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