On Nov 27, 5:44*am, Jeff Liebermann wrote:
On Fri, 27 Nov 2009 01:58:09 -0800 (PST), "

wrote:
The "virtual" compass requires you to move since it bases direction on
differential GPS readings. OK in a car, semi-OK on foot, worthless if
the going is slow since the delta of distance is not significant
enough to get a good calculation.

Actually, it's not too horrible for determining the general direction
of travel. *However, totally useless for determining the azimuth of
something from a fixed location.

The newer GPSs have sensors in them. To null out the nearby magnetic
field, you need to slow spin it around. Two revolutions generally. Or
you can spin your body around and look like an idiot. The trouble is
the GPS has to be level unless you have a 3-D compass. [Garmin
doesn't, Magellan does. Too many complaints about Magellan gear, so
you just put up with Garmin.] The GPS has a threshold where it will
shift from magnetic sensor to virtual compass. You can really screw
yourself up if you don't know about this.

I spent some time trying to get sane readings from the fluxgate
"magnetic" compass in a borrowed Garmin GPS. *The rotation method
worked quite well, until I moved. *When standing next to a vehicle, it
was nearly useless.

I wish I could turn the damn sensor off at times.


Your technique is fine if you spot from the same location. If you are
on the move, you realy do need a compass.

Ummm.... orbital satellite spotting from a moving vehicle is rather
uncommon and dangerous.

I was speaking in the general sense of navigation.

Occasionally I'll see
something off in the distance.[OK, OK, in *denied access area.]

Sigh. *I suppose the alien technology that they're hiding is better
than a compass.

Anyone that tracks my posts knows I sniff around Groom Lake. Knowing
the terrain makes it easy to know where to point the binocs or
telescope.


Log
your position, take a vector, then study it on google earth or a map.
Other times I am DFing radio signals. Again, the compass does the log.
Log everything in magnetic, then use your mag in magnetic, and you
won't go crazy. Attempt to correct your readings and you are either
correct or you just added twice the adjustment factor to your reading.
Really not a good idea.

Ok. *I'll admit to having added my declination instead subtracted more
than once. *Still, some additional practice and a few sanity checks
against known locations should help.

I've concluded I'm certifiable! ;-)

Aviation long ago decided on doing everything magnetic, probably to
get around the confusion.

Mostly true. *All US runways are magnetic. *So is the VOR. *However,
all sectional maps are based on true north. *GPS navigation devices
can be either way. *Magnetic is a big help when using a magnetic
compass for finding the runway. *

Canadian aviation is really strange. *The northern half uses true
north, while the southern half uses magnetic. *

http://en.wikipedia.org/wiki/Magnetic_declination

Of course, they still use AGL (above ground
level) and MSL (mean sea level) for altitude. You may recall either a
Thunderbird or Blue Angel getting into trouble with this.

Dunno. *However, I've screwed up a few HAAT calculations on FCC
license applications when I was doing them with just a calculator:
http://en.wikipedia.org/wiki/Height_above_average_terrain

I use your spotting technique at times too. It depends on if I can do
the preparation. Your scheme works better in mountain areas, which is
where I use it since I know the various peaks and where the campsite
is located.

It works with any decent road map. *You don't even need to sight the
distant mountain peaks. *Just align the map with the local roadways
and it's already better than a magnetic compass. *The most difficult
part is finding a large flat area on which to place the map. *When
desperate, I use corrugated box cardboard and pins for markers.

Incidentally, I have a crude system of doing TDOA (time difference of
arrival) hyperbolic RDF location on a paper map using two nails and
some string. *I'm slowly scribbling a web page on the subject, so no
details until I'm done.

Those old schemes where you quickly switch between antennas work well
for voice signals. I've found data channels don't do so well. I was
told that it has to do with how the data channels are nearly always at
full modulation. One of these days I'm going to hack two radios so
they share one local oscillator and see if I can DF based on phase
comparison of the last IF output.

Oh, if you want high tech location and mapping assistance, there's GPS
augmented reality.
http://www.youtube.com/watch?v=4Fl718QO_xQ

--
Jeff Liebermann * *
150 Felker St #D * *http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann * * AE6KS * *831-336-2558

On Fri, 27 Nov 2009 11:20:48 -0800 (PST), "
wrote:

Those old schemes where you quickly switch between antennas work well
for voice signals. I've found data channels don't do so well.

A "Homer" works fine for continuous uni-directional data. It doesn't
work well for half duplex such as Wi-Fi. They're really awful for
situations where you hear multiple xmitters with the added bonus of
collisions (i.e. packet data). There are workarounds, but all require
decoding the data first and using a computah to keep track of which
xmitter you're hearing.

I was
told that it has to do with how the data channels are nearly always at
full modulation. One of these days I'm going to hack two radios so
they share one local oscillator and see if I can DF based on phase
comparison of the last IF output.

It will work with one big catch. You'll have to replace the IF
crystal filters with one that has a fairly stable group delay across
the IF bandwidth. If you're working with data that has modulation
components all the way to the edge of the IF filter, you're going to
go through huge variations in phase shift between the carrier and the
filter skirts. Building two matched receivers just adds to the
complexity. OAR (Ocean Applied Research, now part of Cubic) once made
an Adcock antenna DF system that had 3 receivers for measuring the
phase and displaying the direction on a scope.
http://www.cubic.com/cda1/Prod_&_Serv/C4ISR_Prod_&_Sys/DF_Products/df_processors.html
http://www.cubic.com/cda1/pdf/aa1319.pdf

Personally, I think you would do better with multiple remote receivers
and measuring the TDOA (time difference of arrival) at each receiver.
Each burst of RF is time coded and packaged along with signal
strength, GPS phase, and decoded data, and sent off to a central
computah for processing. The time codes are critical as that allows
storage, replay, and post processing. Spewing RF around Area 51 is
probably a bad idea, so post processing make sense. Collect the time
coded data on thumb drives, stuff the data into a computah on your way
out, compute, and replay the captured data later.

Drivel: If you're thinking of doing it with a Doppler ADF, then
please read my previous rants before blundering onward:
http://www.qsl.net/n9zia/doppler_notes1.txt
http://www.qsl.net/n9zia/doppler_notes2.txt
http://802.11junk.com/jeffl/AN-SRD-22/
Those are 15 year old postings about a 35 year old product, but it's
still generally accurate.

Did you ever build and try the 1090 Mhz AMOS/Franklin antenna?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558