Need advice for multilateration setup
On Thu, 26 Mar 2015 12:32:33 -0500
Robert Watzlavick rocket@watzlavick.com 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.
The DSSS allows you to make the integer ambiguity, you have with all
periodic signals low enough that you dont care anymore. Ie. if you
have a PRN that repeates every millisecond, then your you will have
an ambiguity of n*300km, which you can easily resolve. The other advantage
is that you have multiple edges (not just one, when you have a single pulse)
over which you can average, thus getting a better precision.
The downside of this is, that you have not only to solve for position and time,
but for position, velocity and time (or rather frequency of the oscillator).
The idea with the reference station on ground, to sync up all
other stations is quite good. Then you can use simple DVB-T dongles
(google RTL-SDR) as receivers, which you get almost for free on ebay.
But you pay for that in higher calculation complexity. On the other
hand, adding another measurment station is just another PC + USB dongle.
I think that most of the receiver work can be done with gnu radio
as basis. But i have never done any DSSS system in GR, so i cannot
say for sure.
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
Robert;
It seems that a Doppler system should work for you.
But first, you have a problem. If you want to track your rocket
to 100K feet (20 miles) using some form of triangulation then you
need your receiving stations further apart than 1 mile. Your
triangle is too extreme and any measurement error will be greatly
amplified.
Here is what I suggest.
Place a simple transmitter in the rocket of say 100 MHz. It
really should be a legal frequency, 2 meter ham band? The
transmitted frequency is not modulated and should be stable
for the duration of the flight.
The receiving stations should have a very narrow receive filter
on the front end and mix the signal with a local oscillator that is
5 KHz off from the rocket frequency. For example: 100.005 MHz.
A narrow audio filter will help as well. This is results in a
very narrow bandwidth receiver which is very good in rejecting
received noise.
Take the audio signal and feed it into a computer's audio input.
Sample the audio A/D converter as fast as you can and timestamp
each sample. The computer's clock should be synchronized with
your GPS receiver's time.
This system measures velocity relative to your vantage point.
Because distance is the integral of velocity you can calculate
the distance during your flight. Since the initial positions
are known you can calculate absolute position.
If we assume a 100 MHz transmitter and with the speed of light
at 300,000 KM/S you will see about 1/3 of a HZ shift for each 1 M/S
of velocity.
You do not need super stable oscillators. They only need
to be stable for the duration of the flight.
Here is how the flight will be tracked:
Before the flight, the ground stations will receive the 100 MHz
from the rocket and record the offset between the rocket's
oscillator and the local oscillator. Any error will show up
as the 5 KHz being somewhat off. This is not a problem if
it remains constant during the flight.
Before the flight the computer logs the audio input data
with the timestamp. This is the reference data.
When the rocket is launched the computer continues logging
but should notice the shift in frequency. The entire set
of logged data should show the velocity profile for the entire
flight. This can be converted to distance since all of the
initial positions of the ground stations and the rocket are
known. Using the data from all the ground stations you can
calculate the absolute position of the rocket for the entire
flight.
This setup should easily fit within your budget. The crystal
oscillators do not need to be super precise or stable.
They only need to be stable for the duration of the flight
since the system calibrates itself immediately before launch.
Pete.
Robert Watzlavick 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.
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. The idea is to use a transmitter in the rocket and have 4 or more
ground stations about a mile apart each receive the signal.
Multilateration based on TDOA (time difference of arrival)
measurements would then be used to determine x, y, z, and t. With at
least 4 ground stations, you don't need to know the time the pulse was
transmitted. The main problem I'm running into is that most of the
algorithms I've come across are very sensitive to the expected
uncertainty in the time measurements. I had thought 100 ns of timing
accuracy in the received signals would be good enough but I think I need to
get down less than 40 ns to keep the algorithms from blowing up. My desired
position accuracy is around 100 ft up to a range of 100k ft.
There were two different methods I thought of. The first method would
transmit a pulse from the rocket and then use a counter or TDC on the
ground to measure the time difference between a GPS PPS and the pulse
arrival. This is the most straightforward method but I'm worried about
the timing accuracy of the pulse measurement. I should be able to find a
timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma)
so that portion is in the ballpark.
There also seem to be TDCs that have accuracy and resolution in the tens
of picosecond range but they also have a
maximum interval in the millisecond range. I'm not sure I can ensure the
pulse sent from the rocket will be within a
few miilliseconds of the 1 PPS value on the ground. I will have
onboard GPS before launch so in theory I could initialize a counter to
align the transmit pulse within a millisecond or so to the onboard
PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard
that pulse may drift too far away (due to temperature, acceleration,
etc.) for the TDC on the ground to pick it up. Plus an OCXO will add
weight and require extra power for the heater. Another idea would be to
send pulses at a very fast rate, say 1 kHz to stay within the TDC window.
But then I need to worry about what happens if the pulses get too close to
the edge of the TDC window. One other variable is the delay through the RF
chain on the receive end but I figure I could calibrate that out.
The other idea, and I'm not sure exactly how to implement it, would be
to transmit a continuous tone (1 kHz for
example) and perform some kind of phase measurement at each ground
station against a reference. I could use a GPSDO to divide down the
10 MHz to 1 kHz to compare with the received signal but how can I
assure the divided down 1 kHz clocks are synchronized between ground
stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to
each other? I let two Thunderbolts sit for a couple of hours and the
10 MHz outputs seemed to stabilize with an offset of about 1/4 of a
cycle, too much for this application. Another related idea would be to
use the 10 MHz output to clock an ADC and then sample several thousand
points using curve fitting, interpolation, and averaging to get a more
accurate zero crossing than you could get based on the sample rate alone.
Adding a TDC would allow the use of RIS (random interleaved sampling) for
repetitive signals which could generate an effective sample rate of 1 GS/s.
Does anybody have advice or practical experience on which method would
work better?
Thanks,
-Bob
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Your second method is by far the best. But it can be simplified. All you
need is two very stable oscillators, one in the rocket and one some known
fixed location. Then you ground stations can be just dumb recorders that
record both signals. In post processing you compare the relative phases.
Likely the rock has a transmitter already so all you need is a very good
oscillator on the ground. This one transmits to all you ground stations
This technique has. Even been used to analyze serious failures of large
rockets. Transmitters are packed with batteries and continue after the
explosion. They have recovered spin rates and so on of falling derbies.
On Wednesday, March 25, 2015, 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.
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. The idea is
to use a transmitter in the rocket and have 4 or more ground stations about
a mile apart each receive the signal. Multilateration based on TDOA (time
difference of arrival) measurements would then be used to determine x, y,
z, and t. With at least 4 ground stations, you don't need to know the time
the pulse was transmitted. The main problem I'm running into is that most
of the algorithms I've come across are very sensitive to the expected
uncertainty in the time measurements. I had thought 100 ns of timing
accuracy in the received signals would be good enough but I think I need to
get down less than 40 ns to keep the algorithms from blowing up. My
desired position accuracy is around 100 ft up to a range of 100k ft.
There were two different methods I thought of. The first method would
transmit a pulse from the rocket and then use a counter or TDC on the
ground to measure the time difference between a GPS PPS and the pulse
arrival. This is the most straightforward method but I'm worried about the
timing accuracy of the pulse measurement. I should be able to find a
timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2
sigma) so that portion is in the ballpark. There also seem to be TDCs that
have accuracy and resolution in the tens of picosecond range but they also
have a maximum interval in the millisecond range. I'm not sure I can
ensure the pulse sent from the rocket will be within a few miilliseconds of
the 1 PPS value on the ground. I will have onboard GPS before launch so in
theory I could initialize a counter to align the transmit pulse within a
millisecond or so to the onboard PPS. But, once GPS is lost on ascent,
unless I put an OCXO onboard that pulse may drift too far away (due to
temperature, acceleration, etc.) for the TDC on the ground to pick it up.
Plus an OCXO will add weight and require extra power for the heater.
Another idea would be to send pulses at a very fast rate, say 1 kHz to stay
within the TDC window. But then I need to worry about what happens if the
pulses get too close to the edge of the TDC window. One other variable is
the delay through the RF chain on the receive end but I figure I could
calibrate that out.
The other idea, and I'm not sure exactly how to implement it, would be to
transmit a continuous tone (1 kHz for example) and perform some kind of
phase measurement at each ground station against a reference. I could use
a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received
signal but how can I assure the divided down 1 kHz clocks are synchronized
between ground stations? Are the 10 MHz outputs from GPSDOs necessarily
aligned to each other? I let two Thunderbolts sit for a couple of hours
and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a
cycle, too much for this application. Another related idea would be to use
the 10 MHz output to clock an ADC and then sample several thousand points
using curve fitting, interpolation, and averaging to get a more accurate
zero crossing than you could get based on the sample rate alone. Adding a
TDC would allow the use of RIS (random interleaved sampling) for repetitive
signals which could generate an effective sample rate of 1 GS/s.
Does anybody have advice or practical experience on which method would
work better?
Thanks,
-Bob
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--
Chris Albertson
Redondo Beach, California
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. Time nuts easily notice the reversal
in a 1G force on a laboratory oscillator caused by flipping
it on its back for service.
But all is not even close to lost.
If your transmitter is amplitude modulated with a rate that
is a digital division of your crystal's frequency, then you
can remove any G-variation in the crystal's frequency by
observing frequency variations in your modulation.
Doppler will change the carrier frequency with speed, but it
won't change the amplitude modulation frequency.
Otherwise it should work beautifully.
-Chuck Harris
Peter Reilley wrote:
Robert;
It seems that a Doppler system should work for you.
But first, you have a problem. If you want to track your rocket
to 100K feet (20 miles) using some form of triangulation then you
need your receiving stations further apart than 1 mile. Your
triangle is too extreme and any measurement error will be greatly
amplified.
Here is what I suggest.
....
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.
Chris Albertson
Redondo Beach, California
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.
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
Depending on construction of the resonator, an oscillator can have maximum
sensitivities anywhere from 5x10^-8 / g to 5x10^-11 per G. Typical numbers
for “good but not great” parts are in the 5x10^-10 to 2x10^-9 per G.
Since the sensitivity is not the same in every axis, a device with 2x10^-9 in (say)
the X-Y axis might have a 1x10^-10 sensitivity in (say) the Z axis. In something
like a rocket, your acceleration is likely to have a dominant axis. With characterization
data on the individual oscillator, you might be able to reduce the impact by 10:1.
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).
If the oscillator has a 1 ppm / C temperature coefficient, a 20C change will give you
the same (static) frequency shift. If you change temperature quickly (as you would in this
case, you hit outer space in a few seconds) figure a 5 to 10X increase in that shift.
Simply put - temperature will get you before acceleration does in terms of static shift. There
are other things that will be a problem before either of these get in your way.
Most tracking assumes good phase noise on the signal. Oddly enough rockets are not
very quiet devices while accelerating. The same sensitivities that give you the issues from
static acceleration give you phase noise under vibration. It is not at all unusual to see
phase noise degradation of >60 db on physical small platforms doing high levels of acceleration.
Bob
On Mar 28, 2015, at 8:25 AM, Peter Reilley peter@reilley.com 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.
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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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.
Hi
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:
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.
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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.