Atmospheric refraction

Accounting for atmopheric refraction for observers on or above Earth's surface. More...

Classes
class	supernovas::Horizontal
	Horizontal (azimuth, elevation = Az/El) sky coordinates at a geodetic observing location, such as an observatory site, an aircraft, or a balloon. More...
class	supernovas::Weather
	Weather data, mainly for atmopsheric refraction correction for Earth-based (geodetic) observers. More...

Macros
#define	NOVAS_DEFAULT_WAVELENGTH   0.55
	[μm] Default wavelength, for wavelength-dependent refraction models.

Typedefs
typedef double(*	RefractionModel) (double jd_tt, const on_surface *loc, enum novas_refraction_type type, double el)
	A function that returns a refraction correction for a given date/time of observation at the given site on earth, and for a given astrometric source elevation.

Functions
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.
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).
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().
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.
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.
Horizontal	supernovas::Horizontal::to_refracted (RefractionModel ref, const Weather &weather=Weather::standard(), const Time &time=Time::undefined())
	Applies atmospheric refraction correction for these coordinates, returning the result.
Horizontal	supernovas::Horizontal::to_unrefracted (RefractionModel ref, const Weather &weather=Weather::standard(), const Time &time=Time::undefined())
	Undoes atmospheric refraction correction for these coordinates, returning the result.

Detailed Description

Accounting for atmopheric refraction for observers on or above Earth's surface.

SuperNOVAS provides various atmopsheric refraction models. Some use the specified weather parameters, while others use a standard weather for the observer location. Some models are wavelength dependent, so user's can (should) specify a observing wavelength when using them.

Macro Definition Documentation

NOVAS_DEFAULT_WAVELENGTH

#define NOVAS_DEFAULT_WAVELENGTH   0.55

[μm] Default wavelength, for wavelength-dependent refraction models.

It is set to the median wavelength of visible light.

Since

1.4

See also

novas_refract_wavelength()

Typedef Documentation

RefractionModel

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

A function that returns a refraction correction for a given date/time of observation at the given site on earth, and for a given astrometric source elevation.

Parameters

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	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).

Since

1.1

See also

novas_app_to_hor(), novas_hor_to_app()

Function Documentation

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.

As such, make sure that temperature and pressure are defined, e.g. set after calling e.g. make_gps_site(), make_itrf_site(), make_xyz_site(), or similar call that initializes the observing site.

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 values (temperature and pressure are used by this call).
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_wave_refraction(), novas_radio_refraction(), novas_standard_refraction()

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 e.g. make_gps_site(), make_itrf_site(), make_xyz_site(), or similar call that initializes the observing site.

Adapted from FORTAN code provided by Berman & Rockwell 1976.

REFERENCES:

1. 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 angle 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(), novas_wave_refraction(), novas_standard_refraction()

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

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

Referenced by novas_radio_refraction(), refract(), and refract_astro().

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] Source elevation angle 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).

See also

novas_optical_refraction(), novas_radio_refraction(), novas_wave_refraction()

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:

1. 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.
2. The algorithm has been simplified to use fewer variables and simpler logic.
3. 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 e.g. make_gps_site(), make_itrf_site(), make_xyz_site(), or similar call that initializes the observing site.

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:

1. 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."
2. 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:

1. Crane, R.K., Meeks, M.L. (ed), "Refraction Effects in the Neutral Atmosphere", Methods of Experimental Physics: Astrophysics 12B, Academic Press, 1976.
2. Gill, Adrian E., "Atmosphere-Ocean Dynamics", Academic Press, 1982.
3. Green, R.M., "Spherical Astronomy", Cambridge University Press, 1987.
4. Hohenkerk, C.Y., & Sinclair, A.T., NAO Technical Note No. 63, 1985.
5. 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.
6. 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_gps_site(), make_itrf_site(), make_xyz_site(), make_gps_observer(), make_itrf_observer()

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

Referenced by novas_wave_refraction(), and refract().

to_refracted()

Horizontal supernovas::Horizontal::to_refracted	(	RefractionModel	ref,
		const Weather &	weather = Weather::standard(),
		const Time &	time = Time::undefined() )

Applies atmospheric refraction correction for these coordinates, returning the result.

Parameters

ref	refraction model to use, or NULL to skip refraction correction
weather	(optional) local weather parameters to use for the refraction correction. (default: standard atmopshere).
Time of observation and astronomical timescales	(optional) Time of observation, for time-dependent refraction models.

Returns

refracted horizontal coordinates.

See also

to_unrefracted()

References supernovas::Unit::deg, elevation(), supernovas::Validating::is_valid(), supernovas::Time::jd(), supernovas::Spherical::latitude(), supernovas::Spherical::longitude(), and NOVAS_REFRACT_ASTROMETRIC.

to_unrefracted()

Horizontal supernovas::Horizontal::to_unrefracted	(	RefractionModel	ref,
		const Weather &	weather = Weather::standard(),
		const Time &	time = Time::undefined() )

Undoes atmospheric refraction correction for these coordinates, returning the result.

Parameters

ref	refraction model to use, or NULL to skip refraction correction
weather	(optional) local weather parameters to use for the refraction correction. (default: standard atmopshere).
Time of observation and astronomical timescales	(optional) Time of observation, for time-dependent refraction models.

Returns

refracted horizontal coordinates.

unrefracted (astrometric) horizontal coordinates.

See also

to_refracted()

References supernovas::Unit::deg, elevation(), supernovas::Validating::is_valid(), supernovas::Time::jd(), supernovas::Spherical::latitude(), supernovas::Spherical::longitude(), and NOVAS_REFRACT_OBSERVED.