Module: planetNav¶

Executive Summary¶

This module is used to generate noisy ephemeris data, similar to what Module: simpleNav does for spacecraft states. These noisy states can either serve as a stand-in for a filter that estimates the body’s ephemeris or as measurements input into such a filter. This module is most useful for small bodies like comets or asteroids where large uncertainty in the body’s ephemeris is present. This module is not recommended for larger bodies like the Earth or the sun.

The noise present in the planetNav module is designed to mimic the error signals that will be observed in the real navigation system. The true ”noise” present in an orbit determination nav system is always a combination of bias, white noise, and brown noise (or random walk). In order to provide this, a second-order Gauss-Markov process model was added to the simulation utilities that allows the user to configure a random walk process.

Model Functions¶

This module allows the user to set the bounds and the standard deviations for:

The body’s inertial position
The body’s inertial velocity
The body’s attitude with respect to the inertial frame
The body’s angular velocity with respect to the inertial frame expressed in body-frame components

The user can set the noise levels of each of these parameters independently.

Message Connection Descriptions¶

The following table lists all the module input and output messages. The module msg connection is set by the user from python. The msg type contains a link to the message structure definition, while the description provides information on what this message is used for.

Module I/O Messages¶
Msg Variable Name	Msg Type	Description
ephemerisInMsg	EphemerisMsgPayload	Planet ephemeris input msg
ephemerisOutMsg	EphemerisMsgPayload	Planet ephemeris output msg

User Guide¶

To create a planetNav module, first instantiate a planetNav object. Then, set the walkBounds and standard deviation of each state. Afterwards, set the crossTrans and crossAtt boleans based on whether or not position error should depend on velocity or attitude error should depend on angular velocity.

planetNavigation = planetNav.PlanetNav()
planetNavigation.ModelTag = "planetNavigation"
planetNavigation.walkBounds = errorBounds
planetNavigation.PMatrix = pMatrix
planetNavigation.crossTrans = True
planetNavigation.crossAtt = False
scSim.AddModelToTask(simTaskName, planetNavigation)

class PlanetNav : public SysModel ¶

#include <planetNav.h>

This is an auto-created sample C++ module. The description is included with the module class definition.

Public Functions

PlanetNav()¶: This is the constructor for the module class. It sets default variable values and initializes the various parts of the model

~PlanetNav()¶: Module Destructor

void Reset(uint64_t CurrentSimNanos)¶

— Reset function

This method is used to reset the module and checks that required input messages are connect.

Parameters: CurrentSimNanos – The clock time associated with the module call
Returns: void

void UpdateState(uint64_t CurrentSimNanos)¶

— UpdateState

This is the main method that gets called every time the module is updated. Provide an appropriate description.

Parameters: CurrentSimNanos – The clock time associated with the model call
Returns: void

void computeErrors(uint64_t CurrentSimNanos)¶

— Compute the errors to add to the truth

This method sets the propagation matrix and requests new random errors from its GaussMarkov model.

Parameters: CurrentSimNanos – The clock time associated with the model call
Returns: void

void applyErrors()¶

— Add the errors to the truth

This method applies the errors to the truePlanetState

Returns: void

void readInputMessages()¶: This method reads the input messages associated with the planet state

void writeOutputMessages(uint64_t Clock)¶

—Read the input messages

This method writes the aggregate nav information into the output state message.

Parameters: CurrentSimNanos – The clock time associated with the model call
Returns: void

Public Members

Eigen::MatrixXd PMatrix¶

—Write the output messages

— Cholesky-decomposition or matrix square root of the covariance matrix to apply errors with

Eigen::VectorXd walkBounds¶: — “3-sigma” errors to permit for states

Eigen::VectorXd navErrors¶: — Current navigation errors applied to truth

bool crossTrans¶: — Have position error depend on velocity

bool crossAtt¶: — Have attitude depend on attitude rate

EphemerisMsgPayload truePlanetState¶: planet ephemeris msg without noise

EphemerisMsgPayload noisePlanetState¶: planet ephemeris msg with noise

ReadFunctor<EphemerisMsgPayload> ephemerisInMsg¶: planet ephemeris input msg

Message<EphemerisMsgPayload> ephemerisOutMsg¶: planet ephemeris output msg

BSKLogger bskLogger¶: — BSK Logging

Private Members

Eigen::MatrixXd AMatrix¶: — The matrix used to propagate the state

GaussMarkov errorModel¶: — Gauss-markov error states

uint64_t prevTime¶: — Previous simulation time observed