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Simple doppler-shift velocity

D

Dan Major

Jan 1, 1970
0
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?
 
F

Fred Bloggs

Jan 1, 1970
0
Dan said:
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?

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.
 
J

Jim Thompson

Jan 1, 1970
0
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?

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
 
P

Phil Hobbs

Jan 1, 1970
0
Dan said:
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?

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
 
J

John Larkin

Jan 1, 1970
0
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


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

John
 
J

Jim Thompson

Jan 1, 1970
0
And any oscillator is going to be trashed by the acceleration of the
rocket a lot more than that.

John

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

...Jim Thompson
 
J

john jardine

Jan 1, 1970
0
Dan Major said:
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?

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
 
M

Mac

Jan 1, 1970
0
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?

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
 
Y

YD

Jan 1, 1970
0
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

Redshift? :)

- YD.
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Jim said:
It just occurred to me that the easiest way would be to use an
accelerometer and down-link the data.

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

Paul Hovnanian P.E.

Jan 1, 1970
0
Jim said:
The frequency shift will be Vrocket/C(speed of light)*Ftransmitted =
149Hz per GHz of Ftransmitted at 100MPH... not real easy to measure
accurately.

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.
 
D

Dan Major

Jan 1, 1970
0
The frequency shift will be Vrocket/C(speed of light)*Ftransmitted =
149Hz per GHz of Ftransmitted at 100MPH... not real easy to measure
accurately.

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.
 
H

Hal Murray

Jan 1, 1970
0
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.

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?
 
G

GPG

Jan 1, 1970
0
It might
also be a good idea to put filaments along the bottom of the fins,
And making them a tuned length.
If the diameter is sufficient, a corner reflector in the nosecone.
 
R

Rene Tschaggelar

Jan 1, 1970
0
GPG said:
It might
And making them a tuned length.
If the diameter is sufficient, a corner reflector in the nosecone.

For a measurement from a satellite ?

Rene
 
R

Rich Grise

Jan 1, 1970
0
It might
And making them a tuned length.
If the diameter is sufficient, a corner reflector in the nosecone.

If he can find a 10 GHz radar gun, a corner reflector could be pretty
small, like 3 CM or so, I think.

Good Luck!
Rich
 
J

John Larkin

Jan 1, 1970
0
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?


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
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Hal said:
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?

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.
 
J

john jardine

Jan 1, 1970
0
Nicholas O. Lindan said:
Time dilation.

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
 
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