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: [email protected] [mailto:[email protected]] On
Behalf Of David Smith
Sent: Thursday, August 26, 2010 10:10 AM
To: [email protected]
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
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--
Have Fun,
Brooke Clarke
http://www.PRC68.com
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