Propagate

• The Spacecraft to be propagated

• The model(s) used for the propagation (numerical integration, ephemeris interpolation)

• The condition(s) to be satisfied at the termination of propagation

• The direction of propagation (forwards or backwards in time)

• The time synchronization of multiple Spacecraft

• Propagation of STM and computation of state Jacobian (A-matrix)

• Setting unique colors on different Spacecraft trajectory segments through Propagate commands

Introduction

The Propagate command GUI provides an interface to assign Spacecraft to Propagators used for propagation and to define a set of conditions to terminate propagation. The GUI also allows you to define the direction of propagation, the synchronization mode for multiple spacecraft, and whether or not to propagate the STM and compute the A-Matrix.

To follow the examples below, you can load the following script snippet or create a new mission with three spacecraft (named sat1, sat2, and sat3) and two propagators (named prop1 and prop2).

Create Spacecraft sat1 sat2 sat3
Create Propagator prop1 prop2
BeginMissionSequencer

Defining Spacecraft and Propagators

To demonstrate how to define a set of propagators and Spacecraft for propagation, you will set up a Propagate command to propagate a Spacecraft named sat1 using a Propagator named prop1 and Spacecraft named sat2 and sat3 using a Propagator named prop2. You will configure the command to propagate for 1 day or until sat2 reaches periapsis, whichever happens first. You will need to configure GMAT as described in the the section called “Introduction” section and add a new Propagate command to your mission sequence. GMAT auto-populates the Propagate command GUI with the first Propagator in the GUI list and the first Spacecraft when you add a new Propagate command so you should start from this point.

To add a second Propagator to propagate sat2 and sat3 using prop2:

Stopping conditions

Continuing with the example above, now you will configure GMAT to propagate for one elapsed day or until sat2 reaches periapsis.

The Propagate1 dialog should now look like the image below.

Introduction

The Propagate command documentation below describes how to propagate single and multiple Spacecraft to desired conditions forward and backwards in time. To streamline the script examples, the objects numSat, spkSat, numProp, and spkProp are assumed to be configured as shown below. GMAT is distributed with the SPK kernels used in the examples.

          Create Spacecraft spkSat;
spkSat.Epoch.UTCGregorian   = '02 Jun 2004 12:00:00.000'
spkSat.NAIFId               = -123456789;
spkSat.OrbitSpiceKernelName = {'..\data\vehicle\ephem\spk\GEOSat.bsp'};

Create Spacecraft numSat
numSat.Epoch.UTCGregorian = '02 Jun 2004 12:00:00.000'

Create Propagator spkProp;
spkProp.Type       = SPK;
spkProp.StartEpoch = FromSpacecraft

Create Propagator numProp
numProp.Type = PrinceDormand78

BeginMissionSequence
        

How to Propagate a Single Spacecraft

Note: See the the section called “Introduction” section for a script snippet to configure GMAT to execute the examples in this section.

The Propagate command provides a simple interface to propagate a Spacecraft to a stopping condition or to take a single propagation step. To propagate a single Spacecraft you must specify the desired Propagator, the Spacecraft to propagate, and if desired, the stopping condition. The Propagate command supports numerical integrator and ephemeris type propagators. For single Spacecraft propagation, the syntax is the same regardless of propagator type. For example, to propagate a Spacecraft using a numerical integrator, you can use the following script snippet:

          Propagate numProp(numSat){numSat.Periapsis}
% or
Propagate numProp(numSat,{numSat.Periapsis})
        

To propagate a single Spacecraft using a Propagator configured to use an SPK kernel use the following:

          Propagate spkProp(spkSat){spkSat.TA = 90}
% or
Propagate spkProp(spkSat,{spkSat.TA = 90})
        

To take a single propagation step, simply omit the stopping conditions as shown below. The Propagator will take a step based on its step size control algorithm. See the Propagator documentation for more information on step size control.

          Propagate numProp(numSat)
% or
Propagate spkProp(spkSat) 
        

How to Propagate Multiple Spacecraft

The Propagate command allows you to propagate multiple Spacecraft by including a list of Spacecraft in a single Propagator, by including a Formation in a Propagator, and/or by including multiple Propagators in a single command. For example purposes, here is a script snippet that propagates multiple Spacecraft.

          Propagate Synchronized Prop1(Sat1,Sat2) Prop2(Sat3,Sat4)...
Prop3(aFormation){Sat1.Earth.Periapsis}
        

In the script line above Sat1 and Sat2 are propagated using Prop1; Prop2 is used to propagate Sat3 and Sat4; all Spacecraft added to aFormation are propagated using Prop3. The Propagate command configured above propagates all Spacecraft until Sat1 reaches Earth periapsis.

All Spacecraft propagated by the same Propagator are time synchronized during propagation. By time synchronization, we mean that all Spacecraft are propagated across the same time step. The Synchronized keyword tells GMAT to keep Spacecraft propagated by different Propagators synchronized in time during propagation. Time synchronization among multiple Propagators is performed by taking a single step for all Spacecraft controlled by the first Propagator (Prop1 in the above example), and then stepping all other Propagators to that time. When the Synchronized keyword is omitted, Spacecraft propagated by different Propagators are not synchronized in time. In that case, each Propagator takes steps determined by its step size control algorithm without regard to the other Propagators in the Propagate command. Time synchronization is particularly useful if you need ephemeris files for multiple spacecraft with consistent time tags, or if you are visualizing multiple spacecraft in an OrbitView.