How to Enter a Polar Sun Synchronous Orbit for Science and Spy Satellites

[inigma]

Figured this out while doing the SpySat mission in the Age of Satellites campaign.

1. Launch into a 90 degree polar orbit using the Mun for angular reference (since it's inclination is 0 degrees to Kerbin), as close to noon time as possible.

2. Circularize at any altitude above 70km. (Spy satellites like to be close to the ground for camera's sake, so I picked 100km)

3. Adjust solar inclination to noon/midnight by burning anti/nominally at the poles (think of your polar orbit as its own "equator" with the sun as the inclination target). You will have to eyeball it since there is no exact reference point to the sun except the visible daylight termination points on either side of the planet. Adjust your polar plane to slide between those two termination points and you'll be at noon/midnight.

4. Adjust polar orbital inclination relative to the Kerbin's equator or the Mun, to 89.68 degrees to precess the sun synchronus orbit to account for annual drift caused by Kerbin's orbit around Kerbol, so that you are always above Kerbin at all points in your orbit at noon or midnight local time.

This orbit will allow your satellites to always pass over any spot on the planet always at noon or midnight local time, which is perfect for eliminating shadows and thus perfect for spy...er scientific imaging.

Enjoy.

I love orbital mechanics.

Edited June 12, 2013 by inigma

[Mr Shifty]

I'm trying to understand how the orbit processes. I think I'm just having problems wrapping my mind around the situation. Regardless of inclination, its hard for me to see how the orbit doesn't change its orientation relative to the sun throughout the Kerbin year.

[inigma]

The polar orbital plane doesn't change (think of it as a gyroscope), but the location of the ground relative to the sun does change. You have to precess by a small degree to keep up with the seasons or else your orbit will eventually slide off the noon plane over Kerbin to where in 3 months you wont be orbiting Kerbin overhead at noon, but rather at the day/night terminator.

Edited June 11, 2013 by inigma

[purpletarget]

The problem here is that there is no orbital procession in KSP. IRL SSO Sats use the bulge at the equator of the earth, and take advantage of the perturbations caused by that deferential in the gravitational field to torque the orbit by about a degree a day in order to maintain the orbit in a sun-relative position. Kerbin's gravitational field is perfectly spherical, so there are no such perturbations produced.

The polar inclination you indicate would make no difference. I think what you're proposing is a powered orbital adjustment to maintain the sun relative position....which is probably a good role for an ion engine because nothing else will have enough dV in it to allow that kind of annual change for any useful Satellite mission time. However, you'd still have to go back to the flight about once a day to make the adjustments.