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dGPS, etc.
A few thoughts from George Davidson of South Africa (ZA).
South Africa has an extremely low magnetic field of about 26,000 nT
which means
that most commercial mags do NOT work here because;
a) the original Geometrics global map placed us at 29,000 nT and
mags were tuned
to that; and
b) precession time is so short that after 300 mS the mags count
noise. Normally
average noise on good systems is +/- 5 nT even when running
east-west, which makes
it impossible to find a cannon that weight a ton at 10 metres (33
ft. approx).
AQUASCAN (UK) mags do work well but only after we redesigned the
timing system and
filled the probe with JET A1 as water never worked. The Jet A1 is
used for tur-
bine choppers here and is not as volatile whereas the aviation fuel
we tried pre-
vious was VERY volatile and when it leaked it dissolved glues and
silicone seal-
ants. [Jet A1 is the same as Jet A except for the addition of low
temperature
additives. When discussing the various jet fuels all values are
NOMINAL unless
you are dealing with a SPECIFIC fuel from a SPECIFIC manufacturer.
All are
volatile although some are less volatile than others. Generally
speaking do NOT
consider jet fuels unless you really know what you are doing. Do
NOT even look at
JP-4 because it is the most dangerous of the bunch and in fact is
being phased out
of use in the U.S. Air Force, I assume at least partially for that
reason. - das]
I use SCHOTT laboratory glass bottles to hold the Jet A1 and then
slide it into
the solenoid, which in turn is slid into a PVC pipe and re-entrable
dual component
resin is poured in. Unfortunately this won't help for toroidal
designs. Make
sure that the resins used are pretested for any ferrous components
because one I
used once WAS magnetic. I think Scotch [This might be 3-M in North
America. -
das] makes proper resins but we use local stuff as it is cheaper.
[George - Can you explain "re-enterable dual component resin" as I
do not think we
use that term in North America? It sounds like you can reuse it and
maybe it is
not so hard to unstick when you have to change fluid? - das]
[George - I wonder what method you use to pretest!!! - das]
Toroids do not seem to work very well here either, perhaps the angle
of inclina-
tion is too low. I am speak here of "round" toroid designs as
generally discussed
on this forum. The Geometrics toroid works well because its design
is an elon-
gated toroid of 15 milliHenry. Unfortunately it pulls 5 amps at 24
volts which
flattens most batteries, that you can carry on an inflatable, in
less than a work
day. [George - I think I disagree, but read Jim Koehler's section
on batteries
and then let me have your thoughts, please. - das]
The AQUASCAN mag takes in GPS Lat/Lon data and echo sounder depth
readings so that
one can download the data to a personal computer and draw 3-D
magnetic maps as a
post processing exercise.
We us dGPS [differential GPS] with a base station which averages its
position over
a 3 hour period and then knows exactly where it is. The GPS data is
exchanged be-
tween base and portable via a modem and VHF link using a sequence
called RTCM that
contains (I assume) the corrective data for the portable GPS at its
current loca-
tion after the base has received the current data from the portable.
So far,
NO-ONE will tell me how to generate an RTCM sequence as it is
apparently propri-
etary secret. With this method we get 5 to 10 meter accuracy.
I have tried running 2 GPS units simultaneously and feeding BOTH
outputs into the
COM ports of a personal computer, and subtracting the difference but
the error is
as bad as a single unit, even with two IDENTICAL units. The GPS
signal tends to
wander around an oval with Lat/Lon doing sine waves of their own
frequency carving
out a Lissajous figure which crosses true position from time to
time. [I have had
other reports stating that repeatability of standard units in
northern Europe
sometimes has deviations of as much as 700 meters. - Data from
North American
locations in respect to the degree of repeatability would be greatly
appreciated.
das]
However for ROUGH magging we have gotten excellent results from the
model of the
GARMIN which has software which corrects (in a way) the position,
especially if
you are doing long, straight lines. This software result is
predicated on the as-
sumption that you CANNOT suddenly be 10 meters to starboard or port
of your previ-
ously indicated position.
A few thoughts from George Davidson of South Africa (ZA).
South Africa has an extremely low magnetic field of about 26,000 nT
which means
that most commercial mags do NOT work here because;
a) the original Geometrics global map placed us at 29,000 nT and
mags were tuned
to that; and
b) precession time is so short that after 300 mS the mags count
noise. Normally
average noise on good systems is +/- 5 nT even when running
east-west, which makes
it impossible to find a cannon that weight a ton at 10 metres (33
ft. approx).
AQUASCAN (UK) mags do work well but only after we redesigned the
timing system and
filled the probe with JET A1 as water never worked. The Jet A1 is
used for tur-
bine choppers here and is not as volatile whereas the aviation fuel
we tried pre-
vious was VERY volatile and when it leaked it dissolved glues and
silicone seal-
ants. [Jet A1 is the same as Jet A except for the addition of low
temperature
additives. When discussing the various jet fuels all values are
NOMINAL unless
you are dealing with a SPECIFIC fuel from a SPECIFIC manufacturer.
All are
volatile although some are less volatile than others. Generally
speaking do NOT
consider jet fuels unless you really know what you are doing. Do
NOT even look at
JP-4 because it is the most dangerous of the bunch and in fact is
being phased out
of use in the U.S. Air Force, I assume at least partially for that
reason. - das]
I use SCHOTT laboratory glass bottles to hold the Jet A1 and then
slide it into
the solenoid, which in turn is slid into a PVC pipe and re-entrable
dual component
resin is poured in. Unfortunately this won't help for toroidal
designs. Make
sure that the resins used are pretested for any ferrous components
because one I
used once WAS magnetic. I think Scotch [This might be 3-M in North
America. -
das] makes proper resins but we use local stuff as it is cheaper.
[George - Can you explain "re-enterable dual component resin" as I
do not think we
use that term in North America? It sounds like you can reuse it and
maybe it is
not so hard to unstick when you have to change fluid? - das]
[George - I wonder what method you use to pretest!!! - das]
Toroids do not seem to work very well here either, perhaps the angle
of inclina-
tion is too low. I am speak here of "round" toroid designs as
generally discussed
on this forum. The Geometrics toroid works well because its design
is an elon-
gated toroid of 15 milliHenry. Unfortunately it pulls 5 amps at 24
volts which
flattens most batteries, that you can carry on an inflatable, in
less than a work
day. [George - I think I disagree, but read Jim Koehler's section
on batteries
and then let me have your thoughts, please. - das]
The AQUASCAN mag takes in GPS Lat/Lon data and echo sounder depth
readings so that
one can download the data to a personal computer and draw 3-D
magnetic maps as a
post processing exercise.
We us dGPS [differential GPS] with a base station which averages its
position over
a 3 hour period and then knows exactly where it is. The GPS data is
exchanged be-
tween base and portable via a modem and VHF link using a sequence
called RTCM that
contains (I assume) the corrective data for the portable GPS at its
current loca-
tion after the base has received the current data from the portable.
So far,
NO-ONE will tell me how to generate an RTCM sequence as it is
apparently propri-
etary secret. With this method we get 5 to 10 meter accuracy.
I have tried running 2 GPS units simultaneously and feeding BOTH
outputs into the
COM ports of a personal computer, and subtracting the difference but
the error is
as bad as a single unit, even with two IDENTICAL units. The GPS
signal tends to
wander around an oval with Lat/Lon doing sine waves of their own
frequency carving
out a Lissajous figure which crosses true position from time to
time. [I have had
other reports stating that repeatability of standard units in
northern Europe
sometimes has deviations of as much as 700 meters. - Data from
North American
locations in respect to the degree of repeatability would be greatly
appreciated.
das]
However for ROUGH magging we have gotten excellent results from the
model of the
GARMIN which has software which corrects (in a way) the position,
especially if
you are doing long, straight lines. This software result is
predicated on the as-
sumption that you CANNOT suddenly be 10 meters to starboard or port
of your previ-
ously indicated position.