This is a small segment of the "Active Nodes" printout showing Nodes 2 to 100 are running. I use this to see who is running and on-time and who might be shut down or out of service for some reason.  Note a new column called "Avg. Ambient mGa". 

Node Id Event Type Compass (deg) mGa Avg. Ambient mGa Pressure (in/Hg) Date Time
2 status 305 1 0 28.42 2018-11-27 21:36:35
4 alert 358 564 0 28.98 2018-11-27 21:36:12
6 status 268 8 0 29.69 2018-11-27 21:36:55
7 alert 81 1440 0 29.13 2018-11-27 21:36:25
9 status 86 2 10049.9 28.43 2018-11-27 21:36:56
100 status 128 1 0 29.18 2018-11-27 21:36:40

The ambient reading is only valid for nodes running the new software which hopefully will be all of them once the issues are worked out.  Any node running the old software will only show 0.  The Ambient reading as been part of the node software for the last 2 version at least.  It is used as the average normal reading of the background magnetic field. As you can see above, Node 9 is running the new software.

The row in the table labeled mGa is the result of subtracting the current field reading from the average ambient reading (unsigned number).  This should normally be a low number if no disturbances are in the area.  In other words the mGa will indicate the variance from what is normal for that node and location.  If the mGa jumps beyond the trigger setting (deviation), this indicates an anomaly.  The mGa reading is NOT the current magnetic field reading.  It ONLY indicates the variance in milligauss from the norm.  Everybody's "normal" will be different depending on their location and background noise.  While the old nodes currently calculate and use the ambient background reading they do not transmit this value to the server.  The new software will.  This was needed, our chief software consultant believes, to help verify that the node is setup in a quiet location and the sensor is working properly.

That's why if you take an E-M meter to measure the area right near your node, it won't match the reading you see on the pop-up or the spreadsheet. And it is called "Average Ambient mGa"

If you look at a normal spreadsheet and an AlertStart line, the line below it will be a status line and it will give numbers right before the alert, and these are what I call ambient or background readings. Not to be confused with our topic of discussion here.

Fran Ridge
MADAR Director