ErrorModel

ErrorModel — Used to specify measurement noise for simulation and estimation, and to apply or estimate measurement biases.

Description

An ErrorModel is assigned on the ErrorModels field of an instance of GroundStation or a spacecraft-attached Receiver to model biases and noise, and optionally to estimate biases on each measurement type provided by the ground station or receiver. An error model must be specified for each data type employed by each tracking station or receiver, but a single instance of ErrorModel may be used by multiple ground stations or spacecraft receivers.

An error model is only assigned to a receiver if GPS_PosVec data is employed. The GPS_PosVec observation type models position estimates provided by an on-board GPS receiver. Since this type of data is not derived from ground station measurement modeling, the error model for GPS_PosVec data is specified on the ErrorModels field of a Receiver resource instead. The receiver must be attached to the corresponding Spacecraft object. Error models for all other observation types should be specified on the ErrorModels field of the relevant ground station resources. Error models cannot be assigned on receivers attached to ground stations.

The ErrorModel is used by both the simulator and the estimator. For a data simulation run, the ErrorModel specifies the measurement type and noise employed when generating the simulated measurement. A bias may optionally be applied to the simulated observations.

For an estimation run, the ErrorModel specifies the observation type, presumed observation noise, and an optional bias to be applied to the observation. An observation bias may also be estimated by adding the keyword Bias to the ErrorModel.SolveFors list. If the SolveFors list is empty, no bias will be estimated. The SolveFors list is ignored by the simulator.

The ErrorModel resource does not currently support application or estimation of biases for the GPS_PosVec data type.

Fields

Field

Description

Bias

The constant bias associated with the measurement. For simulations, this bias is added to the measurement. As shown below, the units used depend upon measurement type, ErrorModel.Type.

Data Type	Real
Allowed Values	Any Real number
Access	set
Default Value	0.0
Units	See Remarks section
Interfaces	script

BiasSigma

Standard deviation of Bias. This field, which only has a function if both (1) BatchEstimatorInv.UseInitialCovariance = true and (2) Bias is a solve-for parameter, is used to constrain the estimated value of Bias. As shown below, the units used depend upon measurement type, ErrorModel.Type. This parameter is not implemented for GPS_PosVec data.

Data Type	Real
Allowed Values	Real > 0
Access	set
Default Value	1e+70
Units	See Remarks section
Interfaces	script

NoiseSigma

One sigma value of Gaussian noise. For simulations, if Sim.AddNoise = true, this noise is added to the measurements. For estimation, this value is used to as part of the batch processing algorithms to calculate the measurement type weighting. As shown below, the units used depend upon measurement type, ErrorModel.Type.

Data Type	Real
Allowed Values	Real > 0
Access	set
Default Value	103
Units	See Remarks section
Interfaces	script

SolveFors

List of parameters to estimate. This parameter is not implemented for GPS_PosVec data.

Data Type	StringArray
Allowed Values	{} or {Bias}
Access	set
Default Value	{}
Units	N/A
Interfaces	script

Type

Measurement data type.

Data Type	Enumeration
Allowed Values	DSN_SeqRange, DSN_TCP, GPS_PosVec, Range, RangeRate
Access	set
Default Value	DSN_SeqRange
Units	N/A
Interfaces	script

Remarks

Units for Bias, BiasSigma, and NoiseSigma

The following table shows the units to be used for Bias, BiasSigma, and NoiseSigma for each measurement data type that GMAT supports.

GMAT Measurement Type	Units
DSN_SeqRange	Range Units
DSN_TCP	Hertz
GPS_PosVec	Kilometers
Range	Kilometers
RangeRate	Kilometers/sec

Deprecated Measurement Type Names

This version of GMAT deprecates the DSNRange/Range_RU and Doppler/Doppler_HZ measurement type names. These have been replaced by the DSN_SeqRange and DSN_TCP types. These new names are employed identically in the GMAT Measurement Data (GMD) data file, the ErrorModel.Type parameter, and the TrackingFileSet.AddTrackingConfig parameter. Scripts employing the deprecated measurement type names will still work in this version of GMAT, but future versions will remove this support. Users are encouraged to update their scripts to use the new names.

The new data type names employ the same name in the GMD file, error model, and tracking file set tracking configuration, eliminating the need for a mapping between the names employed in each resource. For those still using the deprecated data type names, the following table provides a guide.

Deprecated GMD File and TrackingFileSet.AddTrackingConfig Measurement Type Name	Deprecated ErrorModel Measurement Type Name
DSNRange	Range_RU
Doppler	Doppler_HZ

Examples

This example shows how to create an error model for DSN Sequential Range observations and illustrates estimation of a range bias parameter.

%   Create an ErrorModel
%   Measurement noise is in Range Units
 
Create ErrorModel RangeModel;
  
RangeModel.Type       = 'DSN_SeqRange';
RangeModel.NoiseSigma = 11.;
RangeModel.Bias       = 0.;
RangeModel.SolveFors  = {Bias};
 
%   Assign it to a ground station
 
Create GroundStation DSN;
 
DSN.ErrorModels = {RangeModel};

BeginMissionSequence;

This example shows how to create an error model for on-board GPS observations.

%   Create an ErrorModel
%   Measurement noise is in kilometers. Bias estimation is not permitted.
 
Create ErrorModel PosVecModel;
  
PosVecModel.Type       = 'GPS_PosVec';
PosVecModel.NoiseSigma = 0.010;
 
%   Assign the error model to a receiver and add that receiver to a spacecraft.
 
Create Antenna GpsAntenna;
Create Receiver GpsReceiver;

GpsReceiver.Id             = 800;
GpsReceiver.PrimaryAntenna = GpsAntenna;
GpsReceiver.ErrorModels    = {PosVecModel};

Create Spacecraft Sat;

Sat.AddHardware = {GpsReceiver, GpsAntenna};

BeginMissionSequence;