ISEE magnetometer (Some Notes and Errors)


                   

***** Error History *****

1. Magadan induction magnetometer has a GPS problem from Sept.22, 2009. 
The 64Hz sampling may not be corrected by GPS signal.  The PC clock 
is corrected by GPS.  So timing accuracy would be about 1-second. 
This problem was fixed by a new GPS receiver on Jan.12, 2010. 

2. Fluxgate magnetometer at Moshiri was damaged by lightening from 
June 2009.  A new fluxgate magnetometer was installed at Moshiri on Oct. 15, 
2009.  

3. VLF antenna construction occurs near Kagoshima fluxgate magnetometer at September 28 
- November 2009.  The data may become noisy sometimes due to the construction. 

4. Time tag of Moshiri induction magnetometer occasionally become unstable and 
   shift maximum 10-100s for the data before October 26, 2009. (See (b) below). 

5. A 4-28MHz transmitter was working at Magadan for Jan.12-Mar.7, 2010, 
   Jan.13-Feb.29, 2012, and May 2-31, 2012.  This causes pulse noise 
   with an interval of 5-min on the induction magnetometer data.

6. Sept.7, 2009-June 2 0110UT, 2010: RIK  Time tag data was shifted 
   a few min to 2-3 hours due to the lack of clock correction.

7. April 1-30, 2010: PTK,  Time tag is somewhat irregal??

8. July 13-Sept.14, 2011: MSR, GPS time correction did not work on the 
   PC for the induction magnetometer.  The time was shifted 9 second 
   when we fixed this problem on Sept. 14, 2011.  

9. Moshiri: Sinusoidal noise with a period of tens of seconds sometimes 
   appears in all three components after January 2010.  

10. Moshiri: Z-component of fluxgate magnetometer data shows strange 
   offset variations, possibly because offset adjustment circuit is 
   not correctly working, noticed on May 30, 2013. 

11. At Athabasca, we noticed that the windows clock run without 
   GPS clock and network clock.  It was 3.5 sec different from 
   correct clock at Sept.12, 2013, and corrected using the network clock. 
   This may be due to windows did not refer to network clock since 
   July 2013.  The GPS clock could not generate time correction signal 
   since September 2012.   Thus we set the windows to refer to the 
   network clock since then.    The GPS clock was replaced to a new 
   GPS clock at 19 UT on November 19, 2013. 


(a) Caution about the time tag of 64Hz sampled digital data 
    (Oct. 12, 2009  K. Shiokawa) 

The recording of the induction magnetometer data is done by a Windows PC, 
which has a clock with a unit of 1-second.  The PC clock is corrected 
through a network or GPS every 10 minutes.  The PC clock runs freely during 
the 10 mintues.  So if the PC clock proceeds/delays for more than 0.5 
seconds in that 10 minutes (which usually do no happen), the time tag may 
become wrong with a multiple of 1 second. 

The sampling of the magnetometer data is done by using 64Hz trigger signal 
generated by a GPS receiver.  If the GPS receiver fails to receive the radio 
wave from the GPS satellite, it runs freely by an internal clock.  
But usually the receiver receives GPS radio wave.  

The PC recording is first into a PC memory and then written into a 
hard disk every 4 seconds.  Since the sampling is not by the PC clock 
but by the GPS clock, the start of the time tag cannot be exactly 
at 00h00m00sUT.  

In the 4th channel, the 1PPS signal from GPS is recorded as a time standard. 
If the timing accuracy better than 1-sec is needed, this GPS 1PPS pulse 
should be used to correct the time tag of the data file. 



(b)  Further Caution about the timing error of 64Hz sampled data
     (Oct.26, 2009   K. Shiokawa) 

On Oct.26, 2009, we found a problem in the Windows PC clock at Moshiri. 
The Windows PCs automatically correct their clock to Windows time server every 7 days.
We found that when this correction by the Windows finish successfully, the 
Windows clock becomes unstable and start to shift about 10ms per 4 seconds.   
(about several tens/hundreds seconds per day).   Usually we turn off 
this automatic correction (click the clock of the windows and see the 
internet time option).  But at Moshiri, this was ON by some unknown reason.  

Once the clock becomes unstable, it lasts until we restart the PC. 
Since the Windows clock is corrected by GPS clock every 10 min via 
RS-232C, the time tag of the file may not shift much.  But if GPS 
satellites was not available for a certain time, the time tag error 
becomes large because of the unstable clock.  The 64Hz sampling 
and the GPS 1PPS signal at the 4th channel 
should be correct.   You may find the occation when the GPS 
correction was made on the clock, by checking a sudden jump 
(more than 2 seconds) of 4-sec time tag in the file.

This error would not happen at Magadan, since the PC is not connected 
to the network.  At Sata, the network condition is not good, so this error 
would occur less frequently.  For Paratunka, we confirmed that the 
automatic correction of Windows clock is off (Oct.29, 2009).  So this 
error would not happen. 

Please be careful if you want to compare the data with accuracy of 
less than 10-100 sec.  




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