VisibilitySegment.RefractionModelType Property

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Description

Specifies the refraction model to be used in computing this VisibilitySegment. Most of the available refraction models use the VisibilitySegment.RefractionFrequency value as an input. Descriptions of the models and a list of references are provided in the Help File.

 

The "Analytic - Chapman" and "Analytic - TRORD" models are only allowed when the observing object is a GroundStation.

 

The "Analytic - TRORD" model uses the local-mean coefficients for each month specified by the GroundStation.RefractivityNs property of the observing GroundStation.

 

The "Analytic - TRORD" and "Numeric - Exponential" models do not use the VisibilitySegment.RefractionFrequency setting.

 

The following refraction models are available.  Note that these models are used only for calculating refraction through the Earth's atmosphere.  Currently, refraction is not modeled for any other celestial bodies.

 

Refraction Model

Frequency Band

Description

Accuracy

Runtime

None

N/A

No refraction.

N/A

N/A

Numeric - Single Chapman Profile

Visible

Tropospheric refraction effects are modeled using the Edlén model.1234

Ionospheric refraction effects are not modeled.

In the visible spectrum, this model matches the "Numeric - Multiple Chapman Profiles" model, and their results will be identical.

Very high

Fast

Radio

Tropospheric refraction effects are modeled using the International Telecommunications Union (ITU) model.5

Ionospheric refraction effects are modeled using the International Reference Ionosphere (IRI) and Chapman electron density profile at the latitude and longitude location of the minimum-height point along the ray path, and applies the Chapman profile from that location along the whole ray path.6

High

Fast

Numeric - Multiple Chapman Profiles

Visible

Tropospheric refraction effects are modeled using the Edlén model.1234

Ionospheric refraction effects are not modeled.

In the visible spectrum, this model matches the "Numeric - Single Chapman Profile" model, and their results will be identical.

Very high

Fast

Radio

Tropospheric refraction effects are modeled using the International Telecommunications Union (ITU) model.5

Ionospheric refraction effects are modeled using the International Reference Ionosphere (IRI) and Chapman electron density profile at multiple points along the ray path, in order to determine the electron density at the height of the ray path.6

Very high

Slower

Analytic - Chapman

Any

This Earth refraction model is based on analytic Chapman theory.7  This refraction model is only allowed when the observing object is a GroundStation.

Medium

Very fast

Analytic - TRORD

N/A

Tropospheric refraction effects are modeled based on the TRORD refraction model.8 This model is only allowed when the observing object is a GroundStation.  This model uses the local-mean refractivity coefficients for each month specified by the GroundStation.RefractivityNs property of the observing GroundStation.  The VisibilitySegment.RefractionFrequency setting is not used with this model.

Ionospheric refraction effects are not modeled.

Medium

Very fast

Numeric - Exponential

N/A

This simple exponential model calculates a long-term global mean refractive index as a function of height.9  The VisibilitySegment.RefractionFrequency setting is not used with this model.

Medium

Very fast

 

References:

1."The refractive index of air," B. Edlén, Metrologia 2, 71-80 (1966)

2."An updated Edlén equation for the refractive index of air," K.P. Birch and M.J. Downs, Metrologia 30, 155-162 (1993)

3."Correction to the updated Edlén equation for the refractive index of air," K.P. Birch and M.J. Downs, Metrologia 31, 315-316 (1994)

4.NIST Engineering Metrology Toolbox at http://emtoolbox.nist.gov/Wavelength/Documentation.asp

5."Recommendation ITU-R P.453-10 (02.12), The radio refractive index: its formula and refractivity data", Section 1, February 2012

6."The Theory of Scintillation with Applications in Remote Sensing", Charles L. Rino, January 2011

7."Goddard Trajectory Determination System (GTDS) Mathematical Theory", Section 7, July 1989

8."Software Requirements Specification for Tracking and Orbit Determination (TRORD) CPCI 202 of the Command and Control Segment Math Appendix", September 1992

9."Recommendation ITU-R P.453-10 (02.12), The radio refractive index: its formula and refractivity data", Section 2, Equation 8, February 2012

 

Timing Precision Mode

This page describes functionality in nanosecond timing precision mode.

 

Attributes

Type:

integer

Access:

read/write

Default Value:

0 ( None )

Valid Values:

Value

Label

0

None

1

Numeric - Single Chapman Profile.

2

Numeric - Multiple Chapman Profiles.

3

Analytic - Chapman.

4

Analytic - TRORD.

5

Numeric - Exponential

 

Syntax

myVariable1 = myVisibilitySegment1.RefractionModelType;

 

myVisibilitySegment1.RefractionModelType = myVariable1;

 

See also

VisibilitySegment Object

Coverage and Contact Analysis Guide

Visibility Calculators Guide