C++ Module: planetEphemeris

Executive Summary

The planetEphemeris module uses classical heliocentric orbit elements to specify a planet’s position and velocity vectors.

By default, every output position and velocity is heliocentric. The optional zeroBase setting translates all output states to one of the bodies propagated by the module. This provides the same translational-origin concept as spiceInterface.zeroBase while retaining J2000-aligned coordinate axes.

Optionally, the planet’s orientation can also be specified through a right ascension angle, a declination angle, and a local sidereal time at epoch. The planet rotation about its third (polar) axis can be specified as well. If the planet attitude information is incomplete or missing, then an inertially fixed zero planet orientation is modeled. The module accepts a vector of classical orbit elements and orientation information and outputs a corresponding series of planet ephemeris messages. The module PDF Description contains further information on the model and its use.

Reference Origin Selection

Set zeroBase to a configured planet name to express every output position and velocity relative to that body:

ephemeris = planetEphemeris.PlanetEphemeris()
ephemeris.setPlanetNames(
    planetEphemeris.StringVector(["earth", "moon"])
)
ephemeris.planetElements = planetEphemeris.classicElementVector(
    [earthElements, moonElements]
)
ephemeris.zeroBase = "earth"

With this configuration, the Earth output has zero position and velocity, and the Moon output is the heliocentric Moon state minus the heliocentric Earth state. Planet names, timestamps, attitudes, and attitude rates are unchanged. Leaving zeroBase empty preserves the default heliocentric outputs.

Module Assumptions and Limitations

Warning

zeroBase only works for a body configured in the same PlanetEphemeris instance. Separate PlanetEphemeris instances do not exchange states. Selecting a name that was not supplied to setPlanetNames() stops initialization with an error.

The module propagates each body’s orbit independently as a two-body heliocentric orbit. The zeroBase translation changes only the output coordinate origin; it does not create a hierarchical or mutually coupled planet system.

User Guide

Configure all bodies that must share an output origin on one module instance, provide one classical-element set per body, and set zeroBase before initializing the simulation. Consumers of the resulting messages, including gravity models, navigation modules, and Vizard, must use states expressed relative to that same origin.

Message Connection Descriptions

The following table lists all module input and output messages. The user sets module message connections from Python. The message type links to the message structure definition, while the description explains its use.

Module I/O Messages
Msg Variable Name	Msg Type	Description
planetOutMsgs	SpicePlanetStateMsgPayload	vector of planet spice state output messages.

class PlanetEphemeris : public SysModel 

#include <planetEphemeris.h>

planet ephemeris class

Public Functions

PlanetEphemeris(): This constructor initializes the variables.

~PlanetEphemeris(): Module deconstructor

void Reset(uint64_t CurrentSimNanos)

void UpdateState(uint64_t CurrentSimNanos)

This update routine loops over all the planets and creates their heliocentric position and velocity vectors at the current time. If the planet orientation information is provided, then this is computed as well. The default orientation information is a zero orientation.

Parameters:: CurrentSimNanos – The current clock time for the simulation

void setPlanetNames(std::vector<std::string> planetNames): add list of planet names

Public Members

std::vector<Message<SpicePlanetStateMsgPayload>*> planetOutMsgs: — vector of planet state output messages

std::vector<ClassicElements> planetElements: — Vector of planet classical orbit elements

std::vector<double> rightAscension: [r] right ascension of the north pole rotation axis (pos. 3-axis)

std::vector<double> declination: [r] Declination of the north pole rotation axis (neg. 2-axis)

std::vector<double> lst0: [r] initial planet local sidereal time angle (pos. 3-axis)

std::vector<double> rotRate: [r/s] planet rotation rate

std::string zeroBase = {}: — Output origin body; empty preserves heliocentric states

BSKLogger bskLogger: — BSK Logging

Private Members

std::vector<std::string> planetNames: — Vector of planet names

double epochTime: [s] time of provided planet ephemeris epoch

int computeAttitudeFlag: — flag indicating whether planet orientation is provided

int zeroBaseIndex = -1: — index of the body selected as the translational origin