# Tag Info

Accepted

### Why does a satellite with a higher mass fall slower?

That's a great software-based experiment! What is this about? It's about drag and Newton's 2nd law of motion! $$F = \frac{dp}{dt} = ma$$ but in the context of orbital mechanics. We can re-arrange ...
• 149k

### Why does a satellite with a higher mass fall slower?

From Force=Mass*Acceleration for a given starting drag force (from the constant drag area, altitude and velocity) increasing the mass will reduce the acceleration (deceleration in this case) slowing ...
• 22.8k
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### How open is NASA's "open source" GMAT software?

GMAT has been released under the Apache license since R2013a, so I'd say it's about as wide open as you can get.
• 17.3k
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### Differences between numerical propagators

You should check to see what propagator GMAT is using. A constant step size RK4 method is going to drift from the true solution of the ODE over time, so if GMAT is using an adaptive step size method ...
• 1,403
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### Spherical solar radiation model (GMAT)

You can find a ton of information by looking in the "General Mission Analysis Tool (GMAT) Mathematical Specifications" (located at ...
• 5,291
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### Trying to run NASA GMAT on Ubuntu 20.04, I get the error "no main manifest attribute, in gmat.jar"

GMAT is written primarily in C++. gmat.jar is an api wrapper to allow java code to access the core functionality as explained at http://gmat.sourceforge.net/docs/...
• 301

### Modelling ISS orbit in GMAT

The reason my GMAT model was not giving the correct results was that I changed the Epoch without changing the TA - user @notovny pointed that out which was very helpful. I used TLE-tools to get the ...
• 141

### How to model the gravity of Titan in GMAT

A google search for "saturn titan gravity field" turns up Durante et al. 2019. Its Table 2 lists coefficients up to degree 5.
• 1,124

### GMAT - Locating inter-spacecraft contact instead of spacecraft to ground station

I am not familiar with GMAT, but there is another route to solve this challenge using an extensively validated open-source solution. You can use the Orekit Astrodynamics library to build one Moon-...
• 710
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### Why does targeting in the VNB frame lead to a different maneuver magnitude than targeting in an inertial frame?

It's not about the frames; you just haven't fully constrained the orbit that results from the burn. I was able to get the DVs to match using your script by just adding an inclination constraint. The ...
• 741
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### Earth geopotential effect on orbit propagation

To answer your questions, Do the terms (J2, etc.) change with time? Yes, they do. The Earth is still rebounding from the end of the last glaciation, and the Earth's rotation rate is ...
• 75.1k
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### GMAT: Finite burn targeting, solving for burn duration and quaternion

First off: Chris you may already know some/all of my response due to your background. I will attempt to both answer your GMAT question and provide insight to others who may not be as familiar with ...
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### User help forum for GMAT (General Mission Analysis Tool) users?

Yes, there is an alternative forum. Try https://gmat.atlassian.net/wiki/spaces/GW/overview . I just found this link today, by a contorted path through the help files of multiple obsolete versions at ...
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### Is there a parameter that describes the angle between the spacecraft's position and the sun angle or direction of the Earth's motion?

This question has some really good answers that explain gravity assists and how they could be conceived. If you're setting up a simulation, this report probably has everything you need, theory, ...
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### Brouwer-Lyddane mean semi major axis bias

I'll expand on my comment: I'm not familliar with the software, but if you start the satellite at the same everything except time (12 hours difference) then the satellite will be starting in a ...
• 149k
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### GMAT atmosphere

Here is a NASA repository of space weather data, including f10.7. https://omniweb.gsfc.nasa.gov/form/dx1.html I would bet GMAT gets its data there or a similar site. EDIT Apologies everyone for the ...
• 111

### Analysing a phasing maneuver using GMAT- how do I reliably quantify their "phase difference"

From what I understand, you need to ensure that both spacecraft are phased by 180 degrees. There are different orbital elements used to assess the phasing of spacecraft. The main one is the true ...
• 6,180

### Differences between numerical propagators

Your numerical integrator, RK4, is inappropriate for the task. The order is far too low for the accuracy you want. It's also fixed step size. You haven't done your research on this. Error ...

### Differences between numerical propagators

The previous answers contain some good general information, but TBH I don't find it very helpful to the question. The most fundamental sanity check with a numerical solver is: how does the result ...
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### Ноw to configure mission sequence for propagate until desired date in GMAT?

One way to do this is through some built-in GMAT function code. Here's a snip from a functional script: ...
• 96
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### Low-thrust maneuver validation in GMAT

One option is to build a "ThrustHistoryFile" to model your maneuver. That file contains a time history (in seconds) of either the thrust (in N) or the acceleration (km/s), and, optionally. ...
• 96

### Circularization of an orbit on gmat

Your approach seems wrong. In order to circularize an orbit, you have to raise your perigee until it's equal to your apogee. It is implausible to do that only by varying the duration. I suggest you ...
• 181
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### Change in True Anomaly over time

The jumps from near 0 to near 360 are just a branch cut. If the true anomaly were plotted from -180 to 180, the true anomaly would look more like a sine wave. Periapse, and therefore true anomaly, ...
• 816

### Difference between distance from Mars (Rmag) and B-plane R coordinate

A good overview of what the B-plane is and how it's defined is available from the FreeFlyer guide. The "B" in B-plane is the hyperbolic impact parameter, the semi-minor axis of the hyperbola ...
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### Error in Propagating equatorial orbit using GMAT

In the InternalODEModel of your script, you have J2 enabled. Set the degree to zero and the problem should be fixed. ...
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### How to design a lunar flyby trajectory in GMAT?

If you know how to use the Lunar B-plane to target a gravity assist, GMAT can target a Lunar Gravity Assist using its Differential Corrector, i.e. Target and Achieve BdotR and Achieve BdotT. B-Plane ...
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### How to design a lunar flyby trajectory in GMAT?

Are you required to use 2018a instead of 2020a? If not, I would recommend 2020a. The VF13 pluging allows for SQP optimization: this is needed because in a low-thrust optimization scenario, your ...
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### How to create a satellite intersection/conjunction in GMAT?

I had read many years ago in one book the rough empirical rule for orbital maneuvering and rendezvous: suppose that two spaceships follow the same traectory (circular) with distance between them, say, ...
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### How to create a satellite intersection/conjunction in GMAT?

For more complicated scenarios like these, you probably need to use GMAT's Python interface. I would recommend the following: Setup both spacecraft to be propagated together: I'm not sure this is ...
• 6,180