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From YouTube: Simple Orbit Tutorial | GMAT (NASA's General Mission Analysis Tool)

Description

In this video, we're going to do a simple orbit tutorial for the General Mission Analysis Tool (GMAT) software from NASA. Some of the topics that we'll cover is changing the properties of spacecraft, of propagators, of the orbit viewer, adding additional events to the mission sequence, animating the orbit, and panning around in the orbit viewer to get a better shot of the orbit of course.

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Channel Description:
Since the beginning, man had looked to the sky only to have remained earth-shackled. Aerospace engineering changed all that in a very short 100+ years. The airplane has come a long way since the 1903 Wright Flyer and aerospace engineering along with it. If you look at a modern-day fighter jet it’s no wonder that nowadays aerospace engineering curriculum can be a daunting task. This leads to 19 credit hour semesters being the norm, fast pace classes, missed content, less interactive lessons, lower comprehension due to shorter discussions, and never enough time to read the textbooks. Combine this with thick accents, difficult concepts, and professors who care more about research than teaching and engineering education turns into a game of stamina instead of a game of wit. At the Science of Flight it is believed that excellence in engineering starts with excellent education and tSoF is dedicated to improving aerospace engineering education for the student as well as the working professional.

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In this video, we're going to do a simple orbit tutorial for the general mission analysis.

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Tool, (gmat) software from nasa..

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Some of the topics that we'll cover is changing the properties of spacecraft, of propagators,.

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Of the orbit viewer, adding additional events to the mission, sequence, animating, the orbit,.

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And panning around in the orbit viewer to get a better shot of the orbit of course.

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Let's get to it.

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For the first step,, let's go ahead and start the gmat (general mission, analysis, tool), application.,.

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So, look for a shortcut like this.

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when you start it up, it's going to open up the welcome page.

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You can go ahead and click that “x” in the top right to close the welcome page.

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If this is the first time that you're opening up gmat (general mission, analysis, tool) it'll,.

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Have some default objects and default properties already opened up in the project viewer on.

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The left. so,, let's start editing some of those objects for the tutorial that we want to do.

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Starting with the spacecraft, right-click on “defaultsc” in the spacecraft, folder,.

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And when you right-click on that you'll see the menu pop up, and we're going to click.

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“Rename”, and that's going to open up this window here with an edit text, box.

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We're going to type in the new name, and so let's just go ahead and call that “sat1”.

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For satellite 1., so, type that in the box and click “ok”.

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Now that we renamed the satellite,, let's go ahead and open up the spacecraft properties, window.

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You can do that by double-clicking on the object in the left project, viewer, window.,.

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We renamed it sat1, so find sat1 and double-click on it.

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With the properties window, open, you'll notice that it has a bunch of tabs at the top.

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Each of those tabs have a group of different properties of the spacecraft.

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For this tutorial, we're going to just focus on a simple orbit, and so we're going to stay.

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Here on the “orbit” tab and adjust those parameters, only.

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In future, videos we'll cover some of the other tabs and properties in those tabs.

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Let's get back to the tutorial and entering in the orbital elements.

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As a reminder,, we're on the spacecraft properties window for sat1.

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We're first going to start with specifying the epoch format and the actual epoch.

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On the orbit tab,, if you look on the left,, the first selection box gives you the option.

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To choose the format that you want to specify the epoch.

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For our tutorial,, we're going to use “utcgregorian” so find that in the list and click on it.

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With the format specified,, we can go ahead and actually enter in the epoch. Time.

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Now for utc gregorian, it's very important to get the format right.

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This basically takes the text entered into epoch and breaks it up into the different.

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Pieces, so if you get format wrong, it's going to have an error and it will tell you if you.

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Entered it, incorrectly. so, go ahead and choose your epoch. Time.

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Moving on to the other orbital parameters or orbital elements, you'll notice that the.

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Default state type is “cartesian”., we're actually wanting to specify our orbit using “keplerian”, so in the selection.

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Box for state type go ahead and find keplerian and select it.

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You'll see that the values in the right column, change from cartesian elements to keplerian.

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Elements and as you can see, they have the units there on the right of the box. So you.

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Know which units to use. so, go ahead and fill in the orbital parameters and then, when you're done with that click, “ok”.

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With that, we're done editing the spacecraft properties.

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Let's move down and start editing the default propagator’s properties.

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We're first going to change the name of the “defaultprop”, so in the tree on the left.

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(If, the propagators folder is minimized, go ahead and click on the “+” sign. Next to.

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It to expand it out so that you can see the default propagator) right-click on it and.

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Then in the menu that appears go ahead and click “rename”.

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That will open up a window with a edit text field where you can type in the new name.

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I'm going to go ahead and call this one. “Earthpointprop”.

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so,, when you typed in the new name, go ahead and click “ok” and you'll notice in the.

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Tree on the left that the name has been updated.

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So, now we're going to go ahead and double click on it. In the tree view to open up the.

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Propagator properties window.

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this window is split into two columns.. The first column contains different variables or settings for the integrator of the propagator.

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And the second column contains the parameters or variables or settings for the force modeling.

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That gmat (general mission analysis, tool) does.

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For this tutorial, we're going to leave the integrator properties alone and just let it.

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Use the default values.

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we're also going to leave the central body on “earth”, because that's what we want.

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To orbit around. however,, we do need to change the primary body from “earth” to “none”.

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This is because we are doing a point mass example, and we don't want to have,, or rather.

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We don't care, about,, a gravity model or a drag model.

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So,, when you click “none”, you'll notice, that the gravity and drag models get grayed out.

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Now,, we need to add earth to the list of point masses that the propagator, or rather force.

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Model, takes into account., in the right hand, column towards the bottom, you'll notice that there is a button that.

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Has the label “select” on it next to the point masses, area.

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If you click on that,, it will open up this window.

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This window is used to specify which of the available bodies you want to use as a point.

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Mass in your example. to use something out of the list of available bodies in the point mass force modeling,, we.

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Need to move it from the “available bodies” column to the “selected bodies” column.

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So, to add a body to the model, we need to find it here in the left column and then click.

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This top button (which is a right arrow) to move it from the “available bodies” to.

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The “selected bodies”., if we wanted to remove a point mass from our force model,, we would select it here in the.

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Right column, in the “selected, bodies” column, and then click the left arrow to move.

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It back into the “available bodies” list.

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In this tutorial, we're just going to have earth as our point mass, so select, “earth”.

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And deselect anything else that might already be in the “selected bodies” column and.

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When you're done with that, go ahead and click “ok”., when you do that, you'll notice that the point masses that are being modeled shows up in.

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A list next to the button that you clicked to open the window.

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And that's going to be it for the propagator properties, so go ahead and click “ok”.

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On this window to save the changes to the propagator.

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The default object has two other output objects that will appear when we actually run the project.

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I show them here on the left..

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The orbit view is a 3d modeler that allows us to zoom in, zoom out, allows us to pan.

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Or rotate, to get a better view of the body, we're orbiting, of the body that is being.

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Orbited, of the orbit path..

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It also allows us to hit play and see an animation of our project.

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The bottom one is a ground track plot that allows us to see how our satellite flies over.

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The earth. we're going to update some of the properties of the orbit viewer (the top one here).

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So, in the tree on the left, under the output folder, if you expand that you should see a.

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“Defaultorbitview”.

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go ahead and double-click on that, and that will open up the properties window for that.

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Object and you'll see this window.

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there's a lot going on in this window and it's all about making that orbit. Viewer.

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We have different options for how we plot things..

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We have different options for what objects we view or where we view from (this would.

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Be adjusting the camera), along with some other drawing options.

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So,, let's go ahead and on the left column we're going to go ahead and turn off the “xy plane”.

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You don't really need to do that.. I just think it looks better with it turned off,, at least for earth.

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We're also going to go to the top right and adjust what objects we view.

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Of course,, we want to make sure that the spacecraft that we're actually doing this project for.

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Is shown in this so go ahead and make sure our spacecraft, or sat1, gets moved from the.

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List of “available spacecraft” into the “selected spacecraft”.

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Likewise, we're going to go ahead and include “earth” as a plotted, celestial object.

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You can add other ones in there if you want to, but we're just going to do earth for right.

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Now and then click “ok” when you're done with that.

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In a later video,, I'm going to go ahead and talk about the “view definition” and how.

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We can move the camera around to see better shots of the orbit.

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So, stay tuned for that video..

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Up to this point, we've been working on the “resources” tab, which allows us to add.

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Objects and modify their properties.

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however, now we're going to switch gears and we're going to work on the “mission” tab.

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And start specifying what we want the objects to actually do. But first, let's start with.

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An example. right here I have a picture of the apollo 8 lunar mission, sequence.

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Apollo 8 was an orbital mission around the moon.

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so,. As you can see in this mission sequence,, the first part is launching into the earth.

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Orbit and then from there they’re going to do a burn to establish the earth orbit.

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On looks like number 2 there., then on number 3 they do a burn to go to a translunar, trajectory,, meaning from earth.

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To the moon. then,, when they get to the moon,, they do another burn to go into a lunar. Orbit.

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Then they sat there for a bit and did whatever they need to. While they were orbiting, it.

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When they were done,, they went ahead and went into another burn to leave the lunar orbit.

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And head back to earth., that's number 6, there. and then, when they were getting close to earth of course, on number 7 is when they.

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Start doing their burns to re-enter the earth's atmosphere and land, the craft.

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So,, as you can see by this mission, sequence, really there's multiple different. If you want.

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To call them “orbits”,, there's different burns,, there's different actions that are.

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Going on for the entire mission sequence., and so that's what we are wanting to do here on the “mission” tab- is we're wanting.

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To build all of these different actions in the proper order, so that when we simulate.

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Our project we get the correct behavior from our spacecraft.

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And so, from our example,, it shows seven different events in the mission sequence.

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If we're transferring that over into gmat (general mission, analysis, tool), we'd, have.

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Seven different items here in the mission sequence in order to try and build out that.

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Apollo 8 lunar mission, and maybe we'll do that for a future video just as a tutorial.

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That'd be pretty fun., so, go ahead and click on the “mission” tab and, let's start building out our mission. Sequence.

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So,: let's go ahead and start editing the mission sequence in our simulation.

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The default simulation already has an event in there called “propagate1”.

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Let's go ahead and double-click that to open up the properties window and when you'll do.

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That you'll see a couple things in here.

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you'll, see in the “spacecraft list”. What object the event is for, and with its propagator.

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You'll see how it's going to move that object, and then towards the bottom. You'll have the.

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List of stopping conditions.

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the way that it works is that if any of these become true, it will stop that event and move.

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To the next event in your list. so,, if you have a dozen different stopping conditions, whichever one happens, first is.

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What's going to stop it and move it on to the next event.

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Then, if you look at the bottom, you have an option to override what color is plotted in.

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The orbit view- and this is helpful when you have multiple events- you can see what those.

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Events look like compared to other events in your mission, sequence.

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So,: let's go ahead and start updating this event for our tutorial and using the “earthpointprop”.

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To propagate the sat1 is what we want, but we're going to go ahead and change the stopping.

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Conditions for this event., if you look, the initial stopping condition, is “elapsed seconds” with a value of looks.

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Like 12,000., this would cause our satellite to orbit the earth using the “earthpointprop” for 12,000.

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Seconds until it would stop and move on to the next event., what we're going to do is we're going to change the stopping condition.

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To do that,, let's click on these dots to the left of the parameter we want to use for our.

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Stopping condition and when you click on those three dots, it's going to open up this dialog.

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Here, where it will allow us to choose the object and parameter for the stopping condition.

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Some of these parameters, you need to specify what the “central body” is.

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For instance, periapsis is the closest that the orbiting object comes to the central body.

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And so we need to specify what central body we're talking. About.

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On this sim, it would be “earth”.

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so,, go ahead and make sure you select sat1 as the object and periapsis as the parameter.

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That we're wanting to use as the stopping condition and then use the right arrow to.

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Move it to the “selected values” and then go ahead and click “ok”.

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When you do that, you'll notice that the list of stopping conditions is updated and that.

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Periapsis doesn't have a condition to check for.

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This is because this particular parameter is a location, parameter, right,. So that means.

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That it will stop this event in the mission sequence when it gets to that location and.

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Then move on to the next event. so, when you're done changing the stopping condition, click “ok” on the “propagate1”.

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Properties window. for this simulation, we're going to actually add another event to our mission. Sequence.

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If you right-click, on “propagate1” and on the pop-up menu, if you choose “insert,.

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After”, it will give you a list of different events that you can choose.

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Go ahead and find “propagate” down there towards the bottom and click on it.

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When you do that, you'll see that in our mission sequence, a second event has been added called.

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“Propagate2”. so,, the way this mission sequence is going to work is that it will run “propagate1”.

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Until it encounters the first stopping condition that becomes true and then it'll run, the “propagate2”.

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Event until it reaches its first stopping condition.

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So,: let's go ahead and double-click on that “propagate2” and update its stopping condition.

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And properties.

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When the property window opens for the second event, again, you'll see that it shows the.

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Spacecraft sat1 and propagator “earthpointprop”. So it's going to move the sat1 using the “earthpointprop”,.

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Which is what we're looking for. and the default, stopping condition again is elapsed seconds and that's what we want to.

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Change so go ahead and click on those three dots to the left of “sat1.elapsedsecs”.

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And that will open up this dialog. for the second event,, we're going to choose the stopping condition as the location of apoapsis.

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So, in the “object list” we're wanting to create the stopping condition we're going.

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To choose sat1. we're going to scroll down “object properties” to find apoapsis and again, since this is a.

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Location of which the satellite is furthest from the body, it's orbiting, (the central body).

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We need to specify what that “central body” is for the stopping condition which is earth.

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So, make sure that says earth then use the right arrow to pass that parameter to the.

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“Selected values” we want to use as a stopping condition.

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If there's anything else in that “selected values” list use the left arrow to remove.

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It from the list and when you're done with that click, “ok” and you'll notice in the.

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List of stopping conditions that it changed from “sat1.elapsedsecs” to “sat1.earth.apoapsis”.

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And since it's a location,, there is no stopping condition. Just whenever the satellite gets.

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To that location, it's going to stop.

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and in this one, just for this tutorial,, so we can see the difference between the two.

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Events and how they work together. I went ahead and chose to override the color for that segment.

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And make it bright, green and then go ahead and click “ok” on this “propagate2”.

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Window to save the changes that we made to this event in the mission sequence.

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Alright,, we're done making changes and updating both the objects and the events that we want.

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To occur, and so now comes the point where we want to actually run our project to simulate.

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The mission sequence. so, to run the simulation. You have the buttons here in the toolbar.

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You've got the play: button, there's the pause, button, and stop button, and so, if you hit.

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Play then you'll notice that these two windows appear inside of the workspace area and those.

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Appear because we had them in the “outputs” folder.

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We had a default orbit viewer that we just did the properties for then we had the default.

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Ground track plot. after the simulation has finished, running, we're actually able to animate the results.

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To see the satellite travel over the ground track plot or to see the satellite orbit around.

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The earth to visualize the mission sequence.

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And we'll check that on the next slide, but real quick, there's also here on the top right.

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This “gui modified”.- this simply means that you've made changes to the script file using this gui. But you.

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Haven't saved it yet so before you close out of any of this, after you're done, checking.

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Out the results, make sure that you save it. If that's what you're wanting to do.

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So, I went ahead and made the orbit viewer bigger and I'm going to click the animation.

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Button a couple times here so that you can see the satellite moving around the earth.

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I'll go ahead and pan it and rotate to get a better view of the orbit and click animation. Again.

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So,, as you can see, there's that red loop around that it does that's the first event.

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And then, when that event hits its stopping condition at periapsis, the second event starts.

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And that's where we're getting that green line that goes to apoapsis.

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And, of course, I can zoom in closer, and I can zoom out just to get a better shot, not.

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Only of the orbiting object, but also the object being orbited.

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So,, this is pretty cool., go ahead and play with it and check it out and that's where I'm going to end it for this video.

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If you liked what you saw- and you want to see some more: like, comment, subscribe.

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I'll see you in the next video.