Simple doppler-shift velocity

And any oscillator is going to be trashed by the acceleration of the rocket a lot more than that.

John

Reply to
John Larkin
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The frequency shift will be Vrocket/C(speed of light)*Ftransmitted =

149Hz per GHz of Ftransmitted at 100MPH... not real easy to measure accurately.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

It just occurred to me that the easiest way would be to use an accelerometer and down-link the data.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
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Reply to
Jim Thompson

This is pretty hard to do for a police radar. You can get really cheap and accurate crystal oscillators intended for GPS, cell phones, and such truck. Don't know how they handle shock, though. An easier method might be to make a transponder that echoed a tone burst after a short delay, e.g. via a SAW filter. You could look at the phase shift between the transmit and received bursts. The good news about this is that it keeps all the accuracy on the ground, and only relative accuracy is important.

Cheers,

Phil

Reply to
Phil Hobbs

Or store it and download it to a PC once the rocket has been recoverd.

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Paul Hovnanian P.E.

How about an ultrasonic piezo transmitter on the rocket and a directional microphone on the ground with a frequency counter. I don't know if the range will suit the application for any reasonable transmitter power.

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Paul Hovnanian P.E.

I was pondering over on the model rocket newsgroup about a simple way to measure the velocity of a model rocket. One poster had come up with the idea of using a "surplus" police radar set on e-bay. The problem with this is the extremely small RCS of a paper-and-balsa rocket traveling away from the radar. My idea was to put a small fixed-frequency transmitter in the rocket. A ground-based receiver tuned to the same frequency would notice a doppler shift as the rocket accellerated away. Questions:

1) is this practical? 2) could the error signal from a phase locked loop be used to indicate the doppler shift? 3) can this be easily/cheaply (under $50) implemented?
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Reply to
Dan Major

Why limit yourself to "reasonable" transmit power?

How about a whistle run my the rocket going through the air or something similar run by the rocket engine?

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Reply to
Hal Murray

A Doppler detector only gives you radial velocity- that is the frequency shift proportional to the time rate of change of straight line distance to the rocket.

Reply to
Fred Bloggs

Redshift? :-)

- YD.

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Reply to
YD

this

a

Maybe flash a couple leds on the rocket at a known rate and then video the event. Calculate speed from a frame by frame replay. regards john

Reply to
john jardine

The only PRACTICAL way I can envision anything like this working is to use a PLL-based transponder on the rocket, and homodyne detection on the ground equipment. As long as the PLL stayed locked, you would get your doppler shift, just like you expect.

I don't know what frequency bands, if any, would be legal for that application. FCC issues aside, the ideal would be to use a band compatible with the aforementioned surplus police radar, so that the radar itself could be the detector. But the PLL will require time to lock, so this will only work if the police radar is CW, and I don't know if it is.

Another option is to increase the RCS of the rocket by, for example, putting a thin filament of wire the length of the tube. It might also be a good idea to put filaments along the bottom of the fins, and bond these electrically to the filament on the tube.

Another thing to keep in mind is that they use radar to measure the velocity of tennis balls and baseballs, so maybe the rocket RCS is sufficient even without my suggested enhancements.

--Mac

Reply to
Mac

It might

And making them a tuned length. If the diameter is sufficient, a corner reflector in the nosecone.

Reply to
GPG

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

Well.....snot! I got lazy and didn't do the math. Mod-rocks and mid- power rockets *usually* don't go above mach 1. A special class called mach-busters are designed to do just that (but since the rockets are small and very fast moving, they are almost never seen again). There are telemetry and data recorders available, but I was looking for something cheap and easy. The rocket transmitter would also have the benefit of being a tracking transmitter (homing beacon). There are several bands of frequencies that could be used at low power - the 27 and 49 MHz Citizen's Band, 2 meter (would need ham license), 433 MHz, 900 MHz, 1.5 GHz and 2.4 GHz. Some of these are "remote control" frequencies, others are "cordless phone" frequencies. Well, thanks for all the answers. I'll keep thinking about this, but it just doesn't look like it's practical.

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Reply to
Dan Major

Time dilation.

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Reply to
Nicholas O. Lindan

For a measurement from a satellite ?

Rene

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Reply to
Rene Tschaggelar

A few of my engineering guys launch rockets on weekends, out past Livermore somewhere. They use a PIC-based board that includes an accelerometer and a barometric-pressure sensor, with data stored in flash. The accelerometer is used to calculate speed and to deploy the drogue chute (apparently there's a usable signal at apogee somehow) and the barometric estimates max altitude and deploys the main chute close to the ground.

Apparently there's a standard board for this; I can get details if you're interested. But these are *not* itty bitty rockets!

John

Reply to
John Larkin

That might work as well, but I know less about how the aerodynamics of such a whistle might affect a rocket than the electronic solution.

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Paul Hovnanian P.E.

the

Well go on then! ... I know I was bored at the time and it's a crap idea anyway but what glaring bollock did I drop that you guys instantly noticed? :-)

Spent ages trying to figure why the idea cant' work, (well, the past 30 seconds). Simplisticly ... . If an insanely bright led, flashes once per second, for say a 20ms period then only particular video frames will catch the led as being lit. The 25 frames per second provides a timebase. No ?.

regards john

Reply to
john jardine

video

[clip bollockspeak] And 30 seconds later realise I've no distance reference :-(
Reply to
john jardine

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