Need advice for multilateration setup
Hi
Calibrating the G sensitivity of the oscillator can be done much
more easily by simply rotating it 360 degrees while carefully reading
out the frequency. If you want the full vector, you will need to rotate
it through two circles, with the plane of one 90 degrees out relative to the other.
The net result is that you get a 2G change in acceleration in each axis. Measure
the frequency to 1x10^-10 every 10 degrees and you have what you need. You
will need to keep the temperature / voltage / whatever stable enough that you
don’t have more than 1x10^-10 drift through the process. That’s the main reason
for taking two readings at the the same angle, one at the start and one at the
end of the process.
Far easier to do in a static fixture on the ground than to extract it from telemetry
after the fact. The temperature outside your rocket is dropping at around 3C for
every 1,000 feet you go up. At 10G’s your are going through 1,000 feet pretty quick.
Just the 3 C in the first 1,000 feet will move your frequency 3 ppm while you are trying
to measure a 2x10^-8 shift.
============
So, if you put a double oven in the rocket and put a thermal shield around it, (possibly
using the lead acid batteries you are powering it with) - you could get around the
thermal shift to some degree. Of course the extra 20 or 30 pounds of weight might
impact your weight budget a bit :)
============
Bottom line is still the same, you don’t need to worry about the acceleration impact
on the static frequency. You do need to worry about it’s impact on phase noise and your
carefully worked out modulation scheme. This does not just apply to amateur rockets and
working out the RF systems on them. Some fairly large defense systems have run into this
issue pretty hard.
Bob
On Mar 28, 2015, at 10:34 PM, Bill Hawkins bill@iaxs.net wrote:
An idea occurred (always a surprise):
The rocket's acceleration increases from 1 g as the mass of fuel is
ejected energetically, according to f=ma, with pretty constant force
from the motor. At some point, the fuel and oxidizer tanks are empty
(MECO), causing the acceleration to revert to 1 g or less, depending on
altitude. The change from max acceleration to free flight offers an
opportunity to calibrate the effect of max g on the oscillator. The
velocity is almost unchanged at that point, so the change in Doppler
shift comes only from the effect of acceleration on the oscillator. It
should be possible to use linear interpolation for the effect of
acceleration during powered flight, since f=ma is a first order
equation.
Bill Hawkins
-----Original Message-----
From: Bob Camp
Sent: Saturday, March 28, 2015 6:22 PM
The point being that, to even get acceleration into the picture, you
need have impossibly high accelerations .
At 10 G, your oscillator needs to be temperature stable to < 0.01C to
even see the acceleration. If you are climbing 100K feet during the
acceleration phase the oscillator will see a lot more than that.
Bob
On Mar 28, 2015, at 5:01 PM, Jim Lux jimlux@earthlink.net wrote:
On 3/28/15 10:27 AM, Bob Camp wrote:
So If the rocket continuously accelerates at 10,000 G's, you will
get a 20 ppm shift with typical sensitivity. If you do this for very
long, you will also get into time dilation issues.
(you hit 0.1C in < 2 minutes).
10,000G is more like an artillery shell.
A large amateur rocket might be more like 20-30G maximum.
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I want to thank everybody for their help on this. Thanks to the list, I
have plenty of ideas that I can prototype so I'll keep you posted what I
end up trying and how well it works eventually.
-Bob
On 03/25/2015 09:27 PM, Robert Watzlavick wrote:
I want to develop a tracking system for an amateur rocket that can
allow me to track the rocket even if onboard GPS is lost (as is
typical during ascent and sometimes during descent) or if telemetry is
lost.
Hi,
On 03/26/2015 01:25 PM, Attila Kinali wrote:
On Wed, 25 Mar 2015 21:27:35 -0500
Robert Watzlavick rocket@watzlavick.com wrote:
I'm working on a project that I could use some advice on and also might
be of interest to the list. If it's not appropriate for the list, my
apologies.
The gods have apporved of your request. You may speak now.
;-)
I want to develop a tracking system for an amateur rocket that can allow
me to track the rocket even if onboard GPS is lost (as is typical during
ascent and sometimes during descent) or if telemetry is lost.
Given you can synchronize the clocks of the ground stations well
enough, then the rest is "easy". Then you can get away with having
a simple signal generator that only needs an XO. Or you can go
for a TCXO to make your signal processing life easier.
What you need to do, is actually the same as GPS does: Create a
direct spread spectrum signal and track it on all ground stations.
The DSSS has the advantage over the single pulse, that it's more
resilient against noise and interference. The disadvantage is, that
you have to have more complicated hardware. One viable way would be,
that you have precisly synchronized sampling systems (e.g. SDR's like
the bladeRF which can take an external clock) and then feed the data
to a PC where you do the heavy lifting. Then you don't need to build
custom hardware at least.
Also, if the precision by the DSSS signal is not good enough, you can
apply various tricks from the GPS world, like carrier phase tracking, etc.
I think this is a good idea, and it is relatively straight-forward to do.
You can observe both code and carrier phase this way, given that the
transmitting radio is coherent with the code generation clock. Doppler
also pops out of the tracking station.
A good coding-gain reduces the need for a strong transmitter.
The issue might be the allowed width of the signal being transmitted,
forcing the chipping rate down.
Cheers,
Magnus
Remember that you can actually let each base-station transmit at a
different code, and you can then monitor them that way. You could even
keep them frequency and phase locked or just monitor it and adjust it in
the post-processing. Such an approach would be a nice complementary
solution to the GPS/GNSS receivers. Also, it's "more of the same" which
helps in knowing your system.
Cheers,
Magnus
On 03/26/2015 06:32 PM, Robert Watzlavick wrote:
Thanks for the suggestion. Does the DSSS make it easier to correlate between ground stations? I'm not sure how to handle the phase offset on the 10 MHz ref clocks.
-Bob
On Mar 26, 2015, at 07:25, Attila Kinali attila@kinali.ch wrote:
On Wed, 25 Mar 2015 21:27:35 -0500
Robert Watzlavick rocket@watzlavick.com wrote:
I'm working on a project that I could use some advice on and also might
be of interest to the list. If it's not appropriate for the list, my
apologies.
The gods have apporved of your request. You may speak now.
;-)
I want to develop a tracking system for an amateur rocket that can allow
me to track the rocket even if onboard GPS is lost (as is typical during
ascent and sometimes during descent) or if telemetry is lost.
Given you can synchronize the clocks of the ground stations well
enough, then the rest is "easy". Then you can get away with having
a simple signal generator that only needs an XO. Or you can go
for a TCXO to make your signal processing life easier.
What you need to do, is actually the same as GPS does: Create a
direct spread spectrum signal and track it on all ground stations.
The DSSS has the advantage over the single pulse, that it's more
resilient against noise and interference. The disadvantage is, that
you have to have more complicated hardware. One viable way would be,
that you have precisly synchronized sampling systems (e.g. SDR's like
the bladeRF which can take an external clock) and then feed the data
to a PC where you do the heavy lifting. Then you don't need to build
custom hardware at least.
Also, if the precision by the DSSS signal is not good enough, you can
apply various tricks from the GPS world, like carrier phase tracking, etc.
HTH
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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Remember, that if you have 4 receivers you get X, Y, Z and T of the
source, and in this case T will be the phase-drift of the rocket. So, if
logged with sufficient precision, the stability of the on-board clock
may not become as important as the fact that it is there and has
reasonably good phase-noise. That however, might be an issue for
sounding-rockets, but can be addressed to some degree by mounting.
Cheers,
Magnus
On 03/28/2015 01:25 PM, Peter Reilley wrote:
Some crystal oscillators specify their sensitivity to G forces.
Here is one:
http://www.abracon.com/Precisiontiming/AOCJYR-24.576MHz-M6069LF.pdf
Available here:
http://www.digikey.com/product-detail/en/AOCJYR-24.576MHZ-M6069LF/535-12627-
1-ND/4989033
Others specify shock and vibration limits but say nothing about
frequency stability.
Pete.
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of Chris
Albertson
Sent: Friday, March 27, 2015 9:55 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Need advice for multilateration setup
On Fri, Mar 27, 2015 at 10:29 AM, Chuck Harris cfharris@erols.com wrote:
The biggest problem I see is the crystal oscillator in the rocket is
going to notice the G forces during acceleration in a pretty big way.
But all of the ground stations will see the same frequency shift on the
rocket's transmitter. I think this can be backed out in processing.
Someone needs to write the equations and post them here.
Jim,
On 03/28/2015 10:01 PM, Jim Lux wrote:
On 3/28/15 10:27 AM, Bob Camp wrote:
Hi
So If the rocket continuously accelerates at 10,000 G’s, you will get
a 20 ppm shift
with typical sensitivity. If you do this for very long, you will also
get into time dilation issues.
(you hit 0.1C in < 2 minutes).
10,000G is more like an artillery shell.
A large amateur rocket might be more like 20-30G maximum.
Also, it's not 10000 G for very long, it's the fireing moment, which is
critical for any oscillator flying with it. The impact moment is somehow
less important as it is intended to self-destruct most of the times.
Cheers,
Magnus
I have an amateur radio license (mostly CW/HF and some VHF/UHF
experience) and I've written some driver software for an IQ demodulation
board but I have to admit, I would have no idea how to begin setting up
that system as initially described by Attila and expanded by you and
others. I have a rudimentary understanding of the modulation schemes
involved but I don't fully understand how the various codes mentioned
fit in. I've poked around a bit at some articles on PN codes and I can
see how data would be transmitted but I think I'm missing something key
that allows you to extract positions, velocities, etc. out of the
various links. I think I have some more reading to do :)
Thanks,
-Bob
On 04/03/2015 06:08 AM, Magnus Danielson wrote:
I think this is a good idea, and it is relatively straight-forward to do.
You can observe both code and carrier phase this way, given that the
transmitting radio is coherent with the code generation clock. Doppler
also pops out of the tracking station.
A good coding-gain reduces the need for a strong transmitter.
The issue might be the allowed width of the signal being transmitted,
forcing the chipping rate down.
Cheers,
Magnus
To head off a bunch of replies - I think I stumbled upon what is being
suggested. To extract the pseudorange, you have to figure out the
offset of the locally generated PN code against the one that is
received. In this reverse GPS case, I assume each ground station would
have to start their local PN codes at the same time? Then you would be
able to get the pseudoranges at each ground station and use those values
for the multilateration equations. You still would have an uncertainty
of one clock cycle since the phases of the local clocks at the stations
wouldn't be aligned but several folks have suggested ways around that.
-Bob
On 04/03/2015 10:12 PM, Robert Watzlavick wrote:
I have an amateur radio license (mostly CW/HF and some VHF/UHF
experience) and I've written some driver software for an IQ
demodulation board but I have to admit, I would have no idea how to
begin setting up that system as initially described by Attila and
expanded by you and others. I have a rudimentary understanding of the
modulation schemes involved but I don't fully understand how the
various codes mentioned fit in. I've poked around a bit at some
articles on PN codes and I can see how data would be transmitted but I
think I'm missing something key that allows you to extract positions,
velocities, etc. out of the various links. I think I have some more
reading to do :)
Thanks,
-Bob
On 04/03/2015 06:08 AM, Magnus Danielson wrote:
I think this is a good idea, and it is relatively straight-forward to
do.
You can observe both code and carrier phase this way, given that the
transmitting radio is coherent with the code generation clock.
Doppler also pops out of the tracking station.
A good coding-gain reduces the need for a strong transmitter.
The issue might be the allowed width of the signal being transmitted,
forcing the chipping rate down.
Cheers,
Magnus
We essentially propose that you mimic the GPS system.
The original GPS birds are relatively stupid.
In GPS, the core clock produces 10,23 Mhz (modern GPS rubidiums output a
different frequency, but that is not the point here), for C/A code it is
divided down with 10 to produce the C/A chipping rate of 1,023 MHz and
considering that the Gould-codes being used is 1023 chips long, they
will wrap around every 1 ms. The same 10,23 MHz is then used to produce
the carrier frequency which is 154 * 10,23 MHz. The produced PRN
sequence alternate between +1 and -1 and when mixing this with the
carrier frequency a BPSK signal is produced which is amplified and
transmitted. A second carrier is also produced as 120 * 10,23 MHz.
This is on either side of the amateur 23 cm band. That's also the first
band where you have bandwidth enough to fool around with stuff like this
without breaking the bandplan.
Cheers,
Magnus
On 04/04/2015 05:12 AM, Robert Watzlavick wrote:
I have an amateur radio license (mostly CW/HF and some VHF/UHF
experience) and I've written some driver software for an IQ demodulation
board but I have to admit, I would have no idea how to begin setting up
that system as initially described by Attila and expanded by you and
others. I have a rudimentary understanding of the modulation schemes
involved but I don't fully understand how the various codes mentioned
fit in. I've poked around a bit at some articles on PN codes and I can
see how data would be transmitted but I think I'm missing something key
that allows you to extract positions, velocities, etc. out of the
various links. I think I have some more reading to do :)
Thanks,
-Bob
On 04/03/2015 06:08 AM, Magnus Danielson wrote:
I think this is a good idea, and it is relatively straight-forward to do.
You can observe both code and carrier phase this way, given that the
transmitting radio is coherent with the code generation clock. Doppler
also pops out of the tracking station.
A good coding-gain reduces the need for a strong transmitter.
The issue might be the allowed width of the signal being transmitted,
forcing the chipping rate down.
Cheers,
Magnus
Hi Bob,
The actual receiver logic is that you have some sampling point in time,
the tracking phase of a channel is being sampled. As you do for multiple
channels, the relative phase of each channel is sampled.
In order to extend this phase into a pseudo-range, one needs to guess
how many integer multiples of the code there is from each GPS to the
receiver. A bunch of multiples is assumed from the orbit, as there is at
least the delay of the shortest distance, and then you can make a rough
estimate by the sub-code phase of the birds, as they hook up like a set
of clock-work gears. That gives you a first approximate guess, which
might be wrong, but as we try to make it fit, we can solve this equation
and out pops a first rough estimate, from that we can then maintain a
correct guess from then on.
For your rocket, you have a known stable situation at the launch-pad.
That cuts out the guess-work, as at that point, you can assume that
there is no multiple as your measurement nodes are within range.
Cheers,
Magnus
On 04/04/2015 05:51 AM, Robert Watzlavick wrote:
To head off a bunch of replies - I think I stumbled upon what is being
suggested. To extract the pseudorange, you have to figure out the
offset of the locally generated PN code against the one that is
received. In this reverse GPS case, I assume each ground station would
have to start their local PN codes at the same time? Then you would be
able to get the pseudoranges at each ground station and use those values
for the multilateration equations. You still would have an uncertainty
of one clock cycle since the phases of the local clocks at the stations
wouldn't be aligned but several folks have suggested ways around that.
-Bob
On 04/03/2015 10:12 PM, Robert Watzlavick wrote:
I have an amateur radio license (mostly CW/HF and some VHF/UHF
experience) and I've written some driver software for an IQ
demodulation board but I have to admit, I would have no idea how to
begin setting up that system as initially described by Attila and
expanded by you and others. I have a rudimentary understanding of the
modulation schemes involved but I don't fully understand how the
various codes mentioned fit in. I've poked around a bit at some
articles on PN codes and I can see how data would be transmitted but I
think I'm missing something key that allows you to extract positions,
velocities, etc. out of the various links. I think I have some more
reading to do :)
Thanks,
-Bob
On 04/03/2015 06:08 AM, Magnus Danielson wrote:
I think this is a good idea, and it is relatively straight-forward to
do.
You can observe both code and carrier phase this way, given that the
transmitting radio is coherent with the code generation clock.
Doppler also pops out of the tracking station.
A good coding-gain reduces the need for a strong transmitter.
The issue might be the allowed width of the signal being transmitted,
forcing the chipping rate down.
Cheers,
Magnus