Using doppler radar to record rocket speeds

we have measured R/C Airplanes with them, dead head on.

I guess you would need to get under the rocket, and also, the top end might not be their, ie, I don't think MPH was making them read out 400-900 mph.

An RC airplane has a larger profile than most model rockets.

I wonder if the smoke trail will cause a problem?

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

kaplow snipped-for-privacy@encompasserve.org.TRABoD (Bob Kaplow) wrote in news: snipped-for-privacy@eisner.encompasserve.org:

Has a lot more metal in it,too.You need something for radar reflectivity.

It shouldn't,IMO.

Of course,the farther off axis for the radar gun,the slower the radar will indicate compared to actual,IIRC,it's called cosine error.

This is a fairly old unit. It might have a hard time with a rocket. Remember that to accurately measure speed with dopler radar, you must be as close as possible to directly behind or in front of the target. There isn't a lot of difference up to about 8 degrees, but after that the measured speed will be significantly LESS than actual. At 90 degrees to the target, the measured speed will be zero, because there is no change in distance from the target to the radar as the target moves. The target will also need to be relective. A 29 mm RMS might be big enoungh for a modern police radar, but they are very small from behind. An Applewhite saucer would be a good target, especially if the rear surface was reflective foil or mylar.

Just thinking out loud, but it seems like using radar to catch the back-end of a rocket going away is not the best way to do this. The RCS is extremely low, as is the radar albedo. What might be a better solution, if one is only interested in the velocity, would be to have a fixed-frequency transmitter in the rocket, and a receiver tuned to the same frequency on the ground. A simple frequency deviation measurement would give you the doppler shift. The more simple the receiver, the better, as you would not want something that would track the off-frequency signal. Some FM receivers use a phase-locked-loop to correct for tuning errors. This correction signal could also be used to measure doppler shift.

Any reflector inside the BT is still only as big as the BT from the rear. Those units were made to catch a return signal from a large metal and glass vehicle. Modern units will accurately measure the speed of a baseball, but that is significantly denser than a balsa/cardboard rocket, unless of course it has an RMS. The trick would be to get a flat surface even a few inches across perpendicular to the radar, hence my suggestion of a saucer. A cone fin might work. I guess the only way to find out would be to field test. I don't have access to an old stationary unit, but I might be able to get a hold of a handheld eventually. I also don't know the top end of measurable speed. Three digits, but the last might only be capable of displaying a "one" since cars rarely exceed 199 MPH on the highway. (Thank God.) I'll have a look at one and see.

Which is still bigger on radar than the BT.

The slotted disc at night is still the easiest cheap way.

even if it could display 399, it most likely does not have the calibration or resolution at the top end to account for it.

MPH would have traded off the resolution of the time constant to be accurate at the > 199 range.

Very good idea Dan, it could drive an op amp to display on a pic controller.