_obs, or the error from ephemeris().

See also

enum novas_planet, grav_planets(), grav_undo_planets(), set_planet_provider(), set_planet_provider_hp()

Since

1.1

Author

Attila Kovacs

References light_time2(), make_planet(), novas_debug(), NOVAS_DEBUG_EXTRA, NOVAS_DEBUG_OFF, novas_get_debug_mode(), NOVAS_PLANETS, and NOVAS_SUN.

obs_posvel()

int obs_posvel	(	double	jd_tdb,
		double	ut1_to_tt,
		enum novas_accuracy	accuracy,
		const observer *restrict	obs,
		const double *restrict	geo_pos,
		const double *restrict	geo_vel,
		double *restrict	pos,
		double *restrict	vel )

Calculates the ICRS position and velocity of the observer relative to the Solar System Barycenter (SSB).

Parameters

	jd_tdb	[day] Barycentric Dynamical Time (TDB) based Julian date.
	ut1_to_tt	[s] TT - UT1 time difference. Used only when 'location->where' is NOVAS_OBSERVER_ON_EARTH (1) or NOVAS_OBSERVER_IN_EARTH_ORBIT (2).
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
	obs	The observer location, relative to which the output positions and velocities are to be calculated
	geo_pos	[AU] ICRS position vector of the geocenter w.r.t. the Solar System Barycenter (SSB). If either geo_pos or geo_vel is NULL, it will be calculated when needed.
	geo_vel	[AU/day] ICRS velocity vector of the geocenter w.r.t. the Solar System Barycenter (SSB). If either geo_pos or geo_vel is NULL, it will be calculated when needed.
[out]	pos	[AU] Position 3-vector of the observer w.r.t. the Solar System Barycenter (SSB). It may be NULL if not required.
[out]	vel	[AU/day] Velocity 3-vector of the observer w.r.t. the Solar System Barycenter (SSB). It must be distinct from the pos output vector, and may be NULL if not required.

Returns

0 if successful, or the error from geo_posvel(), or else -1 (with errno indicating the type of error).

Author

Attila Kovacs

Since

1.3

See also

novas_make_frame()

References ephemeris(), geo_posvel(), NOVAS_AIRBORNE_OBSERVER, NOVAS_BARYCENTER, NOVAS_EARTH_INIT, NOVAS_OBSERVER_IN_EARTH_ORBIT, NOVAS_OBSERVER_ON_EARTH, NOVAS_OBSERVER_PLACES, NOVAS_SOLAR_SYSTEM_OBSERVER, and tt2tdb().