Why launch window for launch to GEO?

[Meithan]

Yesterday I watched India launch another of their navigational satellites into orbit. The satellite is intended to operate in geostationary orbit, and the launch rocket put it on a transfer orbit, as usual.

What got me thinking is that they said the launch window was 15 minute long, and I got thinking that I don't quite understand why this is so. If the intended orbital position is geostationary, it means it does not move relative to the Earth's surface. So I don't see why it matters when the satellite is launched. You can launch at any time and the target orbital spot will always be at the same place relative to the launch site. Other than things like solar illumination, I don't see why there's a launch window at all. Anyone?

[Kryten]

Solar illumination isn't something you can shrug off. GSO sats aren't going to be built for long periods of eclipse, both in terms of power and thermal control.

[Mitchz95]

Maybe they were concerned about it hitting something on its way up? I remember hearing that the launch of the Dawn space probe was delayed because they had to wait for the ISS to get out of the way.

[Meithan]

Solar illumination isn't something you can shrug off. GSO sats aren't going to be built for long periods of eclipse, both in terms of power and thermal control.

Perhaps. But 15 minutes sounds like an awfully short window. If the crux is that the transfer orbit to GEO occurs in sunlight, I'm sure the window would be considerably larger (a few hours, at least?). No, this short window makes me think that the reason has to do with orbital mechanics, but I just don't see it.

Maybe they were concerned about it hitting something on its way up? I remember hearing that the launch of the Dawn space probe was delayed because they had to wait for the ISS to get out of the way.

Wouldn't this make for a short "don't launch" window, instead of a short "do launch" window?

[linkxsc]

Perhaps on the sunlight argument. After launching they had 15 minutes before it was going to move in the earths shadow. Ofcoure they could just wait a while for it to come around again, but it might be nighttime on the next few orbits, while its still in contact with the ground station. Though i dunno, i didnt read anything about that particular launch.

[dharak1]

My guess is mission planning. If they launched later they may need more Dv to make orbit on the spot they wanted. They might also then have to make an extra orbit for the sat to get ahead or behind in its orbit before being locked into geo with a burn. It would probably also a matter of doing it in one burn with no engine restarts to lower risk.

[Meithan]

Perhaps on the sunlight argument. After launching they had 15 minutes before it was going to move in the earths shadow.

Uhm, I'm not convinced. 15 minutes sounds too short compared to Earth's rotation period. Besides, as it climbs towards GEO altitude, a spacecraft will quickly exit the Earth's shadow, no matter when it was launched.

Though i dunno, i didnt read anything about that particular launch.

In fact it's not particular of this launch. A recent SpaceX launch to GEO (SES-8) also had a shortish ~1 hour window that repeated daily:

SpaceX SES-8 launch attempts

What orbital mechanics detail am I missing here?

My guess is mission planning. If they launched later they may need more Dv to make orbit on the spot they wanted. They might also then have to make an extra orbit for the sat to get ahead or behind in its orbit before being locked into geo with a burn.

Well the point is that a spot in geostationary orbit is stationary with respect to the surface of the Earth, so it holds a constant relation to the launch site. Thus, no matter when you launch, your target orbital position is always in the same spot relative to the launch site. So I don't see why there is a launch window at all.

[linkxsc]

Well a couple things. Earths rotation doesnt have much to do with it. Especially since leo and gto are going around earth significantly faster than it rotates.

1 thing that comes up though, is that there is a window in which its optimal for burning to get to a particular gso position. So thats probably what they were meaning.

[Meithan]

Well a couple things. Earths rotation doesnt have much to do with it.

If the window repeats daily, such as in the SpaceX example, then this is definitely something related to Earth's rotation (such as waiting for the launch site to pass through some orbital plane or for the phase angle to a target to be correct -- but none of these seem to apply for a launch to GEO).

Especially since leo and gto are going around earth significantly faster than it rotates.

Speeds in LEO and GTO sure, but not speeds in GEO, that's my point: things in GEO rotate at the same rate as the Earth.

You can picture a launch to GEO with a specific target longitude as a rendezvous with an empty spot in space that's in GEO orbit. But since GEO has a 24 h period, that "rendezvous target" is always in the same place relative to the Earth: it doesn't seem to orbit (relative to Earth's surface), it's just motionless with respect to the launch site. So why would launching at a different time make any difference?

[Tommygun]

Well the starting transfer orbit for the IRNSS-1b and I assume for the IRNSS-1c as well wasn't a circle but an elliptical orbit.

It was then boosted up to GSO over time and several elliptical orbits. So you are trying to get the timing right with smaller elliptical orbits to match up to a specific place on a larger circular one.

Once in orbit, IRNSS-1B deployed its power-generating solar panels, established communications with ground stations and started to acquire three-axis attitude control. Over the coming days, IRNSS-1B is set for a total of five burns using its Liquid Apogee Motor. The first two burns will be two in-plane maneuvers performed on orbits 5 and 8 to raise the apogee of the orbit to Geosynchronous Altitude. The next step will be the circularization of the orbit and the adjustment of the inclination which will be accomplished by three maneuvers performed out of plane at apogee.

This way, the perigee will be raised and the inclination will be increased to 31 degrees. Once in its Geosynchronous Orbit inclined 31 degrees, IRNSS-1B will undergo checkouts before starting to transmit navigation signals, waiting for the rest of the IRNSS satellites to launch that will boost the system to its full capability.

http://www.spaceflight101.com/pslv-c24-launch-updates-irnss-1b.html

Edited October 17, 2014 by Tommygun

[QuesoExplosivo]

If the launch site is not at the equator, then launching to an equatorial orbit without mid-course trajectory adjustment would only be possible twice a day.

[benkh]

Navigational satellites have to be in the right orbit relativ to each other don't they?

[Bunsen]

If the launch site is not at the equator, then launching to an equatorial orbit without mid-course trajectory adjustment would only be possible twice a day.

Um, how does the position of the launch site relative to the equator change with time of day? Think of it in an Earth-fixed reference frame -- the only things I can see changing are distant objects, like the sun and moon, and therefore their gravitational influences. Tidal forces certainly factor in to orbital calculations, and are more significant for larger-radius orbits than for lower ones, so maybe that's what sets the launch window? I wouldn't think that would be large enough to make a go/no go decision versus a small correction to burn times.

[Meithan]

Well the starting transfer orbit for the IRNSS-1b and I assume for the IRNSS-1c as well wasn't a circle but an elliptical orbit.

It was then boosted up to GSO over time and several elliptical orbits. So you are trying to get the timing right with smaller elliptical orbits to match up to a specific place on a larger circular one.

Navigational satellites have to be in the right orbit relativ to each other don't they?

But my point is that there's no timing involved because the target spot in the geostationary orbit is motionless relative to the Earth's surface and, hence, launch site. If they waited 5 minutes, 30 minutes, 2 hours, 6 hours, 1000 hours more to launch, it'd be still in the same place relative to the launch site. So why does it matter *when* they decide to launch?

If the launch site is not at the equator, then launching to an equatorial orbit without mid-course trajectory adjustment would only be possible twice a day.

Indeed, launching directly into an equatorial orbit is impossible if the launch site is not on the equator, so midcourse corrections are necessary to deliver a satellite to GEO. But I don't see why you assert it's only possible twice a day, Could you elaborate?

[Meithan]

By the way, it seems i'm not the first person bothered by this question:

r/askscience: Why is there a launch window for a geosynchronous orbit.

But none of the answers given there strike me as solving the issue either. The basic argument remains: relative position of the target spot being constant with respect to the launch site. I don't see how any of the answers provided so far explain this point.

[Brotoro]

I think the problem comes from picturing things in the rotating-along-with-the-Earth reference frame. The problem with that is that the TRANSFER ORBIT is NOT stationary in that frame. The rocket has to get the payload to the perigee of the transfer orbit when the target point on the geosynchronus orbit is at the outer "Start" point on my diagram. Then the payload will end up at GEO altitude when the target point arrives there. So the need to insert the payload into the base of the transfer orbit determines the launch time.

Oh...and while it may look from the diagram like there's only one particular section of GEO that you could easily reach from a given launch site, you could solve that by letting the payload loop around its transfer orbit multiple times, since it would be arriving at different points on GEO each time it gets to apogee.

Edited October 17, 2014 by Brotoro

[KerikBalm]

I still don't see it...

"Payload start" will always be in the same position relative to launch,

[Alchemist]

We can't even choose the point of entering transfer orbit - the parking orbit is minimal inclination for the given launch site and the transfer burn is done when passing over equator so that the apoapsis is in the equatorial plane (plane change is performed during circularization). Fortunately the commsats are built with the idea of adjusting and maintaining their positions so it's not much of a problem if it gets too far from the desired spot - just put it in orbit with slightly different period and let it do the correction on itself. (or bielliptic transfer could be used to get exactly right position if needed)

The only reason I can find is avoiding getting too close to other satellites in lower orbits (and "every day at the same time" is just this case since periods of most satellites are proportional to day).

[YNM]

A possibility is regarding the fact that GEO are heavily used - many debris, active and inactive sats. Other possibilities includes crossing the Earth's shadow : while the sat is able to hold the "night" it experiences out there in intended operation, it might not so while launching and in transfer. Having a portion of the transfer crosses the shadow might be a problem that the batteries aren't fully charged.

Might also be mix of these and other reasons (like debris fall site, noise, etc.)

[Tommygun]

I don't know if this helps to visualize it:

Edited October 17, 2014 by Tommygun

[Meithan]

I think the problem comes from picturing things in the rotating-along-with-the-Earth reference frame. The problem with that is that the TRANSFER ORBIT is NOT stationary in that frame.

I still don't see it... "Payload start" will always be in the same position relative to launch,

KerikBalm feels exactly like I do regarding this. I understand what you're saying Brotoro, and you're right. But no matter how complicated the transfer to GEO ends up being, the point is that the moment in time chosen to start the whole affair shouldn't matter because the relative configuration between the Earth, the launch site, and the target spot in GEO never changes. There's complete time symmetry in this respect. Launching at any other time would make the whole deal look exactly the same, except rotated with respect to Earth's inertial frame -- but the point is that the target spot also rotated in that time in the exact same amount, so the whole series of maneuvers, as complicated as they are, would end up reaching the same longitude in GEO.

I'm pretty convinced by now that the reason for there being a launch window has nothing to do with the actual trajectory with respect to the Earth. Something else is involved here, and my only guess so far is that it has to do with the position of the Sun (which completes a revolution relative to the Earth in almost 24 hours -- that would explain why the launch windows repeat almost exactly every 24 hours, like in the SpaceX SES 8 mission example).

And I found this in the Google Books preview of the Handbook of Geostationary Orbits by E.M. Soop:

The sidereal angle of the transfer orbit apogee is constrained by the launch window. It defines the daily time interval allowed for launch and is usually a midday and/or midnight window, meaning that the direction from the Earth to the transfer orbit apogee is approximately aligned with the Earth/Sun direction, Figure B. The reason is that on a spin stabilised spacecraft the spin axis, which is aligned with the motor firing direction, must stay orthogonal to the Sun direction with a certain margin, for thermal reasons. A similar constraint may apply to a three-axis stabilised spacecraft, where a certain Sun-Earth geometry in a given part of the transfer orbit is imposed by the attitude control system.

Could this be it ???

[cantab]

EDIT: On second thoughts, this doesn't explain things after all.

Inclination.

Most real rockets aren't launched from the equator. A rocket launched from a non-equatorial launchpad on an efficient ascent to Low Earth Orbit can reach orbits with minimum inclination equal to the latitude of the launch site.

To then make the plane change to a zero-inclination geostationary orbit using the lowest delta-V, you want to make that plane change at GTO apoapsis and combine it with the GTO-GEO circularisation burn.

This in turn means that you want to make your LEO-GTO transfer burn at the AN/DN of your low orbit, and you want to reach GTO apoapsis at the right time to burn into your desired site in GEO. And that's what creates the launch window.

Edited October 17, 2014 by cantab

[GoSlash27]

Meithan,

Could this be it ???

Sounds reasonable to me.

Best,

-Slashy

[linkxsc]

I still don't see it...

"Payload start" will always be in the same position relative to launch,

Yes but the end point wont be. Think of it this way. Say they were trying to put the satellite directly above the launch point, keep it right over their space center for w/e reason. Theyd have to launch, and wait for the sat to orbit the planet, until it was just the right angle off from the target point. Then there would be a window where they could preform the burn. If they get in that window, they can make the burn and get to gso where they want. But if they miss it, they could perhaps still make it there, but it would take a lot more fuel in course corrections.

[KerikBalm]

Wut? that makes no sense.

I'm going to go with Meithan's explanation.