How is Speed measured in Space?

[Epic DaVinci]

Just a thought, with no air to record speed, how does a space craft know how fast it is going?

Does it rely on radar echos from celestial bodies?

does it Triangulate is position / speed with known positions of the Planets etc etc?

or is there a GPS system that can work in space as it does on the ground?

[Themohawkninja]

We don't use air to record speed on Earth as far as I know. On Earth we normally use chronographs (two sensors about a foot apart that measure the time it took for the object to pass from one point to the other), or radar (which measure the time it takes for the waves to bounce from the object to the sensor).

In space, however, if you know how far an object is away from you, you can measure it's parallax (the observed change in distance over time. It's why farther objects appear to move slower than nearer objects when you are in a car). You can tell how far the celestial object is away from you by using things like Cephid variables which have a given absolute magnitude, so you can tell how far it is by how dim it appears.

[TheSaint]

Actually, aircraft use differential pressure to measure their air speed, I think that is what the OP was referring to.

On the space shuttle, inertial navigation units provide both velocity and location measurements. (http://www.kearfott.com/images/stories/pdf/DATASHEETS_KGN_NJ/SPACE/hains_imu_for_space_shuttle.pdf) These are verified by star tracker navigation units (http://www.ballaerospace.com/page.jsp?page=104).

[Seret]

Aye, aircraft traditionally use pitot tubes for airspeed. Although these days the navigation system will be using inertial with GPS updates at waypoints, so there will be multiple inputs into the system. Inertial does tend to drift over time, so you need some way of correcting that periodically.

[Nuke]

star trackers give a pretty solid orientation reference. ground tracking stations can always relay the relative coordinates of the space craft. inertial guidance works for when you are out of line of sight or range of ground tracking. not sure if sattelite based tracking is used but i dont see why not. thing is you use as many data sources as possible, to correlate it, check less reliable sources against more reliable ones, and get the most accurate solution possible from all the data. to get velocity what you do is measure your position twice with some time in between, and i think the math is (p2-p1)*t = v, where p1 is the first position, p2 is the second position (both vectors in a given frame of reference), and t is the time between them, giving you v, the velocity vector (the magnitude of which can be called speed).

Edited August 6, 2013 by Nuke

[Bunsen]

Just a thought, with no air to record speed, how does a space craft know how fast it is going?

Does it rely on radar echos from celestial bodies?

does it Triangulate is position / speed with known positions of the Planets etc etc?

or is there a GPS system that can work in space as it does on the ground?

GPS works in low Earth orbit. Further out than that, I believe standard practice is to use a combination of celestial observations (planets and such) and Earth-based measurements with radar or radio transponders (sorta like radar, but the signal that goes back to Earth is amplified by electronics on the spacecraft, not just a passive reflection). Radar on board the spacecraft is normally only used for short-range work like rendezvous, because long-range radar takes rather a lot of electrical power.