What launch vehicle, spacecraft and target orbit parameters decide the time of launch to GTO for satellites destined for GEO?

The target orbital slot is always in the same relative position to the launch site so it seems like the launch should be possible anytime?

Some example launches and their windows (all from the Cape):

  • SpaceX - SES-9 - 23:35 to 01:06 UTC
  • SpaceX - JCSAT-14 - 05:21 to 07:21 UTC
  • ULA - MUOS-4 - 09:59 - 10:43 UTC

I know there has to be a window - range, people at work, exclusion zones for ships and aircraft - nothing of these can stay ready indefinitely.


2 Answers 2


When I was a bit more active supporting GTO launches, 15 years ago, the following constraints helped shape the launch window (not in any order and I may have forgotten some important points completely):

  1. Sensor viewing angle to ensure attitude acquisition coming off the launch vehicle (Sun and Earth sensors, may have been superseded by direct use of a star-tracker in the most advance cases these days)
  2. A period for orbit determination before and after each apogee manoeuvre, at least a full orbit for the first one
  3. Thermal constraints on parts of the satellite for the time spent in GTO (its more common to re-orient the satellite for each apogee burn now so this may be less significant)
  4. Sensor viewing constraints for guidance prior to and during the apogee manoeuvres
  5. A policy decision to have the opportunity to launch on any day of the year, i.e. helps with managing the effects of short launch delays
  6. The target orbit: the delivery orbit for each launch vehicle is different, compounded by the launch site latitude as this drives the injection inclination, whether a supersynchronous or plain GTO is required, whether the satellite is eventually aiming for pure GEO or an initial non-zero combination of inclination and right ascension.

It primarily depends on where they want to end up, relative to where they are launching from. It also depends something on the launch profile, so it helps to compare like launching companies. Here's a few other interesting bits, about the desired location.

  • SES-9 108.2 E
  • JCSAT- 154 E

Okay, so there's a 46 degree difference, which corresponds to about 2.5 hours, which is only about half the difference. There's a lot that goes in to these GTO orbits to get them quickly to the desired location, and that affects the target orbit. It's really hard to know exactly why without knowing the exact flight trajectories, but I can tell you that the launch for a GTO satellite will be at the same time of the day, but will vary depending on the eventual destination. This is because the initial orbit isn't a 24 hour orbit, but rather something intended to get the satellite in to the correct orbit quickly.

  • $\begingroup$ "there's a 46 degree difference" - thats the part I don't understand - there will be the same difference any time of the day because both target points and the launch site are rotating with the same angular velocity and stay in the same position relatively to each other. So I thought that the slot insertion is made possible by timing the circularization burns and orbit phasing long after the launch. $\endgroup$
    – jkavalik
    May 6, 2016 at 6:54
  • $\begingroup$ The initial orbit isn't a 24 one, meaning there time of day matters. $\endgroup$
    – PearsonArtPhoto
    May 6, 2016 at 10:01
  • 1
    $\begingroup$ It isn't 24 hours but given the same launch site and similar launch trajectory ("go east"), they should end in a very similar elliptical orbit no matter the hour of day. Or I am just missing something really basic. $\endgroup$
    – jkavalik
    May 6, 2016 at 19:05

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