Module: locationPointing

Executive Summary

This module generates an attitude guidance message to make a specified spacecraft pointing vector target an inertial location. This location could be on a planet if this module is connected with Module: groundLocation for example, or it could point to a celestial object center using EphemerisMsgPayload, or a spacecraft location using NavTransMsgPayload.

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
scAttInMsg	NavAttMsgPayload	input msg with inertial spacecraft attitude states
scTransInMsg	NavTransMsgPayload	input msg with inertial spacecraft translational states
locationInMsg	GroundStateMsgPayload	input msg containing the inertial point location of interest
celBodyInMsg	EphemerisMsgPayload	(alternative) input msg containing the inertial point location of a celestial body of interest
scTargetInMsg	NavTransMsgPayload	(alternative) input msg with inertial target spacecraft translational states
attGuidOutMsg	AttGuidMsgPayload	output message with the attitude guidance
attRefOutMsg	AttRefMsgPayload	output message with the attitude reference

Detailed Module Description

The inertial location of interest is given by \({\bf r}_{L/N}\) and can be either extracted from locationInMsg when a location on a planet is provided, celBodyInMsg when a celestial body’s ephemeris location is provided (for pointing at the Sun or the Earth), or scTargetInMsg when pointing at another spacecraft. The vector pointing from the satellite location \({\bf r}_{S/N}\) to this location is then

\[{\bf r}_{L/S} = {\bf r}_{L/N} - {\bf r}_{S/N}\]

Let \(\hat{\bf r}_{L/S}\) be the normalized heading vector to this location.

The unit vector \(\hat{\bf p}\) is a body-fixed vector and denotes the body axis which is to point towards the desired location \(L\). Thus this modules performs a 2-degree of freedom attitude guidance and control solution.

The eigen-axis to rotate \(\hat{\bf p}\) towards \(\hat{\bf r}_{L/S}\) is given by

\[\hat{\bf e} = \frac{\hat{\bf p} \times \hat{\bf r}_{L/S}}{|\hat{\bf p} \times \hat{\bf r}_{L/S}|}\]

The principle rotation angle \(\phi\) is

\[\phi = \arccos (\hat{\bf p} \cdot \hat{\bf r}_{L/S} )\]

The attitude tracking error \({\pmb\sigma}_{B/R}\) is then given by

\[{\pmb\sigma}_{B/R} = - \tan(\phi/4) \hat{\bf e}\]

The tracking error rates \({\pmb\omega}_{B/R}\) are obtained through numerical differentiation of the MRP values. During the first module Update evaluation the numerical differencing is not possible and this value is thus set to zero.

Using the attitude navigation and guidance messages, this module also computes the reference information in the form of attRefOutMsg. This additional output message is useful when working with modules that need a reference message and cannot accept a guidance message.

Note

The module checks for several conditions such as heading vectors being collinear, the MRP switching during the numerical differentiation, etc.

User Guide

The one required variable that must be set is pHat_B. This is body-fixed unit vector which is to be pointed at the desired inertial location.

The user should only connect one location of interest input message, either locationInMsg, celBodyInMsg or scTargetInMsg. Connecting both will result in a warning and the module defaults to using the locationInMsg information.

This 2D attitude control module provides two output messages in the form of AttGuidMsgPayload and AttRefMsgPayload. The first guidance message, describing body relative to reference tracking errors, can be directly connected to an attitude control module. However, at times we need to have the attitude reference message as the output to feed to Module: attTrackingError. Here the B/R states are subtracted from the B/N states to obtain the equivalent R/N states.

The variable smallAngle defined the minimum angular separation where two vectors are considered colinear. It is defaulted to zero, but can be set to any desired value in radians.

By default this is a 2D attitude control module in attitude and a 2D rate control. In particular, the rates about the desired heading axis are not damped. By setting the module variable useBoresightRateDamping to 1, the body rates about about the desired heading angle are added to the rate tracking error yielding a 3D rate control implementation.

Functions

void SelfInit_locationPointing(locationPointingConfig *configData, int64_t moduleID)

This method initializes the output messages for this module.

Parameters:

configData – The configuration data associated with this module
moduleID – The module identifier

Returns:

void

void Update_locationPointing(locationPointingConfig *configData, uint64_t callTime, int64_t moduleID)

This method takes the estimated body states and position relative to the ground to compute the current attitude/attitude rate errors and pass them to control.

Parameters:

configData – The configuration data associated with the module
callTime – The clock time at which the function was called (nanoseconds)
moduleID – The module identifier

Returns:

void

void Reset_locationPointing(locationPointingConfig *configData, uint64_t callTime, int64_t moduleID)

This method performs a complete reset of the module. Local module variables that retain time varying states between function calls are reset to their default values. Check if required input messages are connected.

Parameters:

configData – The configuration data associated with the module
callTime – [ns] time the method is called
moduleID – The module identifier

Returns:

void

struct locationPointingConfig

#include <locationPointing.h>

This module is used to generate the attitude reference message in order to have a spacecraft point at a location on the ground.

Public Members

double pHat_B[3]: body fixed vector that is to be aimed at a location

double smallAngle: rad An angle value that specifies what is near 0 or 180 degrees

int useBoresightRateDamping: [int] flag to use rate damping about the sensor boresight

double sigma_BR_old[3]: Older sigma_BR value, stored for finite diff

uint64_t time_old: [ns] prior time value

double init: moudle initialization counter

double eHat180_B[3]: — Eigen axis to use if commanded axis is 180 from pHat

NavAttMsg_C scAttInMsg: input msg with inertial spacecraft attitude states

NavTransMsg_C scTransInMsg: input msg with inertial spacecraft position states

GroundStateMsg_C locationInMsg: input msg with location relative to planet

EphemerisMsg_C celBodyInMsg: input celestial body message

NavTransMsg_C scTargetInMsg: input msg with inertial target spacecraft position states

AttGuidMsg_C attGuidOutMsg: attitude guidance output message

AttRefMsg_C attRefOutMsg: attitude reference output message

BSKLogger *bskLogger: BSK Logging.