Macros
#define	NOVAS_DEFAULT_WAVELENGTH 0.55
	[μm] Median wavelength of visible light.

Functions
double	novas_inv_refract (RefractionModel model, double jd_tt, const on_surface *restrict loc, enum novas_refraction_type type, double el0)

double	novas_optical_refraction (double jd_tt, const on_surface *loc, enum novas_refraction_type type, double el)

double	novas_radio_refraction (double jd_tt, const on_surface *loc, enum novas_refraction_type type, double el)

int	novas_refract_wavelength (double microns)

double	novas_standard_refraction (double jd_tt, const on_surface *loc, enum novas_refraction_type type, double el)

double	novas_wave_refraction (double jd_tt, const on_surface *loc, enum novas_refraction_type type, double el)

double	refract (const on_surface *restrict location, enum novas_refraction_model model, double zd_obs)

double	refract_astro (const on_surface *restrict location, enum novas_refraction_model model, double zd_astro)

Detailed Description

Date: Created on Jun 27, 2024

Author: Attila Kovacs and G. Kaplan

A collection of refraction models and utilities to use with novas_app_to_hor() or novas_hor_to_app().

See also: novas_app_to_hor(); novas_hor_to_app().

Function Documentation

◆ novas_inv_refract()

double novas_inv_refract	(	RefractionModel	model,
		double	jd_tt,
		const on_surface *restrict	loc,
		enum novas_refraction_type	type,
		double	el0
	)

Computes the reverse atmospheric refraction for a given refraction model. Thus if a refraction model takes observed elevation as an input, the reverse refraction takes astrometric elevation as its input, and vice versa.

Parameters

model	The original refraction model
jd_tt	[day] Terrestrial Time (TT) based Julian data of observation
loc	Pointer to structure defining the observer's location on earth, and local weather
type	Refraction type to use for the original model: NOVAS_REFRACT_OBSERVED (-1) or NOVAS_REFRACT_ASTROMETRIC (0).
el0	[deg] input elevation for the inverse refraction model.

Returns: [deg] Estimated refraction, or NAN if there was an error (it should also set errno to indicate the type of error).

See also: refract_astro(); itrs_to_hor()

Since: 1.1

Author: Attila Kovacs

References novas_inv_max_iter, and NOVAS_REFRACT_OBSERVED.

◆ novas_optical_refraction()

double novas_optical_refraction	(	double	jd_tt,
		const on_surface *	loc,
		enum novas_refraction_type	type,
		double	el
	)

Returns an optical refraction correction using the weather parameters defined for the observer location.

Parameters

jd_tt	[day] Terrestrial Time (TT) based Julian data of observation (unused in this implementation of RefractionModel)
loc	Pointer to structure defining the observer's location on earth, and local weather
type	Whether the input elevation is observed or astrometric: NOVAS_REFRACT_OBSERVED (-1) or NOVAS_REFRACT_ASTROMETRIC (0).
el	[deg] Astrometric (unrefracted) source elevation

Returns: [arcsec] Estimated refraction, or NAN if there was an error (it should also set errno to indicate the type of error).

See also: novas_app_to_hor(); novas_optical_refraction(); NOVAS_STANDARD_ATMOSPHERE(); refract(); refract_astro()

References NOVAS_WEATHER_AT_LOCATION.

◆ novas_radio_refraction()

double novas_radio_refraction	(	double	jd_tt,
		const on_surface *	loc,
		enum novas_refraction_type	type,
		double	el
	)

Atmospheric refraction model for radio wavelengths (Berman & Rockwell 1976). It uses the weather parameters defined for the location, including humidity. As such, make sure the weather data is fully defined, and that the humidity was explicitly set after calling make_on_surface().

Adapted from FORTAN code provided by Berman & Rockwell 1976.

REFERENCES:

Berman, Allan L., and Rockwell, Stephen T. (1976), NASA JPL Technical Report 32-1601

Parameters

jd_tt	[day] Terrestrial Time (TT) based Julian data of observation (unused in this implementation of RefractionModel)
loc	Pointer to structure defining the observer's location on earth, and local weather. Make sure all weather values, including humidity (added in v1.1), are fully populated.
type	Whether the input elevation is observed or astrometric: NOVAS_REFRACT_OBSERVED (-1) or NOVAS_REFRACT_ASTROMETRIC (0).
el	[deg] source elevation of the specified type.

Returns: [deg] Estimated refraction, or NAN if there was an error (it should also set errno to indicate the type of error). An error is returned if the location is NULL, or if the weather parameters are way outside of their resonable ranges, or if the elevation is outside the supported [-1:90] range.

See also: novas_optical_refraction(); make_on_surface(); on_surface

References on_surface::humidity, novas_inv_refract(), novas_radio_refraction(), NOVAS_REFRACT_ASTROMETRIC, NOVAS_REFRACT_OBSERVED, on_surface::pressure, and on_surface::temperature.

◆ novas_refract_wavelength()

int novas_refract_wavelength ( double microns )

Sets the observing wavelength for which refraction is to be calculated when using a wavelength-depenendent model, such as novas_wave_refraction().

Parameters

microns [μm] Observed wavelength to assume in refraction calculations

Returns: 0 if successful, or else -1 (errno set to EINVAL) if the wavelength invalid (zero, negative, or NaN).

Since: 1.4

Author: Attila Kovacs

See also: novas_wave_refraction(); NOVAS_DEFAULT_WAVELENGTH

◆ novas_standard_refraction()

double novas_standard_refraction	(	double	jd_tt,
		const on_surface *	loc,
		enum novas_refraction_type	type,
		double	el
	)

Returns an optical refraction correction for a standard atmosphere.

Parameters

jd_tt	[day] Terrestrial Time (TT) based Julian data of observation (unused in this implementation of RefractionModel)
loc	Pointer to structure defining the observer's location on earth, and local weather
type	Whether the input elevation is observed or astrometric: NOVAS_REFRACT_OBSERVED (-1) or NOVAS_REFRACT_ASTROMETRIC (0).
el	[deg] Astrometric (unrefracted) source elevation

Returns: [deg] Estimated refraction, or NAN if there was an error (it should also set errno to indicate the type of error).

See also: novas_app_to_hor(); novas_optical_refraction(); NOVAS_STANDARD_ATMOSPHERE(); refract(); refract_astro()

References NOVAS_STANDARD_ATMOSPHERE.

◆ novas_wave_refraction()

double novas_wave_refraction	(	double	jd_tt,
		const on_surface *	loc,
		enum novas_refraction_type	type,
		double	el
	)

The wavelength-dependent IAU atmospheric refraction model, based on the SOFA iauRefco() function, in compliance to the 'SOFA Software License' terms of the original source. Our implementation is not provided nor it is endorsed by SOFA. The original function has been modified slightly, such as:

Out-of-range weather parameters will return with an error (errno set to EINVAL), unlike the SOFA implementation, which sets minimal or maximal allowed values for these.
The algorithm has been simplified to use fewer variables and simpler logic.
The SOFA function this implementation is based on returns A/B coefficients, whereas this implementation returns the refraction correction angle.

The refraction is calculated for the observing wavelenth previously set via novas_refract_wavelength(), or for visible light at 550 nm by default.

The function uses the weather parameters defined for the location, including humidity. As such, make sure the weather data is fully defined, and that the humidity was explicitly set after calling make_on_surface().

According to the documentation of SOFA's iauRefco() function, the model has the following accuracy for elevation angles between 15 and 75 degrees, under a range of typical surface conditions:

	worst	RMS
optical/IR	62 mas	8 mas
radio	319 mas	49 mas

NOTES:

From the SOFA documentation: "The model balances speed and accuracy to give good results in applications where performance at low altitudes is not paramount. Performance is maintained across a range of conditions, and applies to both optical/IR and radio."
The model is divergent in the observed direction of the horizon. As such, it should not be used for calculating refraction at or below the horizon itself.

REFERENCES:

Crane, R.K., Meeks, M.L. (ed), "Refraction Effects in the Neutral Atmosphere", Methods of Experimental Physics: Astrophysics 12B, Academic Press, 1976.
Gill, Adrian E., "Atmosphere-Ocean Dynamics", Academic Press, 1982.
Green, R.M., "Spherical Astronomy", Cambridge University Press, 1987.
Hohenkerk, C.Y., & Sinclair, A.T., NAO Technical Note No. 63, 1985.
Rueger, J.M., "Refractive Index Formulae for Electronic Distance Measurement with Radio and Millimetre Waves", in Unisurv Report S-68, School of Surveying and Spatial Information Systems, University of New South Wales, Sydney, Australia, 2002.
Stone, Ronald C., P.A.S.P. 108, 1051-1058, 1996.

Parameters

jd_tt	[day] Terrestrial Time (TT) based Julian data of observation (unused in this implementation of RefractionModel)
loc	Pointer to structure defining the observer's location on earth, and local weather. Make sure all weather values, including humidity (added in v1.1), are fully populated.
type	Whether the input elevation is observed or astrometric: NOVAS_REFRACT_OBSERVED (-1) or NOVAS_REFRACT_ASTROMETRIC (0).
el	[deg] observed source elevation of the specified type.

Returns: [deg] Estimated refraction, or NAN if there was an error (it should also set errno to indicate the type of error), e.g. because the location is NULL, or because the weather parameters are outside of the supported (sensible) range, or because the elevation is outside of the supported (0:90] range, or because the wavelength set is below 100 nm (0.1 μm);

Since: 1.4

Author: Attila Kovacs

See also: novas_refract_wavelength(); novas_optical_refraction(); novas_radio_refraction(); make_on_surface(); on_surface

References on_surface::humidity, novas_inv_refract(), NOVAS_REFRACT_ASTROMETRIC, NOVAS_REFRACT_OBSERVED, novas_wave_refraction(), on_surface::pressure, and on_surface::temperature.

◆ refract()

double refract	(	const on_surface *restrict	location,
		enum novas_refraction_model	model,
		double	zd_obs
	)

Computes atmospheric optical refraction for an observed (already refracted!) zenith distance through the atmosphere. In other words this is suitable to convert refracted zenith angles to astrometric (unrefracted) zenith angles. For the reverse, see refract_astro().

The returned value is the approximate refraction for optical wavelengths. This function can be used for planning observations or telescope pointing, but should not be used for precise positioning.

NOTES:

The standard temeperature model includes a very rough estimate of the mean annual temeprature for the ovserver's latitude and elevation, rather than the 10 C everywhere assumption in NOVAS C 3.1.<.li>

REFERENCES:

Explanatory Supplement to the Astronomical Almanac, p. 144.
Bennett, G. (1982), Journal of Navigation (Royal Institute) 35, pp. 255-259.

Parameters

location	Pointer to structure containing observer's location. It may also contains weather data (optional) for the observer's location. Some, but not all, refraction models will use location-based (e.g. weather) information. For models that do not need it, it may be NULL.
model	The built in refraction model to use. E.g. NOVAS_STANDARD_ATMOSPHERE (1), or NOVAS_WEATHER_AT_LOCATION (2)...
zd_obs	[deg] Observed (already refracted!) zenith distance through the atmosphere.

Returns: [deg] the calculated optical refraction or 0.0 if the location is NULL or the option is invalid or the 'zd_obs' is invalid (<90°).

See also: refract_astro(); hor_to_itrs()

References NOVAS_NO_ATMOSPHERE, NOVAS_RADIO_REFRACTION, novas_radio_refraction(), NOVAS_REFRACT_OBSERVED, NOVAS_REFRACTION_MODELS, NOVAS_WAVE_REFRACTION, novas_wave_refraction(), and NOVAS_WEATHER_AT_LOCATION.

◆ refract_astro()

double refract_astro	(	const on_surface *restrict	location,
		enum novas_refraction_model	model,
		double	zd_astro
	)

Computes atmospheric optical refraction for a source at an astrometric zenith distance (e.g. calculated without accounting for an atmosphere). This is suitable for converting astrometric (unrefracted) zenith angles to observed (refracted) zenith angles. See refract() for the reverse correction.

The returned value is the approximate refraction for optical wavelengths. This function can be used for planning observations or telescope pointing, but should not be used for precise positioning.

REFERENCES:

Explanatory Supplement to the Astronomical Almanac, p. 144.
Bennett, G. (1982), Journal of Navigation (Royal Institute) 35, pp. 255-259.

Parameters

location	Pointer to structure containing observer's location. It may also contains weather data (optional) for the observer's location. Some, but not all, refraction models will use location-based (e.g. weather) information. For models that do not need it, it may be NULL.
model	The built in refraction model to use. E.g. NOVAS_STANDARD_ATMOSPHERE (1), or NOVAS_WEATHER_AT_LOCATION (2)...
zd_astro	[deg] Astrometric (unrefracted) zenith distance angle of the source.

Returns: [deg] the calculated optical refraction. (to ~0.1 arcsec accuracy), or 0.0 if the location is NULL or the option is invalid.

See also: refract(); itrs_to_hor()

Since: 1.0

Author: Attila Kovacs

References novas_inv_max_iter, NOVAS_RADIO_REFRACTION, novas_radio_refraction(), NOVAS_REFRACT_ASTROMETRIC, and refract().

Macros

Functions

Detailed Description

Function Documentation

◆ novas_inv_refract()

◆ novas_optical_refraction()

◆ novas_radio_refraction()

◆ novas_refract_wavelength()

◆ novas_standard_refraction()

◆ novas_wave_refraction()

◆ refract()

◆ refract_astro()