## Circular Orbit Question

Post here for assistance in troubleshooting a problem using GMAT

### Circular Orbit Question

Hi,

I'm trying to create a circular orbit around the Earth at ~300km. To set this up I have chosen the keplarian option under the 'state type' with a SMA of 6678km, an eccentricity of 0 and an inclination of 0. I have 2 propagate commands under the mission tree, the first is a prop to periapsis followed by a prop to apoapsis.

When I run this the command summary gives contradictory periapsis altitudes. On the prop to perigee command summary I get a 280km altitude and the prop to apoapsis returns a 300km periapsis altitude.

I am new to GMAT and I imagine this is a trivial issue but would be thankful for an explanation and a resolution.

Thanks alot,

Neil
Neil

Posts: 2
Joined: Thu Apr 10, 2014 11:07 am

### Re: Circular Orbit Question

This probably has to do with the force model.

When you report "periapsis altitude", it's calculating the two-body periapsis altitude, calculated from the osculating elements at the current epoch. But you're propagating in a complex force model with non-spherical central body gravity, so your actual periapsis altitude will be different.

To confirm, set your propagator to use two-body gravity (set Primary Body to "None", and add Earth as a Point Mass at the bottom). Then rerun your script. Note that you'll need to give it some small eccentricity (0.00001 or such) so that periapsis and apoapsis are actually defined.
Joel J. K. Parker
Flight dynamics engineer, GMAT team
http://gmatcentral.org
jjkparker

Posts: 617
Joined: Thu Jan 07, 2010 9:48 pm
Location: NASA Goddard Space Flight Center, Greenbelt, MD

### Re: Circular Orbit Question

Hi,

I see - I've made the changes and getting the expected result.

Thank you very much - impressed with the quick response!

Neil
Neil

Posts: 2
Joined: Thu Apr 10, 2014 11:07 am