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ISS orbit

The ISS orbits the Earth at 51.6° to the Equator, following the direction of the Earth’s rotation from west to east. The Earth itself is tilted at 23.4° to the plane of its orbit around the sun (sun vector), so the ISS is orbiting at 75° to the sun vector. The ISS’s altitude varies between 320 to 410 km, and it takes 92 minutes to circle the Earth. The orbit inclination offers good coverage of most of Earth’s surface.

Below is a screenshot from TsUP’s ISS orbital tracking page showing the ISS’s current position. As the map is laid out flat in a two-dimensional view, the orbital ground track appears as a wavy line (sine curve), but it is in reality straight as the Station circles the Earth. Similar tracking maps are projected onto the wall screens at Houston MCC and TsUP, Moscow. The TsUP map shows latitude & longitude, as well as period (time to complete one orbit), degree of orbit, maximum & minimum heights, and the cities in which the main control centers are (or will be) located: Houston (Хьюстон), Ottawa (Оттава), Paris (Париж), Moscow (Москва), Baikonur (Байконур), Tokyo (Токио).

TsUP ISS ground track

Orbital precession

The ISS completes around 15½ orbits per 24 hours at a varying height of 350-400 km. It does not orbit over the same spot every 24 hours, though, as the Earth is also turning eastward. After one orbit, the Earth has rotated approximately 15° of longitude (at the Equator). The ISS’s ground track will thus be further behind on a map projection by 15°.

From Star-Crossed Orbits by James Oberg:

It takes about 90 minutes for the satellite to complete one full circuit. When it gets back to its starting point, Earth’s surface has moved eastward. It moves 360 degrees in 24 hours, or 15 degrees per hour. So after each satellite circuit of 1.5 hours, the Earth’s surface has moved about 1.5 times 15 degrees, or 22.5 degrees, further to the east.

This precession also means that passes over Russian ground sites will also move forward by a certain amount each 24 hours (an hour or so, as best as I can figure out). It is also the reason why the Station sighting times for cities vary from one day to the next, or don’t appear at all for a week or so.

Orbit inclination

Here is a question and answer from NASA’s “Ask MCC” segment which explains why 51.6° was chosen:

From: Patrick Donovan, of Cameron Park, Calif.

To: John Curry, flight director

Question: Why is the space station in a 51.6-degree inclined orbit instead of something less or something more?

Answer: Good question, Patrick!

The short answer is that 51.6 degrees is the lowest inclination orbit into which the Russians can directly launch their Soyuz and Progress spacecraft. Both of these vehicles serve an important role in ISS operations.

The Soyuz – there is always one attached to ISS – serves as an escape vehicle in the event the ISS would need to be abandoned in an emergency. The Progress spacecraft is basically a cargo version of the Soyuz and is used to bring up fresh food and supplies to the ISS.

Ideally, one would want to launch due east from a launch site to maximize the cargo-to-orbit capability for a given launch vehicle. This is because the Earth, rotating from west to east, gives rockets a “free” head start in the right direction. Launching due east from Kennedy Space Center would place the shuttle in a 28.5-degree inclination orbit. Notice that the inclination is the same as the latitude of KSC. Launching due east from Russia’s main launch site, Baikonur, would place spacecraft in a 45.6-degree inclination orbit – the launch site latitude. However, doing so would also drop the lower stages of the boosters on China. To avoid this, the Russians crank up the minimum inclination to 51.6 degrees.

Although the shuttle does trade some payload capacity for propellant needed to make up the difference between launching at 28.5 degrees vs. 51.6 degrees, doing so allows the Russians to participate in the ISS program. It also has the added benefit to Earth Sciences since ISS flies over more of the Earth’s surface – about 75 percent, which covers about 95% of the inhabited lands – at the higher inclination orbit.

I hope this answers your question, Patrick. Thanks for asking MCC!

– Jim Cooney, ISS Trajectory Operations Officer (TOPO), Orbit 3 (planning shift) for STS-112

Latitude & longitude

For those (like me) who haven’t done any geography since their school days, here is a quick reminder of what latitude & longitude (as indicated on the TsUP map above) are!

Both latitude & longitude are divided into degrees (°), then minutes (′) and seconds (″). Degrees can also be expressed as decimal co-ordinates. (Degrees calculator at CSGNetwork.com)

Co-ordinates for main control centers (all in the Northern Hemisphere, Latitude-Longitude):

ISS control centers - longitudes and latitudes
ISS Control Center Latitude Longitude
Baikonur, Kazakhstan 47°22′N 63°25″E
Moscow, Russia 55°4′48″N 38°3′E
Houston, USA 29°27′36″N 95°13′12″W
Paris, France 48°31′12″N 2°12′E
Tokyo, Japan 35°24′N 139°27′36″E
Ottawa, Canada 45°15′N 75°25′12″W

Orbital decay

At ~350-400 kilometers altitude, the ISS is above most, but not all, of the Earth’s atmosphere, and tenuous though it is at the Station’s height, some drag is produced as atoms collide with the ISS. The Station’s orbit decays by a few hundred meters each 24 hours. The decay is variable as solar activity causes changes in the density of the outer atmosphere. Periodic reboosts are thus necessary every few months, usually undertaken by a Progress cargo ship docked to the Russian segment, or an Orbiter when docked to Destiny.

ISS orbit data is posted at the end of each daily On-Orbit Report at Spaceref. Here is data for 4 November 2004:

ISS Orbit (as of this morning, 6:37 a.m. EST [= epoch]):

  • Mean altitude – 358.1 km
  • Apogee height – 364.0 km
  • Perigee height – 352.3 km
  • Period – 91.70 min.
  • Inclination (to Equator) – 51.64 deg
  • Eccentricity – 0.0008684
  • Solar Beta Angle – 32.8 deg (magnitude increasing)
  • Orbits per 24-hr. day – 15.70
  • Mean altitude loss in last 24 hours – 135 m
  • Revolutions since FGB/Zarya launch (Nov. 98) – 34,036