When using a GPS for primary reference point:
Measuring from the antenna you enter the Y distance in meters to the back of the
boat, plus the amount of cable out. That becomes the reference for the shifted point.
Note that in MagMap2000 and in the Survey Wizard what I am about to describe to
you is slightly different in that the “tow point” (point where the cable leaves the back
of the boat) is used as the reference point. This point would be shifted in both the x
(lateral shift which we will use later in this discussion as 5 meters to the right of the
antenna) and the Y longitudinal shift. In the following calculations, the GPS antenna
is used as the primary reference point in calculating the positions of each
magnetometer, doing the interpolation in real time and then storing the data in the
Interpolator file as latitude longitude for each “shifted” point.
So once you have the magnetometers listed in the Configure Devices (concatenated
together on the wing, one serial line coming to the computer, 5 magnetometers’ data
concatenated into a single string), then go to Output Devices and select Interpolator.
You will see this menu:
Click the button that says “Add/remove Interpolator device” it must be checked. Let
us say that from the GPS to the back of the boat is 10 m and from the back of the boat
to the array is 60 m, then put 70m in the “magnetometer array offset”. Do not use the
Tow Point/GPS offsets dialog because in order to use this, you need a very accurate
gyrocompass to get the ship’s orientation compared to the direction of travel. To
proceed, next click on Positions and Add…
You will see a dialog box that opens for mag#1. Under TYPE you will see three
options, GPS (used for rigid GPS to sensor connections like on a land cart), Shifted
(correct for this purpose because we are adding “shifted” magnetometer positions
from the longitudinal line from the GPS antenna extending backwards) and Target
which has to do with using a Track Point II set up under TRK. We are not using this
and are going to either use a real GPS or recalculated positions using your USBL.
This discussion will come later, for now are are using an actual GPS antenna mounted
on the boat and an array some meters behind the boat.
So select Shifted.
Now as I mentioned before, the tow point on the back of the boat is shifted 5 meters
to the right of the GPS antenna. Let us say that the sensor separation on the wing is
1.5 meters such as it is on our G-882TVG. Then each magnetometer is offset 0.75
meter (left or right) from where the tow cable enters the array. But the actual
reference point is offset 5 meters to the left because we did not have the opportunity
to enter the lateral offset of the GPS antenna to the towpoint earlier. We will enter it
now.
From the reference point we must move 4.25 meters to the right which means +90
degrees! So we add 4.25m at 90 degrees and say ok. Now we add another shifted
point only this time it is shifted 5m +0.75 m or 5.75 m at 90 degree so it would look
like this.
Let us say that in your particular case, you too have a horizontal (lateral) offset of the
GPS antenna from the tow point of 5m. Then since your sensors are separated by
0.25 m, mag #1 will have a shifted entry of zero offset (any angle, doesn’t matter),
Mag #2 will have a shifted entry of 2.5m and 90 degrees, etc like this:
Then in the recorded interpolator file, each line of data at 10 Hz will have an entry of
lat long for each shifted magnetometer including the Mag #1 with no shift. These
positions are calculated on the fly using the MagLog dragging algorithm.
When using a USBL as pseudo-GPS position:
Now if you are going to substitute the USBL target positions for the GPS positions,
then you can assume that you will have RIGID relationship between the sensors and
the “GPS Antenna” which is really the USBL target. So in this case you will use GPS
as the reference point.
For simplicity sake, let us say that the USBL target is 5 meters ahead of the array.
Then the first GPS Position (referenced from the USBL target) is (root 5^2+5^2 =
root 50) 7.07 meters at an angle of 225 degrees. Do you see this? The second sensor
is 5.59m at an angle of 206.56 (inv tangent of 0.5 (or 26.56) + 180). Sensor 3 will be
at 5m at 180 degrees, sensor 4 will be 5.59m at 153.44 degrees and sensor 5 will be at
7.07 m at an angle of 135 degrees.
One very important thing to remember: do not assume that the sensors are numbered
right to left or left to right as #1, #2, #3 etc. This depends on how you set up the
devices and how they are wired in the concatenation box! You should actually set up
a graphic display in MagLog so that you can show each magnetometer trace
operating, and then take a small magnetic object and wave it next to each mag so that
you can absolutely determine the “number” of the magnetometer.
One other point, you may have to set up the magnetometers in XS3 transmission
mode depending on the speed of the operation and the number of magnetometers.
They should all be able to come up the com port at 19.2KBaud but if, because of very
long cables, you need to slow down the transmission you may need to switch to
compressed binary XS3. MagLog decompresses XS3 on the fly so it would be
transparent to you.
There is more information in the MagLog manual chapter 17, pages 169-202