Question orbit mfd accuracy - Orbiter-Forum

apeonaut · 03-09-2011, 03:04 PM

hi all,

i sucessfully docked with the dg4 at the iss, so i basically know what to do. i tried to follow the shuttle procedure for docking. i aligned planes and started to catch the station on a lower orbit. for the rendevouz i tried to match my apoapsis with the station's apoapsis. but after my burn at the periapsis my actual height at the apoapsis is not as calculated (about 5-10km less) and the position of the apoapsis is also rotating all the time.
is there any way to do these maneuvers more accurately?

**C3PO** · 03-09-2011, 03:07 PM

Is "non-spherical gravity" on?

**Grover** · 03-09-2011, 03:07 PM

the reason behind the changes is probably advanced simulation, from things like non'spherical gravity sources and the like, turning them off will leave your orbit alone, and it wont decay over time like it does in real life (best to do this as you learn, then put it on when you know your way around)

good luck! (and welcome to the forum i guess)

EDIT: Ninja'd

apeonaut · 03-09-2011, 03:28 PM

yes, 'non-spherical gravity' is on and turning it off feels a bit like cheating to me.

i think transx could to the job much better as the rather simple orbit mfd. i'll give it a shoot after the shuttle landing.

**C3PO** · 03-09-2011, 03:30 PM

TransX doesn't account for it either, sorry.

**Urwumpe** · 03-09-2011, 04:00 PM

Please note that Orbit MFD displays so-called "osculating elements". That elements are only a local approximation with Kepler orbit elements for the real trajectory.

This means: The data displays and the ellipse painted are not the real trajectory, but only an approximation, that is only valid in your current position. By having many short osculating element segments in series, you can approximate the real trajectory.

These osculating elements are calculated pretty simple: you just assume that there is only this planet and you and you have almost no mass, also this planet is just a point source of gravity (no non-spherical gravity, no multi-body problems). The more these factors have an effect on you (eg near Lagrangian points or at the rim of the sphere of influence), the bigger the difference between osculating elements and true elements.

Also, maybe it gives you a better feeling, that in real spaceflight, the problem to calculate the orbit trajectory is not much easier. There are some correction terms there to correct the biggest known influences, but all effects including measurement inaccuracy mean that you need a long list of radar measurements to get the calculated trajectory better than 1 km deviation from the really measured one.

**Goth** · 03-09-2011, 04:09 PM

Indeed, as it is explained in the course of the very nice scenario tutorial "DG-ISS Sync Doc" on the Tutorials folder.

**Urwumpe** · 03-09-2011, 04:12 PM

AFAIR, you already open a bottle of champagne after launch, if the payload is within 5 km of the target orbit after orbit insertion, that is already a bullseye for launch providers.

**Tex** · 03-09-2011, 05:32 PM

Quote:

Originally Posted by apeonaut

yes, 'non-spherical gravity' is on and turning it off feels a bit like cheating to me.

I remember when Orbiter didn't even have a nonspherical gravity sources option. Learning with it off makes things a lot less frustrating, believe me, you have plenty to deal with while learning the ropes. Activating it later, once you come to grasp with things, makes for a new fun challenge.

apeonaut · 03-09-2011, 08:10 PM

thx, you saved my a lot of time in trying to find my mistake in my rendevouz maneuvers. i guess, i underestimated how difficult it is to make an "exact" approach. now i feel pretty satisfied with my results.

03-09-2011, 03:04 PM	#1
apeonaut Orbinaut	orbit mfd accuracy hi all, i sucessfully docked with the dg4 at the iss, so i basically know what to do. i tried to follow the shuttle procedure for docking. i aligned planes and started to catch the station on a lower orbit. for the rendevouz i tried to match my apoapsis with the station's apoapsis. but after my burn at the periapsis my actual height at the apoapsis is not as calculated (about 5-10km less) and the position of the apoapsis is also rotating all the time. is there any way to do these maneuvers more accurately?

03-09-2011, 04:00 PM	#6
Urwumpe Donator	Please note that Orbit MFD displays so-called "osculating elements". That elements are only a local approximation with Kepler orbit elements for the real trajectory. This means: The data displays and the ellipse painted are not the real trajectory, but only an approximation, that is only valid in your current position. By having many short osculating element segments in series, you can approximate the real trajectory. These osculating elements are calculated pretty simple: you just assume that there is only this planet and you and you have almost no mass, also this planet is just a point source of gravity (no non-spherical gravity, no multi-body problems). The more these factors have an effect on you (eg near Lagrangian points or at the rim of the sphere of influence), the bigger the difference between osculating elements and true elements. Also, maybe it gives you a better feeling, that in real spaceflight, the problem to calculate the orbit trajectory is not much easier. There are some correction terms there to correct the biggest known influences, but all effects including measurement inaccuracy mean that you need a long list of radar measurements to get the calculated trajectory better than 1 km deviation from the really measured one. Last edited by Urwumpe; 03-09-2011 at 04:06 PM.

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03-09-2011, 03:07 PM	#2
C3PO Donator	Is "non-spherical gravity" on?

03-09-2011, 03:07 PM	#3
Grover Saturn V Misfire	the reason behind the changes is probably advanced simulation, from things like non'spherical gravity sources and the like, turning them off will leave your orbit alone, and it wont decay over time like it does in real life (best to do this as you learn, then put it on when you know your way around) good luck! (and welcome to the forum i guess) EDIT: Ninja'd

03-09-2011, 03:28 PM	#4
apeonaut Orbinaut	yes, 'non-spherical gravity' is on and turning it off feels a bit like cheating to me. i think transx could to the job much better as the rather simple orbit mfd. i'll give it a shoot after the shuttle landing.

03-09-2011, 03:30 PM	#5
C3PO Donator	TransX doesn't account for it either, sorry.

03-09-2011, 04:09 PM	#7
Goth Occasional orbinaut	Indeed, as it is explained in the course of the very nice scenario tutorial "DG-ISS Sync Doc" on the Tutorials folder.

03-09-2011, 04:12 PM	#8
Urwumpe Donator	AFAIR, you already open a bottle of champagne after launch, if the payload is within 5 km of the target orbit after orbit insertion, that is already a bullseye for launch providers.

03-09-2011, 05:32 PM	#9
Tex O-F Administrator	Quote: Originally Posted by apeonaut yes, 'non-spherical gravity' is on and turning it off feels a bit like cheating to me. I remember when Orbiter didn't even have a nonspherical gravity sources option. Learning with it off makes things a lot less frustrating, believe me, you have plenty to deal with while learning the ropes. Activating it later, once you come to grasp with things, makes for a new fun challenge.

03-09-2011, 08:10 PM	#10
apeonaut Orbinaut	thx, you saved my a lot of time in trying to find my mistake in my rendevouz maneuvers. i guess, i underestimated how difficult it is to make an "exact" approach. now i feel pretty satisfied with my results.