Slightly OT - GPS-Based Accurate Direction Finding
On Thu, 26 Aug 2010 11:04:01 +1000
David Smith david@smithfamily.net.au wrote:
The system seems to work by taking the raw satellite phase information
from two separate GPS systems and crunching the data to come up with an
azimuth figure. Has anyone heard of a (Open Source?) program that could
be used to do these calculations?
AFAIK there is none out there (at least i've never seen one).
But it should be not too dificult write the software yourself.
The bigger problem is to get GPS receivers that provide you
with accurate phase information. Especially with a small baseline
you either need to sync the clocks of the two receivers or have
receivers with precise TCXO and take some additional samples to
calculate the frequency difference/drift. The precision you get
will mostly depend on the baseline and the number of samples you
use. The longer you have time to measure, the smaller your error-band
will be.
As GPS modules, you could use something like the LEA4-T/LEA6-T which
provide you the phase information you need.
Attila Kinali
--
If you want to walk fast, walk alone.
If you want to walk far, walk together.
-- African proverb
Stanley Reynolds wrote:
You still need to DF the VOR station to get your direction. guess any radio that
receives 108 to 117 MHz is good for that.
You don't need to DF the VOR if you decode the signals.. the decoded
signal (i.e. the phase difference between the 30 Hz AM and FM
modulations) tells you what direction the VOR is relative to you (or
more properly, which direction you are relative to the VOR).. however,
on the ground, you're doing real well if you're within 5-10 degrees
(multipath, diffraction and refraction are not your friends here).
You'd be better off with a $10 magnetic compass and have better accuracy.
bg@lysator.liu.se wrote:
Does anyone know how laser gyroscopes are developing?
Laser gyroscopes - as in Ring Laser Gyroscopes or as in Fiber Optic
Gyroscopes?
RLGs are a standard commercial product. Several years back I was
walking through the Honeywell plant in St Paul, MN, and they had a
display case of at least a dozen RLGs that they've made over the past
few decades.
Attila Kinali wrote:
On Thu, 26 Aug 2010 11:04:01 +1000
David Smith david@smithfamily.net.au wrote:
The system seems to work by taking the raw satellite phase information
from two separate GPS systems and crunching the data to come up with an
azimuth figure. Has anyone heard of a (Open Source?) program that could
be used to do these calculations?
AFAIK there is none out there (at least i've never seen one).
But it should be not too dificult write the software yourself.
The bigger problem is to get GPS receivers that provide you
with accurate phase information. Especially with a small baseline
you either need to sync the clocks of the two receivers or have
receivers with precise TCXO and take some additional samples to
calculate the frequency difference/drift. The precision you get
will mostly depend on the baseline and the number of samples you
use. The longer you have time to measure, the smaller your error-band
will be.
If you get a bit "closer to the metal" you could use two GPS L1 samplers
running off a common clock, and do the PN code acq and track, which
would give you carrier phase. If you do the nav solution, you know the
"look angle" to the various SVs, which would tell you the phase
differential vs azimuth.
I believe that there are open source codes out there to do the
processing. The data rate isn't all that high.. the GPS samplers are 1
bit. There's certainly lots of papers from grad students on this kind
of thing.
If you aren't concerned about acquisition time, and you can get the
ephemeris from somewhere else (so your initial guess for acquisition
isn't too far off), acquiring the signal isn't difficult.
Thanks for all the interesting responses.
Some background - I'm needing an accuracy of 1 degree or better. The
experiments are using digital communication modes and sometimes aircraft
scatter so signals are regularly inaudible and often non-existent, so
peaking "by ear" is not usually an option.
I've paced out direction using a handheld GPS (GPSMap 60CSX) and this
gives reasonable results if there's a reasonable baseline. It's a bit
impractical when operating from a firetower though!
Using Sun/Moon/Stars is difficult when there's cloud. We've tried using
Sun RF Noise, but accuracy declines significantly when the sun is high
in the sky.
VOR is an interesting suggestion, but a very sharp (and large) antenna
would be needed and multi-pathing may cause problems.
So, my interest turns back to a GPS-based solution and the military
units suggested by Brooke look perfect ... except that they are most
likely a restricted export and unavailable to us Down Under.
Other links on Brooke's site have lead me to many papers researching
GPS-based attitude systems. I note that the Uni of Calgary have
developed a package called HEADRT+ that can take raw measurements from
several GPS mounted on a small baseline and produce attitude
information. This is the sort of thing I'm after, but I get the
impression that licensing costs are high.
As Atilla says, the software is probably not that fundamentally
complicated. However, the devil is possibly in the detail of aligning
sample timing, positioning ...
Any other suggestions?
Regards,
Dave
Check out http://www.surveying.org/ to establish direction to landmarks. Place a
compass on top of the map using a graphics program that uses overlays.
Stanley
Hi Dave:
You can get the Polaris Guide (civilian version of the DAGR). A fire
tower might have a good view of the GPS satellites. To use the single
receiver North Finding function you would start at the top of the tower
and instead of walking to the second point you could slide the GPS
receiver down a sloping line to someone on the ground at a point also
with a good view of the sky. This would give you an accurate bearing
(much better than 1 degree). The problem is the Polaris Guide is pricey,
but you might be able to find a used late model PLGR or DAGR.
Have Fun,
Brooke Clarke
http://www.PRC68.com
David Smith wrote:
Thanks for all the interesting responses.
Some background - I'm needing an accuracy of 1 degree or better. The
experiments are using digital communication modes and sometimes
aircraft scatter so signals are regularly inaudible and often
non-existent, so peaking "by ear" is not usually an option.
I've paced out direction using a handheld GPS (GPSMap 60CSX) and this
gives reasonable results if there's a reasonable baseline. It's a bit
impractical when operating from a firetower though!
Using Sun/Moon/Stars is difficult when there's cloud. We've tried
using Sun RF Noise, but accuracy declines significantly when the sun
is high in the sky.
VOR is an interesting suggestion, but a very sharp (and large) antenna
would be needed and multi-pathing may cause problems.
So, my interest turns back to a GPS-based solution and the military
units suggested by Brooke look perfect ... except that they are most
likely a restricted export and unavailable to us Down Under.
Other links on Brooke's site have lead me to many papers researching
GPS-based attitude systems. I note that the Uni of Calgary have
developed a package called HEADRT+ that can take raw measurements from
several GPS mounted on a small baseline and produce attitude
information. This is the sort of thing I'm after, but I get the
impression that licensing costs are high.
As Atilla says, the software is probably not that fundamentally
complicated. However, the devil is possibly in the detail of aligning
sample timing, positioning ...
Any other suggestions?
Regards,
Dave
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To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Have Fun,
Brooke Clarke
http://www.PRC68.com
Back in the very late 80's we at Magnavox did propose a system, for the
military, utilizing 4 GPS antennas to give us azimuth, roll, and, pitch.
This other information at the time seemed important in case a soldier would
jump up or down from the HMMWV possibly loosing the satellite after initial
acquisition. Of course, stabilizer legs on the vehicle were a simpler
solution. This was to point a 45 GHz satellite antenna. The GPS antennas
were separated about 1 meter apart in more or less a square configuration. I
believe even 3 would have worked. The concept and theory looked real good on
paper, however, it was unfortunately never built due to lack of funding. I
do not recall if it was ever proprietary or otherwise, as the concept at the
time seemed fairly obvious to us. As pointed out, later solutions were
simply a flux gate compass and adequate tracking/pointing algorithms.
Magnavox had many years of experience with the flux gate compass technology
considering the millions of sonobouys that were built. Just an FYI - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ 07731
732-886-5960
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of David Smith
Sent: Thursday, August 26, 2010 10:10 AM
To: time-nuts@febo.com
Subject: Re: [time-nuts] Slightly OT - GPS-Based Accurate Direction Finding
Thanks for all the interesting responses.
Some background - I'm needing an accuracy of 1 degree or better. The
experiments are using digital communication modes and sometimes aircraft
scatter so signals are regularly inaudible and often non-existent, so
peaking "by ear" is not usually an option.
I've paced out direction using a handheld GPS (GPSMap 60CSX) and this
gives reasonable results if there's a reasonable baseline. It's a bit
impractical when operating from a firetower though!
Using Sun/Moon/Stars is difficult when there's cloud. We've tried using
Sun RF Noise, but accuracy declines significantly when the sun is high
in the sky.
VOR is an interesting suggestion, but a very sharp (and large) antenna
would be needed and multi-pathing may cause problems.
So, my interest turns back to a GPS-based solution and the military
units suggested by Brooke look perfect ... except that they are most
likely a restricted export and unavailable to us Down Under.
Other links on Brooke's site have lead me to many papers researching
GPS-based attitude systems. I note that the Uni of Calgary have
developed a package called HEADRT+ that can take raw measurements from
several GPS mounted on a small baseline and produce attitude
information. This is the sort of thing I'm after, but I get the
impression that licensing costs are high.
As Atilla says, the software is probably not that fundamentally
complicated. However, the devil is possibly in the detail of aligning
sample timing, positioning ...
Any other suggestions?
Regards,
Dave
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi Mike:
Trimble made a number of orientation GPS systems that used multiple
antennas.
The problem with a mag compass is that the Earth's mag field is stronger
in the up/down direction (in N. America) than it is horizontally. So
either the compass needs a bubble level or a 3-axis accelerometer and
correction firmware. Also they don't work well near ferrous metal.
PS the Polaris Guide has a built-in mag compass with digital readout to
1 degree.
http://www.prc68.com/I/DAGR.shtml#Comp
I find that it needs to be calibrated every time I want to use it. This
requires holding it level and turning in a circle.
A practical solution may be to get a used transit, like the Leitz 115A.
It's made of brass (as are all the transits I know of) and has a nice
mag compass. It also has an adjustable offset feature so that the
compass will read true bearings.
http://www.prc68.com/I/Leitz115A.shtml
It could be setup on the ground at a distance from the tower about equal
to the tower height. It also should be possible, by moving the transit,
to get the transit on the bearing line between the tower and the
target. This has the advantage that the person operating the transit
can also help align the antenna.
Have Fun,
Brooke Clarke
http://www.PRC68.com
Mike Feher wrote:
Back in the very late 80's we at Magnavox did propose a system, for the
military, utilizing 4 GPS antennas to give us azimuth, roll, and, pitch.
This other information at the time seemed important in case a soldier would
jump up or down from the HMMWV possibly loosing the satellite after initial
acquisition. Of course, stabilizer legs on the vehicle were a simpler
solution. This was to point a 45 GHz satellite antenna. The GPS antennas
were separated about 1 meter apart in more or less a square configuration. I
believe even 3 would have worked. The concept and theory looked real good on
paper, however, it was unfortunately never built due to lack of funding. I
do not recall if it was ever proprietary or otherwise, as the concept at the
time seemed fairly obvious to us. As pointed out, later solutions were
simply a flux gate compass and adequate tracking/pointing algorithms.
Magnavox had many years of experience with the flux gate compass technology
considering the millions of sonobouys that were built. Just an FYI - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ 07731
732-886-5960
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of David Smith
Sent: Thursday, August 26, 2010 10:10 AM
To: time-nuts@febo.com
Subject: Re: [time-nuts] Slightly OT - GPS-Based Accurate Direction Finding
Thanks for all the interesting responses.
Some background - I'm needing an accuracy of 1 degree or better. The
experiments are using digital communication modes and sometimes aircraft
scatter so signals are regularly inaudible and often non-existent, so
peaking "by ear" is not usually an option.
I've paced out direction using a handheld GPS (GPSMap 60CSX) and this
gives reasonable results if there's a reasonable baseline. It's a bit
impractical when operating from a firetower though!
Using Sun/Moon/Stars is difficult when there's cloud. We've tried using
Sun RF Noise, but accuracy declines significantly when the sun is high
in the sky.
VOR is an interesting suggestion, but a very sharp (and large) antenna
would be needed and multi-pathing may cause problems.
So, my interest turns back to a GPS-based solution and the military
units suggested by Brooke look perfect ... except that they are most
likely a restricted export and unavailable to us Down Under.
Other links on Brooke's site have lead me to many papers researching
GPS-based attitude systems. I note that the Uni of Calgary have
developed a package called HEADRT+ that can take raw measurements from
several GPS mounted on a small baseline and produce attitude
information. This is the sort of thing I'm after, but I get the
impression that licensing costs are high.
As Atilla says, the software is probably not that fundamentally
complicated. However, the devil is possibly in the detail of aligning
sample timing, positioning ...
Any other suggestions?
Regards,
Dave
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.
--
Have Fun,
Brooke Clarke
http://www.PRC68.com
On Fri, 27 Aug 2010 00:09:31 +1000
David Smith david@smithfamily.net.au wrote:
Some background - I'm needing an accuracy of 1 degree or better. The
experiments are using digital communication modes and sometimes aircraft
scatter so signals are regularly inaudible and often non-existent, so
peaking "by ear" is not usually an option.
1 Degree isnt that hard. Even a standard magnetic compas reaches that.
I've quite an old one at home with prismas for aiming with which it
is possible to get better readings than 0.5°. Of course, you'd have
to take the deviations from true north into account. But magnetic maps
should be readily available (at least they were when i was a scout).
As Atilla says, the software is probably not that fundamentally
complicated. However, the devil is possibly in the detail of aligning
sample timing, positioning ...
You don't have to allign sample timing. It is helpfull though, to get
better precision/accuracy. It is enough that you use the relative
phase differences of the SV on each GPS module. This way you can get
rid of the unknown sample phase difference of the GPS modules.
(the principle is the same like normal GPS measurement using a 4th
satelite to get rid of the unknown "time").
I just had a look at the LEA6-T protocol specs. The RXM-RAW message
provides you with a phase difference (in L1 cycles) and a frequency
offset (doppler, in Hz) and pseudorange (in meter) for each SV that
is being tracked. With this you should be able to eliminate the
phase and frequency difference/drift between the two modules.
Using a long enough measurement time, you should be able to get
to well below a degree of heading accuracy. I know that the ETH
in Zürich uses LEA6-T modules to get positioning resolutions
better than 2mm with single GPS modules, though they have to collect
more than an hour of phase data for this.
Attila Kinali
--
If you want to walk fast, walk alone.
If you want to walk far, walk together.
-- African proverb