Longitude of Ascending Node - unit of measurement

[expert_vision]

I have a contract that requires me to put a satellite in a certain orbit and I don't understand one of the given orbit parameter, specifically, longitude of ascending node. I understand what it is, but I don't know how to interpret the given value.

LAN is measured in degrees, but in reference to what?

Below is a screenshot of my satellite in a lower circular orbit positioned at (visually) the same longitude as the AN of the targeting orbit, where I am supposed to burn in normal direction to obtain the designated orbit inclination. But the LAN value given in the contract (28°) doesn't match my vessel longitude (73° W) shown in Engineer Redux mod.

LAN is measured in degrees in reference to what?

Edited January 16, 2015 by expert_vision

[LethalDose]

The longitude of the AN is given in degrees, in reference to some arbitrary, but consistent, celestial location.

Your best best is to try to get your initial transfer AP near either the AN or DN on the target orbit.

[OhioBob]

LAN is measured in reference to some defined point in space. In real life it is the direction of the vernal equinox. I don't know what it is in KSP.

I generally don't worry about the value of the longitude of ascending node and argument of periapsis when I'm completing a contract. I just visually match the shape and inclination of the orbit display and make sure I'm traveling in the right direction. The contracts are forgiving enough that that does the job.

[expert_vision]

Oh, I see.

I did complete the mission visually, I was just curious about this orbit mechanic and a bit intrigued as to why the longitudes don't match.

[OhioBob]

The longitude of your satellite in the surface display is Kerbin longitude, the latitude-longitude coordinate system on the surface of the planet (I suppose we can call it keographic coordinates). The longitude displayed in the contract is celestial longitude, which is a different coordinate system. In the real world, geographic coordinates use Earth's equator as zero latitude and Greenwich, England as zero longitude. Real world celestial coordinates use the ecliptic as zero latitude and the direction of the vernal equinox as zero longitude. In KSP, Kerbin's equator lies within the ecliptic plane, so keographic latitude and celestial latitude are one in the same. Longitude is different, however. Keographic longitude rotates with the planet, while celestial longitude is fixed in relation to the stars. The direction of zero celestial longitude is not clearly defined and, as far as I know, does not correspond to any identifiable feature. However, you can get a rough idea of its direction by going to the map view and zooming out until you see the orbit of Eeloo. The celestial longitude of Eeloo's apoapsis is 130 degrees and its periapsis is 310 degrees. From this you can estimate the direction of zero longitude (longitude increases going counter-clockwise).

Edited January 16, 2015 by OhioBob

[OhioBob]

I just observed something. I went to the map view and estimated the direction of zero celestial longitude to see if I could find any distinguishing feature to define it. It looked like a pretty bland part of space with nothing special in that direction. However, when I reversed my view and looked in the direction of 180^o celestial longitude, I was looking right smack in the middle of the thickest and brightest part of the milky way. Perhaps it is the galactic center that defines the celestial coordinate system, with 0^o being directed away from the center.

[LethalDose]

I think it's also worth noting that your current orbit's LAN is provided on the KER display in the OP. It's the second to last line of output in the "orbital" section above.

[expert_vision]

Yes, LAN in KER is pretty much useless. I just added to see if after I finish the contract the LAN is 28°, and indeed it was 30.5°.

LAN would be useful in KER if I would have the celestial longitude of the vessel in orbit, instead I only have the planet longitude.

[OhioBob]

LAN would be useful in KER if I would have the celestial longitude of the vessel in orbit, instead I only have the planet longitude.

Agreed, I would like to have that as well.

[LethalDose]

Well, KER does provide the option to display true anomaly You can just use the argument of the periapsis as an offset to calculate the vessel's position vector in terms of the celestial constant instead of its PE.

Obviously, doing this on the fly, in your head, every second would be a headache, but you don't need to. You only need to figure out the value where you need to burn and watch for when you're getting close.

And yes it'd be great if KER did this arithmetic for us, but I'm personally very satisfied with that we even have a mod that can give us the information to begin with. Remember, the dev's couldn't be bothered to include this information in stock because "numbers aren't fun."

Edited January 17, 2015 by LethalDose
missing word

[OhioBob]

KER does the option to display true anomaly

Where is that? I've never seen it. Maybe I'm using an outdated version of KER (0.6.2.12).

Edited January 17, 2015 by OhioBob

[LethalDose]

Where is that? I've never seen it. Maybe I'm using an outdated version of KER (0.6.2.12).

It's under orbits. You're definitely using an outdated version, the latest is 1.0.14, but I found that in 1.0.13.