EclipseLocator

Data configuration

The EclipseLocator implementation is based on the NAIF SPICE toolkit, which uses a different mechanism for environmental data such as celestial body shape and orientation, planetary ephemerides, body-specific frame definitions, and leap seconds. Therefore, it is necessary to maintain two parallel configurations to ensure that the event location results are consistent with GMAT's own propagation and other parameters. The specific data to be maintained is:

See SolarSystem and CelestialBody for more details.

Search interval

The EclipseLocator search interval can be specified either as the entire ephemeris interval of the Spacecraft, or as a user-defined interval. If UseEntireInterval is true, the search is performed over the entire ephemeris interval of the Spacecraft, including any gaps or discontinuities. If UseEntireInterval is false, the provided InitialEpoch and FinalEpoch are used to form the search interval directly. The user must ensure than the provided interval results in valid Spacecraft and CelestialBody ephemeris epochs.

Run modes

The EclipseLocator works in conjunction with the FindEvents command: the EclipseLocator resource defines the configuration of the event search, and the FindEvents command executes the search at a specific point in the mission sequence. The mode of interaction is defined by EclipseLocator.RunMode, which has three options:

Search algorithm

The EclpseLocator uses the NAIF SPICE GF (geometry finder) subsystem to perform event location. Specifically, the following call is used for the search:

This function implements a fixed-step search method through the interval, with an embedded root-location step if an event is found. StepSize should be set equal to the length of the minimum-duration event to be found, or equal to the length of the minimum-duration gap between events, whichever is smaller. To guarantee location of 10-second eclipses, or 10-second gaps between adjacent eclipses, set StepSize = 10.

For details, see the reference documentation for the function linked above.

Report format

When WriteReport is enabled, the EclipseLocator outputs an event report at the end of each search execution. The report contains the following data:

The report makes the distinction between an individual event and a total event.

Event location with SPK propagator

When using the SPK propagator, you load one or more SPK ephemeris files using the Spacecraft.OrbitSpiceKernelName field. For the purposes of event location, this field causes the appropriate ephemeris files to be loaded automatically on run, and so use of the Propagation command is not necessary. This is an easy way of performing event location on an existing SPK ephemeris file. See the example below.

Perform a basic eclipse search in LEO:

SolarSystem.EphemerisSource = 'DE421'

Create Spacecraft sat
sat.DateFormat = UTCGregorian
sat.Epoch = '15 Sep 2010 16:00:00.000'
sat.CoordinateSystem = EarthMJ2000Eq
sat.DisplayStateType = Keplerian
sat.SMA = 6678.14
sat.ECC = 0.001
sat.INC = 0
sat.RAAN = 0
sat.AOP = 0
sat.TA = 180

Create ForceModel fm
fm.CentralBody = Earth
fm.PrimaryBodies = {Earth}
fm.GravityField.Earth.PotentialFile = 'JGM2.cof'
fm.GravityField.Earth.Degree = 0
fm.GravityField.Earth.Order = 0
fm.GravityField.Earth.TideModel = 'None'
fm.Drag.AtmosphereModel = None
fm.PointMasses = {}
fm.RelativisticCorrection = Off
fm.SRP = Off

Create Propagator prop
prop.FM = fm
prop.Type = RungeKutta89

Create EclipseLocator el
el.Spacecraft = sat
el.Filename = 'Simple.report'
el.OccultingBodies = {Earth}
el.EclipseTypes = {'Umbra', 'Penumbra', 'Antumbra'}

BeginMissionSequence

Propagate prop(sat) {sat.ElapsedSecs = 10800}

Perform an eclipse event search from a Mars orbiter, with Phobos, Earth, and Moon eclipses:

          % Mars orbiter with annular eclipses of Earth and Moon.

SolarSystem.EphemerisSource = 'SPICE'
SolarSystem.SPKFilename = 'de421.bsp'

Mars.NAIFId = 499
Mars.OrbitSpiceKernelName = {'../data/planetary_ephem/spk/mar063.bsp'}

Create Spacecraft sat
sat.DateFormat = UTCGregorian
sat.Epoch = '10 May 1984 00:00:00.000'
sat.CoordinateSystem = MarsMJ2000Eq
sat.DisplayStateType = Keplerian
sat.SMA = 6792.38
sat.ECC = 0
sat.INC = 45
sat.RAAN = 0
sat.AOP = 0
sat.TA = 0

Create ForceModel fm
fm.CentralBody = Mars
fm.PrimaryBodies = {Mars}
fm.GravityField.Mars.PotentialFile = 'Mars50c.cof'
fm.GravityField.Mars.Degree = 0
fm.GravityField.Mars.Order = 0
fm.Drag.AtmosphereModel = None
fm.PointMasses = {}
fm.RelativisticCorrection = Off
fm.SRP = Off

Create Propagator prop
prop.FM = fm
prop.Type = RungeKutta89

Create CoordinateSystem MarsMJ2000Eq
MarsMJ2000Eq.Origin = Mars
MarsMJ2000Eq.Axes = MJ2000Eq

Create Moon Phobos
Phobos.CentralBody = 'Mars'
Phobos.PosVelSource = 'SPICE'
Phobos.NAIFId = 401
Phobos.OrbitSpiceKernelName = {'mar063.bsp'}
Phobos.SpiceFrameId = 'IAU_PHOBOS'
Phobos.EquatorialRadius = 13.5
Phobos.Flattening = 0.3185185185185186
Phobos.Mu = 7.093399e-004

Create Moon Deimos
Deimos.CentralBody = 'Mars'
Deimos.PosVelSource = 'SPICE'
Deimos.NAIFId = 402
Deimos.OrbitSpiceKernelName = {'mar063.bsp'}
Deimos.EquatorialRadius = 7.5
Deimos.SpiceFrameId = 'IAU_DEIMOS'
Deimos.Flattening = 0.30666666666666664
Deimos.Mu = 1.588174e-004

Create EclipseLocator ec
ec.Spacecraft = sat
ec.OccultingBodies = {Mercury, Venus, Earth, Luna, Mars, Phobos, Deimos}
ec.Filename = 'EarthTransit.report'

BeginMissionSequence

Propagate prop(sat) {sat.ElapsedDays = 2}

        

Perform eclipse location on an existing SPK ephemeris file:

          
            SolarSystem.EphemerisSource = 'DE421'

Create Spacecraft sat
sat.OrbitSpiceKernelName = {'../data/vehicle/ephem/spk/Events_Simple.bsp'}

Create EclipseLocator cl
cl.Spacecraft = sat
cl.OccultingBodies = {Earth}
cl.Filename = 'SPKPropagation.report'

BeginMissionSequence