flybyODuKF

Executive Summary

This module filters position measurements that have been processed from planet images in order to estimate spacecraft relative position to an observed body in the inertial frame. The filter used is a square root unscented Kalman filter, and the images are first processed by image processing and a measurement model in order to produce this filter’s measurements.

The module inherits from the SRuKF interface and implements only the few virtual methods.

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
navTransOutMsg	NavTransMsgPayload	navigation translation output message
opNavFilterMsg	FilterMsgPayload	output filter data message containing states and covariances
opNavResidualMsg	FilterResidualMsgPayload	output measurement data message containing residuals
opNavHeadingMsg	OpNavUnitVecMsgPayload	opnav input message containing the unit vector towards the target

Module models

The measurement model for the filter is functionally contained in the center of brightness converter module. The message read in therefore contains the predicted measurement:

\[H[X] = \frac{X[0:3]}{\|X[0:3]\|} = \hat{r}\]

The dynamics modules used are point mass, single body gravity, propagated with and RK4 integrator

\[F[X] = \dot{X} = - \frac{\mu}{X[0:3]^3}X[0:3] = - \frac{\mu}{r^3}r\]

where \(r\) is the position of the spacecraft center of masss relative to the central body of gravitational parameter \(\mu\).

Module assumptions and limitations

Interface methods implemented

Method Name	Method Function	Class specifics
customReset	perform addition reset duties in child class	assert message link and convert gravitational parameter units
readFilterMeasurements	read the specific measurements in a child class	read opnavHeading message
writeOutputMessages	write the specific measurements in a child class	write Nav message and reconvert units
measurementModel	add a measurement model for the inputs in child class	read opnav heading message and normalize
propagate	add a dynamics model for the inputs in child class	use two body gravity and and rk4 to propagate

User Guide

This section is to outline the steps needed to setup a flybyODSRuKF converter in Python.

1. Import the module:

   from Basilisk.fswAlgorithms import flybyODuKF
   

2. Create an instantiation of converter class:

   flybyOD = flybyODuKF.FlybyODuKF()
   

3. Setup SRuKF general parameters:

   flybyOD.alpha = 0.02
   flybyOD.beta = 2.0
   

4. Setup SRuKF measurement parameters:

   flybyOD.muCentral = 1
   flybyOD.measNoiseScaling = 1
   

5. Setup initial state and covariances:

   flybyOD.stateInitial = [[1000.*1e3], [1000.*1e3], [1000.*1e3], [0.], [1.*1e3], [0.]]
   flybyOD.covarInitial =[ [10., 0., 0., 0., 0., 0.],
                            [0., 10., 0., 0., 0., 0.],
                            [0., 0., 10., 0., 0., 0.],
                            [0., 0., 0., 0.01, 0., 0.],
                            [0., 0., 0., 0., 0.01, 0.],
                            [0., 0., 0., 0., 0., 0.01]]
   

6. Setup process noise:

   sigmaPos = 0.01
   sigmaVel = 0.0001
   flybyOD.processNoise = [[sigmaPos, 0., 0., 0., 0., 0.],
                     [0., sigmaPos, 0., 0., 0., 0.],
                     [0., 0., sigmaPos, 0., 0., 0.],
                     [0., 0., 0., sigmaVel, 0., 0.],
                     [0., 0., 0., 0., sigmaVel, 0.],
                     [0., 0., 0., 0., 0., sigmaVel]]
   

7. Subscribe to the messages, primarily the measurement message:

   flybyOD.opNavHeadingMsg.subscribeTo(cobConverter.opnavUnitVecOutMsg)
   

Class FlybyODuKF

class FlybyODuKF : public SRukfInterface

Top level structure for the flyby OD unscented kalman filter. Used to estimate the spacecraft’s inertial position relative to a body.

Public Functions

void setCentralBodyGravitationParameter(double mu)

Set the gravitational parameter used for orbit propagation

Parameters:

double – muInput

Returns:

void

double getCentralBodyGravitationParameter() const

Get gravitational parameter used for orbit propagation

Returns:

double muCentral