Need advice for multilateration setup
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
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
What's your budget?
Put a white-rabbit switch (3.5keur) in the middle, and install a mile of
single-mode fiber to each rx-station. Then use TDC or FDEL SPEC-cards
(1.5keur each) at the RX-stations to time-stamp the incoming pulse. <1 ns
systematic and <50 ps RMS random error should be doable. The systematic
constant error in time-stamp for each rx-station can maybe be calibrated
out in the TDOA-algorithm? The FDEL-card can time-stamp up to 100 kEdges/s
(that results in a ca 4 Mb/s datastream).
Anders
On Thu, Mar 26, 2015 at 4:27 AM, 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
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.
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
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.
Budget is a concern but not an overriding concern. I'd like to keep the whole system around $1k. I was planning on making it as portable as possible with each ground station being self contained and sending their data to the launch site over a serial RF modem at 9600 baud. I agree though - fiber connections would make it a lot easier.
-Bob
On Mar 26, 2015, at 08:41, Anders Wallin anders.e.e.wallin@gmail.com wrote:
What's your budget?
Put a white-rabbit switch (3.5keur) in the middle, and install a mile of
single-mode fiber to each rx-station. Then use TDC or FDEL SPEC-cards
(1.5keur each) at the RX-stations to time-stamp the incoming pulse. <1 ns
systematic and <50 ps RMS random error should be doable. The systematic
constant error in time-stamp for each rx-station can maybe be calibrated
out in the TDOA-algorithm? The FDEL-card can time-stamp up to 100 kEdges/s
(that results in a ca 4 Mb/s datastream).
Anders
On Thu, Mar 26, 2015 at 4:27 AM, 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
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.
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.
Sounds over complicated. Why not use an onboard triple-axis accelerometer? A few mm of real-estate, milliamp consumption, up to 16g, 600+ samples a sec. The code is probably already available.
Le 26 mars 2015 à 03:27, Robert Watzlavick rocket@watzlavick.com a écrit :
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
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.
"Ceux qui sont prêts à abandonner une liberté essentielle pour obtenir une petite et provisoire sécurité, ne méritent ni liberté ni sécurité."
Benjimin Franklin
On 3/25/15 7:27 PM, 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.
The key is that you don't need real time position.. a few seconds or
minutes delay is probably ok, right?
So transmit a PN code modulated onto a carrier from your rocket at some
convenient frequency that's legal. Drive the PN shift register from
your carrier, divided down, so there's an integer number of carrier
cycles per chip.
Receive that signal and digitize it on the ground at a suitably high rate.
Post process the sampled data to recover the timing of the PN (and carrier).
To compensate for the receiver variability, simultaneously transmit a
signal with a different PN code, at the same frequency (roughly) as the
rocket's transmitter.. The receiver will receive both, but the signal
from your ground reference transmitter isn't moving, so you can use the
"non-rocket" signal as a calibration reference.
What's your budget?
The transmitter can be very cheap.
The receiver is going to be the pricey part, depending on how it's
implemented. A sort of "brute force" approach would be to use a USRP
and a portable PC at each receiver site.
Hi Bob:
There are many ways of doing this.
To test artillery shells they have a GPS front end in the shell and transmit the IF. A receiver at the gun is locked to
the satellites prior to firing. You would want one of the 10 Hz update rage GPS receivers for this.
Another method is to transmit a pulse of RF from the ground. When the rocket receives the pulse it sends out a pulse.
When the receiver sees that pulse it makes another pulse. The repetition rate depends on the range (and fixed delays in
the circuits).
Doppler was used to determine the orbit of Sputnik. (Note: the transmitter was near a WWV frequency so the beat note
was the Doppler.) If the rocket has a stable CW transmitter and you have a few receivers in known locations on the
ground and record the Doppler for each receiver you can work out the path.
A blinking light on the rocket and video cameras on the ground. Hollywood uses reflective dots on an actor's face and
body which are watched with video cameras in a "motion capture" setup.
A 3-axis accelerometer in the rocket and 3 channels of telemetry.
&Etc.
Mail_Attachment --
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
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
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.
I've already integrated an onboard IMU (Analog Devices ADIS16xxx) but
they have a lot of drift, especially in a high-g environment. I plan to
record the raw IMU data to a flash card and assuming I can recover the
card intact, I'll use it to tune a Kalman filter algorithm for the
future version that will have active control.
I understand your point - it is a complicated solution but that's some
of the fun of the project, trying out new ideas and learning new concepts.
-Bob
On 03/26/2015 01:10 PM, Mike Cook wrote:
Sounds over complicated. Why not use an onboard triple-axis accelerometer? A few mm of real-estate, milliamp consumption, up to 16g, 600+ samples a sec. The code is probably already available.
Le 26 mars 2015 à 03:27, Robert Watzlavick rocket@watzlavick.com a écrit :
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 t
oo 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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On 03/26/2015 01:56 PM, Jim Lux wrote:
The key is that you don't need real time position.. a few seconds or
minutes delay is probably ok, right?
Seconds are probably ok, minutes might be a little long. PCs are pretty
fast though these days for signal processing I would think.
To compensate for the receiver variability, simultaneously transmit a
signal with a different PN code, at the same frequency (roughly) as
the rocket's transmitter.. The receiver will receive both, but the
signal from your ground reference transmitter isn't moving, so you can
use the "non-rocket" signal as a calibration reference.
Now I didn't think of that - so you're saying to send another signal
from a central ground station to all the receivers and then have them
use that as a relative reference? Since I'll know where each ground
station is, I should be able to subtract off the TOF so each station has
a common reference point. That's a pretty cool idea.
What's your budget?
I was thinking in the $1k range so that would be about $200 per ground
station. A couple of controllers I was considering for the ground
stations include the Netburner MOD54415 (same one I'm using for the
flight computer) or the BeagleBone Black. Both of those are under $100
and have counter/timers onboard although I have to see what the max
clock rate is. As long as the channel-to-channel delay wan't too bad, I
think using a 12-bit ADC to digitize the two signals would work because
you can interpolate to get a higher-resolution zero crossing.
-Bob
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
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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
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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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and follow the instructions there.
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.
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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and follow the instructions there.
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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and follow the instructions there.
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