# Launch Azimuth and in plane launch windows

#### garyw

Moderator
Tutorial Publisher
Hi all,

As I've now got a bit more time on my hands I'm playing around with orbiter again and still trying to get my head around some of the math.

I've got the formula for launching Northeast/on the ascending node for an ISS flyover but what I don't understand is how to change that forumla so that it can give me a heading for the southeast/descending node heading.

Also, is there a formula for calculating the in plane launch window opportunities for both AN and DN launch options?

#### garyw

Moderator
Tutorial Publisher
Thanks, nice to be back!

#### Thorsten

##### Active member
I've got the formula for launching Northeast/on the ascending node for an ISS flyover but what I don't understand is how to change that forumla so that it can give me a heading for the southeast/descending node heading.

Southward launch azimuth into the same inclination is (180 - northward azimuth) (at least to analytic accuracy before you correct for Earth rotation).

Also, is there a formula for calculating the in plane launch window opportunities for both AN and DN launch options?

Not analytically to the accuracy you'd probably need - it depends on what inclination you're targeting, how long the launch takes and how the spacecraft maneuvers what longitude of the ascending node you'll end up (I even find it makes a difference of 0.1 deg whether I roll the Shuttle to heads up orientation or not...).

If you know the relation between targeted inclination and reached longitude of the ascending node for one launch with a craft, it's easy to compute it for all launch times by using the known rotation of Earth - but the relation itself is pretty non-linear (and, as I said, depends on the fine-print).

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