Spacecraft Navigation

Spacecraft Navigation — There are a number of Spacecraft fields that are used exclusively to support GMAT's navigation capability.

Description

When using GMAT's navigation capabilities, certain Spacecraft parameters can be "solved-for." As discussed in the Spacecraft Ballistic/Mass Properties section, the Spacecraft ballistic and mass properties include the coefficient of reflectivity, Cr, and the coefficient of drag, Cd. As discussed in the Spacecraft Orbit State section, you can specify the CartesianState, i.e., the X, Y, Z position (km), and the Vx, Vy, Vz velocity (km/s) of a Spacecraft. As part of GMAT's navigation capability, GMAT can ingest measurements and estimate ("solve-for") values for Cr, Cd, and either the CartesianState, or KeplerianState.

Fields

Field Description

AddHardware

List of Antenna, Transmitter, Receiver, and Transponder objects attached to a Spacecraft

Data Type	Antenna, Transmitter, Receiver, or Transponder object
Allowed Values	Any user defined Antenna, Transmitter, Receiver, or Transponder object
Access	set
Default Value	None
Units	N/A
Interfaces	script

CdSigma

Standard deviation of the coefficient of reflectivity, Cd. This field is only used if the UseInitialCovariance field of the BatchEstimatorInv resource is set to True and Cd is being solved for.

Data Type	Real
Allowed Values	Real > 0
Access	set
Default Value	1e70
Units	dimensionless
Interfaces	script

CrSigma

Standard deviation of the coefficient of reflectivity, Cr. This field is only used if the UseInitialCovariance field of the BatchEstimatorInv resource is set to True and Cr is being solved for.

Data Type	Real
Allowed Values	Real > 0
Access	set
Default Value	1e70
Units	dimensionless
Interfaces	script

OrbitErrorCovariance

State 6x6 error covariance matrix. If CartesianState is estimated, this must be a Cartesian covariance. If KeplerianState is estimated, this must be a Keplerian covariance. Regardless of choice of spacecraft coordinate system, the covariance must be specified in the EarthMJ2000Eq coordinate system.

This field is only used if the UseInitialCovariance of the BatchEstimatorInv resource is set to True.

Data Type	Real Matrix
Allowed Values	6x6 positive definite symmetric Array
Access	set
Default Value	6x6 diagonal matrix with 1e70 in all diagonal entries.
Units	For Cartesian elements: covariance matrix where position is specified in km and velocity in km/s. (Thus, first three diagonal elements have units km^2 and last three diagonal elements have units (km/s)^2) For Keplerian elements: covariance matrix in km and degrees (For example, the SMA element of the matrix has units km^2 and the INC element has units deg^2). The order of Keplerian elements is (SMA, ECC, INC, RAAN, AOP, MA). See the Remarks section for additional notes.
Interfaces	script

SolveFors

List of fields to be solved for. This list must at least include either CartesianState or KeplerianState (but not both). For example, Cr cannot be the only parameter solved for.

Data Type	StringArray
Allowed Values	CartesianState, KeplerianState, Cr, Cd
Access	set
Default Value	None
Units	N/A
Interfaces	script

Remarks

When estimating CartesianState, the input OrbitErrorCovariance matrix must represent a Cartesian covariance, and when estimating KeplerianState the OrbitErrorCovariance must represent a Keplerian covariance. Note that Keplerian covariance input employs Mean Anomaly (MA) instead of True Anomaly. The current release of GMAT only supports input of Keplerian orbit elements using TA and does not permit explicitly setting an initial MA.

For more details, see the section called “UseInitialCovariance Restrictions” in the Batch Estimator resource.

Examples

Solve for Cr and the spacecraft Cartesian state.

          Create Spacecraft Sat
Create BatchEstimatorInv bat
Sat.SolveFors = {CartesianState, Cr}
%User must create a TrackingFileSet
%and set up bat appropriately

BeginMissionSequence
RunEstimator bat

Solve for Cd and the spacecraft Cartesian state assuming that the a priori information is included in the estimation state vector.

          Create Spacecraft Sat
Sat.SolveFors = {CartesianState, Cd}

Create BatchEstimatorInv bat
bat.UseInitialCovariance= True  
%User must create a TrackingFileSet
%and set up bat appropriately

Create Array Initial_6x6_covariance[6,6]

BeginMissionSequence
Initial_6x6_covariance = ...
       diag([1e-6 1e70 1e70 1e70 1e70 1e70]) %X pos known very well
Sat.OrbitErrorCovariance = Initial_6x6_covariance
Sat.CrSigma = 1e-6   %Cr known very well

RunEstimator bat