For air drag, set the following parameters:
Drag Model  

Parameter  Description 
Use 
Select among:

WillUseAirDrag 
This is a readonly field, with a boolean (true/false) value that reflects the result of the selection made in the Use field:

AtmDensityModel 
Select one of the following:
The MSIS models are available at the Office of Naval Research website. The JacchiaBowman models are available at sol.spacenvironment.net/~JB2008/index.html. 
LowAltAtmDensityModel  Select an atmospheric density model to be used when the altitude of the spacecraft goes below the minimum modeled altitude of the primary atmospheric density model given by the AtmDensityModel setting. Options are None, MSISE 1990 and NRLMSISE 2000. The MSIS models both extend downwards to the surface of the Earth. This setting is useful for cases where a Jacchia atmospheric density model is desired for normal operations, but OD is desired up to reentry. The density computed by the low altitude density model is blended with the density computed by the primary density model during a region above the minimum altitude supported by the primary density model as specified through the setting DensityBlendingAltRange. 
DensityBlendingAltRange  Used in conjunction with the LowAltAtmDensityModel setting to specify the range of altitudes during which the density computation will consist of a weighted average of the primary density model result and the low altitude density model result. For a value of 30 Km, the density will be computed solely from the primary model as long as the altitude of the spacecraft is at least 30 Km above the minimum altitude supported by the primary model. Between the minimum altitude supported by the primary model and 30 Km above the minimum altitude supported by the primary model, the density is computed as a weighted average of the two models. Below the minimum altitude supported by the primary model, the density is computed solely by the low altitude density model. 
EstimateDensity  A correction to the local atmospheric density (δρ/ρ) will be estimated by the filter if set to true and drag effects are modeled. Also controls if the local atmospheric density can be perturbed by the simulator. 
EstimateBallisticCoeff  A correction (ΔB/B) to the nominal ballistic coefficient, B, will be estimated by the filter if set to true and drag effects are modeled. Also controls whether the ballistic coefficient can be varied by the Simulator. The estimate flag must also be set to true if this parameter is to be used as a Least Squares Consider State. When used as a consider state it is modeled as a constant. 
Model  Select either "Spherical" and enter the SpecMethod, CD, Area, and BallisticCoeffModel parameters immediately below, or select DragPlugin for a user defined Atmospheric Drag Plugin model. 
SpecMethod  Specifies the method for input of ballistic coefficient
information, where B = Cd A / M.

CorrectionType 
Specify the estimation solve to be used in estimating corrections to acceleration due to atmospheric drag. Choose between:
See the equations below. Available when "Spherical" Model is selected. 
CD  Specify the drag coefficient (C_{D}) to be used in
calculating acceleration due to atmospheric drag. See the equations
below. Available when "Spherical" Model is selected. 
Area  Crosssectional area of the spacecraft in the selected distance
unit squared, for computation of atmospheric drag. Available when "Spherical" Model is selected. 
BallisticCoeff 
Specify the ballistic coefficient (C_{D} A_{D} / M) to be used in calculating acceleration due to atmospheric drag. See the equations below. Available when "Spherical" Model is selected. 
BallisticCoeffModel 
Identifies the Stochastic sequence to be used to represent the Ballistic Coefficient correction (ΔB/B). Note in the following models the B "const" value is either entered directly or computed based on CD, Area values.
Available when "Spherical" Model is selected. Note: Constant is a readonly field whose value is defined by the selected SpecMethod. 
DensityCorrHalflife  Enter the time that it takes for dr/r to decay to one half its value in the absence of measurements. dr/r is the estimated correction to the atmospheric drag, where the latter is calculated via the CIRA 1972 model. This is the usersupplied value t associated with exponential halflife in the GaussMarkov processes used in ODTK. 
DensityCorrInitialEstimate  This property contains the initial condition for the density correction, dr/r, where r is the atmospheric density. While it is possible to specify this value manually, it is more typically updated during the Transfer to Satellite operation on the LeastSquares object if the LeastSquares process has been set to estimate a density correction. 
DensityCorrSigmaScale  Enter a multiplicative scale factor for the atmospheric density
uncertainty computed by ODTK. The computed density uncertainty is
based on the historical performance of the CIRA 1972 (Jacchia 1971)
atmospheric density model considering the current satellite state.
NOTE: A nominal value of 1.0 is recommended for most applications; use of other values should be considered experimental. 
DensityRatioRoot  Enter the reciprocal of the exponent to be applied to the ratio
of the local atmospheric density divided by the baseline density.
The density ratio is computed as (current density / baseline
density)^(1/DRR) where DRR is the DensityRatioRoot. The density
ratio is then used to drive the atmospheric density process noise
model. The local density is computed using the current solar flux
and geomagnetic activity while the baseline density is computed
using average conditions.
NOTE: A nominal value of 1.0 is recommended for most applications; use of other values should be considered experimental. 
DensityRatioIncreaseThreshold  Specifies a threshold for the stepwise change of a ratio of the density evaluated at perigee at the current solar and geomagnetic conditions to an evaluation of the density at mean solar and geomagnetic conditions. When this threshold is exceeded in the filter, the process noise for atmospheric density changes from the baseline model to the active sun model to open up the covariance under conditions of high solar activity. This process is described in Chapter 11 of ODTK Orbit Determination: Theorems & Equations. Recommended values are 0.1 when polynomial smoothing of Ap and F10.7 is enabled and 1.0 when polynomial smoothing of Ap and F10.7 is not enabled. 
SunPosMethod  Specifies the algorithm to be used in the computation of the position of the Sun for input to the atmospheric density model. Options are to compute the true position of the Sun or the apparent position of the Sun to an observer at the center of the Earth. The latter option is consistent with the use of the apparent Sun position for drag in HPOP in STK. 
UseInVariationalEquations  Set to true to have drag accelerations included in the variational equations for the propagation of covariance. It is recommended that this flag be set to true for low altitude satellites. 
AddProcessNoise  This controls the addition of white process noise in two components (specified via the OutOfPlaneFraction and the InPlaneFraction attributes) normal to the Earth fixed velocity direction. This is useful when significant drag accelerations exist in these directions, since the spherical drag model cannot account for such accelerations. Process noise along the Earth fixed velocity direction is added when the ballistic coefficient is estimated. 
OutOfPlaneFraction  When drag process noise is added, the acceleration noise added in the direction normal to the Earth fixed velocity and normal to the satellite position vector is computed as the magnitude of the nominal drag acceleration multiplied by the OutOfPlaneFraction. For example, to add white noise equal to 50% of the nominal acceleration in the Out Of Plane direction, specify a value of 0.5. 
InPlaneFraction  When drag process noise is added, the acceleration noise added in the direction normal to the Earth fixed velocity and in the plane defined by the satellite position and the Earth fixed velocity is computed as the magnitude of the nominal drag acceleration multiplied by the InPlaneFraction. For example, to add white noise equal to 50% of the nominal acceleration in the In Plane direction, specify a value of 0.5. 
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' ' Example Spherical model represented by a Scalar Gauss Markov sequence ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' drag.Model.Type = "Spherical" drag.Model.SpecMethod = "Mass Area Cd" drag.Model.CD = 2.0 drag.Model.Area.Set 25,"m^2" drag.Model.BallisticCoeffModel.Type = "GaussMarkov" ''''Constant Value determined by CD, Area, and Mass drag.Model.BallisticCoeffModel.InitialEstimate = 0 drag.Model.BallisticCoeffModel.Sigma = 0.2 drag.Model.BallisticCoeffModel.HalfLife.Set 2880,"min" ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' ' Example Spherical model represented by a Vasicek sequence ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' drag.Model.Type = "Spherical" drag.Model.SpecMethod = "Ballistic Coeff" ''''' CD implied by BallicCoeffModel constant, Area, and Mass drag.Model.Area.Set 18,"m^2" drag.Model.BallisticCoeffModel.Type = "Vasicek" drag.Model.BallisticCoeffModel.LongTerm.Constant = 0.06 drag.Model.BallisticCoeffModel.LongTerm.Sigma = 0.25 drag.Model.BallisticCoeffModel.ShortTerm.InitialEstimate = 0 drag.Model.BallisticCoeffModel.ShortTerm.Sigma = 0.05 drag.Model.BallisticCoeffModel.ShortTerm.HalfLife.Set 60,"min"
The airdrag acceleration estimate has the form:
where B is the ballistic coefficient, C_{D} is the drag coefficient, A is the drag area, m is the spacecraft mass, is the speed of the spacecraft with respect to the atmosphere (where the atmosphere rotates with the Earth), and is calculated according to:
The latter equation defines the method to map the filter estimate at perigee height h_{p} to the optimal estimate of atmospheric density, at current height h, for use in trajectory propagation. is the current estimate of the ballistic coefficient that is constructed from the nominal value of B and the current state estimate , where b = DB/B.
The variable K in the airdrag acceleration estimate is the 3x1 matrix of components of the KingHele unit vector K on the inertial vector basis [i]. The latter vector is defined by:
where
with (the velocity of the spacecraft with respect to the corotating Earth atmosphere) defined as:
See ODTK Orbit Determination: Theorems & Algorithms.
A correction to atmospheric density is estimated using a GaussMarkov process, wherein the user defines the halflife. The sigma is calculated internally based on the atmospheric model. However, you have the opportunity to define an initialization period, in which you define a sigma and halflife that is applied for a specified length of time, at the end of which the normal process begins.
Drag Model Initialization  

Parameter  Description 
Duration  Length of time of the initialization period, applied from filter start time. Not used when restarting. Setting the initialization period to 0 turns off initialization. 
DensityCorrSigma  Value of the sigma during the initialization period. Following the initialization period, the sigma is calculated internally. 
DensityCorrHalfLife  Value of the halflife during the initialization period. Following the initialization period, the normal halflife is used. 