# Feasibility - can you record the movement of a model glider through the air?

Hi, I am a complete novice so this question is entirely conceptual.

I would like to track the movement of an object. Not where the object is in the world, just how the object moves relative to a baseline position. It will move forward, perhaps 25 feet before stopping but with rotation and altitude changes of relatively small degree, say from 3/4 inch to 5 inches.

I found a sensor that has a 9-axis accelerometer, magnetometer and a gyroscope, which is what I think is needed (NB I am a complete novice).

Has anyone had programming experience that suggests this idea is possible? I've had around 8 conversations with unis, electronic shops, and Intel (!), and the consensus was 'maybe but you'd need programming to adjust for the error that develops the longer it travels.

Any insights or links to things to read much appreciated.

Thanks.

Most important, do you want to record the movement from inside the glider, or from outside, or both? Good results most probably require a combination of both methods.

Just to explain the problems of on board data acquisition: imagine you are the pilot or a passenger, and want to determine the the movement with closed eyes. Your feelings correspond to the information obtained from a 9DOF (accel+gyro) sensor. Then it will be almost impossible to determine the height, speed and heading of the glider, you'll only notice a shock (acceleration) on start and on touch down, and nothing special in between. In math terms the initial acceleration can be integrated into a speed, which again can be integrated over time into a distance. Even the smallest error in the acceleration will result in a wrong speed and distance computation. The same for the height, where a small decrease of gravity (Z axis) will indicate a constant loss of height. All that only for a straight flight route, with no curves and no wind affecting the speed and heading.

More sensors can be added, like a magnetometer which reports the absolute heading, wind speed sensors for the relative speed, and a height sensor for the relative height over ground. The determination of the absolute speed requires the same sensors at stationary positions along the flight route, at least one when no turbulences are assumed during the flight time. The same for the height, where in the simplest case the height over a flat surface equals the absolute height.

Now it would be helpful if you disclose the exact purpose of your flight monitoring, perhaps there exist better methods for collecting the required information. See e.g. instrument landing system (ILS).

" It will move forward, perhaps 25 feet before stopping "

Why does it only go such a short distance?

It can be done (look up INS - Inertial Navigation System), however, to get decent results, the gyros and accelerometers are going to be very expensive to give the detail you want. The gyros can give roll, pitch and yaw etc. because they create their own "frame of reference" - the trick is using gyros that are good enough to maintain that frame over time without drifting. The acceleration (in x,y,z) is used to then determine speed and distance relative to the start (or where they were reset) and you need to be able to read them very accurately or you miss out on subtle changes where the object is changing speed or direction with a slow rate of change. The longer the time being spanned and the slower the changes, the harder it is to maintain an accurate location without really good hardware. The fact that the integrators used for the accels are going to be in the digital domain (in your processor), gives hope for much better results than older analog integrators that are subject to leakage in the capacitors etc. Just be careful not to overflow the integrator - which rocket was that years ago that self destructed on take-off when the acceleration (if I remember correctly) overflowed causing the auto-destruct to think it had gone to zero? It was a European rocket of some kind from what I remember.

gpsmikey:
It can be done (look up INS - Inertial Navigation System), however, to get decent results, the gyros and accelerometers are going to be very expensive to give the detail you want.

This would be the case if sensor errors accumulated over a long time, but the application as stated is a glider that only travels a short distance. Presumably the duration of flight is only going to be a couple seconds at most so I would expect that a MEMS sensor would be reasonably good from the brief description we’ve seen.

I am very grateful you have all taken the time to reply. Thank you.

The answers are all clear - if slightly at odds sometimes - and I need to reflect on what you’ve told me.

I can clarify in some instances: it must be just an internal sensor, there will be no external measure; it will travel longer than 25 ft but back again; the glider was just the easiest way for me to convey a concept i.e. tracking the small movements of a self-directed object over a distance in one direction, before it reverses course.

Supplementary question: I assume a GPS would not be helpful because (again) I don’t care where the object is in the world; and my understanding is that a typical GPS cannot measure small changes.

If you’ve any suggestions for links to read before I bother anyone with more questions, they would be welcome.

Thanks again Angus

Have you looked into the concepts of differential GPS? Dwight