Thanks for that.
Its not about latitude on Earth the rockets are taking off from its the angle of which the ISS orbits the earth. If I recall where I read it I think it said inclination was 51 degrees, which was ideal for both the yanks and Russians and prevented the Russians from having to drop the 1st stage back on China and allowed a fast approach and emergency exit should it be required back to Earth.
The ISS uses an inclined equatorial orbit, which means that latitude is everything when it comes to launch sites.
On each orbit (every 93 minutes) the ground track of the ISS:
- starts at the equator
- heads north-east up to a latitude of 51.64°N
- heads south-east to cross the equator and go all the way down to 51.64°S
- heads north-east again towards the equator, crossing about 2550km to the west of its previous ascending (northbound) crossing of the equator
Any launch site within those latitudes can launch directly to an ISS approach orbit without cross-range manoeuvres, they just need to wait until the time when there is an ISS orbit path crossing overhead. Cross-range manoeuvres require carrying extra fuel, and every kilogram of fuel carried is a kilogram of payload which cannot be carried.
There are a number of advantages to the other launch sites used for ISS missions - Cape Canaveral, Yoshinobu, Wallops Island, Guiana. All four are located next to the coast so they don't have the flight path restrictions that apply to Baikonur, and all four are much closer to the equator which allows for heavier payloads to be carried