The Global Density Correction (*.gdc) file is an ASCII formatted file with a header section. The file identifies the baseline density model and the parameterization to be used in providing corrective information for the baseline model followed by a series of Parameter sections. Each Parameter section provides a name and describes a model for the identified parameter. Models are further defined through specification of a series of Coefficients and a realization of the model is provided as a table of time ordered Coefficient values. For example, the baseline model may be identified as "Jacchia 1970" with corrections in terms of minimum exospheric temperature (Tc) and inflection point temperature (Tx) adjustments. Tc and Tx adjustments are therefore the Parameters and each Parameter is allowed to have its own model. We can specify Tc adjustments as coefficients of a 2x2 spherical harmonic expansion, and Tx adjustments as scalars. A truncated example of this scenario is provided below.
Information inside the (*.gdc) file is presented in terms of keyword-value pairs where the keywords and values are separated by an equals sign. Anywhere in the file, a # symbol in the first column of a line indicates a comment.
Header Information
| Keyword | Value Description |
|---|---|
| Version | Version of file format (1.0 or 2.0) |
| ReferenceModel | Specifies the baseline atmospheric density model. Valid choices are: Jacchia 1970, Jacchia-Bowman 2008 |
| ReferenceModelParameterization | Specifies the parameterization of the model being used to generate corrections to the baseline atmospheric density model. Valid choices are: Tc_Tx - Additive corrections willl be provided to the baseline model computations of the nighttime minimum exospheric temperature (Tc) and the inflection point temperature (Tx) Tinf - Additive corrections will be provided to the baseline model computations of the exospheric temperature (Tinf). DeltaRhoOverRho - Multiplicative corrections will be provided to the baseline model computations of the atmospheric density (Rho). In the case of a plugin density model, a custom parameterization name can be specified. In such a case, the parameterization name specified here and the names of the parameters must match those advertised by the plugin. If a plugin specifies one of the parameterizations listed above, the parameter names must match those listed in the next table for the selected parameterization. |
| TimeScale (Added in Version 2.0) | Specifies the time scale which will be used in the specification of times in the Data sections of the file. Valid choices are: UTC. |
| TimeFormat (Added in Version 2.0) | Specifies time formatting which will be used in the specification of times in the Data sections of the file. Valid choices are: ISO - YMD - (Ex: 2012-10-15T00:00:00.000) |
Parameter Definition
| Keyword | Value Description |
|---|---|
| Name | Name of the parameter. For Tc_Tx parameterization, valid names are dTc and dTx. For Tinf parameterization, valid name is dTinf. For DeltaRhoOverRho parameterization, valid name is dRhoOverRho. In the case of the plugin density model, a custom parameterization name can be specified. In such a case, the parameterization name and the names of the parameters must match those advertised by the plugin. |
| Parameterization | Identifies the model used for representing corrections to the specified parameter. Valid choices are: SphericalHarmonics and Scalar |
| MaxDegreeAndOrder (SphericalHarmonics only) | Specifies the maximum degree and order of the spherical harmonic expansion used to represent corrections to the identified parameter. Latitude and longitude type angles used in the spherical harmonic formulation are measured in a coordinate system with the same X-Y plane as the Earth-Fixed reference frame but has the X axis pointed in the direction of the Sun. |
| Normalized (SphericalHarmonics only) | Specifies if the spherical harmonic coefficients in the Data sections of the file have been normalized. Valid choices are: Yes, No. |
| Dimension | Specifies the dimension of the specified parameter. Valid choice is: Temperature |
| Unit (Added in Version 2.0) | Identifies the unit of the specified parameter. Valid choice is Kelvin |
Coefficient Definition
| Keyword | Value Description |
|---|---|
| Name | Name of coeffient. For SphericalHarmonics models, valid names are C0,0, C1,0, C1,1, S1,1, etc. The order of the coefficients is also important for SphericalHarmonics models. Zonal terms (Cn,0) are listed first in order of increasing degree followed by tesseral terms in order of increasing degree and increasing order within a set of terms of the same degree. An example order of terms for a spherical harmonic representation with a maximum degree of 2 would be: C0,0, C1,0, C2,0, C1,1, S1,1, C2,1, S2,1, C2,2, S2,2. For Scalar models, the name must be the same as the parameter name. |
| InterpOrder | Specifies the order of Lagrange interpolation to be used when computing the coefficient value at times between the node times provided in the Data section for the associated parameter. |
| Dimension | Optional parameter: Specifies the dimension of the specified coefficient. Inherits value from Parameter specification if not provided in Coefficient specification. Valid choice is: Temperature |
| Unit (Added in Version 2.0) | Optional parameter: Identifies the unit of the specified coefficient. Inherits value from Parameter specification if not provided in Coefficient specification. Valid choice is: Kelvin |
Example File
#This is an example GDC file
Version = 2.0
ReferenceModel = Jacchia 1970
ReferenceModelParameterization = Tc_Tx
TimeScale = UTC
TimeFormat = ISO-YMD
Begin Parameter
Name = dTc
Dimension = Temperature
Unit = Kelvin
Parameterization = SphericalHarmonics
MaxDegreeAndOrder = 1
Normalized = Yes
Begin Coefficient
Name = C0,0
Dimension = Temperature
Unit = Kelvin
InterpOrder = 1
End Coefficient
Begin Coefficient
Name = C1,0
Dimension = Temperature
Unit = Kelvin
InterpOrder = 1
End Coefficient
Begin Coefficient
Name = C1,1
Dimension = Temperature
Unit = Kelvin
InterpOrder = 1
End Coefficient
Begin Coefficient
Name = S1,1
Dimension = Temperature
Unit = Kelvin
InterpOrder = 1
End Coefficient
Begin Data
2014-11-13T00:00:00.000 5.79745 -2.22667 -1.71809 0.17622
2014-11-13T03:00:00.000 3.40140 -1.89432 -1.81519 0.00976
2014-11-13T06:00:00.000 2.11753 -2.05718 -2.57061 0.85561
2014-11-13T09:00:00.000 4.27057 -2.47303 -4.66296 2.71135
.
.
.
End Data
End Parameter
Begin Parameter
Name = dTx
Dimension = Temperature
Unit = Kelvin
Parameterization = Scalar
Begin Coefficient
Name = dTx
Dimension = Temperature
Unit = Kelvin
InterpOrder = 1
End Coefficient
Begin Data
2014-11-13T00:00:00.000 3.06574
2014-11-13T03:00:00.000 3.04006
2014-11-13T06:00:00.000 2.38814
2012-11-13T09:00:00.000 1.74034
.
.
.
End Data
End Parameter