# Satellite Velocities: Relative or Absolute?

Are the reported satellite velocities, such as for the ISS at 17,450 mph relative to the surface of the earth, which is turning, or to some fixed point without the rotational velocity. In other words, is the ISS moving at the above velocity relative to a point on the earth's surface?

• For reference, the difference in speed between surface-relative and fixed reference is roughly 400 m/s or 900 mph. Jul 27, 2017 at 21:46
• @RussellBorogove Depending on latitude and course, of course. Jul 28, 2017 at 6:48
• @RussellBorogove If you're launching to orbit from the ground, that 400 m/s difference will make or break your flight - so you'd better know which it is! Jul 28, 2017 at 9:16
• In ECE the speed of a circular Earth orbit with a radius of 6378 + 400 km is about 17,150 mph. I don't think there is any site that reports 17,450 mph. That would put it at an altitude of only 170 km. That could be the speed of a vehicle after first stage cut-off and before second stage ignition, or you may have copied the number incorrectly.
– uhoh
Jul 28, 2017 at 12:08

All velocities are relative. In this case, the orbital velocity is in an Earth-Centered Inertial (ECI) framework, so it's relative to a non-rotating set of axes whose orientation is fixed with respect to the stars, and with it's center at the center of the earth.

The OP's question can be restated as "Are satellite velocities stated in an ECEF (Earth Centered, Earth Fixed) frame, or an ECI (Earth Centered Inertial) frame?"

As indicated by David Hammen's answer to What is the frame of reference for orbital speed?, the choice of frame of reference is somewhat arbitrary. However, partly by convention, orbital velocity for a satellite in orbit around the Earth is defined in an Earth-Centered Inertial (ECI) frame of reference, relative to a non-rotating set of axes whose orientation is nominally fixed with respect to the stars, and with it's origin at the center of the Earth. (For reference, see Frame of reference section of the Wikipedia article about Orbital state vectors)

Note that using an ECI frame for orbital parameters, rather than an ECEF frame, makes figuring out how to transition to another frame of reference much less mathematically complicated, e.g., going to the Moon where an LCI (Lunar Centered Inertial) frame makes more sense. This is most likely why the convention of using the ECI frame for orbital velocities was chosen over the more obvious (to an observer on the ground) ECEF one.

• There is no ambiguity if you read the question correctly - but it is possible to misread the question, as I stated I had done in the expansion of my answer. I also explained why I misread it, and how to improve the question to avoid the apparent ambiguity. Furthermore, I've added a (mathematical) reason why choosing an ECI frame makes sense where the choice is arbitrary, rather than simply stating that's what's used without any supporting evidence. With all due respect, may I suggest you step back a moment and try to see the perspective that led to my initial misunderstanding? Jul 28, 2017 at 15:45
• In the interest of reducing friction, I've removed my explanation of why I felt the edited Tristan answer can be read as somewhat ambiguous, leaving my additional points and information more readily visible. I still feel the discussion of why it appeared to be ambiguos is valid guidance toward writing better questions, but perhaps it would find a more receptive audience in a psychology or language use discussion forum... Jul 29, 2017 at 2:01
• Looks good! Sometimes the rigid structure of SE, (e.g. answer posts must only be answers to the question as asked, comments should only be made as comment posts) feels overly limiting. But it has evolved over time as a system that works remarkably well to build a treasure-trove archive of questions and answers helpful to both the OP and to all future readers. I refer to this wave/particle duality as "both a floor-wax and a dessert topping."
– uhoh
Jul 29, 2017 at 2:37