# How do I measure speed accurately without any turning wheels?

Consider a pod (about 50 ft long and 15 square feet wide) that levitates using air pressure that shoots through the bottom and travels through a tube that's at low air pressure (about 100 Pa). It travels using linear induction. If you're wondering, this problems comes from the hyperloop tube specification. The pod will be traveling anywhere from 0 to 800 MPH.

I need to measure the speed of the pod traveling through the tube at every point

Approaches I have considered:

1. Using an accelerometer and integrating over time
2. Using a pitot tube

I've heard that an accelerometer can be bad for measuring velocity because an error bias can propagate.

I heard that pitot tubes were used for airplanes, but I don't know if they're very accurate or work at the low air pressures I have.

What approach should I use to measure the speed of my pod?

• The linear induction system can and almost inevitably will have periodic structures which will allow you to derive very precise location information from the track. I wouldn't worry about this "problem", at all, the folks who will design that part of the system will take care of that for you. Commented Oct 28, 2015 at 1:11
• @CuriousOne The updated hyperloop tube spec says "Every 100 feet, a 2-inch wide reflective circumferential stripe will be applied to the inner circumference of the tube." under the navigation system. Do you think this will suffice for determining the velocity? Commented Oct 28, 2015 at 1:19
• IMHO, that would be more than enough, but I would say that it's probably just a backup system to much finer resolution readily achieved by the drive mechanism. Don't get me wrong, but the Hyperloop, so far, strikes me still as the product of people who like to sing in the shower but have very little (if any) experience with the design of large systems. This "problem" and its ad-hoc solution in the specification (a document which is NOT supposed to have a fixed solution to the demands that it makes) is one of those little hints that are giving this away. Commented Oct 28, 2015 at 1:23
• It appears that is the least of their problems. Commented Sep 13, 2016 at 4:30